SEDIMENTARY CYCLES AND GEOCHEMISTRY OF THE INDIDURA FORMATION CENOMANIAN/TURNONIAN (NE MEXICO): CAUSAL RELATIONSHIP TO OCEANIC ANOXIC EVENT 2

High-resolution lithostratigraphic data from C/T sediments assigned to the Indidura Formation in the Sierra de Parras, near Parras de la Fuente (Coahuila, Mexico), show persistent interbeds of soft laminated marlstones and hard highly laminated biocalcilutites 5 to 200 cm thick. The sequence is almost barren of macrofossils except for few inoceramids, and contains calcareous and clay particles, organic matter up to 3.6% together with disseminated pyrite.

SEM imaging and petrographic analyses reveal that main calcareous constituents consist of cyanobacterial precipitates as microspheroids with a size range of 5 to 40 μm, more consistently around 20 μm. Microspheroids are particularly abundant from the middle to the upper part of the section (nannofossils zones CC-12 and CC-13).

C-org stable isotope values show an enrichment of about 2 ‰ in the lower part of the section, zones CC-10 and CC-11, that coincides with the global positive excursion associated with OAE-2, but return to pre-event conditions in the CC-12 zone, followed by a smaller excursion in the upper part of CC-12 to CC-13 zone. Geochemical anomalies in Mo, V, and Cr indicate that most of the unit accumulated under anoxic to dysoxic conditions, as the absence of bioturbation also suggested.

Identification of zones CC-10 to CC-13 further allowed the calculation of compacted sedimentary rates between 6.2 and 10.1 cm/ka. Integrated temporal variations in TOC and CaCO3 contents, field scale measurements, and lamina counting - grayscale intensities of thin sections examined by continuous wavelet analysis, show correlation of the sedimentary fluctuations to Milankovitch and solar cycles.

Climatic forcings that enhanced or diminished cyanobacterial microspheroid productivity generated bed couplets and lamination of the sequence. Cyanobacterial blooms further decreased the oxygen content, causing exclusion, preservation of primary sedimentary structures, and geochemical anomalies, which are coincident with global perturbations associated with OAE2.