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

16 Pages·1990·3.027 MB·English

by  Vega, et al.

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 STRACT
The studied outcrops are approximately locat ed i n
the prt:.ent intertidal area of Pehuen-Có Beach, at
abou t 10 km t o the SE of th~ c i t y of Bah1 a Blanca
( 1 a t . 3 9º S a n d 1 o n g • 6 i• 3 5 ' W ) •
Deposits appear discontinuedl y, 4 to 5 km long
and 100 to 150 m wide, and thei r t hickness is less
than m. Lateral f acies re lat ionships , s ma 11
ver ti cal sequences and paleoflow data wer e
described with certain accur acy in a p 1 a ne ta b 1 e
map at l :2000 sea le.
Ten faci e s have been determined :
1) Cross - bedded quart zit ic conglomerate , mi grati ng
to the S and SE.
2 ) Cl ayey sandstone with subor dinated quartzitic
c lasts .
3) Pelites with dessication cracks and b ir d
ichni tes .
T hes e un i t s corre spond to a longitudi nal bar
and a se quence of a11uv ia1 pl ai n f acies f ro m
fluvial deposits .
The remai ni ng fac ies are :
4 ) Pl anar cross - bedded sandstone , mi grating to t he
w.
5) Trough cros s - bedded sandstone , migrating to the
W and subor dina tely to the E and S.
6) Pelites (mudstone and ~i ltstone ) , l oca lly a
mudstone with ripple symmetrica l to the top (E-W
oriente d r i dges).
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inclinada migrando al sur o sureste.
7) Parallel laminated sandstone.
2) Arenisca arcillosa con c l astos cuarclticos
8) Sandstone with truncated wave-ripple laminae,
subordinados.
migra ti ng to the NW and E. 3) Pelitas con grietas de desecación e icnitas de
9) A conglomerate of calcareous sandstone, remains
aves. Corresponden a una secuencia de facies
of biogenic structures, quartzites , clayey
barra longitudinal y llanura de inundación de un
sandstone, coquines and remnants of shelly sands .
depósito fluvial.
10) Sandstone , partially conglomerated, with low
Las fac ies restantes son :
a n g 1 e c r o s s be d d i n g , p l u n g i n g 4 • to 8° S .
4) Arenisca con estratificación entrecruzada planar
l t corresponds to 3 faci~s sequences generated migrando al o.
in a shallow marine environment, partia l ly
5) Arenisca con estra ti fi cae ión entrecruzada en
represented by deposits which range from foreshore artesa migrando al O y subordinadamente al E y S.
(facies 7 and 10), high shoreface (facies 4 • 5 • g ) •
6) Pelitas {fangolitas y limolitas), localmente
low shoreface {facies 8) and through offshore
limolitas con ondulitas simetricas al techo
{facies 6). These sediments were mainly generated (crestas con rumbo E-0).
by the action of high energy waves·.
7) Arenisca con laminación paral e l a .
lhP ~rrangement { from base to e e i l i n g ) of 8) Arenisca con "truncated wave-ripple laminae"
facies sequence ; s b • fac i es 9 + 4 + {7)+5+8 migrando al NE y al E.
+ 6 facies 6 + 4 + 5 and O: facies 9 + (10) + 4 + 9) Conglomerado de aren1· sca ca l cilrea, restos de
5 . estructuras biogénicas, cuarcitas, areniscas
The evolution of the integrated stratigraphic
arcil l osas . coquinas y restos de conchillas.
column indicates the existence of two relative sea 10) Arenisca, conglomerildica en parte, coro
1 e ve l rise {sequence 8 and O) and a relative sea
estratificación entrecruzada de bajo angulo,
1eve1 lowering during the deposition of sequence C bu za n d o 4• a 8º a 1 S • C o r r e s p o n d en a 3 s e cu en c i a s d e
in the are a . facies generadas en un ambiente marino somero
representado por depósitos que van desde el
foreshore (fac ies y 10), · shoreface alto (facies
RESUMEN
4,5 Y g), shoreface bajo {facies 8) y offshore
{facies 6) . Estas sedimentitas fueron generadas
Los afloramientos estudiados se ubican principalmente por acción del oleaje bajo
principalmente en la zona intertidal actual de las
condiciones de alta energ1a.
