Spatial Analysis in Karst Geomorphology: An Example from Krk Island, Croatia [1st ed.] 9783030614485, 9783030614492

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Springer Theses Recognizing Outstanding Ph.D. Research

Ela Šegina

Spatial Analysis in Karst Geomorphology: An Example from Krk Island, Croatia

Springer Theses Recognizing Outstanding Ph.D. Research

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Ela Šegina

Spatial Analysis in Karst Geomorphology: An Example from Krk Island, Croatia Doctoral Thesis accepted by Graduate School, University of Nova Gorica, Nova Gorica, Slovenia

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Author Dr. Ela Šegina Geological Information Centre Geological Survey of Slovenia Ljubljana, Slovenia

Supervisors Prof. Dr. Martin Knez Karst Research Institute Research Centre of the Slovenian Academy of Sciences and Arts Postojna, Slovenia University of Nova Gorica Nova Gorica, Slovenia UNESCO Chair on Karst Education Vipava, Slovenia International Joint Research Center for Karstology Yunnan University Kunming, China Prof. Emerit. Dr. Čedomir Benac Faculty of Civil Engineering University of Rijeka Rijeka, Croatia

ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-3-030-61448-5 ISBN 978-3-030-61449-2 (eBook) https://doi.org/10.1007/978-3-030-61449-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Supervisors’ Foreword

Karst geomorphology as a science of the surface, and speleology as a science of the underground, has developed rather independently as individual spheres driven by different mechanisms and characterized by different features. The concept of surface denudation was the breakthrough that induced the integration of underground features onto the karst surface. Only then, the essence of the karst system’s three dimensionality has been fully adopted, bringing numerous new insights into the concepts deeply rooted in the traditional karst geomorphology. This thesis is a comprehensive research of karst surface founded on such a new perception of a karst system. It is based on a large data set of the variety of karst surface features acquired remotely and supported by the extensive fieldwork. The research employs contemporary GIS techniques and modern approaches in spatial analysis, yet it is aware of their limitations. Karst surface features had been discussed in the context of local environmental settings, comprising the detailed overview on geological, geomorphological, hydrological, speleological and climatological data of the study site. Morphometric and distributive analyses have served as a tool for classification of surface features, some of them being fully discussed or even recognized for the first time. This study is the first comprehensive, yet detailed investigation of the karst surface of Krk Island in Croatia. It gives insights into the local karst surface features, processes and overall evolution of the karst surface in the study area. More importantly, it presents a methodological example of the holistic approach in karst geomorphology that can be adopted in the research of any karst landscape. Finally, the most valuable outcome of this thesis that concerns karst geomorphology on a global scale is the discussion on the principles valid in modern research, as well as a presentation of yet undefined karst surface features. Hruševo, Slovenia Rijeka, Croatia September 2020

Prof. Dr. Martin Knez Prof. Emerit. Dr. Čedomir Benac

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Abstract

The intriguing spatial variability of surface features on Krk Island has stimulated the research of this karst area located in the coastal zone of the Dinaric karst in Croatia. Field inspection, orthorectified aerial photos (0.5 m resolution) and a topographic map (1:5000) were used for the detection and delineation of karst surface features appearing on the island with the area of 405.5 km2. This method resulted in the identification of several yet undefined types of surface features occurring on karst, requiring the revision of the existing classification and re-establishment of a new classification system compatible with the particular field reality. Several morphologic and distributive parameters that had been calculated for each reclassified type of surface feature provided insight into the surface features elementary characteristics, their spatial variability and the correlation to the other types of surface features and to the recent karst relief. This analysis based on a large, accurate data set, contributed to the general knowledge on karst surface features, the conditions of surface features in Dinaric karst and to the understanding of the karst surface evolution on Krk Island. Keywords Karst geomorphology Dinaric karst Adriatic Sea





GIS
Spatial analysis
Krk Island


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Parts of this thesis have been published in the following journal articles: • ŠEGINA, Ela, BENAC, Čedomir, ŠUŠTERŠIČ, France, KNEZ, Martin, ČAR, Jože (in press). Some aspects on the formation of linear features on karst surface, an example from Krk Island. Geologia Croatica. • ŠEGINA, Ela, BENAC, Čedomir, RUBINIĆ, Josip, KNEZ, Martin. Morphometric analyses of dolines: the problem of delineation and calculation of basic parameters. Acta Carsologica, 2018, 47/1, 23–33. https://doi.org/10.3986/ ac.v47i1.4941 • ČERU, Teja, ŠEGINA, Ela, KNEZ, Martin, BENAC, Čedomir, GOSAR, Andrej. Detecting and characterising unroofed caves by ground penetrating radar. Geomorphology: an international journal of pure and applied geomorphology, 2018, 303, 524–539. https://doi.org/10.1016/j.geomorph.2017.11.004 • ČERU, Teja, ŠEGINA, Ela, GOSAR, Andrej. Geomorphological dating of pleistocene conglomerates in Central Slovenia based on spatial analyses of dolines using LiDAR and ground penetrating radar. Remote sensing, 2017, 9/12, 1–23. https://doi.org/10.3390/rs9121213 • ČERU, Teja, GOSAR, Andrej, ŠEGINA, Ela. Application of ground penetrating radar for investigating sediment-filled surface karst features (Krk Island, Croatia). In: 2017 9th International Workshop on Advanced Ground Penetrating Radar (IWAGPR), Edinburgh, UK 28–30 June 2017: proceedings. Red Hook, NY: IEEExplore. cop. 2017, 1–6. • ČERU, Teja, ŠEGINA, Ela, KNEZ, Martin, BENAC, Čedomir, GOSAR, Andrej. Možnosti in omejitve metode nizkofrekvenčnega georadarja za raziskavo kraških pojavov - primer meritev na otoku Krku. In: ROŽIČ, Boštjan (ed.). Treatises, reports, 22nd Meeting of Slovenian Geologists, Ljubljana, November 2015, (Geološki zbornik, 23). Ljubljana: Univerza v Ljubljani, Naravoslovnotehniška fakulteta, Oddelek za geologijo, 2015, 30–34. • ČERU, Teja, ŠEGINA, Ela, KNEZ, Martin, BENAC, Čedomir, DOLENEC, Matej, GOSAR, Andrej. Application of ground penetrating radar for unroofed caves detection. In: NOVAK, Matevž (ed.), RMAN, Nina (ed.). Book of abstracts, 5th Slovenian geological congress, Velenje, 3–5. 10. Ljubljana: Geološki zavod Slovenije. 2018, 37. • ŠEGINA, Ela, BENAC, Čedomir, KNEZ, Martin. Fluviokarst forms: examples from the island Krk (Croatia). In: ROŽIČ, Boštjan (ed.), VERBOVŠEK, Timotej (ed.), VRABEC, Mirijam (ed.). Abstracts and field trips, 4th Slovenian geological congress, Ankaran, 8–10. October. Ljubljana: Naravoslovnotehniška fakulteta. 2014, 67–68.

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Acknowledgements

Many people contributed to this dissertation, and I wish to thank to: Martin Knez and Čedomir Benac for the leadership, pleasant company, and moral and technical support; France Šušteršič and Josip Rubinić for unofficial leadership and Jože Čar for the pleasant correspondence and fruitful discussions. Čedomir Benac, Teja Čeru, Mladen Juračić, Martin Knez, Nina Peca, Andrej Pipan, Alain Piquerez, Josip Rubinić, Uroš Stepišnik, Miomira Šegina, Mitja Šegina, Matej Šircelj, France Šušteršič, Martina Tekavec, Nadan Tudor and Timotej Verbovšek for sustained company on the field trips. Marko Budić, Marjeta Car, Jože Čar, Nadia Dunato Pejnović, Goran Durn, Lidija Galović, Silvio Giorgolo, Bojana Horvat, Neven Hržić, Stjepan Husnjak, Mladen Juračić, Blaž Komac, Tvrtko Korbar, Ivana Lončarić Trinajstić, Ljerka Marjanac, Čedomir Miler, Maja Oštrić, Nina Peca, Lucijan Plevnik, Alain Piquerez, Boštjan Rožič, Josip Rubinić, Andrija Rubinić, Igor Ružić, Ugo Sauro, Tadej Slabe, Uroš Stepišnik, Tilen Šetina, Tomaž Verbič, Timotej Verbovšek, Maja Vrčkovnik and Paul Williams for the discussion and accessibility to different materials. Matej Dolenec, Urša Klun, Nina Zupančič and Helena Grčman for sedimentological analysis; Mateja Zadel for the assistance in the Chemical Analytical Laboratory and Irena Trebušak for the lectureship. Lidija and Mladen Kopasić, Željka Dobrinčić, Ivana Lončarić Trinajstić, Hajrudin Mulaosmanović, Darko Volarić, and Rada and Čedomir Benac for pleasant staying on Krk Island. My dears Miomira and Mitja Šegina, and Andrej Pipan for their precious support. Special thanks to Teja Čeru and France Šušteršič for being my fellows during the entire project. Thanks to France Šušteršič for his enthusiasm, his doubts and answers, a detailed review of the text and the insight to his unpublished materials. The research was funded by the Fund of donors for postgraduate studies in Mathematics and Science at the Slovenian Academy of Science and Arts, the Slovene Human Resources and Scholarship Fund, the University of Nova Gorica and the Municipality of Domžale.

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Introduction

The karst surface duality on Krk Island was first exposed in the study published by Benac et al. (2013). They noticed the exclusive occurrence of karst depressions in one and abandoned surface streams in the other karst areas. This peculiar spatial heterogeneity of surface features was the starting point for the present research. The main idea was to implement spatial analysis accessible by the modern computer-based programs to the entire karst surface to reveal the variability of karstification conditions, processes or mechanisms that may have contributed to the geomorphic heterogeneity. Even though the approach seemed straightforward, three main problems arose: (i) lack of clear definitions of karst surface features, (ii) unknown reliability of detecting and delineating karst surface features and (iii) presence of surface features with linear geometry in relatively pure karst conditions. Wandering around the field, practical questions such as: is this topographically unclosed depression also a doline? or: is the edge of this depression here or there? or: how to classify this linear depression? revealed numerous inconsequentialities of theoretical background in karst geomorphology, as well as the deficiencies of methodological approaches, applied so far. The critical use of spatial analysis and the importance of the quality and consistency of input data are stressed out in the first part of this study. Here, the methodological deficiencies in obtaining and processing spatial data in karst geomorphology are discussed, and several new approaches are introduced to overcome these obstacles. An overview of regional settings of the study area, enriched by the supplementary investigations that filled up numerous gaps in so far existing knowledge on Krk Island, is presented in Chap. 1 as well. Chapter 2 is dedicated to the theoretical background of karst surface geomorphology; to the discussion on reliability, stability and exactness of starting points that are valid in the modern karst research. Rather than searching for surface features that would satisfactorily fit into the traditional classifications, I created a suitable classification after identifying all the existing varieties of surface features.

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Introduction

Based on current knowledge, induced by the new opportunities that modern high-resolution technology provides, and inspired by the discussions with Prof. dr. France Šušteršič, I created a new classification of karst surface features. I discussed and placed in the existent karst context the overlooked and up to now barely discussed linear surface features occurring in relatively pure karst conditions. By fieldwork and examination of several data sources as a topographic map, digital orthophotography and digital terrain model, a great number of surface features have been noted: some were clearly defined; some were classified based on the similarities to the proved features, and some remained noticed but undefined. The current karstological knowledge cannot explain their appearance, yet their existence cannot be denied. Such are large-scale karst surface features that were detected by the manipulation of the 3-D spatial data and up to now remained unnoticed in the global karst research. The results of spatial analysis, including numerous morphologic and distributive parameters calculated for both circular and linear surface features of all dimensions, are presented in Chap. 3. Spatial data were processed by the existent algorithms built in the ArcGIS Desktop 10.2 software. In Chap. 4 of the present thesis, the results of spatial analysis are interpreted and put in the spatial and temporal context of the study site, contributing to the understanding of karst evolution on Krk Island and in wider Dinaric area. The intriguing topics that were revealed during the research are discussed in Chap. 5 and the last chapter of this study. They offer numerous starting points for further research. The main prospects of this study are: (i) to recognize morphologic and distributive characteristics of surface features on the study site, (ii) to understand the nature of their spatial variability, (iii) to reveal the conditions, processes and mechanisms that may have induced such spatial variability and (iv) to contribute to the understanding of the evolution of karst on Krk Island.

