Resumen

PIOVANO, Eduardo Luis. Los parámetros físicos y químicos y su vinculación con el modelo depositacional de la Formación Saldán (Cretácico Temprano), Sierra Chica de Córdoba, República Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1995, vol.2, n.1-2, pp.57-73. ISSN 1853-6360.

The Saldán Formation is an integral part of Cretaceous sequences developed on the eastern Sierras Pampeanas of Argentina (Fig. 1). The deposition of these red beds took place in an intracratonic hemigraben-type basin, intimately related to the Mesozoic extension which determined the formation of Argentine sedimentary basins The sedimentological investigation of the Saldán Formation allowed the definition of a depositional palaeoenvironment associated to alluvial fans. Several facies were recognized (Table l): a) Facies l or massive matrix supported brecchias, interpreted as non-cohesive debris flows; b) Facies 1a or lenticular sandstones, assimilated to residual fluid flows which follow debris flows and deposit sediments during their decreasing stage; c) Facies 2, 2a and 2b named conglomeratic clast-supported facies, associated to sheet floods and, to a lesser extent, to stream channels; d) Facies 3 or laminated sheet sandstones, related to sheet floods during decreasing flow stages; e) Facies 4 or very fine sandstones, and sheet and lenticular siltstones, deposited during the period previous to the final stages of the flow; f) Facies 5 or massive and laminated mudstones, related to sedimentation in perennial ponds; g) Facies 3a or massive sandstones, corresponding to sandy mudflow deposits; h) Facies 6 or fine sandstones with trough cross-stratification; i) Facies 7 or fine sandstones with tabular cross-stratification, and j) Facies 8 or sandstones with planar stratification, associated to accretional growing of dunes under wanning stages. Up to four facies sequences have been defined in the Saldán Formation, all related to depositional environments typical of alluvial fans (Table 2). The sequence of facies A corresponds to proximal settings dominated by debris flows. The facies sequence B represents sedimentation conditions in intermediate sectors of alluvial fans, which are characteristic of water-laid deposits. The facies sequence C is associated to sheet flood deposits in distal sectors of the alluvial fan, and to particle setting in residual water bodies. The facies sequence D is herein interpreted as channel filling deposits in shallow and wide ephemeral streams, which cut through sheet flood beds. These sets of sequences define two sequences which are separated from one another by a period of volcanic reactivation. The signature of the volcanic episode in the upper sequence is recognized by the presence of basaltic boulders and hydrothermal minerals. The facies arrangement is amply justified by the interpretation of a set of sedimentological parameters. The variability inherent to the dynamic nature of the processes which control the sedimentation in the basin, allowed to determine the correspondence between the spatial organization of facies on one hand, and the chemical and physical parameters on the other. Grain-size distributions plotted on normal probability paper yield the differentiation of means of transport according to the lithofacies analyzed (Table 3). Debris flows are mainly composed of three populations. The first one is related to the gravel-sand framework, while the other remaining populations are part of the matrix. Those lithofacies transported by water-driven processes, show populations associated to bed-load and suspension transport. Distal lithofacies as well as those showing an upward grading, exhibit an increasing amount of suspension population. When the physical parameters are considered (Table 4 and 6), those related to grain-size information allow the classification of the samples according to their depositional setting (Fig. 2). The clustering of samples reflects their situation within the palaeodepositional environment (Fig. 3). The transition from proximal to distal facies is texturally characterized by an improvement of the sorting, a change in the frequency curves from positively skewed and platykurtic to bell-shaped or negatively skewed and leptokurtic. Geochemical parameters (Table 5; e.g., SiO2/A12O3 ratio, K2O and MgO content) correlate with textural parameters (Table 6) and depict the mineralogical and textural maturity of sediments. By using Herron‘s geochemical scheme (Fig. 4) the geochemical results are compared to petrographic and textural classifications (Table 5). The chemical index of alteration (CIA) mainly shows an increase towards distal facies (Fig. 5), where markedly weathered material accumulates (weathering processes more intensive than transport ones). In proximal facies, CIA values are closely related to the transport capacity of the agent. Water-driven deposits show lowest values due to the loss of fine material during high energy flows. The freezing of debris-flows during their deposition allow preservation of fines yielding higher CIA values. Clearly, the mean grain size of distal and proximal facies is correlated to the CIA (Table 6; Fig.5). Moreover, the higher potassium content observed in the upper sequence is probably attributable to the occurrence of a basaltic volcanic event (Fig. 6).

Palabras clave : Cretaceous; Pampean Range Córdoba; Alluvial fan; Texture.

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