Shallow marine and fluvial environments of Quaternary deposits in Pehuen-Có Beach, Buenos Aires, Argentina

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Y1CTOR VEGA & MIGUEL VALENTE

3

Departamento de Geología, Universidad Nacional del Sur, Bahía Blanca, Argentina

SERGIO RODRIGUEZ

.\

Departamento de Geología, Universidad Nacional del Sur, Bahla Blanca & CTC de la Peía de Buenos Aires, Argentina

Shallow marine and fluvial environments of Quaternary

deposits in Pehuen-Có Beach, Buenos Aires, Argentina

AB S TRA C T The i n st udi ed o utc ro ps ar e a pproximately l oca t ed the prt:.ent intertidal area o f Pehuen-Có Beach , at a bou t 10 k m t o the SE of th ~ c i t y o f Ba h1 a B l anca ( 1 a t . 3 9 º S a n d 1 o n g • 6 i• 3 5 ' W ) • Deposits appe ar discont inue dl y, 4 to 5 km lo n g a nd

1 00

to

150 m w id e,

and

thei r

t hickness

is

map

a t l : 2000 sea l e. Ten faci e s h av e

b e en

we r e ta b 1 e

determined :

1 ) Cross - bedd ed q ua rt zit ic co ng lo me r ate , to the S and SE. 2) Cl a ye y s a ndstone with s u bo r din a ted c lasts . 3) Pelites w ith dessication cracks ic hni tes . un i t s corr e spond to T hes e an d a s e qu e n ce of a11uv ia1 fluvial deposits . Th e r e mai n i ng fac i es ar e : 4 )

less s ma 11

than m. Late r al f acies re la t ionships , v er ti cal seq u enc e s and pal e oflo w data described with certain accu r acy in a p 1ane

a

mi gr ati n g q u a rtzitic and

b ir d

longitudi n al

pl ai n

f acie s

bar f ro m

P l ana r

cro s s - bedded

sandstone ,

mi g r ating

to

t he

T r ough

c r o s s - b e dded

sandstone ,

migrating

to

the

w. 5)

W and subo r di na tely to 6) Pelit es ( mu ds to n e mudstone with ripple or ie nt e d

the E and S . an d ~i lt sto ne ) , l oc a lly a symmetric a l t o the top (E - W

r i dges).

51

inclinada 7)

Parallel

S andstone

8)

migra ti ng 9)

laminated sandstone.

to

with the

truncated

of

of calcareous sandstone, structures, quartzites ,

biogenic

sandstone, coquines Sandstone ,

10)

ang1e

c r oss

and

remnants

of

shelly

partially conglomerated,

be d d i n g ,

pl ungi ng

4•

to

8°

3)

remains

A conglomerate

clayey sands . with

represented by deposits which range from (facies 7 and 10), high shoreface (facies through and low shoreface {facies 8) {facies 6). These sediments were mainly

foreshore

in

a

shallow

faci~s

3

to

marine

partia l ly

4 • 5• g) • offshore generated

action of high energy waves·. of eei l i ng) base to lhP ~rrangement { from { 7 ) + 5 + 8 b • fac i es 9 + 4 + facies sequence ; s + (10) + 4 facies 9 + + 6 facies 6 + 4 + 5 and O: by

the

5 .

The

evolution indicates

column 1 e ve l

rise

1eve1

lowering

in

are a .

the

stratigraphic the integrated existence of two relative sea

of the

8

{sequence

and the

during

O)

and

a

relative

deposition

sea

of sequence

Pelitas

C

con

RESU MEN

playas

del

km

sureste

al

posi e i ón

balneario de

g e o g r él f i e a

1 o n g • 6 i• 3 5 ' O • Los depósitos

por

4

a S km de

Pehuen-Có,

la

ciudad es

se

largo

de

situado Bahia

a p ro x i ma dame n te

observan y

100

a

ubican

en forma 150 m de

a

de unos

Blanca. l a t . 3 g• S

las

70

Su y

saltuaria, ancho ,

con

que

latera les verticales

de

no

superan

facies,

las

cuarclticos

de

longitudinal

y

llanura

4)

e

icnitas

de

facies

desecación secuencia de

inundación

de

un

de

fluvial.

Las fac i es Arenisca con

migrando

restantes

son :

estratificación

planar

entrecruzada

o.

al

5) Arenisca con estra ti fi cae ión en entrecruzada artesa migrando al O y subordinadamente al E y S. 6) Pelitas {fangolitas y limolitas), localmente limolitas (crestas 7) 8)

Arenisca

ondulitas

con

al

l aminación

con

simetricas

al

E.

Conglomerado

de

· aren1sca

estructuras

techo

para l e l a .

"truncated

y

NE

al

E-0).

rumbo

Arenisca

migrando 9)

con con

wave -r ipple ca l cilrea,

biogénicas,

laminae" restos

cuarcitas,

arcil l osas . coquinas y restos de 10) Arenisca, conglomerildica

conchillas. en parte,

estratificación

entrecruzada

de

bu za n d o facies

S • C o r r e s p o n d en a en un ambiente

4• a 8º a 1 generadas

Y

por

g),

{facies

Estas

evolución indica

estadios

de

de

la

By

de

D)

y

l a

la

desde

alto

columna para

offshore generadas

oleaje

bajo

estratigrilfica el

élrea

relativo

del

descenso

relativo

secuencia

el

(facies

y

8)

fueron

del

existencia un

van

bajo {facies

sedimentitas

ascenso

depositación

que

coro

angulo,

s e cu en c i a s d e marino somero

· shoreface

por acción alta energ1a.

integrada {secuencia

10),

y

shoreface

6) .

principalmente condiciones de La

depósitos

(f a c ies

bajo 3

de

areniscas

ni ve 1

de de

durante

dos mar la

C.

relaciones

m. Las p e q u eñ a s se e u en e i a s de paleo flujos fueron los datos y base mapa precisión en un cie r ta con ubicados, levantado con plancheta a escala 1:2000. Se determinaron 10 facies: estratificación con cuarc1tico Conglomerado l )

espesores

grietas

barra

4,5

actual

c l astos

una

representado

se

sureste.

a

foreshore

estudiados afloramientos Los principalmente en la zona intertidal

o

con

Cor res ponden

low

S. generated

corresponds

sur

aves.

depósito

sequences environment,

l t

al

arcillosa

subordinados.

E.

and

NW

laminae,

wave-ripple

migrando

Arenisca

2)

INTROOUCTION Frenguelli herein

(1928)

studied.

fossiliferous

lime

initially described He recognized presenting a

large

the outcrops ver y sandy calcareous

q3 52

crust in the form of thick roots. lt belongs to a st r eam bed deposit during the "lujanense " stage, followed by a mobile sedimentation during the "Querandinense" stage, within a fluvial valley which was drowned during the "Preensenadense" stage. Mouzo et al . (1975) indicate the presence of beach rock which includes the tubular structures des c ribed by Fre n guelli (1928) . Mouzo (1980) concludes that the organic st r uctures found in the beach sandstone correspond to fossil traces be l onging to the Callianassids . Subsequently, Mouzo et al. ( 1989) define a coarse to medium grained sandstone with parallel criss-cross structure , which contains what they defined · as upper low - beach and biogenic structures in the high beach "in si tu" . Then, they loca te a compact, p a rtially carbonati c c emented sandstone. Without finding any relationship with the deposits previously discovered , they analyze a quartzitic conglomera te in which they observe a marked strearn imbrication suggesting a fluvial origin and conclude that the tu be formation process and the subsequent cementation of the sandstone must ha ve occurred after the deposition of the fluvial conglomerate. They analyze these structures defined as galleries built by Callianassids - and pro pose that the l ack of the upper and intermediate sections of the in sit u Callianassid tubes and the subsequent cementation of the sandstone i ndicate an ero si ve process occurred in this area due to a sea leve l change or to a coastline migration in t hi s location. The purpose of this paper is to demonstrate by means of a facies ana l ysis that : 1) The studied sedirnents were deposited in a shallow fluvial and marine environment. 2) The relationship between both deposits and t he i r present location provide evidence of relative sea l evel changes.

STUDY AREA ANO

FIELD METHODOLOGY

beach , The study area is located in the Pehuen-Có of the in " Playa del Barco", at 70 km to the SE .,l s (Figure 1). city of Bah fa Blanca l t approximately at lat. 39º S a n d 1 o n g . 6 i• 3 5 ' W w i t h with reference to the an East-West variation •) shoreline. i n t he outcrops are located Most of t he i ntertidal area (Figure 2 ) . They can be present observed in an area 4 to 5 km long and 1 oo to 150 m w i de . Thickness, which makes possible to analyze small vertical facies re1ationships, is generally less than 1 m. The outc r ops a.ppear discontinuously due to the intense e r osive process and they are constantly covered by beach s ediments , frequently swept by storm action . Due to the dynamic s o f t h e area, field work was carried out in different times of the year . Thus, all the information obtained was included in a plane table map at sea le l :2,000. In this way, it was possible to loca te, with sorne accuracy , s mall vertical sequences , latera l facies relationships and paleostream data . FACIES OESCRIPTION FACIES 1 completely formed by Reddish - orange conglomera te, quartzitic clasts, which mean si ze ranges from l to Equi dimensiona 1 and cm with a maximurn of 15 cm. 6 oblate shape predominate, with partially sandy and clayey matrix. This presents an incli n ed-type in ne r structure (Fig.3), which sets change due to gr a i n size variations in both clasts and rnatrix. The 'i nc li nation angle i s app r oximate l y 25° Beds are geometrical l y tabular. lt is not possible to know the inner bedding because it remains covered. The maximum thickness observed is 60 cm . In sorne sections, there are "megaclasts" which mean average side size is 50 cm, being equidimensionai, sharp-edged in shape and sandy to clayey-sandy in composi t i on. Sorne cf thern present b i odisturbance

55

54

,.,.

-

f 1

l

F9 F5 "'--'~_,,.._,..,_,, F4

FS

1f

tª

f§~..,.,.

1res

Provincia de Buenos Aires

si -:--~- HWL - - -.¿J:l_

-·-·-·-

.--· --:·--·

_....__ . ·. .

Pehuen-Co

e

conglomerate

wave ripple burrows horizontal V callanasa

DL l .

Location

and vegetal debris. is composed of gaqeries.

map of

the

studied

The biogenic structure observed scarce Callianassid in si tu

Thi s

southwar~

indicate t he existence orientat i on, subordinated

corresponds to a bed tractive unid ir ectional streams, bar grout or lateral migrat ion, inclined bedding.

lo ad deposited which generate evidenced

by

by a i t s

Changes of grain size indicate that the f l ow rate has varied cyclically. Megaclasts indicate th e ex isten ce of small scarpments close to the mentioned deposit which is shown by the size and the lo w rate of erosion i n th eir crests.

56

=

dune line

mud-crack parallel laminfted

~ N

ro

area.

Measu red pa l eocurrents of a prevailing to the SE .

e ship .......-v-

~ · ~·

.. · , · · .

~

SL silt Fl facie.s 1 ELWL exceptional LWL Figure

DL

~·-

~

Sa san4stone

i :;, ;_; .

Figure area.

2. Plan

Oistr i bution of outcrops in the studied view wit h the location of the present are o f the profiles mad e in the area ti des . Four depicted.

FACIES

2

Pale reddish - brown (10R5/4), mean grained c lay ey sandstone, presenting only qua. rt zitic clasts , 0,5 to 4 cm in s i ze . lt lacks internal structure. Bedding

structure

development

of

is the

not

contact with facies is This corresponds generated by

a

v i s i ble

outcrops,

due

15

completely to a bed

decelerating

to cm

the

scarce

thick.

The

trans iti onal . load deposit

st r ea mflow .

57

-

-

3.

Figure

of

contact

40

Ha mmer:

the cm

view

of

over l ying

in

l •

facies

9

fac ie s

which

are

the

ero si ve

Figure

4.

ob se rved.

cracks

a~d

the

and

structure

i n ne r

t yp e

inc lined

FACIE S

General

l ong.

in

The observed biogenic structures are isolated situ Callianassids gal l eries . The paleostream of

th is oran ge They

pin k

these

FACIES

4).

settlement indicate a

formed

isolated

by

m,

mean

criss-cross remnants

clasts

(F ig u re

58

length

being

the

5).

of most

(Figure

shows

6)

a

marked

single

orientation .

This

corresponds

unidirectional migration

of

to

traction

straight

yellowish

Modera te

(lOVR5/4)

brown

bedding .

of

which

shell

follow

sandstone, lt

sand the

5

to

6

cm,

remarka ble

presents as

well

with

gra ined

sorne

7)

as

set slope.

thickness ranges from 20 to 40 cm . In a net inner contact is observed . Set a

fac ie s

westward

a

bed

f lows

crest

load

which

mega

depos i ted generated

by the

ripples.

f A C 1.E S 5

4

Modera te coarse planar cm

l º ªd. of a suspended sub - aerial exposition

sediments

yellowish

sets

laminated parallel desiccation cracks and b ir d Thi s 1O e m. Thickness

(5VR1/2),

present

ichnites (Figure corresponds to the oesicc at ion cracks

of

desiccation Knife : 20 cm

1o n g •

3

Greyish pel ites

o et a i 1 of fac ies 3 where bird ichnites are observed.

sorne plan of

Bed

of

erosive

sand.

Sed and

trough

l ength

following

(10YR5/4) mean

brown of

presents

shelly

13

to

mean

This

observed shaped.

the

a

wi t h

1 5 cm.

sections presents

occasionally up feature

5-

criss - cross

bedded

to

coarse

sandstone

(F igur e

150 cm and a mean sets f ormed by

sorne

l solated set

thickness

is

lami~ar

slope.

30

cm.

It

width

clasts is

Lower

of

remnants are

bar-1

contact

ke is

net .

The observed bingenic structures are galleries

facies bu i 1 t

by

i n

situ

Callianassids;

there

are

ar e as

59

•

5 .

f acies long as

of where

4 •

5 to

continuity observ e d .

6 meters ; s

N

sets

as

Figure 7 . Detai led view of the t r ough crossbedding 5, where the o" facies chara e te r i sti e biogenic s:ructures of these facies a r e observed as w e 11 as the laminar clasts . Hammer 40 cm l ong .

where they have almost compl e t e ly d i sturbed t he inner bed s tr u cture . Pa l eostr e am da t a present e d i n the di a g r am of Figure 6 i n di ca te p r eva i l i ng a westward orientation and subordinately to the E and

N

s. This cor r esponds to a bed load deposited by unidirectional t r active streams '" hich generated mega ripple and wavy ridge mi gration .

n

= 12

Bar - like indicates the

n

= 35 FACIES

F 4

structure presented by this facies existence of a channelled flo w.

6

F 5 Massive p elites (mudstone and mantle - like geometry and

Figu r e 6 . Pa leoflow distribution : F 4, bedding and F S, trough c r os s bedding . data

60

plana r cross n= number of

development . The p re s en t s s y mm e t r i c which wave length is cm .

Crests

are

si 1 ts t one) ha ve a a discontinuous

leve1 of the siltstone top st r a i ght cr os s r i p p 1e ma r k s , 10 cm and its wave amplitude ,

orien t e d

to

th e

E - W.