Contents

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1 Study Area and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Natural Characteristics of Krk Island . . . . . . . . . . . . . . . . . 1.1.1 Lithostratigraphy of Bedrock . . . . . . . . . . . . . . . . . 1.1.2 Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.3 Unconsolidated Quaternary Sediments . . . . . . . . . . 1.1.4 Present Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.5 Hydrology and Hydrogeology . . . . . . . . . . . . . . . . 1.1.6 Degree of Karstification . . . . . . . . . . . . . . . . . . . . . 1.1.7 Human Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Climate Changes and Sea-Level Oscillations in Pleistocene and Holocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Detecting Karst Features . . . . . . . . . . . . . . . . . . . . 1.4.2 Delineating Karst Features . . . . . . . . . . . . . . . . . . . 1.4.3 Spatial Analysis and the Employed Parameters . . . . 1.4.4 Supplementary Methods . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Theoretical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Karst System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Definition and Classification of Karst Surface Features . . 2.2.1 Towards the Classification of Circular Karst Depressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Circular Karst Depressions on Krk Island . . . . . . 2.2.3 Linear Karst Depressions . . . . . . . . . . . . . . . . . . 2.2.4 Towards Classification of Linear Features on Krk Island . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Complete Classification of Karst Surface Features References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Spatial Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Relief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Coastline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Circular Depressions on Karst . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Simple Depressions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Depressions with Additional Mass-Removal Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Regional Negative Undulations . . . . . . . . . . . . . . . . . . . 3.4 Linear Features on Karst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Branching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Longitudinal Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.5 Spatial Correlations of Linear Features to the Lithology of Bedrock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.6 Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.7 Linear Features Orientation as an Indicator of the Regional Structural Grid . . . . . . . . . . . . . . . . . . . . . . . 3.4.8 Drainage Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Spatial Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Simple Depressions Versus Regional Undulation . . . . . . 3.5.2 Simple Depressions Versus Linear Features . . . . . . . . . . 3.5.3 Depressions with Additional Mass-Removal Mechanisms Versus Regional Undulation . . . . . . . . . . . . . . . . . . . . . 3.5.4 Regional Undulation Versus Linear Features . . . . . . . . . 3.5.5 Idealized Regional Undulation Versus Simple Depressions, Depressions With Additional MassRemoval Mechanisms and Linear Features . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Interpretation of Spatial Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Regional Undulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Overview of Calculated Parameters . . . . . . . . . . . . . . . . 4.1.2 General Trend of Spatial Variability . . . . . . . . . . . . . . . 4.2 Depressions with Additional Mass-Removal Mechanisms . . . . . 4.2.1 Overview of Calculated Parameters . . . . . . . . . . . . . . . . 4.2.2 General Trend of Spatial Variability . . . . . . . . . . . . . . . 4.2.3 Formation and Development of Karst Depressions with Additional Mass-Removal Mechanisms on the Example of a String at Location Ponikve-Kimpi-Kaštel . . . . . . . . 4.3 Simple Depressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Overview of Calculated Parameters . . . . . . . . . . . . . . . . 4.3.2 General Trend of Spatial Variability . . . . . . . . . . . . . . .

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Contents

4.4 Linear 4.4.1 4.4.2 4.4.3

Features . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of Calculated Parameters . . . . . . General Trend of Spatial Variability . . . . . Conditions that Induced the Appearance of Features on a Karst Surface . . . . . . . . . . . 4.5 Geomorphic Evolution of Krk Island . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Linear . . . . . . . . . . . . 160 . . . . . . . . . . . . 167 . . . . . . . . . . . . 168

5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 The Existence of Regional Undulations . . . . . . . . . . . . . . . 5.2 The Spatial Variability of Simple Depressions Morphologic and Distributive Characteristics . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Chapter 1

Study Area and Methods

Abstract Krk Island is one of the largest Croatian islands, occupying an area of 405.5 km2 .

1.1 Natural Characteristics of Krk Island Krk Island is one of the largest Croatian islands, occupying an area of 405.5 km2 . It is located in the northern Adriatic Sea between the Istria peninsula and the Vinodol coast, and together with the neighbouring islands of Cres, Lošinj, Rab, Pag and several other small islands constitutes the Kvarner area (Fig. 1.1).

1.1.1 Lithostratigraphy of Bedrock Detailed geological inventory of Krk Island was registered between 1969 and 1970 during the Yugoslavia state field survey. Krk Island is presented on three sheets of the geological map at scale 1:100,000: Crikvenica [1], Rab [2] and Labin [3]. Due to inconsistency induced by several survey campaigns of different authors, generalized and verified geological data after Veli´c and Vlahovi´c [4] and Benac et al. [5] were mostly applied in spatial analyses of this study (Fig. 1.2). Krk Island is located on over 4-km-thick carbonate bedrock [1]. The oldest exposed rocks are several hundred meters thick Lower Cretaceous (Albian) limestones and dolomites that form cores of large anticlines in the central and western parts of the island [1]. They are characterized by thin-layered mudstones, peloid packstones to grainstones, with rare occurrences of thin layers of emersion breccias [5]. They are overlaid by Albian–Cenomanian dolomites and diagenetic breccias occurring in minor outcrops [1, 3]. Most of the island is built of over 200 m tick Upper Cretaceous (Cenomanian) carbonates containing rudists and index foraminifera [6– 8]. These bright limestones, almost white in colour are often recrystallized and thickly bedded (60–120 cm), and belong to mudstones, foraminiferal bioclasts, intraclast grainstones and rudist bioclast floatstones [5]. Paleogene foraminiferal limestones © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 E. Šegina, Spatial Analysis in Karst Geomorphology: An Example from Krk Island, Croatia, Springer Theses, https://doi.org/10.1007/978-3-030-61449-2_1

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1 Study Area and Methods

Fig. 1.1 Kvarner area and location of Krk Island

and siliciclastic rocks mainly occupy the major syncline structure crossing the entire island from Omišalj to Baška. Short and narrow stretches of both units are also found related to other minor synclines, namely along the eastern and western coasts of the island and in Stara Baška in the SW [1, 2]. Small outcrops also appear in isolated zones over the entire island. They are preserved inside larger karst depressions or compressed within the tectonic structures. The lower part of the foraminiferal limestones consists of mudstones and wackestones. Packstones prevail in the upper part of the limestones [9]. Eocene siliciclastic rocks consist of marls in the lower part and flysch on top. The thickness of the siliciclastic rocks spatially varies. It increases from the north (320 m) [6] to the south (750 m) [8]. According to the borehole data, the thickness of recently exposed siliciclastic rocks in the main syncline extending from Omišalj to Baška varies from approximately 35 m in the centre [10, 11] to 118 m in the south of the island [12]. Oligocene–Miocene carbonate breccias overlay Cretaceous and Paleogene rocks in the SW and W. They are presumed to be a part of the Jelar formation [5, 9] linked to the late-orogenic uplift of the Mt. Velebit anticline [13]. Depressions have been filled by the Quaternary deposits of fluvial, colluvial and lacustrine origin [1– 3]. Borehole drilling revealed over 60 m of Quaternary deposits in the bottom of Ponikve [14]. Pleistocene breccias and breccioconglomerates are preserved in the

1.1 Natural Characteristics of Krk Island

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Fig. 1.2 Lithostratigraphical map of Krk Island according to Veli´c and Vlahovi´c [4] and Benac et al. [5]. Bauxite deposits after Šušnjar et al. [1], Mamuži´c et al. [2], and field inspection

major syncline Omišalj-Baška [15] and in the minor syncline Stara Baška [15–17]. They are deposited on flysch and marls that are filling both syncline structures. Isolated outcrops of Quaternary breccias are preserved on the flanks and in the floors of abandoned surface streams.

1.1.1.1

CaCo3 Content in Rock Samples

The dissolution rate of carbonate rock generally decreases with the increase of impurities in carbonate rocks [18]. The highest limit of insoluble content for worth karstification has been estimated to 20–30%. However, the purity of carbonate rocks is only one among lithological, and the lithology is only one among the geological factors that control the rock liability to the karstification factors [19]. Other characteristics as type and degree of carbonate rock fissuring [20] can prevail over rock purity and mask its influence in the process of dissolution.

4

1 Study Area and Methods

The results of complexometric titration indicate very high and uniform CaCO3 content in all lithostratigraphic units outcropping on Krk Island. Negligible differences prevent evaluation of the possible correlation between CaCO3 content and distribution, size or shape of karst features (Table 1.1). New data on CaCO3 content for all lithostratigraphic units on Krk Island improve the data from state geological mapping of Yugoslavia in 1969 and 1970 [1, 2, 3] (Fig. 1.3). Concerning lithostratigraphic properties of rock in the context of karstification, it is important to stress (i) high and uniform CaCO3 content characteristic for all exposed lithostratigraphic units, and (ii) the occurrence and the duration of terrestrial phases inducing karstification processes. The sedimentation in the study area has been interrupted by two fairly long terrestrial periods (Fig. 1.4). Nearly 20 Ma long terrestrial period is presumed to exist between the Cretaceous and Paleogene. Since the Eocene up to the present, the approximately 35 Ma long terrestrial period occupying the entire Krk Island enabled the deposition of fluvial, colluvial and lacustrine sediments. For fairly long periods, the carbonate rocks of high purity were exposed to the sub-aerial conditions and subject to the processes of karstification.

1.1.2 Tectonics Krk Island is a part of the External Dinarides with the major orographic axis and geological structures of the Dinaric strike (NW–SE to NNW-SSE) [4, 21] (Fig. 1.5). Its early tectogenesis is related to the subduction of the Adriatic carbonate platform beneath the Dinaric during the Paleogene and Neogene [22]. During the larger part of the Jurassic and Cretaceous, the area of Krk Island was a part of the Adriatic carbonate platform. Platform disintegration was initiated in the Cenomanian, when Krk Island emerged and remained under terrestrial conditions until the start of a new marine transgression in the Paleogene [23]. Intense younger tectonic movements destroyed most of older structural forms but based on preserved Cretaceous structures in neighbouring Istria [24, 25], it is presumed that compressional tectonic stress with similar orientation (WNW–ESE) also affected the area of Krk Island [5]. However, the folding structures in the Cretaceous rocks were formed before the deposition of the Paleocene–Eocene sediments which is evidenced in the numerous erosive remnants of flysch that are angular-discordant to the Cretaceous ˇ [26]). During the Eocene, wide basins were formed under rocks of various ages (Car the influence of the regional stress of NE-SW orientation [27]. Uplift and re-working of sedimentary masses originating from the Internal Dinarides provided sources of clastic deposits that were deposited in such basins during the Paleocene and Eocene [13]. In the Pliocene, the tectonic phase finished with the orogenic uplift of Dinarides [28]. The major structures on Krk Island originated from this phase and are characterized by the Dinaric strike (NW–SE) [1–3]. The dominant NW–SE strike has been disturbed by younger diagonal and transverse strike-slip faults during the Pliocene

1.1 Natural Characteristics of Krk Island

5

Table. 1.1 CaCO3 content of samples from Krk Island analysed in Chemical Analytical Laboratory of the ZRC SAZU Karst Research Institute, Postojna. See Sect. 1.4.4.1 for details on methodology Sample id

CaCO3 (%)

Lithostratigraphic unit

27

93.29

Eocene–Oligocene carbonate breccias

30

33.58

Paleogene marls

33

91.18

Paleogene Foraminiferal limestones

10

92.08

Paleogene Foraminiferal limestones

5

93.24

Paleogene Foraminiferal limestones

16

89.48

Upper Cretaceous limestones and dolomites

24

90.78

Upper Cretaceous limestones and dolomites

15

94.24

Upper Cretaceous limestones and dolomites

13

94.89

Upper Cretaceous limestones and dolomites

17

94.89

Upper Cretaceous limestones and dolomites

6

62.66

Upper Cretaceous limestones

23

75.17

Upper Cretaceous limestones

25

83.53

Upper Cretaceous limestones

1

85.38

Upper Cretaceous limestones

9

90.18

Upper Cretaceous limestones

12

90.28

Upper Cretaceous limestones

2

90.33

Upper Cretaceous limestones

19

91.43

Upper Cretaceous limestones

4

91.98

Upper Cretaceous limestones

32

92.18

Upper Cretaceous limestones

29

92.48

Upper Cretaceous limestones

8

92.78

Upper Cretaceous limestones

11

92.89

Upper Cretaceous limestones

22

93.19

Upper Cretaceous limestones

20

94.04

Upper Cretaceous limestones

31

94.29

Upper Cretaceous limestones

28

94.34

Upper Cretaceous limestones

3

94.89

Upper Cretaceous limestones

7

94.99

Upper Cretaceous limestones

21

95.04

Upper Cretaceous limestones

26

97.69

Upper Cretaceous limestones

14

92.18

Cretaceous carbonate breccias

18

93.69

Cretaceous carbonate breccias

6

1 Study Area and Methods

Fig. 1.3 CaCO3 content for lithostratigraphic units on Krk Island: data by Šušnjar et al. [1], Mamuži´c et al. [2] and Šiki´c et al. [3] and new data

1.1 Natural Characteristics of Krk Island

7

Fig. 1.4 Approximate time intervals and exposed sedimentary units on Krk Island modified from Babi´c [9]

and Quaternary under the influence of re-oriented neotectonic regional stress oriented N–S [29, 30]. It reactivated pre-existing structures with the Dinaric strike resulting in a dextral slip [28]. Recent transpression circumstances with the tectonic stress oriented NNE–SSW result in further deformations of Paleocene–Eocene structures and in the formation of strike-slip and reverse faults directed along the folds axes, ˇ [26]). and in weaker faults striking NE–SW (Car At present conditions, Krk Island represents an elevated area between two submerged Paleogene flysch basins to the NE (Vinodol Channel) and SW (Kvarneri´c) [5]. It is characterized by a compressed tectonic structure with folds and faults striking NW–SE to NNW-SSE and with the major deformed syncline Omišalj-Baška filled with Paleogene sediments [1–3]. The displacement of Istria north-eastwards lasting from the Miocene to present resulted in clockwise rotation of the central and northern part of Krk Island, together with other west-Kvarner islands [28]. Morphological characteristics of the relief, tectonic structures and high seismicity indicate still active tectonic activity [31, 32]. The different position of tidal notches along the coast of

8

1 Study Area and Methods

Fig. 1.5 Tectonic map of Krk Island (according to [1–3])

Krk Island [33, 34], and very steep slopes along the eastern side of the island are obvious indicators of recent tectonic activity.

