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Título

Unveiling Sequence-Generating Mechanisms in a Carbonate Lacustrine System: An Integrated Stratigraphic Investigation in the Yacoraite Formation (Salta Group – Argentina)

Texto do resumo

The Yacoraite Formation in the Metán-Alemania Sub-basin is the sedimentary record of a lacustrine system during the Maastrichtian and Danian periods. Previous research has created a hierarchical stratigraphic framework of multiscale sequences. This study reviewed these proposals and sought to identify the main processes that generate these sequences. The reinterpretation of regional studies identified the 1st- and 2nd-order sequences. Regional unconformities bound these large-scale, tectonically controlled units. Reinterpreted as a first-order sequence, the Balbuena supersequence spans 10 million years and represents the complete filling of a Sag phase of the Salta Basin. The sequences known as Balbuena I, II, III, and IV are identified here as subdivisions within the 1st-order sequence and constitute second-order sequences corresponding to the record of the primary paleogeographic shifts in the equilibrium between accommodation and sediment supply at the basin scale. Geochemical analysis of carbon and oxygen isotopes in outcrops at the depocenter of the Metán-Alemania sub-basin identified different phases of lake sedimentation influenced by climatic conditions, corresponding to third-order regressive and transgressive tracts. Increasing isotope values towards the top indicate dry climatic conditions in a progressively shallower and more alkaline lacustrine environment while decreasing values indicate deposition in deeper and more diluted environments under humid paleoclimatic conditions. Facies associations confirm these interpretations, with carbonate-rich facies with strong microbial presence during dry periods and fine-grained siliciclastic facies dominating during wet intervals. The K–Pg boundary transition, identified by a negative anomaly in the δ13C profile, was used as a tie point for the astronomical calibration. Using a high-resolution astronomical time scale (ATS), the duration of 3rd-order sequences was estimated at 1.8 million years. Climate change and eustatic variations during the Cretaceous were likely the result of long-period modulations to obliquity and eccentricity. The primary process of global sea level variation during the Cretaceous greenhouse climate is aquifer-eustasy, characterized by an out-of-phase relationship with lake levels. The stratigraphic record of the 3rd-order tracts and the sea level profile for the same period indicate this relationship. Cyclostratigraphy of the Yacoraite Formation reveals 4th-order sequences corresponding to the long eccentricity cycle (405 kyr), exhibiting correlations over distances exceeding 50 km. Within the 4th order sequences, there are further divides into 5th-order sequences, indicating that they reflect cycles of obliquity (40 kyr) that are only partially recorded due to gaps in deposition. According to this observation, it is estimated that a minimum of 30% of the time involved in Yacoraite Formation development during the Maastrichtian is not preserved. The study conducted in this research integrates methodologies to accurately calibrate sedimentary processes and mechanisms that control deposition within a hierarchical stratigraphic structure. This approach reduces inaccuracies and contradictions commonly found in models uncommitted to temporal and spatial order. Such detailed definitions make the Yacorate Formation an excellent analog for characterizing carbonate lacustrine reservoirs, such as those found in the Pre-Salt.

Palavras Chave

High-resolution sequence stratigraphy; Sequence-Generating Mechanisms; Carbonate Lacustrine System; Yacoraite Formation