ABSTRACT

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 lime 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 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 sediments showing progressive upward shoaling. To the east around Monterrey a bank edge appears, marked by more than 100m of rudists and corals. This facies migrates eastward and rises in the section as the bank expands. The downslope facies in this area (Taraises Formation) is thick and well-developed, contains litho-clastic conglomerates in black micritic matrix, and also has tumbled remains of corals and rudists. An eastern edgeline of the bank occurs at Saddle Mountain, Monterrey and the Sierra Minas Viejas 50 km north east of the city. Still farther east and south of Monterrey in the Sierra de la Silla basinal micritic limestone occurs through the total Lower Cretaceous section.

The extension of this trend northeast into Texas is as yet 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 km. Initial porosity and brecciation, vuggy and cavernous secondary voids, and dolomitization in the bank edge are encouraging signs for subsurface reservoir development.

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