1.1.3 Unconsolidated Quaternary Sediments Unconsolidated Quaternary sediments were roughly evidenced on the geological map at the scale of 1:100,000 [1–3] and briefly described in the Explanatory notes to the same geological map [6–8]. Some peculiar and isolated sediment outcrops stimulated additional research, as sediments outcropping in the SW of Baška Bay

1.1 Natural Characteristics of Krk Island

9

and sand sediments close to Šilo. The formers were investigated by Bognar et al. [35] and Marjanac [15], and the latter by Waagen [36], Grimani et al. [6] and Marjanac et al. [37]. Pedological characteristics of the soil on Krk Island were described by Bogunovi´c et al. [38].

1.1.3.1

Soil

Most of the surface on Krk Island is covered with the soil of polygenetic origin consisting of loess deposits, disintegrated flysch sediments and insoluble residue of carbonate rocks [39]. The thickness of the soil considerably varies depending on tectonic stability [40] and exposure to wind erosion and is under the strong anthropogenic influence [37]. Due to lack of data, soil thickness was determined by field inspection, sediment drilling and DOF examination (Fig. 1.6). Particularly thick soil is filling karst depressions of all sizes. Over 6-m-thick soil was observed at the excavation of a karst depression close to Malinska, and 11.5m-thick soil is completely filling large karst depression and masking several smaller depressions on its bottom close to Omišalj [41]. Sub-soil rock sculpturing exposed in the central part of the island indicates relatively recent soil erosion, while corrugated features located along the eastern coast exhibit long-lasting exposition to sub-aerial conditions.

Fig. 1.6 Estimated soil thickness on Krk Island based on field survey and DOF examination

10

1 Study Area and Methods

Fig. 1.7 Amount of rock fragments on the surface of Krk Island. a—Rock fragments absent on the surface as they are floating in the thick soil (Photo E. Šegina). b—Frequently occurring rock fragments deposited in situ and completely exposed due to soil erosion (Photo A. Pipan). c— Frequently occurring rock fragments built in stony structures (Photo E. Šegina)

1.1.3.2

Residual of Rock Top-Layer Weathering

A considerable amount of angular rock fragments that are being deposited on the surface of Krk Island is a result of the intensive weathering of the uppermost karst rock layer. Rock fragments appear in various quantities, depending upon the amount of preserved soil. The amount of rock fragments on the karst surface was determined by field inspection and DOF examination (Fig. 1.7). In tectonically stable and wind-sheltered areas in the central and western parts, the weathered material is deposited within the soil as floating material (Fig. 1.7a). Towards the east, south and north, the amount of exposed rock fragments gradually increases together with the soil erosion. With the soil erosion or washing down, the weathered residue of an uppermost karst rock layer remains deposited directly on the bedrock (Fig. 1.7b). During the cultivation of the land settlers actively cleaned the debris to reach the underlying soil and built it into dry stone walls (Cro. gromaˇce). An extreme amount of fragments of cm-dm size was cleaned between the towns Punat and Krk (Fig. 1.7c). The calculated mass of debris built in the variety of stony structures at this location is estimated to over 900 kg/m2 and would constitute a 46-cm-thick layer of gravel over the bedrock and soil [42].

1.1.3.3

Slope Talus

Based on the geological map at a scale of 1:100,000, slope talus consists of unconsolidated angular rock fragments of different sizes and is distributed mainly along the limb of a syncline in the SE of Krk Island [6]. Slope talus covers the lower parts of

1.1 Natural Characteristics of Krk Island

11

Fig. 1.8 a—Expected distribution of talus on slopes with inclination >30°. b, c—1: active talus, 2: poorly cemented inactive talus. Photos M. Tekavec and E. Šegina

non-equilibrated slopes located mainly along the E coast and around karst plateaus in the SE and is related to the edges of synclines. In given mechanical properties of rocks exposed on Krk Island, slopes are equilibrated at approximately 30° (Fig. 1.8a, b). Long-lasting deposition of the talus is evidenced in poorly cemented breccias located underneath the active slope talus (Fig. 1.8c).

1.1.3.4

Aeolian Sediments

The presence of loess on Krk Island has been presumed [43], but no recent analysis that would confirm its existence has been performed up to now. The actual extent of loess accumulation on the island is hardly determinable due to mixing with other sources of material, while the degree of preservation varies with the tectonic stability and liability to the wind erosion related to deforestation, cultivation and topography. Five locations of several meters thick presumably loess accumulations are evidenced on Krk Island (Fig. 1.9a). The thickest outcrop of loess is located in the eastern part of the island close to the settlement Polje (Fig. 1.9b) and was also the only one studied so far. The present fossils link it to the loess of central Europe [36] and place it to the Upper Pleistocene [37]. The geochemical properties of a sample taken at the depth of 40 cm at this accumulation exhibit comparable properties to the well-evidenced loess deposits on Susak Island located over 50 km south-west of Krk Island (Fig. 1.9a, c). The loess on Susak

12

1 Study Area and Methods

Fig. 1.9 a—Location of Krk and Susak Islands. Loess largest outcrops on Krk Island are marked with red dots. b—Polje loess outcrop where geochemical analyses were performed (Photo E. Šegina). c—Major elements oxides, trace elements and rare earth elements composition of loess sample from Susak [44] compared to the loess sample from Krk (Polje)

originated from the Po river plain and was dated to MIS 3-MIS 5 (marine isotope stage) [44].

1.1.3.5

Lacustrine Sediments

There is no available literature on sediments of lacustrine origin on Krk Island. However, the sediment composition in the Ponikve depression indicates exchanging environments of swamp and periodic lake during the rise of the sea level in the Holocene. Deposition of clayey sediments resulted in colmatation of voids and the rise of the water table NE of the depression [14]. A similar process probably occurred in the depression Jezero. It presumably resulted in paleolacustrine sedimentation at hanging water table conditions in the wider Jezero and Ponikve depressions, encompassing depression Kimpi-Kaštel and several large karst depressions south-west of Ponikve (Fig. 1.10). There, clayey sediments filled depressions of different shapes and sizes in several levels up to 120 m above m.s.l. Close to Vrbnik, the occurrence of

1.1 Natural Characteristics of Krk Island

13

Fig. 1.10 Presumed lacustrine sedimentation on Krk Island

periodical lake emptying through several ponors was evidenced until the construction of the effluent to the sea in 1948 [45]. Sedimentation of lacustrine deposits is limited to large karst depressions and tectonic structures with floors deep enough to reach the water table or to topographically closed tectonic depressions with floors built of impermeable siliciclastic rocks. Recent lacustrine deposition occurs in lakes Jezero and Ponikve. Periodically flooded are also some areas NE of Jezero with slightly higher elevations of floors (Mali Lug, Veli Lug, Lokviš´ca, Cerovi Vrh, Veli Vrh). Small lakes persist even on extremely small patches of siliciclastic rocks or as man-made ponds on carbonate rocks.

14

1 Study Area and Methods

Fig. 1.11 Cave sediments. a—Caves on Krk Island are mostly free of cave sediments: an example of Vela Jama (Photo A. Rubini´c). b—Bauxite pebbles associated to the unroofed cave in the SE of Krk Island (Photo E. Šegina). c—Flowstone-covered breccias associated with the unroofed cave in the SE of Krk Island (Photo E. Šegina)

1.1.3.6

Cave Sediments

Recently accessible caves are mainly, except the horizontal passage of Biserujka Cave, pelite-free and host few speleothems (the record of the Speleološki odsjek HPD Željezniˇcar Zagreb). Linked to the unroofed caves, cave sediments of all kinds are rare also on the surface. Among the few cases of existing cave sediments, flowstone-covered breccias and bauxite pebbles have been evidenced associated with the unroofed cave on the karst plateau in the SE of the island [46] (Fig. 1.11).

1.1.3.7

Fluvial Sediments

A minor amount of fluvial sediments on karst consisting of cm-dm-sized carbonate pebbles appear sporadically along large trenches. Isolated and scarce accumulations were noticed also in the upper sections of trenches with no visible traces of siliciclastic outcrops in the watersheds. They appear together with partly rounded rock fragments with imprints of rolling. Low degree of roundness and a large diameter of over 15 cm (Fig. 1.12) indicate locally very intense periodical surface runoff restricted to short segments and time periods.

1.1.4 Present Climate The Kvarner area extends deep into the continent between the high mountain ranges of Uˇcka (1400 m) in the West and North Velebit (1300 m) in the East. It is influenced by both the Mediterranean and continental climate: the intrusion of cold air during the winter and convective precipitations in the summer cause the lack of dry period characteristic of typical Mediterranean climate [47].

1.1 Natural Characteristics of Krk Island

15

Fig. 1.12 a—Location of the pebble within the trench of Bunculuka, SE of Krk Island. b—Location of the pebble on the field (Photo E. Šegina). c—Pebble with traces of rolling (Photo E. Šegina)

Mean annual precipitations on Krk Island amount to 1100–1500 mm (for the period 1961–1990) (https://meteo.hr/). Spatial distribution of mean annual precipitations (Fig. 1.13) indicates the influence of the Dinaric mountain range on the increased amount of precipitation in the Kvarner area and gradual decrease of precipitation with

Fig. 1.13 Spatial distribution of mean annual precipitation (adapted from Zaninovi´c [48]). Interpolated data are valid for the period 1961–1990. Author of the map is Melita Perˇcec Tadi´c

16

1 Study Area and Methods

Fig. 1.14 Spatial distribution of mean annual temperature (adapted from Zaninovi´c [48]). Interpolated data are valid for the period 1961–1990. Author of the map is Melita Perˇcec Tadi´c

the distance from the mountains towards the open sea (SW direction). The increased amount of precipitation is characteristic for elevated areas in the central and southern parts of Krk Island, while the general trend of precipitation on the island decreases from the north-east to the south-west. Mean annual air temperature on Krk Island amounts to 11–15 °C (for the period 1961–1990) (https://meteo.hr/). Spatial distribution of mean annual air temperature shows distinctive temperature change along the north-eastern Adriatic coast and Dinaric mountain range. Induced by the influence of the Mediterranean Sea, the considerably higher temperature compared to that in the continent is characteristic for the entire Kvarner area. Spatial variations of the mean annual temperature on Krk Island are directly correlated to the elevation: temperatures are lower in the central part and on high karst plateaus in the south (Fig. 1.14). Due to its high speed and common occurrence, wind named bora (Cro. bura) considerably influences the local environmental conditions. It is katabatic, orographically controlled north-easterly wind (Zaninovi´c [48]). Due to narrow passes in the hinterland, dissected topography and low water depth in the Kvarner area bora develop locally extremely strong gusts with the speed exceeding 50 m/s [49]. Its significance for karstification processes in the area is in soil accumulation and erosion, and consequentially in dictating the type of vegetation cover. Combining the climatologic indicators Krk Island is, as well as the wider Kvarner area, characterized by the mixing influences of the Mediterranean and continental climate, namely by the Mediterranean air temperature conditions and continental precipitation regime. The Adriatic Sea and the topography of the Dinaric mountain range with its altitude and position according to prevalent air-mass circulation are the most important local factors causing a modification of the climate (Zaninovi´c [48]). Within the island, the topography and the distance from the Dinaric Mountains are the most important factors of microclimate modifications.

