Palaeoenvironments of the middle course of the Luján river (Buenos Aires, Argentina) during the last glacial period (OIS 4-2)

Abstract

The sedimentary sequences exposed in cutbanks of the middle course of the Luján river preserve important paleoenvironmental information on the late Quaternary climatic evolution of the north-eastern Pampean region. The objective of this paper is to define depositional units and infer the paleoenvironmental conditions from the analysis of the sedimentary facies and the paleobiological content (mollusks, phytoliths and diatoms) of the sequences dated between ca. 70,000 and 11,000 yr BP, Oxygen Isotopic States (OIS) 4 to 2, of the middle region of the Luján river basin. This paper follows on from previous studies about the reconstructions of the paleoenvironmental and climatic changes in the northern region of the Buenos Aires Province presented by Dangavs and Blasi (1995), Prieto et al. (2004) Fucks et al. (2005), Fucks and Deschamps (2008) and Blasi et al. (2008, 2009a, b).

A late Pleistocene-Holocene cutbank (PT-H) is preserved along the banks of the Luján river where three sections were analyzed (Table 1). The lower exposures, found from the present river level to halfway up the bank show late Pleistocene materials, while the upper portion shows Holocene fluvio-lacustrine and aeolian deposits. The Holocene deposits overlie paraconformably and start with a high organic matter concentration level which was deposited ca. 11,000 ¹⁴C yr BP in a lentic environment of meso-eutrophical characteristic (Prieto et al., 2004). Between the Jáuregui and Manzanares cities the PT-H cut bank is not continuous but alternates with deposits from a previous sedimentary cycle that we informally named "Pampeano" (Ameghino, 1884; Dangavs and Blasi, 1995). This last unit constitutes the present fluvial channel floor and interfluvial deposits of this region.

The development of the cutbanks studied here is likely to be related to a late Holocene fluvial incision of the late Pleistocene-Holocene sediments and aeolian, fluvial and lacustrine deposits that infill blowout depressions (Ameghino, 1880-1881, 1884; Frenguelli, 1925; Dangavs and Blasi, 1995).

Three sections along the middle course of the Luján river were selected for the production of seven detailed stratigraphic profiles (Table 1), for collecting samples for sedimentological and paleobiological analyses (Table 2), and for radiocarbon and IRSL dating (Table 3). The late Pleistocene sedimentary record (ca. <70,000 - <11,000 ¹⁴C yr BP) between the localities of Jáuregui and Manzanares allowed recognition of five facies: F1, F2, F3, F4 and F5 (Table 2), which are either totally or partially present in the studied sequences. The facies were correlated to the units described by other authors (Table 4).

The studied sequence starts with silty gravel and gravelly silt of lacustrine and fluvial nature (facies F1), that grade upward or laterally to sandy silt to silty sand of facies F2 that represent dunes of pellets (Figs. 4 and 5). Facies F2 presents an irregular based and low angle cross-bedding, and is composed of fine sedimentary lithoclasts, quartz and feldspar grains. The biological indicators are represented almost exclusively by phytoliths. This facies would have been produced because of seasonal wind storms that deflated sediments of the marginal platform of the lentic bodies that had been exposed by the retraction of the water body during temperate sub-humid dry to seasonal conditions. Facies F1 and F2 are unconformably covered by sandy gravel, gravelly sand and sand within a sandy silt matrix, included in the facies F3. It forms a massive lenticular stratum with commonly carbonatic clasts ("tosca"), reworked fragments of bones, siliciclastic grains and fine sedimentary intraclasts (Figs. 4-6). Remains of ostracods, diatoms, extinct mollusks and phytoliths are also abundant in facies F3 (Table 5). These deposits were accumulated after successive ephemeral fluvial episodes, because of heavy and concentrated rains of different frequency and caudal from channeled flow in gullies. During each fluvial event, temporary lentic water bodies would have originated because of either damming or overflow, thus producing the accumulation of fine material which, in part, was infiltrated as the matrix of the coarse deposits. The paleobiological records, frequently monospecific, suggest conditions of environmental stress due to abrupt salinity changes, caudal and temperature. These environments were very changeable because of water overflow produced during storms, and with low water levels after high evaporation periods. During facies F3 deposition, distal and local aeolian inputs would have continuously taken place with low sedimentation rates and without aeolian landforms; instead the aeolian materials would have been amalgamated to the alluvial ones. Sedimentation under temperate to cold conditions with alternant sub-humid/humid facies and dry or highly seasonal facies is interpreted here. Subsequently, the sediments included in facies F4 are vitroclastic sandy silt and silt (Figs. 4-6) with higher abundance and diversity of diatoms and phytoliths of Poaceae, Cyperaceae and woody dicots. This facies would correspond to loess deposits that accumulated in flooded lowlands, lentic shallow water bodies, and under temperate to cold and sub-humid to highly seasonal conditions. The succession ends with sandy silt of the facies F5 (Fig. 6), with very scarce biological content. These deposits represent the degradation of the lentic water bodies due to clastic infilling under temperate sub-humid/dry conditions that formed alkaline swamps.

