LITHOLOGIC AND STRUCTURAL CHARACTERISTICS OF THE PALEOCENE-EOCENE ZACATECAS CONGLOMERATE, CENTRAL MEXICO

Paleocene-Eocene continental red beds crop out in central and southern Mexico. They were deposited simultaneously and locally with volcanic rocks in fault-bounded basins. The initial sedimentology and structural studies of these red beds foresee a better understanding of the tectonic evolution of central Mexico during the early Tertiary time.

The study area is located in the state of Zacatecas, in central Mexico. The stratigraphy is composed of the Early Cretaceous Zacatecas Group unconformably covered by the Zacatecas Conglomerate that is in conformable, transitional contact with the Tertiary Volcanic Sequence. The Cenozoic deformation has several extensional stages whose variation of the minimum stress developed structures that change from WNW-ESE and NW-SE to NE-SW.

The Paleocene-Eocene Zacatecas Conglomerate crops-out in the Zacatecas and Guadalupe cities; it is composed of three facies. The lowermost is rich in basaltic and dioritic clasts with a few granitoid clasts. It is interlayered with lithic tuffs. The middle facies includes clasts similar to the lower one; however, the tuffs are pumice and crystals-rich. The bottom of the uppermost facies is rich in granitoid clasts; towards the top their abundance decreases and increases in basalt.

The Zacatecas Conglomerate is limited to the north by the south dipping, WNW-ESE trending Veta La Cantera normal fault. It is a 7 km-long, continuous structure that puts in contact the Zacatecas Group with the Conglomerate. To the south, the basement does not crop out so the boundary is defined by several north-dipping, WNW-ESE trending normal faults. Most strata are north dipping with azimuths parallel to the WNW-ESE faults. In the northern outcrops the strata have dips are 10 to 30° north dipping, whereas in the south they generally dips less than 15°.

On the basis of cross-cutting relationships, the first formed faults are 260° to 320° trending, whereas the successive extensional deformation stages made faults oriented from 340° to 060°.

During the initial stages of the deposit of the Zacatecas Conglomerate, the volcanism occurred simultaneously. The main structural features are related to the San Luis-Tepehuanes fault system; while the post-Eocene deformation stages show a wide range of orientations as a response of the Basin and Range deformation.