1.1 Natural Characteristics of Krk Island

17

1.1.5 Hydrology and Hydrogeology In terms of water balance, Krk Island is the richest of the Kvarner islands. Average annual discharge amounts to 6.6 m3 /s, resulting in the average annual volume of produced discharge of 209.2 Mm3 [50]. There is a great difference in specific annual discharge distribution within the island, ranging between 11.2 and 22.5 l/s/km2 , with an average value of 16.4 l/s/km2 [51]. In recent relief circumstances, the high karst plateaus in the south-east exhibit the highest karstification potential based on calculated water capacity values, followed by the central and the very northern parts of the island. As Krk Island is mainly built of karstified permeable carbonate rocks (94% of the surface), the above estimated annual discharge is being nearly completely transmitted underground. Limestones, dolomitic limestones and carbonate breccias are mostly karstified and permeable, while siliciclastic rocks (marls and flysch) are mostly impermeable. Quaternary sediments of different composition and lithogenesis have variable permeability (Fig. 1.15). The main karst aquifer is partly divided by the NW–SE-oriented zone of impermeable siliciclastic rocks filling the syncline between Omišalj and Baška (Fig. 1.2b). This hydrological barrier is interrupted by the fault zone exceeding between Vrbnik and Negrit cape at Punat (Korbar [52]). Besides this major siliciclastic outcrop, minor patches of impermeable siliciclastic rocks are preserved in tectonically compressed zones or are masked by the Quaternary deposits. They locally redirect and slow down the circulation of groundwater, as well as prevent its fast communication with the seawater (Rubini´c [53]). Due to the mask or small size, such siliciclastic outcrops are not marked in geological maps of small scales. They were evidenced by a field survey. Following the regionalization obtained by the terrain ruggedness index (Sect. 3.1), the lowlands in the north gradually pass on to the highlands in the south-east of the island. Accordingly, the north of Krk Island is characterized by permanently waterfilled karst depressions at the water table and the south-eastern part by the deep trenches of segmental and temporal surface runoff (Fig. 1.16). (1) Lowlands with depressions at the water table. In the lowlands of the northern part of the island, floors of some depressions located close to the coastline are permanently water-filled as they reach the karst aquifer (Fig. 1.17a). Close to the shoreline, also some submerged depressions are visible in the shallow sea. Physical and chemical analysis of permanently water-filled depression near Rudine and the seawater nearby with the mutual distance of only 25 m were performed during this research. The results show virtually no communication of karst aquifer with the seawater despite the proximity (Fig. 1.17b, c). This is due to low hydrological gradient and resulting slow oscillations of groundwater level as a result of high secondary sediment infilling of the voids (Rubini´c [53]). Clay infilling of voids has been detected by borehole drilling in the area of Jezero Lake at depths of 30–35 m and 42–48 m, that is −3 m and −15 m under

18

1 Study Area and Methods

Fig. 1.15 Hydrogeological factors on Krk Island. Annual discharge according to Oštri´c et al. [51], simplified geological map according to Veli´c and Vlahovi´c [4], patches of marls and flysch based on a field survey

the recent sea level [54, 55], as well as in the area of Ponikve depression at depths greater than 10–30 m [56] (Fig. 1.17). (2) Transitional hilly landscape with rare trenches of temporal surface runoff in their lower sections. In terms of relief configuration, the central part of Krk Island presents a transition zone between the two distinct types of karst hydrological surface features. It is characterized by the gentle elevations of maximally 250 m above m.s.l. Shortly after strong rain events, temporal surface runoff is activated in the lowest sections of some short trenches due to an uplift of the water table. The temporal surface streams are draining to the sea (Fig. 1.18). (3) Highlands with trenches of segmental and temporal surface runoff . The southeast of the island consists of two high karst plateaus located mostly between 350 and 500 m above m.s.l. On the contrary to the lowlands of the north, the coastal karst areas located at the foothills of the high karst plateaus exhibit strong communication of karst aquifer with the seawater (Geološki konzalting

1.1 Natural Characteristics of Krk Island

19

Fig. 1.16 a—Karst surface on Krk Island characterized by the two distinct types of hydrological features and the transition between them, with the distinctive correlation to the recent relief configuration. b—Lowlands with depressions at the water table. c—Highlands with trenches of segmental and temporal surface runoff. Photos Ž. Gržanˇci´c

Fig. 1.17 a—Distribution of permanently water-filled and submerged depressions on Krk Island. ˇ Benac). c—Electrical conductivity (σ) and the number of b—Depression 1 on the field (Photo C. chlorides for seawater and water in depression 1 close to Rudine with the location, N of Krk Island

20

1 Study Area and Methods

Fig. 1.18 a—Hilly landscape of central Krk Island with the location of B (Photo Ž. Gržanˇci´c). ˇ Benac) b—Trench with the short periodical stream in its lower section (Photo C.

d.o.o. [57]) [58]. The hinterland recognized as an area of highest water potential (Fig 1.15) with the fast oscillations of water table caused the washing-out of the sediments, resulting in sediment-free voids. They have formed a wider mixing zone between the seawater and karst aquifer (Rubini´c [53]). Relief is characterized by long trenches incised even over 100 m deep. Shortly after strong rain events, temporal segmental torrential surface runoff is activated in some trenches. It occurs: (i) on segments located downwards of the siliciclastic outcrops, (ii) on sections with locally decreased permeability and (iii) in the lowest sections of trenches, where due to the rise of the water table after the strong rain event the water drains directly from karst aquifer through the unconsolidated beach sediments (Fig. 1.22). 1.1.5.1

Karst Aquifer and Water Table

As typical for karst aquifers, the spatial distribution of groundwater levels shows high variability within the Krk Island [59]. Large depressions Jezero and Ponikve located in the northern and central part of the island are characterized by low elevation of the floors that are periodically flooded and represent local drainage base for local surface and underground waters. As well as the trenches, also large depressions were strongly influenced by the past climate changes and oscillations of the sea level. In the recent conditions, Jezero functions as polje on the water table and Ponikve as polje in the hanging aquifer (Fig. 1.19). Jezero (Fig. 1.19a) is a crypto-depression with a bedrock bottom located at − 9 m under the sea level, covered by 2 m of Quaternary sediments [60]. Based on the origin of sediments and the depression morphology, in natural conditions (before the exploitation in 1971) the level of the natural lake extended between 0.95 and 2.10 m above m.s.l. and the lake occupied three times larger area than a recent lake. Water had flown into the crypto-depression from springs located along the southern rim of

1.1 Natural Characteristics of Krk Island

21

Fig. 1.19 Large depressions are recently functioning as poljes. a—Jezero (Photo Ž. Gržanˇci´c). b—Ponikve (Photo Ž. Gržanˇci´c)

the depression (the largest springs being Vrutak and Luˇcica) and in the depression floor [55]. It drained out from the depression through the ponor zone of Ponicalo on the west side of the depression. The confined aquifer at the border of the depression is located at a depth of 35.3 m (−7.6 m below the sea level) [54]. Entering the underground at ponor zone Ponicalo, the water flows to the W and drains to the sea through coastal springs Beli kamik, Dražica and Kijac (Fig. 1.20). Šerko [61] and Nicod [62] classified Jezero as polje.

Fig. 1.20 Jezero aquifer. Hydrological data after Register of springs Hrvatske vode VGO Rijeka and hydrological elaborate [55]. The natural extent of the lake is taken into consideration. Geological data after Šušnjar et al. [1], Mamuži´c et al. [2] and Šiki´c et al. [3]

22

1 Study Area and Methods

Ponikve (Fig. 1.19b) is a depression with a sediment floor located at 11–17 m above the sea level, with over 60-m-thick Quaternary sediments at its base [14]. The bedrock underwork is thus located at least 45 m beneath the recent sea level. In natural conditions, i.e. before the construction of a dam in 1986, the depression was periodically flooded by waters from springs Vela Fontana, Škrilji, Munjˇcel, Mala Fontana, Brajdine and Rakita. The waters formed a stream which sunk in the extensive ponor zone in the north-west of the depression. The lake sprung up in the rain period and lasted nearly the entire year [14]. Waters entering the karst underground at ponor zone drain mostly to the north-west to coastal spring Jaz at Malinska [56], while at the high levels the outflow to coastal spring in Krk south-west of Ponikve is also activated (Giorgolo [63]) (Fig. 1.21). The bulk of groundwater that drains to Ponikve arrives along the structures from the north-east and east. Following the recent relief down to the depth of 10–30 m, the permeability of rocks is, despite the lithological heterogeneity, uniform and high with the velocity of 750–1900 m/day. Voids in the underlying carbonate rocks are colmatated and exhibit low permeability [56]. Ponikve was classified as polje by Nicod [62] and as a transition feature between polje and uvala by Šerko [61].

Fig. 1.21 Ponikve aquifer. Hydrological data after Register of springs Hrvatske vode VGO Rijeka and hydrological elaborate [14, 56], the natural extent of the lake is taken into consideration. Geological data after Šušnjar et al. [1], Mamuži´c et al. [2] and Šiki´c et al. [3]

1.1 Natural Characteristics of Krk Island

1.1.5.2

23

Karst Water Discharge

Water from the main island karst aquifer in major part discharges through the channels formed at the recent drainage base at the sea-coast (coastal springs), as a consequence of relatively stable sea level in the last 5000 years (Correggiari et al. [64]) [65]. The majority of coastal springs are periodical and of low capacity, and their presence has never been evidenced. Major concentrations of coastal springs at the recent sea level are located at settlements Krk and Malinska, both being supplied from the watershed of larger extent in the central part of the island (Ponikve). The minor part of karst groundwater discharges through the channels adapted to the relatively lower sea level in the Pleistocene (submarine springs). Siliciclastic outcrops with their limited permeability also store a minor amount of the water and form hanging aquifers above and separated from the main karst aquifer (Rubini´c [53]). The water from such local and spatially limited aquifers drains through springs of low capacity located along the permeable–impermeable rock contacts (contact springs) [59]. In Rijeka Bay, the submarine springs as activated remnants of the past conditions of lower sea level are noticed at the depth of 20–24 m below the present sea level, together with the entrances to numerous submerged caves [66]. The deepest submerged caves in the Kvarner area are found 45 m below the recent sea level [67]. Some submarine springs on the E coast of Krk Island appear also at the depth of only a couple of meters. At low groundwater level, the discharge commonly occurs through submarine springs located in the mouth of the trenches with temporal and segmental surface runoff. At groundwater level rise, i.e. during and shortly after intense rain events, the temporal surface runoff in the trenches is activated. Such temporal streams could have relatively high transport potential [68]. Close to the coast, water from the karst aquifer drains through unconsolidated beach sediment (Fig. 1.22). The locations of major submarine groundwater discharge detected from infrared satellite thermal images based on water temperature differences support this idea. They are located on the south-western coast at the foothill of the high karst plateau with the highest water potential on the island, expected high gradient and sedimentfree voids (Rubini´c [53]). Both marked locations are related to the trenches with periodically active surface runoff and a siliciclastic zone with a series of ponors in the watershed (Fig. 1.23). It is presumed that increased permeability as a consequence of changed climate conditions in the glacial periods of the Pleistocene (see Sect. 1.2) together with the progressing karstification resulted in a gradual drop of surface runoff into the karst underground. Except for the watershed of Vela baš´canska riˇcina flowing on impermeable siliciclastic rocks and outflowing directly to the sea, the bulk of estimated runoff on Krk Island is involved in the processes of karst denudation of the surface and speleogenesis underground. In the recent relief circumstances, the high carbonate plateaus in the south-east exhibit the highest karstification potential based on calculated water capacity values, followed by the central and the very northern parts of the island (Fig 1.15). Present water circulation is determined by the lithological and structural

24

1 Study Area and Methods

Fig. 1.22 Periodically activated surface runoff in Potovoš´ce. a—Location. b—Watershed. c— Situation on 28.5.2018. d—After the strong rain event on 19.11.2017. Photos E. Šegina

Fig. 1.23 a—Processed Landsat 7 infrared thermal image according to Oštri´c et al. [51] showing presumed sources of groundwater discharge. b—Hydrogeological situation at the sites of detected groundwater discharges

1.1 Natural Characteristics of Krk Island

25

Fig. 1.24 Hydrogeological map of Krk Island based on maps of Habsburg empire (1806–1869), Alfirevi´c [69], Hrvatske vode [55], Hidroinženjering d.o.o. [14, 56], GEOS [70], Rubini´c et al. [50], Veli´c and Vlahovi´c [4], register of springs Hrvatske vode VGO Rijeka [59] and field observations. The natural extents of the lakes are taken into consideration

characteristics of the bedrock, degree of underground karstification, past oscillations of the sea level, recent relief configuration and resulting sediment filling of the voids (Fig. 1.24).

1.1.6 Degree of Karstification The purity of carbonate rocks with mainly >90% CaCO3 content, their wide exposition, a high degree of fracturing, temperate climate with a high amount of precipitation and contact to impermeable siliciclastic rocks induce a high potential of karstification processes on Krk Island. Indeed, the surface is highly karstified, which is being manifested through high depression density, reaching the value of up to 60

26

1 Study Area and Methods

depressions per km2 . However, the existing record of speleological objects is very limited.

1.1.6.1

Underground Karstification

Most of 86 evidenced caves on Krk Island are shallow and small (the record of Speleološki odsjek HPD Željezniˇcar Zagreb and author’s record). With the average length of 17 m, they are located up to 20 m below the surface. The largest cavity has a volume of less than 5500 m3 (Vela Jama). The predominant cave geometry is a shaft (Fig. 1.25). The flowstone deposits in caves are rare, excluding Biserujka cave. Biserujka Cave is the largest horizontal cave, located at the depth of 15 m under the recent surface and approximately 22 m above m.s.l. The channel is non-branched, 5 m wide and less than 100 m long with a keyhole geometry. Approximately 2 m deep secondarily incised bottom indicates a gradual drop in the water table. Towards the south-west, the channel ascends towards the surface and finishes with the terminal breakdown. Resembling a Dinaric doline (in sense of [71]), at the surface, it is recognizable as shallow depression filled with sediment and debris (Fig. 1.26). The NE-SW orientation, dimension and non-branched horizontal morphology of the channel are common for the denuded horizontal passages in several parts of the island, recently located at 85 m above m.s.l. (Unroofed cave 2), 185 m above m.s.l. (Unroofed cave 3), 265 m above m.s.l. (Unroofed cave 4) and 367 m above m.s.l. (Unroofed cave 5) (Figs. 1.25 and 1.26). Frequent phenomena in the Kvarner area are submerged caves located at depths between −20 m and −24 m. It is considered that they were formed at the sea-level stagnations during the lower sea-stand in the Pleistocene [66, 67, 72]. The results of geophysical profiling and borehole drilling in the central area of Krk Island imply that the karstified zone characterized by solutionally enlarged fissures generally follows recent relief down to the depth of 30 m under the surface, even under the large karst depression of Ponikve. Its rocky bottom is located at −45 m below sea level, proving that intensive speleogenesis operated during considerably lower sea-stand [56]. The existence of recently unroofed caves in different locations with poorly preserved cave sediments [46] indicates several storeys of speleogenesis as a consequence of long-lasting karst denudation that encompasses the entire island.

