Our sedimentological study of Lobos Lake sediments has shown that, below modern deposits, there is a clastic and biological record of three different paleoenvironments: two aquatic (freshwater and mixohaline respectively) and one subaereal. This record documents the climatic and geomorphologic changes that affected the basin during its evolution up to its present water level, from the upper Pleistocene to the Holocene.Facultad de Ciencias Naturales y MuseoComisión de Investigaciones Científicas de la provincia de Buenos Aire

Blasi, Adriana Mónica

Dangavs, Nauris Vitauts

Journal of Paleolimnology

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Journal of Paleolimnology 10: 59-66, 1994. 59 @ 1994 KIuwer Academic Publishers. Printed in Belgium. Quaternary ontogeny of a pampean 'laguna' I. Sedimentological  and  biological  characteristics o f  Lobos  Lake sediments, Argent ina * Naur i s  V. D a n g a v s  t & A d r i a n a  M. Blas i  2 l lnstituto de Geormorfolog[a y Suelos, Universidad Nacional de La Plata, Argentina; 2Comisi6n de Investigaciones Cientificas de la Provincia de Buenos Aires, Argentina; Present address: Instituto de Geomorfotog& y Suelos, UNLP. Calle 3 Nro. 584, (1900) La Plata, Argentina Accepted 29 December 1993 Key words: Quaternary, limnogeology, Pampean laguna, sedimentology paleoenvironments, marine sediments Abstract Our sedimentologicat study of Lobos Lake sediments has shown that, below modern deposits, there is a clastic and biological record of three different paleoenvironments: two aquatic (freshwater and mixohaline respectively) and one subaereal. This record documents the climatic and geomorphologic changes that affected the basin during its evolution up to its present water level, from the upper Pleistocene to the Holocene. Introduction Few sedimentological studies of modern pampean 'la- gunas' have appeared in the limnological lite-rature. This paper, on the basis of geologic surveying in the Lobos Lake area, deals with the main sedimentary characteristics of this modern pampean 'laguna' basin, which are described in terms of vertical sedimentary sequences, textural facies and depositional environ- ments. Furthermore, a reconstruction of the basin's development is presented, as it is useful not only for comparison with other Holocene 'lagunas', but also as an analogue to determine the origin of other Quaternary sequences, Geological setting Lobos Lake is an inland lake typical of the Pampean plains of Argentina. It is a permanent water body, re- gionally called 'laguna', corresponding to a third order * This is the seventh in a series of papers published in this issue on 'Paleolimnology in Southern South America'. Dr. C. A. Fern~indez served as guest editor for these papers. or polymictic lake, on the basis of its continuos circu- lation and absence of thermal stratification. The lake, at 22.6 m above sea level, covers an area of 7.6 km 2. It is situated 115 km southwest of Buenos Aires City (37 ° 17'S and 59°07'W) in an aeo- lian plain of low relief and very gentle slope (0.026%). The drainage system of Lobos Lake is 1720 km 2, and is a tributary of the Salado River of Buenos Aires Province (Fig. 1 and 2). The regional climate is humid temperate, with a mean annual temperature of 15.6 °C and a regular dis- tribution of the rainfall through out the year, with an annual mean of 902 mm. However, the hydrographic network is poorly developed, with a 71.3 km long col- lector, Las Garzas Creek and sixteen tributaries, which compose only twenty-two channels with a drainage density of 0.126 km km -2. All these streams have been, to various extents, culturally modified. The lake is fed by rainfall and runoff, The seepage relationship with the groundwater regime has a double character (effluent- influent). Water chemical characteristic show an oligohaline salinity range (2 g 1-I), with scarcely alkaline pH, high total hardness (420 mg 1-1), Mg/Ca: 4.9 and CI/CO3+HCO3: 2.6. The distinctive features of its 60 - G 2  o -Cj~oo Fig. l. Buenos Aires Province, Argentina. Lobos region location map. chemical