Exploring hypotheses about mixed carbonate-siliciclastic successions in lacustrine settings: a case study from the Yacoraite Formation (Salta Basin, Argentina)

The interplay between carbonate production and siliciclastic input in marine mixed depositional systems results in spatially complex distribution of facies. In this study, we investigate this interplay in a lacustrine setting to explore hypotheses for the facies distribution and stratigraphic architecture in such settings. The Yacoraite Formation (Maastrichtian-Danian) is a mixed lacustrine carbonate-siliciclastic succession within the Salta Group in the Salta rift Basin (Argentina). The Yacoraite Fm has been thoroughly characterized in the southern part of the basin (Metán-Alemania sub-basins), whereas the northern sub-basin of Tres Cruces, focus of this study, remains largely understudied. In this project we applied high-resolution stratigraphic and sedimentological analyses to characterize in detail the depositional environment and the stratigraphic architecture of the Yacoraite Fm. Facies are mainly represented by lacustrine marginal and littoral associations. The facies associations, their distribution and stacking pattern are interpreted to reflect deposition in a predominantly shallow water balanced-fill lake basin type. Littoral and sub-littoral facies associations are dominated by oolitic, skeletal and microbial carbonates, frequently intercalated with fine-grained siliciclastic facies, ranging from mudstones and siltstones deposited in mudflats, shoreline sandstone deposits and distal profundal shales. The Yacoraite Fm is tentatively subdivided in two intervals. The lower part (lower 100 m) is characterized by carbonate-dominated facies, showing a marked and regular cyclicity, with metric-scale sequences of carbonate-dominated facies overlying fine-grained siliciclastics and mudstone-wackestone. These cycles are interpreted as shallowing-upwards cycles, composing the regressive hemicycles of metric-scale Transgressive-Regressive (T-R) cycles. These cycles are often asymmetric and result from lacustrine expansion-contraction cycles, controlled by climatically influenced lake-level fluctuations. The middle-upper part (m 100 to 220 circa) is dominated by siliciclastic facies and is characterized by a decrease in regularity of cyclicity, with high frequency T-R cycles being asymmetric and often lacking the transgressive hemicycles. Frequent desiccation cracks and tepees mark the top of the regressive hemicycles in the middle to upper part, indicating repeated sub-aerial exposures. Our observations are in line with the hypothesis that alternating phases of deposition between clastic-dominated facies and carbonate-dominated facies are the result of climatically driven lake-level fluctuations. Carbonate production is enhanced during arid climatic phases (lake contraction), whereas siliciclastic-dominated facies are mainly deposited during humid phases, coeval with an increase of water inflow and sediment input into the lake, corresponding to expansion phases. Based on our sedimentological and stratigraphical analysis the evolution of the lake system has been inferred, with the identification of two lake stages in the evolution of the Yacoraite paleo-lake. A first lake stage is characterized by a perennial lake system that progressively changes into a more rapidly fluctuating ephemeral setting; this shift appears to be gradual as there is no clear stratigraphic expression corresponding to the transition itself. Climate appears to be the primary control on the stratigraphic architecture, with rapid lake-level variations resulting in sharp facies transitions from carbonate to siliciclastic facies and prevalently stratigraphic mixing. Compositional mixing is limited to the littoral facies, due to the local presence of siliciclastic input sources by riverine inflows into the Yacoraite paleo-lake system.