1.1.6.2

Surface Karstification

The observations of road-cuts and quarries on the study site indicate locally high variability of the karst rockhead morphology of Krk Island. Dissected pinnacle–cutter morphology (in sense of [19]) with solutionally enlarged fissures is characteristic for the central and western parts of the island (Fig. 1.27a). These areas are considered recently tectonically stable, and they are covered by the thick soil of polygenetic origin [40].

1.1 Natural Characteristics of Krk Island

27

Fig. 1.25 Caves on Krk Island, according to Cave Association Zagreb and author’s records. The depth of submerged caves is showed by the marker (data according to Benac et al. [34])

Along the entire tectonic structure of Omišalj-Baška extending from north to south of the island, the uppermost layer of karst rock is strongly fissured. Two distinct types of karst rockhead morphology were defined. Where the joints are solutionally enlarged, the soil is packed in cutters and angular to sub-angular rock fragments originating from weathered tops of pinnacles overlay the surface (Fig. 1.27b). Along the eastern coast of the island (Fig. 1.6), the soil is completely absent and weathered rock fragments constitute a thick layer that is covering pinnacles and cutters (Fig. 1.27c). In some areas, the pinnacle–cutter morphology is completely missing and strongly fissured bedrock is extremely levelled (Fig. 1.27d). Rockhead is covered by a thin

28

1 Study Area and Methods

Fig. 1.26 Biserujka Cave and some examples of unroofed caves on Krk Island. See Fig. 1.25 for location

Fig. 1.27 Basic characteristics of karst rock uppermost layer on Krk Island. a—Pinnacle–cutter morphology covered by thick soil. b—Pinnacle–cutter morphology with partly eroded soil and weathered rock fragments. c—Pinnacle–cutter morphology covered by a large number of weathered rock fragments. d—Levelled karst rock missing pinnacle–cutter morphology

1.1 Natural Characteristics of Krk Island

29

layer of soil and weathered rock fragments, probably originating from in situ mechanical weathering of the pre-existing pinnacles. Anthropogenic activities in levelling rough karst surface should also be considered.

1.1.7 Human Impact Before the considerable anthropogenous intervention, Krk Island was entirely forested. The high karst plateaus in the S, approximately located at the elevations between 300 and 500 m above m.s.l., were vegetated by the sub-mountainous community of pubescent oak and hop hornbeam, while the majority of the remaining land was vegetated by the sub-Mediterranean community of downy oak and hornbeam [73]. Population growth and intense agriculture at the end of nineteenth and at the beginning of the twentieth century led to deforestation of most of the land for fields and pasture [74]. Actual vegetation is a consequence of recent human degradation rather than natural conditions (Fig. 1.28a). The then cultivated land is now overgrowing by the shrubberies, while areas exposed to strong NE wind bora are unable to recover. They are lacking continuous soil cover and are vegetated by grass and sporadic bushes (Fig. 1.28b). At some locations, sub-soil rock sculpturing characterized by smooth edges [75] exposed on a bare karst surface indicates relatively recent soil erosion. Settlers modified the hydrological regime by building the effluent to the sea from Vrbniˇcko polje [44] and Jezero depression and widening of the channel connecting the Puntarska draga Bay and the sea at the end of the nineteenth century [74]. Ponikve was a permanent lake in the north-west and periodical lake in the south-east of the depression (Fig. 1.29a). In 1986, a dam was built on the outflow side of the depression

Fig. 1.28 a—Karst bare lands due to human degradation and soil erosion as visible from a satellite image with a location of B (Source Google Maps) b—Recently abandoned cultivated land in the process of overgrowing (Photo U. Stepišnik)

30

1 Study Area and Methods

Fig. 1.29 a—Ponikve depression before major human modifications at the beginning of the nineteenth century (Source map of Habsburg empire (1806–1869) available on https://mapire.eu/en/). b—Ponikve depression after the construction of a dam in 1986

Fig. 1.30 A—Rock fragments as a result of rockhead weathering in a primary location (Photo E. Šegina). B—Rock fragments built in the variety of dry stone walls constructions and terraces (Photo Ž. Gržanˇci´c)

and transformed nearly entire depression to a permanent lake (Fig. 1.29b). Traces of past geomorphic processes are partly anthropogenically modified as well. Settlers have intensively moved rock fragments on the surface to clean the land for agriculture and build dry stone walls (Cro. gromaˇce) and terraces (Fig. 1.30).

1.2 Climate Changes and Sea-Level Oscillations in Pleistocene and Holocene Besides glacioeustatic oscillations of the sea level and neotectonics, climate changes in Quaternary considerably contributed to the morphological evolution of the study area. Several global glaciations have been recognized since the beginning of the Quaternary in 2588 Ma BP [76].

1.2 Climate Changes and Sea-Level Oscillations …

31

Due to the development of a fixed anticyclone over the north European ice sheet and colder sea surface temperatures, climate conditions recorded in pollen and lakelevel records indicate generally warm and humid interglacial conditions and cold and arid glacial conditions in the Mediterranean basin [77]. Global climate changes during the Pleistocene and Holocene were accompanied by considerable sea-level fluctuations of the Adriatic Sea [78]. At the peak of the previous interglacial period in MIS 5.5, some 125,000 years BP, the global sea level was similar to the recent or up to several meters higher [79]. The absence of marine record above the recent sea level in the area yet indicates that the sea probably did not reach the present coastline due to the higher position of the land at that time as the consequence of the post-active tectonic subsidence of the Kvarner area [78]. After the previous interglacial period, the sea level gradually decreased. At the peak of the Last Glacial Maximum (LGM; MIS 2), some 20,000 years BP, Rijeka Bay and Vinodol-Velebit Channel have been under terrestrial conditions with periodical lakes and intense river incision [66, 80]. Only at the peak of LGM for the relatively short period, the sea level in the Adriatic Sea dropped as low as −130 m (Fig. 1.31a). At that time, the sea-coast was approximately 200 km away from the study area. After that, the rapid sea-level rise has occurred, characterized by several melt-water pulses: −100 m at 16 ka BP, − 60 m at 12 ka BP, −40 m at 10 ka BP, −12 to −14 m at 8 ka BP, −5 m at 6 ka BP, −2 to −2.5 m at 4 ka BP and −0.5 to −0.75 m at 1 ka BP [78, 81]. According to this data, the flooding of Rijeka Bay and Vinodol-Velebit Channel has started around 12,000 years BP. Near the eustatic peak of the Holocene sea-level rise at around 5000 BP, there was a period of equilibrium between the regional tectonic subsidence and hydro-isostatic emergence in the northern Adriatic area. These two opposite phenomena caused the relative sea level to remain stable for a few thousand years until the Roman times [65, 82] (Fig. 1.31b). Since then, the sea-level rise in the Adriatic Sea was estimated to 2–3.4 mm/y [83]. Some more detailed investigations of the climate during glacial periods in the Mediterranean show that due to the change of the air-mass circulation during the LGM, the climate was considerably colder than at present, but not considerably dryer [86]. Such specific microclimatic conditions could have also been driven by the interplay of the Mediterranean influence and the orographic barrier of the Velebit mountain range. The skeleton of a cave bear—Ursus spelaeus found in Biserujka cave in the north of Krk Island [87]—links the local climate conditions before the last glacial period to the ones extending from north-west Spain, across central Europe to the Ural, and from Belgium and the Harz region in Germany in the north to central Italy and Greece in the south, where such species was distributed [88]. Moreover, according to the palynological analysis performed on the sediment core from Valun bay on the nearby Cres Island [89], the climate during the LGM at the actual Cres Island coast was comparable to the present sub-alpine Pinus Mugo belt constituting the uppermost tree line and now characteristic for altitudes over 1450 m on Velebit [90]. However, according to the majority of the studies, the glaciations were presumed

32

1 Study Area and Methods

Fig. 1.31 a—Course of the seashore in the Adriatic Sea at the Last Glacial Maximum (MIS 2) app. 18,000 years BP according to [64]. The extent of glaciated areas according to Kuhlemann et al. [84], Žebre and Stepišnik [85]. b—North Kvarner islands at the Last Glacial Maximum. The position of lakes and water flows according to Benac [86]

to be limited to the mountain ranges of the Central Velebit [91], Risnjak-Snežnik [85] and Uˇcka, with the snow-limit at approximately 1300 m above m.s.l. [92] or 1500 m above m.s.l. [93] (Fig. 1.31a). At the peak of the LGM, the wider study area was therefore characterized by sub-alpine-like climate rich in snow [89]. The estimated average monthly temperatures calculated for the nearby town of Senj located at the foothill of the North Velebit indicate 11–15 °C lower temperatures in LGM compared to the recent, with average monthly temperatures under the 0 °C from November till April [92]. The end of the cold period of Late Pleistocene in the area dated to 14,445 ± 145 BP when the sea level was 55 m lower than presently [89], and the coast was located south-west from Susak Island 60 km away from Krk Island. The influence and extent of periglacial and perinival conditions in the area have not been unambiguously established. Yet, the above-mentioned facts indicate frequent temperature oscillations around 0 °C and snow cover during the glacial periods must have caused intense speleogenesis underground and effective weathering processes on the surface with the development of at least periodic surface drainage that dominated over the retained karst denudation.

1.3 State of the Art Even though certain global conditions of karst depression formation related to climate, elevation, slope inclination, lithology and tectonics have been successfully established [94], correlations between the presence and geometry of karst depressions and environmental factors on a regional scale have often been quite variable or even

1.3 State of the Art

33

weak. It was hoped that morphologic studies would reveal direct links between karst landforms and processes, but they have not satisfactorily fulfilled such expectations [95]. This is due to the unclearly defined object of research and lack of questions that should be solved by such methods (see Sect. 2.2). Due to rapid data processing, its applicability to extensive and inaccessible areas, and its apparent objectivity, the use of topographic, aerial and LiDAR data sources in Geographic Information System (GIS) has proved a very promising method in geomorphology. However, its objectivity is subject to the scale employed, the use of correct algorithms, the interpreter’s subjectivity, the precision of scanning and possible measurement and drawing errors. Spatial analysis has been applied to karst surface features relatively recently. Morphologic and distributive analyses have been focused on circular karst depressions and among them applied mainly to small karst depressions [71, 96–113], less to large karst depressions [114], or cockpit karst [115–117]. Spatial analysis of valleys in fluvial geomorphology is more numerous and has far longer tradition, starting with Horton [118, 119, 120], Strahler [121–123] and Hack [124], and followed by numerous modern authors (e.g. [125–131], etc.). However, such methods have seldom been applied to linear features on karst [132] due to scarce appearances of such features in pure karst systems and relatively particular environmental settings required for their evolution. Previously, the most frequently used data sources in the spatial analysis of karst depressions involved the use of topographic maps of various scales [104, 108, 133]. Recently, the detection of karst depressions has become increasingly focused on automation of the process using DEMs of different resolution [110, 134, 135], even though the precision of such methods does not yet surpass classical approaches that use topographic maps or digital orthophotography. Error in detecting karst depressions has already been noted and quantified for topographic maps of various scales by Troester et al. [136], aerial photos by Day [99], and digital elevation models (DEMs), among others, by Obu and Podobnikar [137]. Recently increasing accessibility to high-resolution laser imaging, detection and ranging technology (LiDAR) data is opening the way to new challenges in data processing (Sect. 1.4.3.4). Besides considerably higher resolution of the input data, the same issues of detecting and delineating karst features as discussed here for topographic maps and DOFs, keep spatial analysis on karst extremely difficult. As LiDAR data for Krk Island were not accessible at the time of the present research, topographic maps, digital orthophotography and DTM, supported by the extensive fieldwork, were employed as source data for spatial analysis. The issue of karst depression upper rim definition has already been discussed by several authors [101, 108, 114, 138]. Even though being the most reasonable definition of karst depression perimeter, a principle of “an abrupt change in surface slope” [101] has seldom been used in practice due to its fieldwork requirements [109]. Instead, the focus has been directed towards simplification and automation of the procedure. Previously, the most frequently used methods for karst depression upper rim delineation involved the use of topographic maps of various scales based on the assumption that the uppermost closed contour properly represents the karst depression perimeter [104, 108, 139]. Recently, the delineation of karst depressions

34

1 Study Area and Methods

has become increasingly focused on automating delineation using digital elevation models (DEMs), combining DEMs and satellite imagery, or combining DEMs and diverse algorithms (e.g.[110, 111, 134, 137, 139], Telbisz et al. [140]). The main obstacle of karst depressions morphologic analysis is their irregular geometry. The demand for a certain amount of geometry generalization has led to several approaches which were applied without mathematical examination of their adequacy. Various parameters have been applied especially based on the karst depression upper rim (listed in [138]). Among them, two basic parameters for describing the perimeter shape of karst depressions, namely circularity and elongation, have been the most commonly employed (e.g. [104, 106, 109, 112, 134, 139, 141]), but due to the variants of the parameters, the results have limited comparability. Due to the unsatisfactory results of karst depression distribution and morphologic studies acquired so far, a discussion on method reliability is required. Difficulties may arise at every stage of data processing: (1) definition of karst depression, (2) detection of karst depression, (3) definition of karst depression’s planar shape (delineation), and (4) the use of suitable parameters in morphometric calculations. The issue of defining the research object is discussed in Sect. 2.2, while methodological questions of karst depression detection, planar shape definition and the use of suitable parameters in morphometric and distributive analyses are discussed in Sects. 1.4.1, 1.4.2 and 1.4.3, respectively.