composition, according to Maucha's clas- sification (Maucha, 1932) are: clorate sodic, sulfated, hemi to bicarbonated and hemi-magnesic water. This aquatic environment is located in a basin on- tologically related to hydrologic and aeolian processes with a progressive infitling of detrital and chemical materials. The initial 'wanne' shape was more than 7 m deep, but the present shallow 'pfanne' is only 2 m deep. Bluffs of variable heigth (0.3 to 4 m) composed of upper and late Pleistocene and Holocene sediments, are found around its coastal perimeter. Stratigraphy The regional landscape is developed in Quaternary ter- rains, whose main outcrops are found along the lake bluffs. Four lithostratigraphic units have been recog- nized; three making up bluffs and one along the shore line (Fig. 4). The oldest unit, correlated with the Ensenada For- marion (Riggi et aL, 1986) of middle Pleistocene age, constitutes the lake bedrock. This unit, which is a slightly to strongly consolidated and carbonated, lam- inated or massive sandy siltstone, crops out at basal bluff section and lake beaches. The second unit, the Buenos Aires Formation (Riggi et aL, op. cir.) of late Pleistocene age, is a loess mantle, 0.15 to 3 m thick, unconformably over- lying the Ensenada Formation. It is mostly a friable sandy silt loess, containing scattered caliche nodules (L6esskindel). The uppermost unit, the upper La Postrera Forma- tion (Fidalgo, 1983), blankets the whole landscape. It is a Holocene vitroclastic loess mantle, up to 1 m thick, devoid of caliche nodules. Its thickness dimin- ishes considerably below the alluvial deposits in the valleys outside the lake area. Present soils have devel- oped on this unit. The youngest subaereous unit in the area is the Allu- vium, a discontinous, poorly sorted, gravelly to clayey sediment found in lake beaches and fluvial channels. Besides, it ocurrs as the major conspicuous alluvial deposit at the lake bottom, where it is more than 1.3 m thick, (Blasi & Dangavs, 1992) (Fig. 4). Sedimentary and biological paleoenvironmental record of the lake basin The paleotopography of the primitive lake bed bassin and the vertical sedimentary sequences were studied from cores of 40 boreholes reaching a depth of more than 6 in, which allowed us to analyze 5 m of infilling sediments, (Fig. 3). The infilling sediments, constitute different super- imposed sedimentary bodies (Fig. 4), which represent diverse paleoenvironmental depositional episodes that ocurred in the basin throughout the development to the present water body. Thus, below the modern lake sedi- ments, we have found the clastic and biological record from three different paleoenvironments: two subaque- ous (freshwater and mixohaline) and one subaereal. All sedimentary sequences have been modified by post-depositional intra-sedimentary gypsum precipita- tion that occurred during the maximum desiccation episodes of the basin under semi-arid conditions (Dan- gavs& Blasi, 1992a). 61 f ' ~  / jr '~ / aa  ~ / ~J  \ il S /I "\ f J ( \ N t /  / " \  / '  . • . .  . . . . .  • ~ Genera/ Las Heras /l / I f ® ,fOQUE PE~Z. C/V/CA ZAKS o ~S .SO X F L A N A  T t O I V  ,'-%,-" S T R E A M  / (" LAPCE HIGH W A y  "~ ~ ~ DRAINA G~E LAKE ~,AS/N Fig. 2. Location map of Lobos Lake and its drainage basin. 62 . , . . . .  i , i , ,  , , , , , ,  , , , , ,  • _~/ AT, ';, "~ " ~  P'9- 4 A . r /  • * * / / ~ - e  " " I '  ~ ' ~ ' : ÷ ~ '  / / ~'3 * /--A K , ~  • • " ) .  ~ ' ~  [ l (  , I  ,J f,,,l, \ / /  Z..,, . . " Y F  ¢? / /  ",7" O "  . f"  " Fig. 3. Index maps of Lobos Lake Sample locations. Sedimentary body 1 - Freshwater paleolentic deposit (mudstone facies) Over the basinal bedrock are 2.9 km 2 of freshwater paleolentic deposits, with a maximum thickness of 2.66 m and a volume of 3.1 hm 3. These are com- posed of grey to light grey silicoclastic mudstones (5Y 7/1 - 5Y 6/1), slightly hard to hard, bearing freshwater organic remains and abundant post-depositional lens- shaped gypsum crystals. The biological remains of the mudstones are: Gastropods Littoridina parchappei: adults and young; abundant. Ampullaria canaliculata: valve fragments; frequent to r a r e .  