354-5 Environmental Controls on Organic-Rich Sediment Deposition Across the Jurassic–Cretaceous Transition in Central-Eastern Mexico

Abstract:

Organic-rich sediments of the Pimienta Formation constitute one of the principal hydrocarbon source-rock intervals in central-eastern Mexico. To identify the environmental controls on sedimentation and organic-matter accumulation from the middle Tithonian to the early Valanginian, this study integrates data from the Padhí and Ameca stratigraphic sections. To this end, we applied a multiproxy approach combining sedimentological observations, mineralogical analyses, elemental geochemistry, total organic carbon measurements, spectral gamma-ray data, microfacies analysis, and biostratigraphy.

The studied succession accumulated in outer-ramp to basinal settings during a regional transgression, with local evidence of storm-induced reworking and sediment transport from shallower environments. The underlying Tamán Formation and the lower to middle parts of the Pimienta Formation were deposited under restricted to semi-restricted marine conditions characterized by high surface-water productivity and oxygen-depleted to locally euxinic bottom waters. These conditions favored the widespread accumulation and preservation of laminated, organic-rich calcareous sediments dominated by radiolarians and saccocomids. Regionally, these intervals contain the highest total organic carbon and uranium concentrations and display the strongest total gamma-ray response. Enhanced nutrient delivery through upwelling likely sustained high primary productivity. Geochemical indicators also suggest recurrent volcanic influence, which may have supplied additional nutrients to surface waters. However, a hydrothermal contribution associated with the opening and tectonic evolution of the Gulf of Mexico cannot be excluded. Enrichments in redox-sensitive trace elements further indicate persistent water-column stratification and limited bottom-water ventilation during deposition of the most organic-rich intervals.

In contrast, the upper Pimienta Formation and the overlying Lower Tamaulipas Formation were deposited under conditions of lower surface-water productivity and improved bottom-water oxygenation. These conditions resulted in the accumulation of bioturbated, organic-poor, calpionellid-rich sediments. Regionally, these younger deposits contain lower total organic carbon and uranium concentrations and consequently display a weaker gamma-ray response (Project-SECIHTI:CBF-2025-G-896).

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