1.4 Methods Input data employed in the spatial analysis were acquired from the following sources: (i)

a topographical map at scale 1:5000 with contour lines of 5 m interval created between 1954 and 2010 (Državna geodetska uprava Republike Hrvatske) (ii) orthorectified aerial photos (digital orthophoto or DOF) in resolution 0.5 m/pixel obtained in 2004 (Državna geodetska uprava Republike Hrvatske) (iii) DTM interpolated from the topographical map at scale 1:25,000. The data have been processed with ArcGIS 10.2 software.

1.4.1 Detecting Karst Features Field inspection shows that the variability of karst depression manifestations appearing in the dynamic natural conditions does not fit into the traditional definitions of surface karst features (Sect. 2.2). Widening the karst circular depression definition from the feature with a topographically closed upper rim to the feature exhibiting centric geometry enabled the detection and inclusion of karst depressions with the topographically opened upper rim. Other varieties can be noticed on other karsts such as secondary and double karst depression (not discussed here). All these

1.4 Methods

35

atypical varieties require scientific discussion and cannot be simply “left out of the examined population” [138]. On the contrary, they represent a distinctive component and hold important information on regional environmental settings that most probably influenced the entire population. Surface karst features were detected by combining topographic map at a scale of 1:5000 and DOFs in resolution 0.5 m, verified by several field campaigns between the years 2014 and 2018.

1.4.1.1

Defining Detection Error

Detection error was established for the topographic map, DOF and fieldwork methods, and three types of vegetation cover: bare, bush and forest (Fig. 1.32). It was applied to the most problematic sites where the divergence of results obtained by the two methods was the highest. The calculated detection error thus directly refers only to these problematic sites. The overall accuracy (OA) (Table 1.2) was calculated following the method used in Carvalho Júnior et al. [134]. The results reveal low reliability of topographic maps and high reliability of DOF in detecting karst depressions. The accuracy is the highest on the bare karst surface, where the depressions of d = 5 m can be detected using DOF. Underestimation of actual karst depression presence by both topographic map and DOF on bare karst (Table 1.3; Fig. 1.33) is not a consequence of the size but the depth of the depressions. It must also be considered that various types of vegetation completely mask small and shallow features. Therefore, in a distributive analysis of karst depressions, particular attention should be given to bare karst areas. A topographic map and DOF are least reliable in areas of shrubbery due to the unfortunate combination of a vegetation cover and the lack of the shadow effect. Positive detection error noted in bush and

Fig. 1.32 Example of topographic map (1:5000) detection error identified by rough DOF of 0.5 m resolution: a—Visibility of karst depressions on rough DOF. b—Karst depression missing on the topographic map due to detection error (marked with an arrow)

36

1 Study Area and Methods

Table. 1.2 Overall accuracy of topographic map and DOF calculated for critical test areas and different vegetation types Topographic map

Bare

Bush

Forest

Sum

TP (true positive): number of karst depressions detected correctly 9

1

6

16

FP (false positive): number of karst depressions indicated where none exist

0

0

0

0

FN (false negative): number of unidentified karst depressions

35

41

11

87

OA (overall accuracy)

0.20

0.02

0.35

0.16

DOF

Bare

Bush

Forest

Sum

TP (true positive): number of karst depressions detected correctly 40

32

16

88

FP (false positive): number of karst depressions indicated where none exist

1

5

3

9

FN (false negative): number of unidentified karst depressions

4

10

1

15

OA (overall accuracy)

0.89

0.68

0.80

0.79

Fig. 1.33 Range of topographic map and DOF detection errors and their dependence on vegetation type established for critical areas. Divergence from 0 indicates if the method overestimates (FP— false positive) or underestimates (FN—false negative) the real number of karst depressions

forested areas is a result of the shadow effect. Even though highly useful in depression detection in the case of low and uniform forests, in this case, the shadow effect may be misleading. Topographic maps give very reliable but incomplete data, while DOF gives data of variable reliability (depending on vegetation type) but generally much better accuracy. Combining both methods does not improve the minimal overall accuracy acquired solely using DOF but is recommended for cross-checking.

1.4 Methods

37

Table. 1.3 Comparison of methods reliability. Relative values (%) calculated for critical areas for a total number of existing karst depressions (bare—44, bush—42, forest—17) Bare (%)

Bush (%)

Forest (%)

Karst depressions detected by topographic map and DOF but not detected in the field

0

0

0

Karst depression detected by neither topographic map nor DOF but detected in the field

9.1

21.4

5.9

Karst depressions detected by a topographic map or DOF

90.9

78.6

94.1

Karst depressions detected by a topographic map and in the field but not by DOF

0

2.4

0

Karst depressions detected by a topographic map only (topographic map error)

0

0

0

Karst depressions detected by DOF and in the field but not 70.5 by a topographic map

76.2

58.8

Karst depressions detected by DOF only (DOF error)

11.9

17.7

2.3

Out of karst depressions detected on bare karst areas (1262 karst depressions), 33% are small karst depressions with d < 15 m indicating that small features are an important part of the database and should not be excluded from the distributive and morphologic analysis of karst depressions. Along with their quantitative contribution, they are especially significant for their location and their correlation to relief configuration.

1.4.1.2

Method Applied for Karst Surface Features Detection

As DOF appears nearly five times more reliable in karst surface features detection in comparison to a topographic map at the same scale, the prime importance of detecting surface features on the test site was given to the information acquired from DOF. Detection of karst features by topographical map and DOF was performed based on previous field examination which resulted in the following assumptions (valid for the surface on carbonate rocks) (Table 1.4; Fig. 1.34):

1.4.2 Delineating Karst Features 1.4.2.1

Defining Delineation Error

The main obstacle for the consistent description of karst depressions planar shape is the irregularity of their perimeters and insufficient scale of data sources applied for their delineation. To employ the most accurate method for karst surface features determination, all the available data sources were discussed: a topographic map at

38

1 Study Area and Methods

Table. 1.4 Assumptions based on fieldwork, employed for detection of karst surface features Assumption

Source

Figures

All closed contour lines encircling a measurably lower elevation are Topographic map 35a circular karst depressions In the forest, all circular areas with richer vegetation or shadow-effect are circular karst depressions

DOF 35b

On bare karst, all circular to sub-circular, or irregular and elongated DOF areas of considerably thicker soil and richer vegetation, superficially not being manifested as depressions, are not only areas where settlers cleared the land, but are negative structures in the shallow sub-surface filled-up with sediments. Those sediment bodies are often enclosed by dry walls and on DOF with no 3-D information resemble karst depressions. The additional geophysical method was employed to prove the occurrence of sub-soil karst features (see Sect. 1.4.4.3). The sub-soil inspection indicated that such features were denuded segments of horizontal caves, denuded vertical shafts filled-up with sediments, or structures of epigenic karstification—cutters, and were not defined as circular karst depressions Point labels for altitude on flat surfaces mark circular karst depressions that are shallower than contour interval of the topographic map (5 m)

Topographic map

On slopes, curved contour lines with a lower central altitude than the altitude downslope represent circular karst depressions

Topographic map

On slopes, curved contour lines with a higher or equal central altitude relative to the altitude downslope represent depressions of an amphitheatral geometry. Only those occurring individually are circular karst depressions

Topographic map

Zones with thicker soil arranged in a string within the linear features are terraced floors of trenches and are not treated as circular karst depressions

DOF

All linear negative structures are detected as linear karst depressions irrespective of ground plan pattern and profile

Topographic map

35c

35d

35e 35f

35g

35h

scale 1:5,000 with contour lines with 5 m interval, DOF in resolution 0.5 m and field measurements. The limitations of those methods are presented in Šegina et al. [142]. Traditionally applied procedures based on contour concept (topographical maps, automatic extraction from DEM) are unsuccessful when the karst depression upper rim varies in altitude (Fig. 1.35a, b) and when contour interval is too wide to capture actual karst depression upper rim (Fig. 1.35c). Field examination on the test site indicates that even karst depressions located on a very flat surface are bounded by a vertically irregular rim. Šegina et al. [142] concluded that there is no single approach that would work optimally in all circumstances and that combining several spatial data sources gives

1.4 Methods

39

Fig. 1.34 Detecting circular and linear depressions by combining DOF and topographic map. Blue line—circular karst depression. Green line—linear karst depression. See 0 for description

40

1 Study Area and Methods

Fig. 1.35 a—Karst depression upper rim manually extracted from DEM based on high-resolution LiDAR data and the karst depression upper rim extracted from DEM by common procedures, following the uppermost contour concept. The upper rim elevation in case of particular karst depression vertically varies for even 8 m. b—Karst depression representation on the topographical map: uppermost closed contour vs. karst depression perimeter defined on the field. c—Karst depression perimeter acquired on field occupies an altitude within a contour interval

optimal results. It was established that digitized data are subject to shape and especially noticeable size errors. Perimeters defined by DOF using shadow effect correspond very well to the actual rims determined by the field survey. Other vegetation types as bare karst land and areas covered by bushes and coniferous trees could be reliably mapped by field survey only.

1.4.2.2

Method Applied for Delineating Karst Surface Features

The circular depression planar shape was delineated by DOF in resolution 0.5 m, where visual variations in surface representation that are linked to the change in surface slope (shadow effect, change in vegetation, land use and stoniness of the surface) gave quite a good approximation of the depression upper rim (Fig. 1.36). Even though proved to be slightly overestimating the size of depressions, delineating from DOF was applied as a more accurate method in comparison to the highest closed contour on a topographical map. In bare karst land, the delineating of depressions from DOF is particularly aggravating, as the rim is completely invisible in the uniform stony landscape. The delineation was there performed by extrapolation of ratios measured in several karst depressions. Due to the low reliability of data in bare karst areas, the operation with the information of the presence and location of depressions was preferred over the planar shape size and shape. The course of linear surface features was delineated by the combination of topographic map and DOF due to low 3-D perception on DOF. It followed contour curving on a topographic map and thicker soil accumulations, change in vegetation and shadow effect on DOF (Fig. 1.37).

1.4 Methods

41

Fig. 1.36 Delineating karst depressions from DOF. a—Abrupt change in slope inclination is visible due to a shadow effect. b—Delineation of the same depressions

Fig. 1.37 a—Visibility of linear features extent on DOF. b—Visibility of linear features extent on a topographic map

1.4.3 Spatial Analysis and the Employed Parameters Spatial analyses were employed to the following data set (Table 1.5): A complete list of performed spatial analyses is presented in (Table 1.6): Table. 1.5 Basic statistics of measured parameters for a total of 5867 analysed surface features on Krk Island Total of 4917 circular depressions on karst Min P—perimeter (m) A—area of a ground plan

8.35 (m2 )

Max 12 299.25

Mean 227.55

St. dev 392.29

5.05

11,298,545.82 13,838.22 263,064.75

Total of 950 linear features on karst

Min

Max

L—length (m)

22.54

26,051.95

Mean 727.55

St. dev 1,716.56

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1 Study Area and Methods

Table. 1.6 Spatial analyses performed in this study Analysed feature

Parameter

Method

Relief

Topographic roughness

Terrain ruggedness Summing the change in index [143] elevation between a grid cell and its eight neighbour grid cells

Procedure

Coastline

Coastline ruggedness Sinuosity

The divergence of a line from the Euclidean distance between the endpoints of the curve

Circular depression on karst

Area

Measurement

Measurement of depression’s planar shape bounded by the upper rim

The circularity of a planar shape

Circularity index (see Sect. 1.4.3.1)

The circumference of a circumscribed circle/the perimeter of the depression

The orientation of a planar shape

Elongation index (see Sect. 1.4.3.1)

The orientation of the best-fitting ellipse long axis

Elongation of a planar shape

Elongation index (see Sect. 1.4.3.1)

The numerical eccentricity ε of the best-fitting ellipse

The density of depressions

Density index (see Sect. 1.4.3.1)

The ratio between the depression area and the area of the belonging Voronoi polygon [144]

The density of depressions

Point density (see Sect. 1.4.3.1)

The area of the belonging Voronoi polygon

Type of karst depression distribution

Average nearest neighbour index (ArcGIS 10.2)

The average distance between each feature centroid and its nearest neighbour’s centroid location/expected average distance (with the expected average distance being based on a hypothetical random distribution with the same number of features covering the same total area) (continued)

1.4 Methods

43

Table. 1.6 (continued) Analysed feature

Linear feature on karst

1.4.3.1

Parameter

Method

Procedure

The degree and directional trend of distribution

Standard deviational ellipse (ArcGIS 10.2)

Calculating the standard deviation of the x- and y-coordinates of the features from the mean centre of the population to define the axes of the ellipse

Length

Measurement

Measurement of a complete linear feature’s length

Orientation

Measurement

Orientation measurement of an individual linear feature’s segment

Branching

Strahler–Horton stream number [120, 122]

Assigning a numeric order to segments representing branches of a linear network

Meandering

Sinuosity

The divergence of a line from the Euclidean distance between the endpoints of the linear feature

Drainage pattern

Listed in Assigning a type of Summerfield [145] channel network geometry

Longitudinal profile

Measurement

Measurement of linear feature floor altitude along the main trench

The density of linear features

Line density (ArcGIS 10.2)