61

This facies ge neral ly covers beds whose top presents symmetric ripp l e marks (F i gures 8 and 9) with a wave length of 25 to 30 cm and an amplitude of 2 to 3 cm. -~ The observed biogenic structures belong to horizo ntal tracks contained in the mudstone top , determlned by two associated ichnofo r ms (Figure 9): a) irregularly grouped traces , w i th st r aight"' or gently curved development , with in ne r half-moon shaped structure , which convex side i s oriented in the inferred advance direction, 10 to 15 mm wide; and b) negative relief cylindrical wavy tracks, 2 to 3 mm in diameter. This corresponds to a suspended load which (Figure 10).

Figure

8.

General

view of and

like distribution underlying facies is

facies 6 where the mantle wave reworking of the observed . Hammer: 40 cm long.

was

deposited

by

settlement

The ripp le mark top of the limestone bed sho ws a r eworking due to the wave action as the surfaces covered by this facies do which, in this case , is of highe r intensity . Track "a" is interpreted as the trace produced by a debris-eating organism when feeding itself while trac k "b" corresponds to the trace produced by an organ~sm when displacing probably a worm.

FACIES

7

Pale

yellowish

brown

(10YR6/2)

parallel laminated sandstone lamination are 5 mm and mm Lower contact is net planar.

fine grained (Figure 11) Bed and thick, r espective l y. This facies has been

found in an unique outcrop whose development is of approximately 9 m; This corresponds to a tractive, hi gh f1 o w deposited bed load.

FACIES

Figure 9. General view of the net contact between facies 6 and the overlying facies 4. Note the action of w aves on the top of facies 6 . Hammer : 40 cm long.

62

B

Pale yellowish brown (10YRB6/2), fine to very fine grained sandstone with level r ippled laminations (see Figure 12) similar to the one described by Campbell (1966) as "truncatcd wave -r ipple laminae" . The maximum thickn~ss is of 10 cm . Paleostream data of

these

ripple

mar ks

indicate

an

E

and

NE

63

(

111

orientation in the migration . corresponds to T hi s by wave act i on , deposited

r i p ple.

wave

asymmetric

and

suspended which

the

bed

load ,

formed

truncations

the

bétween

t he m.

9

FACIES

•)

L i ght

brown i sh

g r ey ,

conglomera te .

determined clayey

near

section size

Mean with Figure

10.

in

mudstone

the

Oetailed view of facies

of

6.

t he Reference:

is

to in

of of

70

and

maximum

situated

and

there

is

A 1. been

at a

the

l ot

of

" megaclasts " w hich

cm . gra i ned

-s -a n d y -=-c l a y e y

shelly sand

shape .

the lower i n e 1 i n a t i o n t o 3• the

ar ea,

equidimens i ona l

that oi

clast-supported mean

deposits

the

coarse

fragments

blanket-li ke

the

of

sandstone

maximum

po l ymictic,

composition ,

th e clasts are included in Ta b 1 e increase of quartzitic clasts has

size of gradual western

The

lt was

bed

limit

s o u t h wa r d a t

The

ma t r i x

deposi ts

are

possible to determine presents a slight the

southern

section

a rea .

It cou l d be observed dueto the fact that the outcrops were semi-submerged in quiet waters. The resting position of the shells is mainly ventral . In

sorne

places,

the

laminar

clasts

position

is

horizontal . leve l

The of

situ

in

bed

load

biogenic structures observed in some t h i s facies are Call i anassid galleries in

the i r

ho r izontal

deposited

run .

by:

This

Tractive

co r responds high

flow

t o rate

$tream with a marked wave action rework or only the product of the latter . The wave action reworked older sediments, concentrating the clasts , r elocating part i a l ly

the

laminae

wash in g

a wa y

in

horizontal

the

fi n e

p osition

mate r ia l

and

(Fig ur e

13 ) . FACIES

Figure of

64

11

facies

Oetai led 7 .

view

Refe r ence :

of 10

the cm .

10

Pal e red partially

matrix-suppor t ed

composi ti on

is

(lOR0/2) s i mi l ar

coarse to

grained

sandstone ,

co n glome r ate .

F a cies

9

wh ich

Clast

proport i on

65

Figure

l2 .

General

Vi

eW of

t he

truncated

r i pp 1e

ne t

th e

10

Note t he a rea o f facies 8. overlying facies 6 . Reference:

wave-

con ta et w i t h cm.

of General Vi e W o f t he ero si ve base l3 . facie s .9 in c;ontact w i t h t he underlying facies 4 or

Figure

5 where

t he

ooserved .

Table and

l.

Characteristics

of

the

clasts

in

facies

biogenic

i n

si t u structures

40

cm

long

Hamme r:

(arrow)

are

9

10.

COMPOSITION

MEAN SIZE

MAX SIZE

laminar

15

so

35

tubular

15

Coquines

5-10

laminar

30

Pelites

5-10

laminar

15

Quartzcites

1-5

Shells

1-5

Liney sandstone

7.

SHAPE

45

Inner tubes and rnolds Calcified and reworked

equidimensional

5

10

4

6

Figure

General view of facies 10 where a gentle slope of the laminae is observed. The w re e k the remnants of a ship points to 14.

sou thward arrow

indicat ed

66

in

Fi g .

2.

Hammer:

40

cm

long.

