ABSTRACT

The outcrops of the folded Sierra Madre Oriental in the Xilitla, San Luis Potosi area, allow the study of contemporaneous, shallow-water, rudistid-reef, forereef, and basinal carbonate sediments.

The “Tamabra” and Mendez Formations represent the forereef and basinal sediments and also the host rocks of the platform-derived exotic blocks. Parts of these formations are composed largely of lithoclastic fragments of rudistid reefs, or shallow-water, fossiliferous carbonates mixed with basinal, fossiliferous micrite containing planktonic foraminifers, radiolarians, calcispheres, and a few ammonites. These sediments show such sedimentary structures as graded bedding, and thin horizontal, and convolute laminations; many of these features are arranged in combinations analogous to Bouma’s sequence of turbidite structures. The exotic blocks are formed mainly of rudistid biomicrite (biolithite?); their stratigraphic thicknesses range from 10 to 95 m.

Stratigraphic correlations between the reef and forereef or basinal sediments were made by using fossils to determine the probable displacement of the exotic blocks; by palinspastic reconstructions of Barremian, Albian, and Maestrichtian times, approximate displacement of 5.5, 3.5, and 1 km, respectively, were inferred.

Although the Valles-San Luis Potosi platform is the counterpart of the subsurface Cretaceous Golden Lane platform, it does not have comparable large displacements of sediments which were derived from the reefs of the latter. These sediments were transported 10-15 km or more from the Golden Lane and are now contained in the “Tamabra Formation.” Perhaps this large displacement has not yet been discovered in the Valles-San Luis Potosi platform, or perhaps the patterns of sedimentation differed, because the tectonic settings of the platforms were different.

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