REMNANTS OF THE CONTINENTAL SLOPE ALONG THE WESTERN EDGE OF PANGEA IN NORTH-CENTRAL MEXICO

Paleogeographic reconstructions for the Triassic to Early Jurassic time in Mexico, include remnants of continental fluvial deposits in northeastern Mexico, known as El Alamar Formation, as well as turbidites to de west, defined as Zacatecas Formation and interpreted as part of the Potosí submarine fan and other possible submarine fans placed to the northwest and southeast along the ancient margin of Pangea. Nowhere between marine facies in Zacatecas and San Luis Potosí and the fluvial facies in Nuevo León and Tamaulipas there are any deltaic or littoral facies recognized. The easternmost Triassic marine rocks were described in Real de Catorce in northern San Luis Potosí and Caopas-Rodeo area in northernmost Zacatecas. In both localities are found fine-grained sandstones and shales with thin to medium stratification, which become chaotic upwards in the sequence, as product of massive flow deposits, consisting of poorly stratified sandstones with floating quartz pebbles to blocks of exotic rocks such as siliceous rocks, compact sandstones and limestones, which show a strong sedimentary deformation, typical for continental slope deposits. In Real de Catorce, interlayered mafic flows and quartz-litharenites (MDA = uppermost Triassic) occur at the uppermost part of the succession and change upwards in to fine-grained yellow and red-purple siltstone layers of a marine marginal facies. Volcanic rocks of the Lower to Middle Jurassic Nazas Formation rest on the marine marginal strata and underlie conglomerate and red sandstones of La Joya Formation. In the Caopas area the Taray Formation (Upper Triassic MDA) and part of the Rodeo Formation show similar deposits underlying the Jurassic Nazas and La Joya formations. To the northwest other poorly studied comparable deposits occur in Santa María del Oro, Durango, as well as to the south in part Queretaro. We propose all of these localities as sediment remains occurring along the ancient continental slope on the western edge of Pangea, further sedimentological stratigraphic studies supported by geochronological data are needed to reconstruct this important paleogeographic element among deep marine Triassic rocks at the west and Triassic fluvial and alluvial deposits at the ancient continental area to the east.