MICROSCOPIC BIOFORMS IN PYRITIC LAYERS FROM THE CENOMANIAN/TURONIAN (UPPER CRETACEOUS) AGUA NUEVA FORMATION, XILITLA, CENTRAL MEXICO: A PRELIMINARY DESCRIPTION

The Agua Nueva Formation (Cenomanian/Turonian) at Xilitla, San Luis Potosi, Mexico, includes laminated limestone beds with pyritic levels up to 1cm thick. Scanning Electron Microscope images of these levels reveal both euhedral, and framboid pyrites. Crystals show closely packed structures forming aggregates with irregular polyhedral forms varying in size from 2μm to 5μm. Framboids are up to 7μm long, and appear as closely packed equigranular euhedral microcrystallites of approximately 1μm in size or less, forming regular honeycomb-like pattern of polygons. Pyritic levels also contain 1 to 2 μm sized oval to spherical biomorphic structures seen in two and three dimensions, which may form aggregates resembling colonies with morphology and size similar to pyritized coccoid bacteria. Such bacteria are also reported in the coeval Indidura Formation in northeastern Mexico (Duque-Botero and Maurrasse, 2005; 2008; Duque-Botero et al., 2008). Similarly, rod-like microstructures are 1μm to 5μm long and slightly less than 1μm wide, and resemble rod-shaped bacteria known as bacillus. Energy Dispersive X-ray Spectrometry (EDS) analysis reveals the same content of iron and sulfur in both coccoids and framboids, whereas a high content of carbon was detected only in the biostructures contrasting with the surrounding pyrite, which suggest their organic origin. The biomorphic structures are interpreted to represent single bacteria and their colonies associated with pyrite formation during early digenesis under anoxia/dysoxia in euxinic conditions that prevailed during the deposition of the Agua Nueva Formation (Blanco-Piñón et al., 2008). References: Duque-Botero, F. and Maurrasse, F. J-M.R. 2005. Journal of Iberian Geology 31, 85-98; and 2008. Cretaceous Research 29, 957-964. Duque-Botero et al. SEPM Spec. Publ. 93, 1-16; Blanco-Piñón et al. 2008. Eos Trans. AGU, 89(23), Jt. Assem. Suppl., Abstract PP24A-04.