MICROBIAL (CYANOBACTERIA?) INDUCED SEDIMENTS FROM THE CRETACEOUS OF NORTHEASTERN MEXICO

Facies successions of the Indidura Formation (Cenomanian-Turonian) in the Parras Mountains, near Parras de la Fuente (Coahuila State, NE Mexico) are composed of alternating beds (30-100 cm) of limestone and marl with conspicuous internal laminae. Standard petrographic study of these sediments in thin sections, acid etched polished rocks and scanning electron microscope examination (secondary and backscatter imaging), reveal an evident pattern of very thin laminae (<3 mm) arranged in nearly even parallel light olive gray (5Y6/1) and brownish black to olive black (5YR2/1 – 5Y2/1) in color. Laminae from either the marly or limestone facies show that the color difference is due to an increase or decrease in the carbonate content, and consist of lower or higher concentrations of sparry calcite filling micro spheres and “micro-ooids”, with diameters between 5 to 100 µm. EDS analysis of these features reveals a composition of low magnesium CaCO3 (Mg 0.43 – 1.3 wt%).

These couplets are consistent with biolaminates known to be of microbial origin, and due to cyanobacterial mats accumulation. It is apparent that the microbial communities that generated the laminae dominated the ecosystem at the exclusion of other organisms as can be deduced by the low diversity observed and the absence of bottom dwellers. Only rare pelecypods (Inoceramus) are found while no other benthic nor nektonic organic skeletons are found within the sequence. A few planktonic foraminifera are observed only within the laminae with low micro sphere content. The sequence of laminae can be interpreted to represent rhythmical cyanobacterial die-off, perhaps related to daily cycles of immense blooms scattered over the basin, and in a calm-low oxygen environment.