playas del balneario Pehuen-Có, situado a unos 70
La evolución de la columna estratigrilfica
km al sureste de la ciudad de Bahia Blanca. Su
integrada indica la existencia para el élrea de dos
p o s i e i ó n g e o g r él f i e a e s a p ro x i ma dame n te l a t . 3 g• S y
estadios de ascenso relativo del ni ve 1 de mar
1 o n g • 6 i• 3 5 ' O •
{secuencia By D) y un descenso relativo durante la
Los depósitos se observan en forma saltuaria,
depositación de l a secuencia C.
por 4 a S km de largo y 100 a 150 m de ancho , con
espesores que no superan m. Las relaciones
latera les de facies, las p e q u eñ a s se e u en e i a s
INTROOUCTION
verticales y los datos de paleo flujos fueron
ubicados, con cier ta precisión en un mapa base
Frenguelli (1928) initially described the outcrops
levantado con plancheta a escala 1:2000.
herein studied. He recognized ver y sandy
Se determinaron 10 facies:
fossiliferous lime presenting a large calcareous
l ) Conglomerado cuarc1tico con estratificación
q3
52
crust in the form of thick roots. lt belongs to a STUDY AREA ANO FIELD METHODOLOGY
str eam bed deposit during the "lujanense" stage,
followed by a mobile sedimentation during the The study area is located in the Pehuen-Có beach ,
"Querandinense" stage, within a fluvial valley in "Playa del Barco", at 70 km to the SE o f t.h, e
which was drowned during the "Preensenadense" city of Bah fa Blanca (Figure 1). l t l s
stage. approximately at lat. 39º S a n d 1 o n g . 6 i• 3 5 ' W w i t h
Mouzo et al . (1975) indicate the presence of an East-West variation with reference to the
beach rock which includes the tubular structures shoreline. •)
des cribed by Frenguelli (1928) . Mouzo (1980) Most o f t he outcrops are located i n t he
concludes that the organic structures found in the present intertidal area (Figure 2 ) . They can be
beach sandstone correspond to fossil traces observed in an area 4 to 5 km long and 1 o o to 150 m
bel onging to the Callianassids. Subsequently, Mouzo w i de .
et al. ( 1989) define a coarse to medium grained Thickness, which makes possible to analyze
sandstone with parallel criss-cross structure , small vertical facies re1ationships, is generally
which contains what they defined ·as upper low-beach less than 1 m. The outcrops a.ppear discontinuously
and biogenic structures in the high beach "in due to the intense er osive process and they are
si tu" . constantly covered by beach sediments , frequently
Then, they loca te a compact, partially swept by storm action . Due to the dynamics of t he
carbonati c cemented sandstone. Without finding any area, field work was carried out in different times
relationship with the deposits previously of the year . Thus, all the information obtained was
discovered , they analyze a quartzitic conglomera te included in a plane table map at sea le l :2,000. In
in which they observe a marked strearn imbrication this way, it was possible to loca te, with sorne
suggesting a fluvial origin and conclude that the accuracy , small vertical sequences , latera l facies
tu be formation process and the subsequent relationships and paleostream data .
cementation of the sandstone must ha ve occurred
after the deposition of the fluvial conglomerate. FACIES OESCRIPTION
They analyze these structures defined as FACIES 1
galleries built by Callianassids - and pro pose that
the lack of the upper and intermediate sections of Reddish-orange conglomera te, completely formed by
the in situ Callianassid tubes and the subsequent quartzitic clasts, which mean si ze ranges from l to
cementation of the sandstone indicate a n ero si ve 6 cm with a maximurn of 15 cm. E qui dimensiona 1 and
process occurred in this area due to a sea leve l oblate shape predominate, with partially sandy and
change or to a coastline migration in t h i s clayey matrix. This presents an incli ned-type in ne r
location. structure (Fig.3), which sets change due to gr a i n
The purpose of this paper is to demonstrate by size variations in both clasts and rnatrix.
