Geochemical and paleoenvironmental record of the early to early late Aptian major episodes of accelerated change: Evidence from Sierra del Rosario, Northeast Mexico

Abstract

The lower to lower-upper Aptian succession of northern Mexico documents the drowning of the shallow-water Cupido/Cupidito carbonate platform system followed by deposition of the deep-water sediments of the La Peña Formation. Using δ13C stratigraphy, geochemical and mineralogical information coupled with previous microfacies, paleontological and total organic carbon (TOC) data from a stratigraphic section, which includes such lithological change, this study identifies distinctive episodes of accelerated global environmental change, and determines the paleoenvironmental conditions conducive to the deposition of TOC-rich intervals. Within the Cupidito unit, the Oceanic Anoxic Event 1a (OAE 1a) is recorded near the base of the section and the Intra-Furcata Negative Excursion in the topmost beds of the unit. The upper part of the section, within the La Peña Formation, is correlatable with the Noire level. Organic-carbon rich intervals occur in the lower and middle part of the OAE 1a, upper part of the Cupidito unit, base of the La Peña Formation, and in the Noire level equivalent. Reducing conditions within the sediment and oxic–dysoxic at the seafloor, locally controlled, persisted both before and during OAE 1a interval in the Cupidito lagoon. Oxygen-depleted conditions (dysoxic–anoxic) were more permanent and stronger during the deposition of the base of the la Peña Formation and the Noire equivalent level. It is proposed here that deposition of the lower-middle part of the OAE 1a and the base of the La Peña Formation was influenced by climate-controlled increases in detrital and accompanying nutrient influx that supplied especially biolimiting nutrients (Fe, P), fostering marine productivity and TOC burial. Upwelling of nutrient-rich deeper waters and minor arrival of nutrients from runoff, both account for the domination of radiolaria and organic-carbon burial during the Noire level equivalent. Record of the OAE 1a time-equivalent facies in the Cupidito lagoonal sediments confirms the diachronism of the Cupidito–La Peña flooding. Heterozoan carbonate production in the Cupidito ramp was uninterrupted during and after OAE 1a. Periodic arrival of detrital components and nutrients during warmer/humid periods accelerated platform drowning, which peaked during the diachronic deposition of the La Peña Formation.