ENVIRONMENTAL DRIVERS OF ORGANIC MATTER DEPOSITION DURING THE JURASSIC-CRETACEOUS TRANSITION IN CENTRAL-EASTERN MEXICO

During the Mesozoic, deposition of Laminated Organic-rich Mudrocks (LOM) was commonly linked to exceptional episodes of accelerated global change (e.g. Ocean Anoxic Events) caused by profound alterations in the ocean-lithosphere-atmosphere system. However, LOM successions were also deposited in the Proto-Gulf of Mexico (P-GoM) during the Jurassic–Cretaceous transition, a time interval of apparent global stability. In particular, the Pimienta Formation accumulated in central-eastern Mexico from middle Tithonian–early Berriasian. This pelagic calcareous LOM is one of the major hydrocarbon source rocks in the country. In this study, sedimentological, petrographic, mineralogical and geochemical analyses are integrated in order to reconstruct the paleoenvironmental conditions that favored the deposition of this unit. In the studied section, the organic-richest interval (TOC: up to 3.9 %) corresponds to the lowermost part the Pimienta Formation (44.5-m-thick.) and is constituted by a Tithonian succession composed of dark pelagic limestone commonly intercalated with shale and bentonite. Moreover, it is characterized by radiolarian-rich wackestone/packstone with a strong gamma ray signal and is highly enriched in redox- (U, V, Ni, Cu and Mo) and productivity-sensitive trace elements (Cd, Zn, Ba). Our dataset suggests that delivery and burial of organic matter in these sediments was favored by both: 1-eutrophic conditions in surface waters linked to the input of volcanic ash and, in a lesser extent, to the injection of biolimiting metals into the marine system associated with magmatic-hydrothermal activity in oceanic ridge segments, and 2- water mass stratification and stagnation of the P-GoM caused by the rotation of the Yucatan block, which resulted in anoxic conditions.