Ostracods The same species found in present lentic environments of Buenos Aires Province. Adults and young. Left and right valves in the same proportions. Charophyta. Abundant. Oogonia, calcified tubes and plates of the same origin; common. Diatoms Related to high vitroclastic ash contents. Rare in the sand fraction but abundant in the silt fraction. Deposition of these sediments is related to subaque- ous and partially subaereal processes during dessica- tion periods, indicating waning water inputs and/or seasonally changing climates. Biological remains are similar to present lake organisms (i.e. oligohaline and alkaline indicators), but in lower numbers and much more broken. This evidence suggests that a marshy environment developed during the late Pleistocene. Sedimentary body 2 - Subaereal paleonvironment (dessiccation surface and clay - pellets facies) The more conspicuous deposits appears in the east sec- tor, as an elongate eolian accumulation 1.7 m thick, which runs parallel to the present eastern coastline. This dune is made up of sand-size, pellet-shaped clay-aggregates, light brown and light green, by break down of the lake's floor dry crust into sand-sized par- ticles, and eolian accumulation due to the action of steady on-shore winds in the warm seasons with strong insolation. 63 This depositional feature is associated with clear signs of weathering (such as strong bioturbation, iron- oxide precipitation, abundant broken bioclasts and iso- lated clay pellets) in the upper 0.4 m of the surrounding paleolenfic deposit. Sedimentary body 3 - Mixohaline paleolentic deposits (siltstone and mudstone facies) This deposit covers an area of 6,25 k m  2 with a max- imum thickness of about 2 m and 9.7 hm 3 of total volume overlaying the Ensenada Formation, Sedimen- tar 5, Body 2 (wheathering crust and clay dune), and Sedimentary Body 1 (paleolentic freshwater deposit). Sedimentites are friable to hard sandy siltstones and sandy mudstones, pale-colored: yellowish-white to slightly green and gray, all in hue 10YR, 5Y and 2.5Y. The mudstone facies were affected by post- depositional intra-sedimentary gypsum precipitation in the form of sand size crystals with discoidal or lens- shape grains or even as microrrosetes. Epigenetic gypsum constitutes 34 to 56% of the mudstone facies, which cover an area of 3.22 km 2. In the sandy siltstone facies, the gypsum content is very low due to the dissolution of crystals (Dangavs & Blasi, 1992a). Biological remains of this unit are: Fish teeth, spines and vertebrae Gastropods A large amount of shells of diverse species, such as: Littoridina parchappei (d'Orbigny), abundant to fre- quent. Littoridina australis Pilsbry, common to fre- quent, that is dominant over L. parchappei near the present mouth. Littoridina conexa Gaillard, frequent to rare, only in ten samples. Littoridina australis crassa Gaillard is rare. Biomphalaria peregrina is frequent. Ampullaria sp. is frequent to rare. Succinea meridion- alis is rare. Ostracods Abundant adults and young with both open and closed valves. Both valves in the same proportions. Typi- cally freshwater forms, common in modern Buenos Aires Province inland waters. The major genera are Cypridopsis, Limnocythere and Elpidium. In smaller numbers there are closed valves with sculpture and valvular vaulting. In some samples, the ostracods re- place Foraminifera and constitute an ostracod-mud. Foraminifera Abundant to rare. Present in all samples, but with low species diversity. They are generally large forms (0.33 mm). Foraminifera taphanocoenosis is dominant in some sites and absolute in others. It is associated with marine organisms such as Littoridina australis, L. aus- tralis crassa and L. conexa. The taxonomic determi- nations made by nomenclatura aperta identify these species: Rotalia beccari parkinsoniana (d'Orbigny) abundant; Elphidium discoidale (d'Orbigny) frequent; Elphidium galvestonense Kornfeld rare; Bucella ex gr. frigida (Cushman) rare; Discorbis nitidus (f. praegeri) (Heron-Allen et Earland)? rare; Cornuspira planorbis Schultze very rare. Generally the dominant genus is Rotalia, except in some samples where it is replaced by E. discoidale. Rotalia - E. discoidale - Bucella - and other species relationships are 92:6:1:1. Diatoms Frustules appear discontinuously in the sand frac- tion; finer sizes were not studied. Their presence is usually related to the vitric ashfall layers. We only identified some of the large forms: Surirella striatula Turpin, Campylodiscus clypeus Ehrenberg, Navicula sp., Caloneis sp., Coscinodiscus sp. and Anomoeoneis sp. Charophyta Oogonia are always present in samples, and especially common in sand-size samples. We could recognize three different types. Also, the gyrogonites were asso- ciated with calcium carbonate plates, tubes, branches and stems related to the calcified thalli of these algae. Regarding the origin and salinity tolerance of these organisms, we have found freshwater and mixohaline- euryhaline (Segerstrale et al., 1959) species together. The Gasteropoda of marine lineage, like Littoridina australis (Pilsbury, 1911), live presently in the Rio de la Plata near Montevideo in an euryhaline habitat. Littoridina conexa is a typically estruary-lagoon form (Gaillard, 1974), and Littoridina australis crassa per- haps could be an ecological variety of L. austratis. Among the Ostracoda, we have found freshwater and mixohaline species together, and perhaps calcareous and siliceous algae also. Elsewhere, Foraminifera populations (Rotalia bec- O~ 4~ ~Sm. a.~.L I ~o~. 151e. V~rMC'°I ~cale Ease (~ Ensenada Frn. (~ t~uen~s Aires Fro. (~ ~B~/gqTen~r B ~Jd2/.2 ~'= L~wer Za Pe~re~ Fro.) (~ Upl~r LO PosBrera Fro; (~ ~/lu~,'um Fro. ~';'~ E"Y"e~ ~f ~J; /. / (~ 5ed~ment-er~ t~od~ I ('= Gc~erre~ Member L~an Fro.) Ol l Rm. B.~C,n. '" I ¢~ = po.~fn'ep=.~t~ional 9~lpsun? RR = R~'.qro~,d Fig. 4. Lobos Lake west-east geologic profile showing stratigraphic features. 65 cari parkinsoniana, Elphidium discoidale) suggest a mixohatine environment, such as a tidal marsh or what Urien (1967) called a 'fluvio-marine zone' in the Rio de la Plata ([named later "Rotalia beccari parkinsoniana biofacies zone' by Boltovskoy & Lena (1974)]. Hence, the freshwater and mixohaline species mix- ture in Sedimentary Body 3 reveals the sa_rne habitat characteristics that occur in coastal lagoons, with salin- ity variations from oligo to mesohaline ranges. These may be related to a broader estuary of the Rio de la Plata that developed toward the south, when the Holocene marine transgression reached the ancient Lobos Lake. Cann & DeDeckker (1981), however, state for Aus- tralia the athalassic origin of this type of population, introduced by sea bird displacements into an environ- ment where saline lakes made possible their survival and reproduction. Likewise, DeDeckker (1988) con- siders that the low species diversity of Foraminifera is typical of non-marine saline lakes. All these possibil- ities are not denied by our study, but in our opinion, based furthermore on considerable Pampean plain re- search, the arguments for direct marine influences are more likely. For example: 1. Modern lentic environment samples (fresh or brackish saline waters) never contain Foraminifera tests. 2. The existence of a fluvio-lacustrine watershed that formerly interconnected with the Rfo de la Plata estuary. 3. The regionally broad distribution of the mixohaline environment sediments. 4. The presence of surficial and related groundwater, whose ionic composition relationships (CI/HCO3; Mg/Ca) suggest sea water influence. 5. The inland geographic location of these sediments is similar to those assigned by various authors to the Holocene Querendinense ingression. 6. The position above sea level (18.4 to 21.5 masl) is not compatible with a Holocene ingression, unless they are considered neotectonic events. 7. The presence of similar mixohatine sediments along the Salado River course up to the Las Flores Chica Lake latitude. 