UPPER CENOMANIAN-LOWER TURONIAN DEPOSITS OF NORTHERN MEXICO: PALEOENVIRONMENTAL IMPLICATIONS BASED ON FACIES AND REDOX-SENSITIVE TRACE ELEMENTS

Cenomanian-Turonian marlstones of the Agua Nueva Fm in San Luis Potosi and Tamaulipas are characteristically C-_org-rich, predominantly dark gray (N3) to black (N1) in color, and contain occasional levels with fish and other vertebrate fossils. Similar facies occur in coeval Indidura Fm at Parras de la Fuente in the Sierra Madre Oriental, southwestern Coahuila (Duque-Botero and Maurrasse, 2005; 2008), but levels with abundant macrofossils are absent. At the field scale these facies contrast with the contemporary Eagle Ford Formation in northern Coahuila where the limestones are essentially very pale orange (10YR 8/2), low in TOC, with limited dark gray levels, but also include levels with macrofossils. Microfacies show similarities at all localities in main carbonate components of the matrix with dominance of coccoid cyanobacteria and fewer filamentous cyanobacteria, irrespective of the TOC values. All sites include well-diversified planktonic foraminifera (15 - 20 %) indicative of sustained oxygenated conditions in the upper part of the water column. By contrast, intermittent levels showing lack of benthic foraminifera, even parallel to sub-parallel alignment of all components in the microfacies, as well as absence of bioturbation, imply periodic oxygen-deficient bottom waters.

Redox-sensitive trace element concentrations (V, Ni, U), and ratios of redox indices (V/(V+Ni), Ni/Co, V/Cr and U/Th) from the three localities further confirm that they went through conditions of severe oxygen deficiency.

Deeper-water facies of the Agua Nueva Fm, as well as the shallower equivalent of the Eagle Ford Fm, indicate sustained conditions favorable for microbial and eukaryotic microplankton productivity in the upper water column. Preservation of C-org-rich deposits in the deeper water environments is related to quasi-constant severe oxygen deficient conditions, whereas the epicontinental site of accumulation of the Eagle Ford Fm experienced only periodic pronounced shallowing of the oxygen minimum zone that may have caused asphyxiation of large nektonic vertebrates.

Duque-Botero, F., Maurrasse, F. J. M: 2005. Journal of Iberian Geology, 2005. 31 (1) 85-98; Cretaceous Research 29 (2008) 957–964.