67

Table 2 . Characteristics of the in situ Callianassid galleries

PIECE

vert.sup.

vert . inf,

horizontal

typical subaerial exposition of lateral deposits of the cha.nnel Facies and 2 represent the channel backfill to facies 3 , which corresponds to associated t he f l oodplain a r e a •

...

Mean external diameter

3,0 cm

6,0

-

9,0 cm

6,0

-

9,0 cm

niean interna! cfiameter

FACIES

1,5

cm

2,5

-

6,0

11

2,5

-

6,0

mean lenght

5,0 cm

20

35

11

10

-

150 cm.

facies sequence is ordered from base to top by T he facies 9. 4 • ( 7) • 5. 8 and 6, presenting a decreasing grain arrangement (Figure 16b). The base exposed in the western section of the area presents con ta et with ero si ve the underlying facies (Figure 3). The contact among facies is net, except far fac ies 4-5 , which is erosive. The up per li mit of the sequence presents a transitional contact with the underly i ng facies sequence C.

•)

present : Fl

F9

11

si

F4

FS

si si

si

S·i si

01

Figure 15. Detailed view of the horizontal section of in situ Callianassid galle ries, in facies s . The wrinkled wall s of the tubes are observed . Reference : 10 cm.

70

SEQUENCE B (Facies 9 + 4 + (7.) + S + 8 + 6)

SCU SS ION

Facies " 9, which base 1s erosive , was generated by wave action in the high shoreface area which reworked older sediments, concentrating clasts and relocating laminae in horizontal position. These reworked sediments are from fluvial and marine depos it s , being evidenced by the presence of tube clasts and inner molds of rounded Callianassids . Facies 4 wa s generated in the high shoreface (surf) by streams parallel to the coast , causing westward sand wave migration (Figure 6a). Facies 5 was al so generated in the surf area, but its paleostream (.Figure 6b) indicates ttiat longshore currents caused ma)nly westward and in a few cases, eastward sand dunes migration . According to Da vis and Fox ( 1972), streams parallel to the coast, under high energy conditions , can cause sedimen t tractive motion. thus simulating a fluvial envl tonment . Southwards of this facies, the paleostream (Figure 6b) corresponds to deposits generated by rip currents. Facies 7, which was fcund in only one outcrop , keeps a lateral relationship wit h facies 4 and S of th is sequence.

71

Table 2 . Characteristics of the in situ Callianassid galleries

PIECE

vert.sup.

vert . inf,

horizontal

typical subaerial exposition of lateral deposits of the cha.nnel Facies and 2 represent the channel backfill to facies 3 , which corresponds to associated t he f l oodplain a r e a •

...

Mean external diameter

3,0 cm

6,0

-

9,0 cm

6,0

-

9,0 cm

niean interna! cfiameter

FACIES

1,5

cm

2,5

-

6,0

11

2,5

-

6,0

mean lenght

5,0 cm

20

35

11

10

-

150 cm.

facies sequence is ordered from base to top by T he facies 9. 4 • ( 7) • 5. 8 and 6, presenting a decreasing grain arrangement (Figure 16b). The base exposed in the western section of the area presents con ta et with ero si ve the underlying facies (Figure 3). The contact among facies is net, except far fac ies 4-5 , which is erosive. The up per li mit of the sequence presents a transitional contact with the underly i ng facies sequence C.

•)

present : Fl

F9

11

si

F4

FS

si si

si

S·i si

01

Figure 15. Detailed view of the horizontal section of in situ Callianassid galle ries, in facies s . The wrinkled wall s of the tubes are observed . Reference : 10 cm.

70

SEQUENCE B (Facies 9 + 4 + (7.) + S + 8 + 6)

SCU SS ION

Facies " 9, which base 1s erosive , was generated by wave action in the high shoreface area which reworked older sediments, concentrating clasts and relocating laminae in horizontal position. These reworked sediments are from fluvial and marine depos it s , being evidenced by the presence of tube clasts and inner molds of rounded Callianassids . Facies 4 wa s generated in the high shoreface (surf) by streams parallel to the coast , causing westward sand wave migration (Figure 6a). Facies 5 was al so generated in the surf area, but its paleostream (.Figure 6b) indicates ttiat longshore currents caused ma)nly westward and in a few cases, eastward sand dunes migration . According to Da vis and Fox ( 1972), streams parallel to the coast, under high energy conditions , can cause sedimen t tractive motion. thus simulating a fluvial envl tonment . Southwards of this facies, the paleostream (Figure 6b) corresponds to deposits generated by rip currents. Facies 7, which was fcund in only one outcrop , keeps a lateral relationship wit h facies 4 and S of th is sequence.

71

FACIES

F

F a

a

e s

e s

SEQUENCE

C (Facies

6

+

4

+

5)

e

el.

l.

The

i s

2

l.r

trans i t i ona1 .

follo ws :

net

4-5.

upper

The

is

ordered

4 and 5 , with ( F i gu r e 1 6 e ) . The

for

an The

con tact

facies

1 imit

from

6 -4

puts

base

to

increasing bas e

the

of

between and

9

in

by size

s e q u e.9 e e

facies

erosive

facies

top

grain is

for

as

facies

contact ,,with

a partially eroding facies.