8. The low species diversity of the paleolake Forminifera benthos thaphanocoenosis is similar to the present assemblage in the Samborombdn Bay coastal plain and Aj6 River. Stratigraphic correlation Lithological and environmental characteristics allow us to place the Sedimentary Body 1 in a stratigraphic position near the Pleistocene- Holocene boundary, and correlate it with the Guerrero Member of the Luj~n Formation (Fidalgo, 1983). The overlying clay dune deposits of Body 2 belong to the lower La Postrera Formation (Fidalgo, op. cit. ). The overlying mixohaline deposits of Body 3 belong to the Lobos Formation (Dangavs & Blasi, 1992b), and was deposited in the Holocene, with a probable age between 6000 to 7000 years B.R It can be correlated with the Querandinense Transgression (Frenguelli, 1957). Both the Lobos Formation and Querandinense are lithostratigraphic records of estu- arine channel deposits in the region along the Salado River up to +15 m bench mark, i.e. 120 km west of the present Samboromb6n Bay coastline. Conclusions All the infilling sediments of Lobos Lake and the strati- graphic sequences of the surrounding terrain represent more than 0. I million years between the upper Pleis- tocene and the Holocene. Our sedimentologic study of Lobos Lake allows us to recognize a development of different environ- ments during the Quaternary in response to climatic and geomorphologic changes, and the existence of an important stratigraphic hiatus between the Sedimen- tary Body 3 and the modern lake sediments, of about 5000 B.R This hiatus imposes a great limitation on our paleoenvironment interpretations. More research on the geological history of the surrounding sedimentary sequences is needed, which should include paleopedologic, paleohydrologic and paleoenvironmental data for elucidating the environ- mental history of Lobos Lake. Acknowledgements Our thanks to Comisi6n de Investigaciones Cienffficas of Buenos Aires Province, Argentina for their support of this project. We also offer our thanks to Drs John R Smol and Mario E. Terrugi for their suggestions in reviewing this manuscript. 66 References Blasi, A. & N. Dangavs, 1992. Sedimentos actuales de la laguna Lobos, Buenos Aires, Argentina. 4 ~ Reuni6n Ar- gentina Sedimentotogfa Actas II: 167-174. La Plata. Boltovskoy, E. & H. Lena, 1974. Foraminfferos del Rfo de La Plata. Publicaci6n H 661 Servicio Hidrograffa Naval, 22 pp. Buenos Aires. Cann, J. & P. DeDeckker, 1981. Fossil Quaternary and liv- ing Foraminifera from athalassic saline lakes, South Aus- tralia. J. Pateontol. 55: 660-670. Dangavs, N. & A. Blasi, 1992a. Presencia de yeso en Lobos, Buenos Aires, Argentina. Revista Museo La Plata, secc. Geol. X (in press). DeDeckker, P., 1988. Biological and sedimentary facies of Australian salt lakes. Palaeogeogr., Palaeoclimatol., Palaeoecol. 62: 237-270. Fidalgo, E, 1983. Algunas caracteffsticas de los sedimentos superficiales en la cuenca del rlo Salado yen  La Pampa Ondulada. Coloquio Intemacional Hidrologfa Grandes Llanuras, Olavarda. UNESCO. Actas II: 1043-1067. Buenos Aires. Frenguelli, J., 1957. Neozoico. In: Geograffa Repfiblica Ar- gentina GAEA. 2:1-113. Buenos Aires. Gaillard, M. C., 1974. Sobre nuevas Littoridina de la Reptiblica Argentina. Neotr6pica 20: 141-143. Maucha, R., 1932. Hydrochemische Methoden in der Lim- nologie. Die Birmengewtisser 12: 1-173. Stuttgart. Pilsbry, H., 1911. Non marine Mollusca of Patagonia. Rep. Princeton University Expedition Patagonia, Zoology 5: 513-653. Riggi, J., R Fidalgo, O. Martinez & N. Porro, 1986. Geologfa de los 'Sedimentos Pampeanos' en el Partido de La Plata. Asociaci6n Geol6gica Argentina Revista 41: 316-333. Buenos Aires. Segerstrale, S. et al., 1959. Simposio sulla classificazione della acque salmastre. Venezia 1958. Archivio Oceono- grafia e Limnologia XI, suppl. 243 pp. Italia. Urien, C., 1967. Los sedimentos modernos del rfo de La Plata exterior. Boletfn Servicio Hidrograffa Naval 4:113-2t3. Buenos Aires. 

Quaternary ontogeny of a pampean ‘laguna’

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Quaternary ontogeny of a pampean ‘laguna’

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