means of a facies ana l ysis that : The 'inc li nation angle is app roximate l y 25°
1) The studied sedirnents were deposited in a Beds are geometrical l y tabular. lt is not possible
shallow fluvial and marine environment. to know the inner bedding because it remains
2) The relationship between both deposits and t he i r covered. The maximum thickness observed is 60 cm.
present location provide evidence of relative sea In sorne sections, there are "megaclasts" which mean
level changes. 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 biodisturbance
54 55
-
,.,.
f
1
l
F9
F 5 "'--'~_,,.._,..,_,, F9~
F4 tFªS 1f FF ~ . • 1f
f§~..,.,. ~ ~~
1res F 5 k"'<--->-""" 11' ~ . e1{f
F4 __
Provincia de Buenos Aires si <ia e p..;~,._,.,_,,,__ lf Sl S1a e1 F2
S Sa C
Sil S:;a,; ;_C; ;.
C--L._,,_-~_--':'-.,=-~~ _o::.,;:~-_-
:-.1'.l:::t
-_-_:cq
--- -- --- - - -- - =- - -
-~- a:>-:--~-HWL - - -.¿J:l_
.--· --:·--· -·-·-·-
_....__. DL
·. . ~·- ..~ ·,· ·~··.
Pehuen-Co
e conglomerate
~ wave ripple
Sa san4stone ~ burrows horizontal ~
SL silt V callanasa
Fl facie.s 1 e ship
ELWL exceptional LWL N
.......-v-mud-crack
DL dune line = parallel laminft ed
ro
Figure l . Location map of the studied area.
Figure 2. Oistri bution of outcrops in the studied
area. Plan view with the location of the present
and vegetal debris. The biogenic structure observed ti des . Four of the profiles made in the area are
is composed of scarce Callianassid in si tu depicted.
gaqeries.
Measured pa leocurrents indicate t he existence FACIES 2
of a prevailing southwar~ orientation, subordinated
to the SE . Pale reddish- brown (10R5/4), mean grained c layey
T h i s corresponds to a bed load deposited by sandstone, presenting only qua.rt zitic clasts , 0,5
tractive unidirectional streams, which generate a to 4 cm in s i ze . lt lacks internal structure.
bar grout or lateral migration, evidenced by i t s Bedding structure is not vi si ble due to the scarce
inclined bedding. development of the outcrops, 15 cm thick. The
Changes of grain size indicate that the f low contact with facies is completely transiti onal .
rate has varied cyclically. Megaclasts indicate th e This corresponds to a bed load deposit
existence of small scarpments close to the generated by a decelerating str eamflow .
mentioned deposit which is shown by the size and
the low rate of erosion i n their crests.
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57
- -
Figure 3. General view of facies l • in which the Figure 4. o et a i 1 of fac ies 3 where desiccation
cinocn ltiancetd otf yp et he i no nvee rr l ysintrgu cftaucriee s a9 nd areth e obseeror vsie dve. cracks a~d bird ichnites are observed. Knife : 20 cm
1 o n g •
Hammer: 40 cm l ong.
The observed biogenic structures are isolated
in situ Callianassids gall eries . The paleostream of
FACIES 3
th is fac ies (Figure 6) shows a marked single
Greyish oran ge pin k (5VR1/2), parallel laminated westward orientation .
pel ites They present desiccation cracks and b ir d This corresponds to a bed load deposi ted by
ichnites (Figure 4). Thickness 1 O e m. T h i s unidirectional traction f lows which generated the
corresponds to the settlement of a suspended l º ªd. migration of straight crest mega ripples.