OISCUSSION

t

Facies

6

indicat es

conditions supp l y.

lf

5

sequence

6, a r r a n g e me n t

3

Sl Sa

facies

f acies

The se

offshore ·

5 4

4

the

without

pelites

area

ver y

thin

top

shows

Th e

siltstone a

wave

i t s

i ndicating

ex is tence

wa v e

action

have

bee n

presence bed

is

in

the

action

of

proximity

p ure

or

sandy

sett l ement material

deposi ted

small

t he

on

a

of

pointed out upper part ,

t he

to

of

which

magn i t ude

shoreface - offshore

11 mit . . 9

Fig u re the

facies

as

e

Sl Sa 16 .

Facies

fac ies

decreasing

Si:?q u ence

sequence s

A,

arran gement ,

A,

B,

S,

C and

and

w he r eas

those a r ea , ( sur f ) shorefac e show ing the existence of

e

Sl Sa O.

Note

O present

sequen ce

S were generated in a similar w ay the hi gh of sequen ce B, in

4 and

Facie s

6

by high

currents .

1ongsho r e energy

conditions .

t h at

a

grain

C is

grain

FACIES

SE.QUENCE

O (Facies

9

+

S)

fr om base

to

+(JO)+

4

increasing . The

facie s

sequence

9,

facies

decreasing base Its

northern

rep r esents

h i gh

inte rpreted as fores hore (wash)

position flow

-

considering

r ate

having area .

b ee n

that

conditions ge nerated

in

transit iona l

for

is

5.

not

the

Facies settlement

1966 ) • 6

indicates

conditions

the

existence

generated

in

the

of

pu re

It

was

bet wee n

f acies

in

sequence

the

outcrop

i n

I t

is

a

in

situ

low

was

5 ,

4-5

and

(F igur e

al so

consolidation

t hey

ca n

Facies

supratidal facies

that

degree

fo r

sequence

of

in

both

4-

co nta cts be

ar

f a cies

are:

facies

4 - 5,

9-10

by

gra i n

facies

dete rmi ne

but

above

observed

Callinassid

to

top a

The

erosiv e

16d) .

p r esent located

16d) .

between

9 -1 0 ,

possib l e

9-10

presenting

(Figure

Contacts

facies

the

topograp hically m)

ordered and

arrangement erosive.

it

Fac ies 8 wa s generated in the 1ow foreshore by the wave actio n . Under these conditions , wave action c an genera te truncated wave-ri pple laminae (Campbell,

is

is

4

(10),

located which

ea

ar e

(apro x .

4 - 5 ther e

sediments

and

galleries

offshore

a rea

72

73

• OISCUSSION

correspo n d ing (fluvial)

This

facies

sequence

is

similar

10

in

to

sequence

facies

an

unique

outcrop,

lateral

re lationship

with

facies

9.

position

w ith

to

B,

Facies

found respect

to 8 southwards 9enerated in the

9

Facies

( f l uVi

a1

in

The

marine) erosive

contact with

facies too

-

as

is

an

the

e xistence

moment

of

the and

Facies

C· O n c

galleries

B and

C,

longshore of

their

where

high

wave found

13 4

w

orientation ,

-

Facies

by

t he

present

generated ,

as

high

shoreface

currents,

thus

suggesting

conditions

at

crests -

parallel

to

(limolite

of

a

paleostream bed

of

N-S

to

orientation

currents

the

a n·~ E -

with

coastline .

top

by

p re s en c e in

an

E-W

1o

B,

of

4 or

action.

a

5

l 7 .

account

the

is

with

E-W

an

were

This

reworked suggested

ma r k s

r i pp 1e

wi t h

approximately .

gentle

southward

ind icating the

top i n

cre st s

s y mm e t r i c

of

Figure

in

presents

t h

facies

posit i on,

t h ¡; s

paleos 1ope. A scheme

wi

wave

presents

layers ,

present ed

level)

mar k s

sequence

bed

Facies

its

hada

5 correspond

r i pple

their

shoreface

which

6

Facies symmetric

at

s

the

Facies migration

5 also represents a southw ard orientat i on . a s s o c i a t e d wi t h r i p c u r r e n t s . .,

B,

thi s

data :

southward

that

poss ibly

4 and

of

the

energy

deposits

is

the

i ndicates

Facies

p.o s i t i o n . In facies

or

t ha t

high

were

in

facies

5,

the se

and

-

deposi ts

sequence

1ud e d

front

whi ch

Figure

facies

4

sequences by

outcrop

9 •

Callianassid area,

(see

of

in

been

action.

