CUPIDO – LA PEÑA FORMATIONS: HIGH-PRODUCTIVITY TO SUPER-PRODUCTIVITY DURING CRETACEOUS ENHANCED GREENHOUSE CONDITIONS

Detailed microfacies analyses of Barremian/Aptian deposits at the La Huasteca Canyon, Nuevo Leon, Mexico, indicate that the uppermost Cupido Formation that lies within the Bedoulian show intermittent levels impoverished or devoid of benthic foraminifera. These levels sharply contrast with the overall Cupido facies, which is typically rich in rudist fragments, benthic foraminifera, but contains only rare ammonite fragments in layers closest to the boundary with the suprajacent La Peña Formation. The succession of the La Peña facies is characteristically a sequence of interbedded calcareous shales, marly calcilutites and calcirudites with abundant thin-shelled ammonites, and few planktonic foraminifera. It also includes intermittent belemnites, small pelecypods other than rudists, and sea urchins, but benthic foraminifera are mostly absent occurring toward younger levels in the Gargasian coincident with highest taxonomic diversity. The Cupido facies may display weak subaqueous flow structures, while isotropic fabric of the La Peña facies and burrow mottling clearly imply accumulation under very low-energy conditions and some degree of oxygenation.

TOC and TIC results from the two facies show that TOC content is consistently higher (up to 3%) within the La Peña Formation, and only about 1% or less in the Cupido Formation. However, neither facies shows a clear correlation between TIC, TOC and carbon isotopic values, although occasionally high TOC levels within the La Peña Formation correlate with enriched isotopic values, thus implying high productivity. We interpret the Cupido facies to indicate a fluctuating high-productivity oxygenated neritic environment punctuated by dysoxic bottom waters starting in the early Bedoulian. A long-term shift occurred in the La Peña facies to a super-productivity hypoxic nerito-pelagic environment punctuated by changes consistent with low oxygenation but that did not develop a well-defined anoxic event as observed elsewhere.