Calculating the density of linear features in the neighbourhood of each output raster cell in length per unit of area

Circularity and Elongation Indices

Despite numerous circularity and elongation calculations performed on karst depressions, their applicability in karst surface morphology is still not evident. The correlation between geological structures and the geometry of karst depressions was traditionally evaluated by the use of elongation index, especially for large [114, 146] but also for small karst depressions [110]. It should be considered though that effective surface reshaping in most cases successfully blurred the structural predispositions involved in the formation of initial negative mass (Sect. 2.1). As the tendency towards centricity is the only stable attribute of a Dinaric doline, the value of the

44

1 Study Area and Methods

Fig. 1.38 Parameters of indices employed in morphologic analyses. a—Circularity index applied for sub-circular shapes. b—Circularity index applied for highly irregular shapes. c—Elongation index

circularity index can only give the evidence of the efficiency and duration of karst surface reshaping (Sect. 2.2.1.2). Its high value in karsts similar to the Dinaric karst is already expected. For that reason, rather than their influence to karst depression shape, the influence of geological discontinuities to the presence, location and size of karst depression should be considered. Circularity describes the (ir)regularity of depression planar shape and defines its divergence from the perfect circle. Due to the general tendency of karst depressions towards centricity, the divergence in circularity is theoretically expected to reveal the local differences in “initial depression-forming” processes or impure karst conditions. Among several expressions describing circularity of karst depression planar shape, the circumference of a circumscribed circle/the perimeter of the depression was employed to determine the circularity of sub-circular shapes [142] (Fig. 1.38a), while the circularity of highly irregular ones was calculated by the perimeter of the depression/the circumference of the largest inscribed circle [142] (Fig. 1.38b). Elongation describes the extension of depression planar shape in a particular direction. It is a dual-aspect parameter that describes the degree of elongation and its orientation. It is mostly presumed that its value correlates with a structural predisposition. Among several expressions describing elongation of karst depression planar shape, the numerical eccentricity ε of the best-fitting ellipse as the optimal method [142] was employed. The best-fitting ellipse was calculated as the standard deviation of the x-coordinates and y-coordinates from the mean centre. The orientation was determined based on the orientation of the best-fitting ellipse long axis (a) (Fig. 1.38c).

1.4.3.2

Density Index

As karst depressions possess a certain planar size, in the distributive analysis it is more convenient to observe them as polygons rather than points. Large karst depressions

1.4 Methods

45

Fig. 1.39 Distance between the centres of karst depressions 1 and 2 is the same as the distance between depressions 2 and 3 despite different distances between them

may appear close to each other, but the points representing their centres are as far apart as the sum of their radii (Fig. 1.39). Thus, the density index was calculated to consider the size of karst depressions as well. I calculated the density of karst depressions as the ratio between the karst depression area and the area of the belonging Voronoi polygon: Ad Av where Ad = area (m2 ) of a karst depression planar shape Av = area (m2 ) of the zone where any location is closer to its associated karst depression than to any other karst depression (Voronoi polygon) In the second density calculation, depressions were treated as points. A single depression was represented by its centre of gravity. The belonging Voronoi polygon was determined for the centre of each depression, and its area was measured representing the density of the features without considering its size.

1.4.3.3

Classification Method

As I was dealing with natural phenomena, the data were classified with the Jenks natural breaks classification method that is based on natural groupings inherent in the data [147]. The method minimizes the class’s average deviation from its mean value and maximizes each class’s deviation from the means of the other groups.

1.4.3.4

Problems of Advanced Spatial Analysis

Due to the inaccessibility of high-resolution LiDAR data for the study site, this source of input data was not taken into consideration at the evaluation of detection

46

1 Study Area and Methods

and delineation errors. Nevertheless, I will point out some pitfalls that remain present at employing such data to karst surface despite its extremely high accuracy: The problem of variability of the karst depression rim on the z-axis. When automatic methods delineate karst depressions by defining their uppermost closed contour or limit them based on pour points, the actual vertically variable upper rim remains undetected. However, high-resolution LiDAR data represent a good source for manual delineation of karst depression rims. The problem of sediment infillings in karst depression. Sediment accumulated in the central part of flat-floored karst depressions masks the shape of the rocky walls of the depression and impedes the determination of the actual karst depression geometry [71]. As the principles governing sediment accumulation have not yet been defined, and since the depth of sediments can considerably vary [148, 149, 150, 151], morphologic analyses based on the depth parameter (i.e. volume, depth) could only be applied as a method for relative comparison within the particular region. If no additional sub-soil methods are applied, morphologic analysis of sediment-floored karst depressions should refer exclusively to the visible part of the karst depression or should be restricted to the two-dimensional ground plan geometry. The problem of the fluvially conditioned interpolation methods. Interpolation methods applied in the generation of surface models out of measured point data are based on the assumption that spatially distributed objects are spatially correlated, meaning that proximity correlates with similarity (fluvial principles). Due to vertical mass removal that characterizes karst surface, planar spatial correlations in a karst system do not exist. The space between measured points can hold whichever value and the interpolations are, theoretically, impossible.

1.4.4 Supplementary Methods Besides the data collected from topographic maps, aerial photography, DTM and fieldwork, additional methods had been used to supplement the available geologic and geomorphic data for spatial analysis. To run spatial correlations of karst surface features morphology and distribution to basic geological proprieties, the CaCO3 content has been measured for rock samples collected on the major lithostratigraphic units of Krk Island as these data have been partially missing in the available literature. Ground-penetrating radar as a non-destructive geophysical method has been applied to display the actual geometry of the karst surface features masked by the unconsolidated sediments. The geochemical analysis was employed to determine loess deposits.

1.4 Methods

1.4.4.1

47

Geochemical Analysis of Sediments

Samples for geochemical analysis for loess determination were extracted at the depth of 40 and 200 cm. The geochemical analysis was performed by the chemistry laboratory Bureau Veritas in Canada by applying the method of inductively coupled plasma mass/emission spectrometry (ICP—MS/ES). A detailed description of the method is presented in Klun [152].

1.4.4.2

Complexometric Titration of Rock Samples

A total of 33 samples from Krk Island (Fig. 1.40) have been analysed in the Chemical Analytical Laboratory of the ZRC SAZU Karst Research Institute in Postojna, Slovenia, to obtain missing basic geologic data for the study area and to find the potential correlation between the existence of karst surface features and CaCO3 content in the rock. The laboratory analysis followed the procedure presented by

Fig. 1.40 Sampling locations of analysed rock samples. Geological map by Veli´c and Vlahovi´c [4]

48

1 Study Area and Methods

Müller [153]: (i) crushing of the rock sample by agate mill, (ii) drying in the oven for 1 h on 120 °C, (iii) weighing, (iv) desiccation, (v) filtration and (vi) titration. Analyses were performed by Mateja Zadel and the author of this doctoral thesis.

1.4.4.3

Ground-Penetrating Radar

Karst surface features are usually at least partly covered by the sediment. Estimations of the depth of sediments in karst depressions are very few, among them achieved by the borehole drilling [154, 155], electric resistance tomography [149, 150, 151], aerial photos in the infrared spectrum [156], and by ground-penetrating radar [46, 157, 158, 159, 160]. The estimated depths of the sediments are very variable, reaching even 30 m [161]. Morphologic analysis based on masked morphology may, therefore, lead to misleading results. To approach the actual morphology of partly visible surface features and to detect completely covered surface features, the ground-penetrating radar (GPR) was employed as a non-destructive geophysical tool for obtaining highresolution images of near-subsurface. A total of 80 profiles were measured during four field campaigns from May 2015 to December 2016 with a total length of 5025 m. The Mala ProEx GPR commonoffset survey method was used. Two different antennae, an unshielded 50 MHz RTA (Rough Terrain Antenna) and a shielded 250 MHz antenna, were employed. See ˇ Ceru et al. [46] for the results of GPR measurements. ˇ The GPR profiles were measured, processed and interpreted by Teja Ceru at the Faculty of Natural Sciences and Engineering in Ljubljana. The results of the GPR surveying served to define: (i) the depth of sediment infillings in various types of karst depressions, namely in Dinaric dolines, unroofed caves and large karst depressions, (ii) the depth of Quaternary deposits and (iii) the detection of caves associated with the existing surface features (Fig. 1.41).

1.4 Methods

49

Fig. 1.41 Surface appearance of some karst surface features that were subject to GPR measurements. a—Sediment-filled vertical shafts, b—sediment-filled horizontal passages of unroofed caves, c—sediment-filled bottoms of large karst depressions, d—sediment-filled bottoms of Dinaric dolines. Photos E. Šegina

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57. Geološki konzalting d.o.o. (2002) Istraživaˇcko bušenje PSB-1, PSB-2 I PSB-3, Stara Baška, otok Krk. Zagreb, p 14 58. Geoaqua d.o.o. (2003) Vodoopskrbni sustav otoka Krka. Zagreb, Stara Baška, p 4 59. Rubini´c, J. (2005) Hidrološke znaˇcajke izvorišta vodoopskrbe na podruˇcju otoka Krka. Hidrološka analiza u svrhu novelacije zaštitnih zona izvorišta vodoopskrbe, p 50 60. Petrik M, Zebec M, Meštrov M (1971) Limnološke karakteristike Jezera na otoku Krku. Krš Jugoslavije 7(6):189–206 61. Šerko A (1948) Kraški pojavi v Jugoslaviji. Geografski Vestnik 19:43–70 62. Nicod J (2003) Les karsts Dinariques—paysages et problèms. Slovenska akademija znanosti in umetnosti, Ljubljana, Slovénie, Croatie, Bosnie-Herzegovine, Monténégro, p 183 63. Giorgolo S (2016) Personal communication 64. Correggiari A, Roveri M, Trincardi F (1996) Late Pleistocene and Holocene Evolution of the North Adriatic Sea. Il Quaternario 9:697–704 64. Pirazzoli PA (2005) A review of possible eustatic, isostatic and tectonic contributions in eight late-Holocene sea-level histories from the Mediterranean area. Q Sci Rev 24:1989–2001 ˇ Juraˇci´c M (1998) Geomorphological indicators of sea level changes during upper 65. Benac C, Pleistocene (Würm) and Holocene in the Kvarner region (NE Adriatic Sea). Acta Geogr Croat 33(1):27–45 ˇ Gržanˇci´c Ž, Šiši´c S, Ruži´c I (2008) Submerged Karst phenomena in the Kvarner Area. 67. Benac C, In: Marjanac T (ed) Proceedings of the 5th international ProGEO symposium on conservation of the geological heritage, pp 12–13 68. Faivre S, Pahernik M, Maradin M (2011) The gully of Potovoš´ca on the Island of Krk—The effects of a short-term rainfall event. Geologia Croat 64(1):67–80 69. Alfirevi´c S (1969) Jadranske vrulje u vodnom režimu Dinarskog primorskog krša i njihova problematika. Krš Jugoslavije 6, JAZU, Zagreb, pp 183–205 70. GEOS (2015) Radni izvještaj o rezultatima hidrogeoloških istraživanja i istražnog bušenja na krškim poljima Kimpi i Kaštel na otoku Krku. GEOS Društvo za geološka istraživanja, projektiranje i inženjering, Rovinj, p 25 71. Šušteršiˇc F (2017) A conceptual model of Dinaric Solution Doline dynamics. Cave Karst Sci 44(2):66–75 ˇ Kovaˇci´c M, Kirinˇci´c M (2001) A submarine cave at the island of 72. Arko-Pijevac M, Benac C, Krk (north Adriatic Sea). Natura Croat 10:163–184 73. Trinajsti´c I (1965) Vegetacija otoka Krka. Ph.D. dissertation, Sveuˇcilište u Zagrebu, p 702 74. Rogi´c V (1961) Osobine i postanak današnjeg pejzaža. Geografski Glasnik 23:67–101 75. Gams I (1971) Podtalne kraške oblike. Geografski vestnik XLIII:27–45 76. Gradstein FM, Ogg JG, Schmitz MD, Ogg GM (2012) The geologic time scale 2012, Vols 1 and 2. Elsevier, Amsterdam, p 1137 77. Rohling EJ, Abu-Zied RH, Casford JSL, Hayes A, Hoogakker BAA (2009) The marine environment: present and past. In: Woodward JC (ed) The physical geography of the Mediterranean. Oxford University Press, Oxford, pp 33–67 78. Lambeck K, Antonioli F, Purcell A, Silenzi S (2004) Sea-level change along the Italian coast for the past 10,000 years. Q Sci Rev 23:1567–1598 79. Benjamin J, Rovere A, Fontana A, Furlani S, Vacchi M, Inglis RH, Galili E, Antonioli F, Sivan D, Miko S, Mourtzas N, Felja I, Meredith-Williams M, Goodman-Tchernov B, Kolaiti E, Anzidei M, Gehrels R (2017) Late Quaternary sea-level changes and early human societies in the central and eastern Mediterranean Basin: an interdisciplinary review. Q Int 449:29–57. https://doi.org/10.1016/j.quaint.2017.06.025 80. Brunovi´c D (2019) Morska i jezerska sedimentacija u potopljenom krškom bazenu: taložni sustav Lošinjskog kanala tijekom kasnog kvartara. Ph.D. thesis. Prirodoslovno-matematiˇcki fakultet, Sveuˇcilište u Zagrebu, p 385. 81. Lambeck K, Antonioli F, Anzidei M, Ferranti L, Leoni G, Scicchitano G, Silenzi S (2011) Sea-level change along Italian coast during Holocene and a protection for the future. Q Int 232:250–257. https://doi.org/10.1016/j.quaint.2010.04.026