(surf)

5

and

w as

i t

the was

facies

generated

has

situ

4

th i s

i s

wave

facies

of

by

4

slope,

o 1 de r

the

I t

fac ies in

its

rework ed

perhaps

fa e i e s

area

by

base

C -

there

that

were

facies

sequence

o n 1 y un d e r 1 i e

facies,

nor thern

and its wave action prove foreshore (wash) area. evidences

a nd

action.

this

a

k~eps

lts

bar

paleocurrent

repr esents

the

paleoflow

s l ope

of

foreshore

orientation

i

s

the

formation. OISCUSSION

THE

AGE

OF

THE

OUTCROPS Taking

(1928)

Frenguelli the

"Lujanense"

lnstead,

Mouzo

and et

much more modern. radiocarbon dated years of

BP.

sedimerits

This

age

COASTLINE

time was

74

order

studied

deposits

(1975)

to

stages.

considered

them

as

E . Farinati (oral communication) shell s obtaining an age of 6950 to

from

the

wh ich to

the

lithological shells

were

4

facies

or

description taken,

5

of

i s

with

Brazilian

a

that position

deposits of

the

to

conclude

ancient

when the studied sedi me nts were depos i t ed, necessary to integrate each facies with

re lation

S au ce

Grande

that

these

mouth

of

location to

the

(Figure deposits

the

cited

of

the

present

it

1 ),

belong Rlo

is to

Sauce

Grande . OF

THE

to

at

it

deposits

and

base to top .(F igure 1 8) .

position

R1o

outlet

The coastline

reasons,

marine

geographical

in

possib le

EVOLUT ION

the

t he

Con sid er i ng fluvial position

event

and

i n coastline was exactly a i t is at present, namely E- W

as

they

(Sug u1o

aforementioned

fluvial

oriented .

ORIENTATION analyze

t ha t t he

facies

transgressive coast

into

concluded

indica te similar

an

correlates

deter mi ned in Martin , 1 976) .

the

"Querandinense•

al.

According

would correspond sequence B or C.

In

i:.ssigned

t he i t i t

s

integrated

Facies enviro nment,

STRATIGRAPHIC columnar

by

facies

sequence may

A,

COLUMN

sec ti on

i s

ordered

sequence

A•

s,

wh i

repres ent

c h belongs different

to

e a sea

f ro m

and

O

fluvia 1 level

75

5 e

shoreface bajo

E s

p

q

INTEGER COLUMN SECTION

ENVIRONMENTS

·I

5

©A

o·

L.~

9

~~

/P

shorel ine

t

Upper

~\

~

i 1

-

L.J

........ ....r-~ "'1;iE ) 1ongshore currents :...--v fJuvial ~rip currents ~ zone swash

Figure paleoflow Wes t .

17.

lnterpretation

scheme .

Coastl i ne

© s

~water

act1on facies 1

CD

of the o r ientation

integrated was

e

depending

on

sediment

supplied

eustatic

variat i ons .

to

such factors as vol u me the ar e a , subsidence rate

of an d

indicate that facies sequence A was sediments of facies sequence its consolidation process. sequence which

is erosive 8

B befare

covered the

onset

by of

initiated with a base (rav in ement

a relative rising of sea l eve l . This re l ative rise which bega n with deposits from high shoreface, reaches to its maximum with the offshore peli tes in the top of facies sequence B . At that mo ment, a relative sea level l owering began, being evidenced by facies sequence C, unti l streams were surface;

76

Swift,

1968)

shoreface

4

rl

6

3

indicates

the

onset

of

Q)

_ QfJsJ.102'.'e___ _

¡..,

Lower

8 5

¡;::¡

u

< ~

r::I

4

~

2

::e:

Uppper

U)

7

u

3

Therefore, the facies sequence A, up to this moment, may belong either to a high or l ow relative sea level. It is remarked that the in si tu Callianassid galleries which are present in facies

Facies conglomerate

Upper

East-

[8 stages,

4

5

lp

shoreface

5

4

ravinement

f sur ace

Flood plain

2 l

FLUVIAL 1

Longitudinal bar

* Stages

sea level . -

Figure

l 8 . Integrated columnar sequences, environment

facies

o

**

Relative variations sea level.

osci l lat i ons . * sea level sea l evel oscillations .

section, and sea

stages;

**

facies, 1 e ve 1 re lative

. 77

sequenc e

B,

is

transgressive 4 .

T he

the

one

today) ,

5.

At

5,

located

the

seashore

7.

the was

sequence

The

19.

Figure

Block

paleogeographic environment

and

established

i n

diagram

showin g

re const ruction facies

of

a

generalized

the

depositional

relationships.

parallel

to

high

shoreface

( sur f)

by

et

al .

p a 1 e o s t r e a ms position

A possibly

the

shallow

generated

generated

during

1987). a l l o w e d.;

(similar

to

t ha t

to

of

in

of

as (Hag

E - W oriented. the deposition of facies 4 and facies sequences B and C , an E-W taking place in this area.

namely moment

mouth

of

coast line

drift

Facies

outlet

system

a na 1y s i s

determ ine

6.

interpreted

t ract

R1o

marine wave

corresponds

Sauce

to

an

o 1d

Grande.

sediments

action

under

were

ma in 1 y

hi 9h

energy

conditions .

8. Finally, reconstruction

a is

scheme

presented

of in

paleogeographic

19.

figure

AC KNOWLEDGEMENTS

coastline . From level

facies

rise

is

sequence

O a

started which,

new

at

sea

The

facies

for

relative

the

base

of

authors his

(Swift,

Coasta1

1968). This relative sea level rising is recorded even in the high shoreface (surf) deposits . The evolution of the stratigraphic column tale.en from facie s sequence 8, is interpreted as

readi n g

sequence

B,

repr~sents

being generated during (Hag et a l . , 1987).

a

a

ravinement

surface

transg res sive

tract

system

CONCLUSIONS l.

