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Climatic Signals in Lacustrine Deposits of the Upper Yacoraite Formation, Western Argentina: Evidence From Clay Minerals, Dolomite, and Radiaxial Fibrous Calcite


Authigenic clay is an important control on reservoir quality in lacustrine carbonates but remains challenging to predict. Lacustrine depositional systems respond to climatic variations in water input, via rainfall, surface runoff and groundwater, and in evaporative water loss. These factors can result in a rapid and frequent change in lake volume, expressed though changing water depth and shoreline position. In the upper portion of the Yacoraite Formation (Early Paleocene) of the Salta Basin (Argentina), lacustrine deposits were identified in the sag phase of rifting. Prior high-resolution stratigraphic studies suggested climatic factors control microbial carbonate sequences within a "balanced fill" lake, with variation in the lake level having a major influence on facies association changes. The aim of this study is to characterize the evolution of facies and mineralogy in the Yacoraite Formation. We focus on the distribution of clay minerals and their response to climatic variations in this lacustrine environment, making a link between the high and the low frequency sequence stratigraphic cycles. The low-frequency transgressive hemicycle of upper portion of the Yacoraite Formation is shown to be comprised of abundant siliciclastic facies combined with coarse-grained agglutinating microbialites, representing the wetter period. Clay minerals associated with detrital sediments such as illite and chlorite occur in high-frequency transgressive period and carbonate facies with Ca-dolomite occur on high-frequency regressive hemicycle. The authigenic clays are regular chlorite/smectite mixed-layers. By contrast, the presence of fine-grained agglutinating microbialites with levels of radial fibrous calcite, dolomite, barite, authigenic magnesian clay minerals (smectite, and locally kerolite) are found in the low-frequency regressive hemicycle, indicating the highest concentration of ions in the lacustrine system under intense evaporation in an arid period. The high-frequency cycles have a well-developed regressive interval, and a thinner transgressive interval. Understanding the conditions which are favorable for formation and preservation of authigenic clays within the lacustrine environment is important information which can help in the evaluation of reservoir quality in petroliferous basins in the Pre-Salt, such as the Brazilian Santos Basin, and elsewhere.