oesiccat ion cracks indicate a sub - aerial exposition
of these sediments f A C 1. E S 5
FACIES 4
Modera te yellowish brown (10YR5/4) mean to coarse
Modera te yellowish brown (lOVR5/4) sandstone, with gra ined criss- cross trough bedded sandstone (Figure
coarse planar criss-cross bedding . lt presents sorne 7 ) w i t h a mean l ength of 150 cm and a mean width of
sets formed by remnants of shell sand as well as 5- 15 cm. This presents sorne sets f ormed by remnants
cm isolated clasts which follow the set slope. Bed of shelly sand. l solated lami~ar clasts are
thickness ranges from 20 to 40 cm. In sorne sections observed following set slope. It is bar-1 ke
a net inner contact is observed . Set plan presents shaped. Sed thickness is 30 cm. Lower contact is
a mean length of 5 to 6 cm, occasionally up to 13 erosive and net .
m, being the most remarkable feature of the facies The observed bingenic structures are galleries
(F igure 5). bu i 1 t by i n situ Callianassids; there are ar e as
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58
•
5 . o f Figure 7 . Detai led view of the t rough crossbedding
facies 4 • where continuity sets as o" facies 5, where the chara e ter i sti e biogenic
long as 5 to 6 meters ; s observed . s:ructures of these facies are observed as we 11 as
the laminar clasts . Hammer 40 cm long .
where they have almost complet e ly d isturbed t he
inner bed structure . Pal eostream dat a presented in
N N the diagram of Figure 6 i ndi ca te a prevai l i ng
westward orientation and subordinately to the E and
s.
This corr esponds to a bed load deposited by
unidirectional t ractive streams '"hich generated
mega ripple and wavy ridge mi gration .
Bar- like structure presented by this facies
indicates the existence of a channelled flow.
=
n 12 n = 35
FACIES 6
F 4 F 5
Massive pelites (mudstone and si 1 ts t one) ha ve a
mantle - like geometry and a discontinuous
development . The leve1 of the siltstone top
Figu re 6 . Paleoflow distribution : F4, planar cross p re s en t s s y mm e t r i c s t r a i g h t c r o s s r i p p 1 e ma r k s ,
bedding and FS, trough cr ossbedding . n= number of which wave length is 10 cm and its wave amplitude ,
data cm . Crests are orien te d to the E-W.
60 61
This facies general ly covers beds whose top
presents symmetric ripp le marks (Fi 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
horizontal tracks contained in the mudstone top,
determlned by two associated ichnofor ms (Figure 9):
a) irregularly grouped traces , wi th straight"' or
gently curved development , with inner 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 was deposited by settlement
(Figure 10).
The ripp le mark top of the limestone bed shows
a reworking due to the wave action as the surfaces
Figure 8. General view of facies 6 where the mantle covered by this facies do which, in this case , is
like distribution and wave reworking of the of higher intensity. Track "a" is interpreted as
underlying facies is observed . Hammer: 40 cm long. the trace produced by a debris-eating organism when
feeding itself while track "b" corresponds to the
trace produced by an organ~sm when displacing
probably a worm.
FACIES 7
Pale yellowish brown (10YR6/2) fine grained
parallel laminated sandstone (Figure 11) Bed and
lamination are 5 mm and mm thick, respectivel y.
Lower contact is net planar. This facies has been
found in an unique outcrop whose development is of
approximately 9 m;
This corresponds to a tractive, h i g h f1 o w
deposited bed load.
FACIES 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
Figure 9. General view of the net contact between
Campbell (1966) as "truncatcd wave-ripple laminae" .
facies 6 and the overlying facies 4 . Note the
The maximum thickn~ss is of 10 cm. Paleostream data
action of w aves on the top of facies 6 . Hammer : 40
of these ripple mar ks indicate an E and NE
cm long.
62 63
(
111 orientation in the migration .
T h i s corresponds to suspended bed load ,
deposited by wave acti on , which formed the
asymmetric wave r i p ple. and the truncations bétween
t he m.