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Chapter 2

Theoretical Background

2.1 Karst System By the geomorphologists, karst had been traditionally observed as a type of relief due to the inaccessibility of underground features [1]. The findings of hypogene karst provoked the ideas that karst system may not necessarily reflect in the landscape [2]. The recognition of karst denudation and its crucial role in surface morphology revealed the inseparable correlation of karst relief to speleogenesis [3, 4]. High diversity of karst features as a consequence of not completely developed karsts (in the sense of Šušteršiˇc [5] around the world has led to localized studies instead of solving basic and general questions of karst and karstification. To overcome this obstacle, Šušteršiˇc (o.c.) developed “The Pure Karst Model” by which he visualized fundamental processes of karstification resulting in basic geomorphic elements of karst surface: centrally organized depressions and intermediate elevations (Fig. 2.1). As the karsts around the world are mostly impure, it is necessary to understand the differences and similarities between the processes of both, karstic as well as non-karstic environments (fluvial, arid, periglacial, glacial, etc.). Considering local conditions, only karstic and fluvial systems will be faced (Fig. 2.2). Both systems depend on the same transport medium: water. However, in a karst system, the mass removal (in its widest sense) is mostly due to chemical, and in a fluvial system, mostly due to mechanical proprieties of the water. The physical proprieties of the eroded residual define the direction of the optimal path for its transport: the particles downhill and the dissolution vertical. The direction of mass transport characterizes the morphology and gives the typical configuration to the surface. Chemical dissolution and mechanical removal are competitive processes that take place in both environments simultaneously as they do not exclude each other [6]. The prevalent efficiency of one or the other defines the landscape morphology and consequently the type of a particular system (Table 2.1). Surface karstification or karst denudation consists of both chemical and mechanical weathering processes, even though in practice only solutional denudation rates are usually considered [7] (Table 2.2). The gravitational processes, such as the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 E. Šegina, Spatial Analysis in Karst Geomorphology: An Example from Krk Island, Croatia, Springer Theses, https://doi.org/10.1007/978-3-030-61449-2_2

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Fig. 2.1 Karst surface is characterized by towards circularity and centricity tending depressions of different sizes. An example from Laški Ravnik, Slovenia. Source Public information of Slovenia, the surveying and mapping authority of the Republic of Slovenia, LiDAR, 2015

Fig. 2.2 LiDAR representation of karst and fluvial surfaces. Source Public information of Slovenia, the surveying and mapping authority of the Republic of Slovenia, LiDAR, 2015

processes of transport and additional mechanical disintegration of the rock, should also be considered. Chemical erosion (corrosion) primarily occurs because of the mineral surface reaction, where the hydrogen cation (H+ ) as a product of carbonic acid (CaCO3 ) and bicarbonate decay, attracts the CO3 2− anion from the surface of the calcite mineral.

2.1 Karst System

59

Table 2.1 Comparison of karst and fluvial systems Pure karst system

Erosion

Transport

Accumulation

Pure fluvial system

Surface

Underground

Surface

Prevalent erosion medium

Water

Water

Water

Property of the medium with the highest erosive capacity

Chemical activity

Chemical activity

Mechanical agents

The process governing the transition into transportable state

Dissolution

Dissolution

Mechanical weathering

Type of eroded mass

Dissolution

Dissolution

Particles

Direction

Vertical

In all directions

Downhill

Resulting morphology

Circular

3-D network

Dendritic

Extent

Planar

3-D canalized

2-D canalized

Type of transported Dissolution material

Dissolution and particles

Particles

The process Gravitation governing transport

Gravitation and hydraulic head

Gravitation

Direction

Vertical

In all directions

Downhill

Condition

When environmental pCO2 < solution pCO2 (travertine formation)

When local environmental pCO2 < solution pCO2 and at critical deposition velocity as a function of particle size (flowstone formation)

At critical deposition, velocity as a function of a particle size

Location

Underground

Underground and outside the system

Outside the system

The general geomorphic tendency of Vertical dissection the system surface when the corrosion base is under the surface, and planation when the corrosion base matches the surface

Declining surface gradients [99–101]

60

2 Theoretical Background

Table 2.2 Processes of karst denudation Karst denudation Chemical weathering

Mechanical weathering

A function of chemical and physical proprieties A function of mechanical proprieties of rock of solvent and solute, environmental factors and and environmental conditions hydrological conditions of dissolution

Secondarily, it is a result of the reaction of CO3 2− with free H+ released at the dissociation of H2 O or the direct reaction with molecules of H2 O and CaCO3 . The total solution rate of calcite is determined by the slowest of the operating processes: (i) the reaction on the surface of the mineral, (ii) transport (diffusion of the released molecules) or (iii) the hydration of CO2 . It depends on the phase and the conditions of the dissolution (temperature, pH, pCO2 , hydrological conditions) [8, 9]. In the laminar flow conditions, the dissolution rate is mainly transport-limited [9, 10] or limited by the hydration of CO2 [11]. After the breakthrough and the establishment of the turbulent flow, the calcite dissolution rates get limited by the surface reaction, and the widening of the fracture proceeds fast and virtually along its entire length [11]. Mechanical erosion is a compound of various physical processes that cause a rock to disintegrate into smaller and transportable fragments, where the oscillation in volume due to temperature and humidity variations or crystallization of salt, causes the material to crack. Gravitational (slope) processes are a consequence of the material potential energy and are independent of any transport mechanism. They encompass mechanical rock disintegration as a consequence of mechanical stress exceeding the internal stability of the rock and the process of gravitational transport. The proportion of chemical and mechanical erosive processes, as well as the role of gravitational processes that are involved in the karst denudation, has not yet been entirely clarified. In order to achieve relatively pure karst morphology, chemical erosion must prevail over mechanical erosion, giving the main character to the landscape. However, the efficiency of the competing processes varies considerably. Recent karst denudation rate in different locations achieved by various methods on diverse carbonate bedrocks was for 1,000 years long exposure estimated to 1.5– 2 cm [12], 4 cm [13], 9.4 cm [14], 2–10 cm [15], 1.1–4.8 cm [16], 0.9–14 cm [17] and 1.8–2.5 cm [18], considering that the recent climate conditions are karstification favouring. On the other side, low rates of recent speleogenetic processes measured in epiphreatic caves in Slovenia and ranging between 0.0061 and 5° − Non-graded linear profile − Siliciclastic watershed and high topographic heterogeneity − Traces of surface fluvial sediments and erosion of recent fluvial activity during heavy rain

Unrecognizable (unroofed horizontal cave or geologic structure)

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2 Theoretical Background

Fig. 2.16 Spatial distribution of linear features on karst by class for the test site of Krk Island

2.2.5 Complete Classification of Karst Surface Features Several types of centric and linear features have developed on Krk Island due to complex geological and climatic guidance. Based on the fieldwork and the discussion in Sect. 2.1, the following complete classification of circular and linear karst surface features was synthesized (Table 2.5). It contains features appearing on pure karst, in not fully realized karst conditions and on karst characterized by the intrusion of torrential surface runoff, valid for the study site but applicable also to the wider karst area. This classification represents the basis for spatial analysis in Part 3.

2.2 Definition and Classification of Karst Surface Features

81

Fig. 2.17 Appearance of karst linear features on Krk Island. a—Linear feature of speleogenetic origin: unroofed horizontal passage. b—A linear feature of geologic origin: karstified bedding plain on overturned anticline. c—A linear feature of tectonic origin: karstified fault. d—Karstified tectonic structure considerably reworked by torrential surface runoff. e—Karstified tectonic structure weakly reworked by torrential surface runoff. f—Karstified tectonic structure considerably reworked by torrential surface runoff

Pure karst features

Linear

Circular

Basic geometry

Active doline [92]

Polje, uvala

Karst depression with additional mass-removal mechanisms

Regional negative undulation

Denuded cave, roofless cave Bogaz

Unroofed epiphreatic cave

Weathered and karstified tectonic structure

Unroofed vadose shaft

Doline, solution doline, Dinaric doline, collapse doline, sinkhole

Other comparable nomenclature

Simple karst depression

Feature

Table 2.5 Classification of karst surface features

Several metres wide Karstification of a and deep straight dyke linear tectonic along the fractures structure

Empty or cave/surface Denudation of a sediment-filled horizontal passage horizontal cavity

Denudation of an empty vadose shaft

Selective karstification Tectonic of regional tectonic structures

Largesta depression of any geomorphic appearance (flooded/dry) Empty or surface sediment-filled vertical cavity

Denudation, additional Speleogenetic and erosive factor and tectonic karstification of the empty centric cavity

Distinctively largera depression of any geomorphic appearance

(continued)

Tectonic

Speleogenetic

Speleogenetic

Speleogenetic tectonic or non-karstic

Denudation and karstification of the empty centric cavity, or karstification of tectonic or non-karstic initial depression

Relatively smalla depression of any origin and geomorphic appearance

Origin of the initial depression

Forming processes

Basic characteristics

82 2 Theoretical Background

Dry valley

Fluvial

Origin of the initial depression

Unrecognizable (tectonic or speleogenetic)

Unfavourable climatic Fluvial or geological conditions preventing effective karst drainage

Headwards retreat of Fluvial spring on a margin of a karst region

Prolongation of an allochthonous stream on karst

Forming processes

Fluvial valley on karst Fluvial reshaping during past climate conditions favouring fluvial processes

Rocky or sediment-covered dyke with torrential surface runoff

Wadi, dolec, etc.

Weathered and karstified tectonic structure or unroofed horizontal cave reworked by torrential surface runoff

Valley appearing at the karst spring

Pocket valley

Basic characteristics Valley terminating at the ponor

Other comparable nomenclature

Blind valley

Feature

size range depends on the local hydrogeological conditions

Linear

Karst features transformed by torrential surface runoff

a Absolute

Linear

Not fully realized karst features

Basic geometry

Table 2.5 (continued)

2.2 Definition and Classification of Karst Surface Features 83

84

2 Theoretical Background

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Chapter 3

Spatial Analysis

3.1 Relief The terrain ruggedness index (TRI) values indicate three major relief units (Fig. 3.1). The relief of the northern part of Krk Island is the least rough. Flat lowlands are located mainly 5–45 m above m.s.l. A slight increase in terrain ruggedness on the NE coast is linked to the minor syncline with the Dinaric strike. In the central part of the island, the lowland passes to the undulating surface located mainly 60–160 m above m.s.l. Increased TRI values in the east of the central part are linked to the main tectonic structure on the island, i.e. the syncline Omišalj-Baška. In the southern part of Krk Island, the relief is characterized by the highest heterogeneity, where two relatively flat plateaus of elevations mainly 240–360 m above m.s.l. are bordered by steep slopes. The local considerable decrease of TRI is linked to the floor of already mentioned syncline Omišalj-Baška, while the slight decrease of TRI on the eastern plateau is a remnant of dissected undulating karst surface.

3.2 Coastline Applying a 10 km interval, calculated values of coastal ruggedness reflect regional structures as synclines and regional undulations. Deep and large bays were formed by the erosion of siliciclastic cores of synclines and by the flooding of regional negative undulations (Soline and Puntarska draga coves) (Fig. 3.2a). At a smaller scale (2 km long coast segments), the coastline is less dissected and consists even of relatively straight sections (Fig. 3.2b). The coastline ruggedness is not linked to the relief configuration (Fig. 3.1) but reflects the frequency of surface features. Straight sections are linked to synclines where siliciclastic cores were partially or completely washed out relatively recently. In Puntarska draga regional negative undulation, the coast appears straight where a large amount of unconsolidated sediments covered and masked the primarily rugged coastline. On the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 E. Šegina, Spatial Analysis in Karst Geomorphology: An Example from Krk Island, Croatia, Springer Theses, https://doi.org/10.1007/978-3-030-61449-2_3

89

90

3 Spatial Analysis

Fig. 3.1 Relief units of Krk Island obtained by the terrain ruggedness index (TRI)

contrary, the coast of regional negative undulation Soline is dissected by submerged depressions (Fig. 1.17). The high ruggedness of the northern and south-eastern coasts reflects the NW-SE-oriented bedding planes. High faulting frequency is indicated in the central part of the island by the high coast dissection between Vela Jana and Sv. Juraj Coves on the west and between Šilo and Vrbnik on the east coast.

3.3 Circular Depressions on Karst Spatial analysis of circular depressions was performed to establish the spatial variability of morphologic and distributive characteristics of such features that would help to explain the processes that may have had operated on the karst surface of Krk Island. The results of the spatial analysis are presented separately for simple depressions, depressions with additional mass-removal mechanisms and negative regional undulations (see Sect. 2.2.1 for the classification).

3.3 Circular Depressions on Karst

91

Fig. 3.2 Coastline ruggedness. a—10 km interval. b—2 km interval

3.3.1 Simple Depressions To reveal the spatial variations in formation and reshaping of simple depressions, I established 13 square zones sized 1 km × 1 km and arranged them over the entire test site to cover all the potential areas where depressions could theoretically exist. The two required conditions were carbonate bedrock and slope inclination of