Throu gh

sequences to the

a

fluvial others

environment, foreshore to 2.

78

the

were

The

analysis

determined , environment

of

10

one

facies, 4 facies of whi ch corresponds

(facies

sequence

A)

and

O) to a shallow marine with deposits which range from the the offshore . (B ,

evolutio n

C,

of

the

integrated

strati9raphic

wish

to

thank

assistance Environments of

the

in

to

the

as

Or .

Leonardo

description

well

as

for

Legarre ta of

his

Marine critical

manuscript.

REFERENCES Campbell,

e.u.

1966.

Truncated

wave - ripple

Laminae.

J. Sediment. Petrol . 36:825-828 . & Fox, W.T. 1972 . Coastal Process and Oavis , R . A. hearshore sand bars. J . Sedim . Petrol., 42: 401-412 . Frenguell i , J. 1928. Observaciones Geológicas en la región Buenos

costanera

sur

An.

Fac.

Aires .

de

la

Cienc.

Pr·ovincia Educ .

de

2 :1- 145.

Para ne!. Hag,

B . U.,

Chronology the

Hardenbol , of

T r iassic.

J .,

fluctuating Science,

Vo l .235:

Vail, sea

P.R.

1987 .

levels

sin ce

1156-1166.

79

Mouzo, F. 198 0. Rastros ("ophiomorphidos") en Pehuen-Có, sobre

provinc i a

prob le mas

Bonaerense , Mouzo ,

F.,

1975 .

Ma r

del

Garza ,

de

M. ,

Argent i na

Callian assidos

sedimentos

Buenos

de l Plata.

Aires .

J . ,

occurrence

structures (Abstract).

at

In :

l itora l es

F. ,

Fa r inati ,

fósi l es de Pehuen - Có,

Zi becchi,

Pehuen-Có

O V •

Geol . Bonaerenses. Bahi~

Sugu io, de

K.

1975. Bol .

J.

de

Buenos

en

la

· Aires .

(Tand i l ,

1985) .

playa

de

e

sua

1989,

util i za.!ªº

paleoambiental. Bol.Do USP., V . 7:17-26 .

Swift ., D. J . P . 1968 . Coastal transgress i ve stratigraphy. Geo l ogy , Vo l 76:444-456 . Callianasso l i toral

&

major

and of

·)

Berrando soil at the Rio Salado, Buenos Aires Province. Argentina ABSTRACT Po l 1 en

and

exposure

Paul i sta

R .J.

Departamento de Ciencias Biológicas, Universidad de Buenos Aires & CONICET, Argentina

las , Jorn,

Actas,

Martín, L. 1976 . Presencia de tubos "Ca llianassa'nas Forma~oes Quaternarias do

Weimer,

RAÚL E. POZNER

E ., & Esposito, G . 1989. Tubos

&

reconstru.tao Geociencias,

4

Pollen and spores from the type locality of Puesto

Symposium

Blanca .

litoral

Jou rn.

R.

CELINA A. FERNÁNDEZ Departamento de Ciencias Biológicas, Universidad de Buenos Aires & CIC, Argentina

sorne beach,

Jnternationa l

Callianassidos pr

Simposio

and

on the Qu at e rnary, s o uthe rn Brazil Bigarella and R . Becker , Edit. Paranaense de Geociencias. 33 : 60 . Mouzo,

de

litoral Atl!ntico Resumen inédito.

Izquierdo,

Beach -r ock

assoc iate d

fósiles

:ioyt, Say,

Shallow

Pa l eonto l ogy,

J . M.

1964 .

Geologic meritic

J G.

Jnst.

erosion Journal Burrows indicator

studied . with

na

a

de

has occur

and

spores

a l ong Samples

the

Las

bur i ed

of

three

the be l ong

Escobas

soil ,

the

obtai ned

Salado

a

to

(marine)

Puesto

at

ha ve

sequence

Formation

an been

beginning over

Berrendo

which So i l ,

. developed. A t the top, a ll uvial sediments on which the present so i 1 develops. T he pallen record shows a rich f l o r a in this present

of

so il level with fungi of spec ia l ly Papulaspora sp .

of o f

steppe

and

well

bryophyta

environment 38(4) :761 - 767 .

samples

R1o

as

shows

freshwater

algae

suggests a

the

ar e

at

the

same

microf lora, of the grass

al so

Phaeoceros

diffe r entia l The presence

d ef in es

contains

a l gae as

impoverishment

indicating diagenes i s . that

such

the edaphic A n giosperms

present

sp .

buried

reco r d

soil

level

destructi on of f~ngi and pe dog enesis

hydromorphic

soi 1 .

d i noflagellates

The

from

as

The

due to freshwater envi\on ment

bur i ed

the

soi 1

parental

sediments.

RESUMEN Muestras perfil

correspondie n tes ubicado

fueron

las

estudiadas

pal ino lógico.

80

en los

a

desde

el

niveles

secuencia

que

p r esenta

Formación

Las

Escobas

tres

barranc a s

en

su

niveles de 1

R1o

punto

de

pertenecen base

(ma rin a) ,

de

vist a a

sedi mentos sobre

los

un

Salado una de

que

la se

81