FACIES 9
•)
Li ght browni sh grey , pol ymictic, clast-supported
conglomera te . The composition , mean and maximum
size of the clasts are included in Ta b 1 e 1 . A
gradual increase of quartzitic clasts has been
determined near the deposits situated at the
western section of the ar ea, and there is a lot of
clayey sandstone equidimensi ona l "megaclasts " which
maximum size is of 70 cm .
Mean to coarse gra i ned -s -a n d y -=-c l a y e y ma t r i x
with fragments of shelly sand The deposi ts are
blanket-li ke in shape . lt was possible to determine
Figure 10. Oetailed view of t he that the lower bed limit presents a slight
in the mudstone of facies 6. Reference: i n e 1 i n a t i o n t o 3• s o u t h wa r d a t the southern section
oi the 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 .
The biogenic structures observed in some
level of t hi s facies are Call ianassid galleries in
situ in thei r hor izontal run . This cor responds t o
bed load deposited by: Tractive high flow rate
$tream with a marked wave action rework or only the
product of the latter . The wave action reworked
older sediments, concentrating the clasts ,
relocating the laminae in horizontal position and
parti a l ly washing away the fi ne mater ia l (Figure
1 3 ) .
FACIES 10
Pal e red (lOR0/2) coarse grained sandstone ,
Figure 11 Oetai led view of the partially matrix-support ed con glomerate . Clast
of facies 7 . Reference : 10 cm . composi ti on is s i mi lar to Facies 9 which proporti on
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Figure l 2 . General Vi e W o f t he truncated wave- Figure l 3 . General Vi e W o f t he ero si ve base o f
r i p p 1 e a rea o f facies 8. Note t he ne t con ta et w i t h facie s .9 in c;ontact w i t h t he underlying facies 4 or
th e overlying facies 6 . Reference: 10 cm. 5 where t he biogenic i n si t u structures (arrow) are
ooserved . Hammer: 40 cm long
Table l. Characteristics of the clasts in facies 9
and 10.
COMPOSITION 7. SHAPE MEAN SIZE MAX SIZE
Liney sandstone 45 laminar 15 so
Inner tubes and
rnolds
Calcified and
reworked 35 tubular 15
Coquines 5-10 laminar 30
Pelites 5-10 laminar 15
Figure 14. General view of facies 10 where a gentle
Quartzcites 1-5 equidimensional 5 10
sou thward slope of the laminae is observed. The
Shells 1-5 4 6 arrow points to the remnants of a ship w re e k
indicated in F i g . 2. Hammer: 40 cm long.
66 67
Table 2. Characteristics of the in situ Callianassid typical subaerial exposition of lateral deposits of
galleries
the cha.nnel
PIECE Facies and 2 represent the channel backfill
vert.sup. vert. inf, horizontal associated to facies 3, which corresponds to .t.. he
f loodplain ar e a •
Mean external
diameter 3,0 cm 6,0 - 9,0 cm 6,0 - 9,0 cm FACIES SEQUENCE B (Facies 9 + 4 + (7.) + S + 8 + 6)
niean interna!
•)
cfiameter 1,5 cm 2,5 - 6,0 11 2,5 - 6,0 11 T he facies sequence is ordered from base to top by
mean lenght 5,0 cm 20 35 11 10 - 150 cm. facies 9. 4 • ( 7) • 5. 8 and 6, presenting a
present: Fl si decreasing grain arrangement (Figure 16b). The base
exposed in the western section of the area presents
F4
si
ero si ve con ta et with the underlying facies
FS si si S·i (Figure 3). The contact among facies is net, except
F9 si far fac ies 4-5 , which is erosive. The upper li mit
of the sequence presents a transitional contact
with the underlyi ng facies sequence C.
01 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
deposits , being evidenced by the presence of tube
clasts and inner molds of rounded Callianassids.
Facies 4 was 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)
Figure 15. corresponds to deposits generated by rip currents.
Detailed view of the horizontal section
of in situ Callianassid galleries, in facies s. The Facies 7, which was fcund in only one outcrop ,
wrinkled keeps a lateral relationship with facies 4 and S of
wall s of the tubes are observed.
th is sequence.
Reference : 10 cm.
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