Abstract: Lower Cretaceous Shelf Margin in Northern Mexico

James Lee Wilson, Giampaolo Pialli

In seven sections in the Monterrey-Saltillo area of Mexico the 800 to 500-m thick Lower Cretaceous Cupido Limestone and the underlying Taraises Formation of shale and black carbonate mudstone of about the same thickness have been studied petrographically. These units apparently represent, in part, complementary facies of carbonate bank and basinal environments. Isopach maps should include both formations if used for paleotectonic interpretation. The formations record a marine transgression in earliest Cretaceous time over a positive element in central Mexico which furnished sands and muds to the east. This transgression was followed in Barremian time by eastward progradation of a carbonate bank out from the positive element. The fully expanded late Cupido bank is overlain by the transgressive La Pena black shale and limestone of late Aptian age. The bank, as developed around Saltillo, consists almost wholly of cyclic grainstone and tidal-flat deposits showing progressive upward shoaling. On the east, around Monterrey, the bank edge is present, and is marked by more than 100 m of rudists and corals. This facies extends eastward and is higher in the section as the bank expands. The downslope facies in this are (Taraises Formation) is thick and well developed, contains lithoclastic conglomerates in black micritic matrix, and also has tumbled remains of corals and rudists. An eastern edge of the bank occurs at Saddle Mountain, Monterrey, and at the Sierra Minas Viejas 50 km northeast of the city. Farther east and south of Monterrey in the Sierra de la Silla b sinal micritic limestone is present throughout the Lower Cretaceous section.

The extension of this trend northeast into Texas is difficult to follow. From outcrop studies around Monterrey-Saltillo and north at Sierra de la Gavia and Bustamante it is possible to predict that the bank margin has a gentle slope over some tens of kilometers. Initial porosity and brecciation, vuggy and cavernous secondary voids, and dolomitization in the bank edge are encouraging signs for subsurface reservoir development.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas