PALEOGEOGRAPHY, FACIES AND PROCEDENCE OF LATE TRIASIC SILICICLASTIC SEQUENCES FROM CENTRAL AND NORTHEASTERN MEXICO

Several isolated outcrops of pre-Cretaceous rocks in central and northeastern Mexico, include late triassic continental sequences, toward the east-northeast (Lower Huizachal Group) and marine sequences, toward the west-southwest (Zacatecas Formation). In several localities of the eastern portion, in Nuevo León and Tamaulipas, the continental triassic sequence is frequently absent, even appearing older rocks whose presence allows to interpret high areas where deposition of triassic sediments did not occurs. In addition it allows to reconstruct a paleogeography and sedimentary facies distribution from the continental part, on the Oaxaquia block, to this time already accreted to North America. To the west, vestiges of the ancient pacific continental margin appears in western San Luis Potosí, which probably extends toward the northwest, to Sonora and toward the south-southwest, to Guerrero. Toward the west, there is a projection to a wide system of submarine fan deposits that extends from San Luis Potosí to Zacatecas. The procedence of clastic components, based in petrographic as well as geochemical data, suggests a possible source at the mentioned Oaxaquia-North America continental block, in some cases with probable participation of local sources. The paleocurrent patterns, agree with this sedimentation model, showing a range of directions toward the northwest and southwest inside the body of the wide submarine fan. In the Huizachal-Peregrina anticlinorium, in Tamaulipas, the continental facies are more than 500 m thick; they are overlaying paleozoic sedimentary rocks and underlying Jurassic continental volcanic and volcaniclastic rocks. The basis of submarine fan deposits from San Luis Potosí and Zacatecas is unknown; a maximum reported thickness is a drilled 4000 m thick sequence, although increased in great measure by structural effects. This sequence is underlying early jurassic volcanic rocks.