LACUSTRINE CARBONATE LITHOFACIES OF THE LATE NEOGENE MAYRÁN FORMATION, NORTHEAST MÉXICO

Lithofacies analysis has been undertaken on a suite of lacustrine deposits from the Late Neogene Mayrán Formation in the Mayrán Basin system, northeast Mexico. Using a combination of hand sample description, optical and electron optical petrography together with whole rock geochemistry nine lithofacies have been identified. These are: ostracode mudstone, clotted mudstone-packstone, gastropod wackestone-packstone, oncoid wackestone-packstone, stromatolites, charophyte wackestone-packstone, macrophyte boundstone, marlstone and crystalline carbonate. They can be distinguished from one another on the basis of their sedimentary structures and the relative proportions of allochthonous (production), autochthonous (detrital) and diagenetic derived components. The lithofacies deposited in both in proximal and distal lake settings, are organized in shallowing upward cycles, and exhibit mostly aggradation geometries. Factors controlling their deposition were lake level variations, basin geomorphology, distance and input of the clastic (both carbonate and siliciclastic) source, clastic dilution, rates of primary production, bottom sediment redox reactions, and bioturbation. Specifically, the ostracode mudstone deposited in sectors farthest down the lake margins away from clastic yield and intense bioturbation in areas of fairly low carbonate mud production. Areas closer to the lake margins commonly contain more diversity of carbonate shelled organisms and calcareous algae. Here primary production was intense and yield of clastic particles was insufficient to overwhelm carbonate production, as a result in sediment accumulation rates where fairly higher. Overall oxic to disoxic conditions prevailed during deposition of these lacustrine carbonates. Moreover pedogenic overprint affected the lithofacies deposited in lake margins, as they were subjected to subaerial exposure during lake level fluctuations. The carbonate lithofacies descriptions enable a detail sedimentological analysis of the Mayrán Formation to relate depositional and post-depositional changes controlling lithofacies variability in a series of carbonate-dominated linked lacustrine subbasins, with a common hydrology but yet different inputs.