The depositional paleoenvironments for the lapse ca. <70,000 - <11,000 ¹⁴C yr BP were interpreted as ephemeral lake-fluvial and aeolian settings in dry-subhumid and semiarid basins. Three major erosion discontinuities were recognized (Fig. 8). The first discontinuity would have occurred during late Pleistocene times, the second one corresponds to the late Pleistocene-Holocene transition and the third discontinuity would have taken place during historical times. Two other discontinuities were observed and are marked by indicators of stability like the presence of a duricrust on the top of the facies F3 and a paleosoil on the top of the late Holocene lake-fluvial deposits (Prieto et al., 2004). These discontinuities allowed recognition of five depositional units (DU) or aggradation events in the middle basin of the Luján river within the late Pleistocene-Holocene sub-cycle (Zárate, 2005).

The DU 1 consists of facies F1 and F2, with ephemeral lacustrine-fluvial and aeolian deposits, respectively. It represents sedimentation in lowlands interconnected by ephemeral streams, which were active during instantaneous current episodes, derived from storms, with the generation of ephemeral lakes and formation of aeolian lunettes towards the end of the flooding cycle. The minimal depositational age of DU 1 was inferred as being ca. 60,000 yr BP.

The DU 2 includes the facies F3 of ephemeral fluvial to ephemeral lacustrine deposits (the later of eutrophic to distrophic characteristics). This sedimentation took place in lowlands interconnected by ephemeral streams during instantaneous current episodes derived from storms and the formation of temporary lakes. Different from the previous DU 1, this multiepisodic accumulation was produced under cold, dry conditions. The inferred depositional lapse was estimated to be between ca. 50,000 yr BP and 32,500 yr BP.

The DU 3 is formed by facies F4 and F5. Its accumulation started with settling in permanent lakes or ponds with variable inputs of aeolian sand and dust, due to wind storms, under temperate and sub-humid climatic conditions (facies F4). Subsequently, these lentic water bodies were degraded by dystrophy (facies F5). The temporal interval of DU 3 accumulation was dated between 37,700 yr BP and 11,000 yr BP.

The DU 4 includes lake-fluvial to marsh deposits of eutrophic to distrophic characteristics, analyzed by Prieto et al. (2004). According to these authors this deposition took place in lowlands interconnected by permanent streams, under humid to sub-humid/dry conditions after 7,000 yr BP. The deposition lapse was dated between ca. 11,000 and 3000 yr BP.

The DU 5 consists of sandy-silty facies corresponding to reworked aeolian deposits that buried a paleosoil of ca. 3,000 yr BP (Prieto et al., 2004). They interpreted this unit as deposited under climatic conditions similar to the present ones since 3000 yr BP and until the incision of the present channel in historical times.

Two aeolian episodes of large magnitude were recognized, presented in the facies F2 (?56,400 ± 6,500 yr BP) and facies F4-F5 (?32,500 ± 4,700 to ?11,000 yr BP). The deposition of distal or local aeolian material could have been continuous, but, during some stages of lower deposition rate, aeolian deposits might have been integrated in the alluvial deposits. During the late Pleistocene, the seasonal regime of these environments would have influenced the aeolian and ephemeral-fluvial deposition processes, with differing degrees of participation in the sedimentation. The present day middle reaches of Luján river may have constituted, at that time, the head of the basin with low order ephemeral or temporary streams, so the influence eustacy is considered negligible (Blasi et al., 2009a). The succession studied here shows that the environmental variability during the OIS 4 to 2 in the northern Buenos Aires province would have been higher than previously interpreted. Episodic alluvial events probably took place under sub-humid climatic conditions during the OIS3, as shown by the paleobiologic records, and aeolian deposition was predominant during most of the OSI2 and, at least, part of the OIS4, evidencing sub-humid/dry climatic conditions. Sedimentological and paleobiological records of the middle course of Luján river allowed the inference that climate was the main driving force for sedimentation.