Abstract

COMBINA, Ana María  and  SANCHEZ, María Lidia. Sedimentologic analysis of La Invernada Fm, Late Pleistocene-Holocene, foothills of the Sierra de Comechingones, Province of Córdoba, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.2, pp.123-135. ISSN 1853-6360.

This work is aimed at analyzing the sedimentology and paleoenviroment of the Late Pleistoceneearly Holocene deposits outcropping along several creeks which drain the eastern foothills of the southern Sierra de Comechingones. To investigate the vertical and spatial relationships between fluvial and aeolian deposits, 18 detailed vertical sections were logged and complemented with several architectural panels and discontinuity surface tracing along 30 km in a NNW-SSE trend (Fig. 1). The facies analysis was accompanied by mineralogical characterization of the sand-sized grain fraction (0.062 mm) and the fossil register. The climatic factors were considered to define the paleoenviromental model. The succession studied is part of La Invernada Fm (Cantú, 1992), presents a maximum thickness of 15 m and overlie discordantly over older quaternary sediments assigned to either Pampeano Fm (Frenguelli, 1955) or Conchancharava Fm (Cantú, 1992). Bones and isolated fossil pieces indicating Lujanense Mammal Age were found within the studied sediments. These fossils belong to the Pampeano and Central Domain, and most of them were interpreted to belong to browsers and grazers. The mineral constituents within the sand fraction are uniform in all deposits studied. However, in considering individual minerals, the observation of differences in the advance of weathering and morphometric properties suggest that at least two populations of minerals coexist within every sample. The light minerals, including quartz, alkali feldspar, plagioclase, and volcanogenic components (volcanic fragments) make up most of each sample (up to 98 %). Whereas heavy minerals usually appear in low concentration, they show a wide spectrum given by combinations of pyroxenes, amphiboles, micas, lithoclast, garnet, apatite, turmaline, oxides, epidotes, and olivines. Mineral constituents reveal the existence of three sources of clastic sediments: crystalline basement rocks (Sierras Pampeanas), acid volcanic rocks, and Cenozoic recycled sediments. Seven sedimentary facies (Table 1) were identified, one of them of aeolian nature and the others related to fluvial systems. According to the characteristics observed the facies were grouped in four facies associations (AF-1, AF-2, AF-3 y AF-4). Facies association 1 (AF-1): constituted by facies A and B or A and C, forms lens-shaped bodies bounded by sharp erosive limits; occasionally, the limits present irregular shapes. This association has been recognized regionally, and is interpreted as channel fills which were high-energy flows rapidly loosing energy, usually known as ephemeral flows (facies A and B) and hyperconcentrated sandy flows (facies C). Sometimes, this association consists only of amalgamated units of facies C. Considered individually, each channel fill represents, thereof, either a unique event or several episodes of channelised floods under ephemeral flow conditions close to the feeder zone. Facies association 2 (AF-2): involves facies B, C, and E continuously covering hundreds of meters, and usually accumulates to form tabular bodies. This association is separated by erosional surfaces that define sharp limits (Photo 8). It has been interpreted to represent a system of braided channels draining the distributing zone of a terminal fan. Recurrence of events led to the stacking of units conforming tabular bodies. Facies Association 3 (AF-3): composed by facies E and F; constitutes tabular units, and within each of them the facies represent cycles generated under highly variable flow conditions and quick loss of energy (Photo 10). This facies association represents a distal terminal fan environment. Facies Association (AF-4): This includes mainly facies D y G. It normally presents a thickness ranging from 2 m to 10 m. This association locally passes to lenticular bodies of the D facies (Photo 9). The facies G corresponds to aeolian sheet deposits (loess), whereas facies D shows the presence of paleosoils formed under saturated environments in zones of impeded drainage. Spatial relationships between both facies (D and G), dissimilar lithological features found in each facies, and the particular geometric configuration of facies D, lead to infer the development of low sinuosity channels contemporary with the loess deposition. The installation of vegetation suggests the association underwent a change in the redox levels that produced the precipitation of iron carbonate and oxide, while the fluctuation of the phreatic level controlled the variations in the Eh and pH lines. The formation of hematite-rich rizhoconcretions implies that the pH value should have varied between 7 and 8 whereas the Eh line may have fluctuated within the range 0.0 and +0.1. The distribution and geometry of the facies associations indicate that the paleoenvironmental evolution went through two distinct stages. The first stage is characterized by an intense aeolian aggradation process with formation of thick aeolian sheets (facies G). The lateral variation of facies associations results from the development of a terminal fan system (AF-1, AF-2 AF- 3) synchronously with the deposition of loess (facies G). In several columns it has been observed an upward transition from non-channeled flows to braided channels with multilateral relationships, an evidence that demonstrates the progradation of the system (Fig. 6a, photo 11). The development of these terminal fan systems happened on a palaeosurface of low gradient (between 2° and 5°E), under arid or semiarid climatic conditions, typical of extraglacier areas. The presence of fauna fossils that include big herbivores, indicates the preponderance of a semiarid regime not so rigorous, as it allowed the coexistence of browsers and grazers The second stage is represented by the AF-4, corresponding to the implantation of a low sinuosity system, of which sectors of abandoned channels are registered (facies D), with profuse vegetal colonization and pedogenetic processes typical of poorly drained areas. The presence of a sandy fluvial system strongly fits in the loessic mantels and with a design stack architectural it suggests an amplification of the area of tributary to the system, with the incorporation of new areas sources under conditions of low generation of accommodation space. The paleoenvironmental evidences and the sandy nature of the channels included in the loessic sections, suggest changes in the sediment supplyaccommodation space relationship that can be associated with a contemporary change in the base level.

Keywords : Sedimentation; Terminal fans; Loess; Palaeoclimatology; Comechingones.

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