RELATIONS OF THE LATE PALEOZOIC TURBIDITES IN CHIHUAHUA TO THE OUACHITAN AND CENTRAL MEXICAN TURBIDITES

The Rara and Plomosas Formations are enigmatic Late Paleozoic turbidites in central Chihuahua that are traditionally defined as foredeep basins related to the Marathon-Ouachita deformation. Both units have Wolfcampian and Leonardian fossils, although these in the Rara are so rare that its age has been somewhat uncertain. These two formations are highly deformed, and the thrusting of the Rara in particular is taken to be the Ouachita Deformation Belt in Chihuahua. These two formations are the first exposure of deformed Paleozoic rocks southwest of the Glass Mountains in West Texas. Since the Ouachita deformation proceeded from east to west as South America - Africa progressively closed with North America, the late age of the Chihuahuan deformation has not been seen as a problem.

However, there are a number of problems in defining the Rara deformation to the Marathon-Ouachita system. The age of thrusting actually is somewhat too late. The deformation in the Plomosas at Placer de Guadalupe seems to be a landslide or gravity decollment. The direction of thrusting in the Rara at the Sierra del Cuervo is from west to east, not east to west or southeast to northwest as should be seen if this was developed from the colliding South American - African plate. Many years ago, a researcher suggested that the Rara showed foreland basin type reverse high angle thrusting, but this does not explain the small scale but pervasive left-lateral shear in the outcrop, which post dates the thrusting.

A better explanation for these basins involves connecting them into the Ancestral Rockies system of uplifted basement blocks with intervening small deep asymetrical basins. Therefore, in the Middle Permian, the Aldama Platform to the west is the basement cored uplift, with the Pedragosa Basin - Chihuahua Trough as the linear asymetrical basin, deepest on the western margin where the Rara Formation is now.

There are a series of small deep basins in Mexico during the Permian. They all have turbidite infills, and are usually very deformed. The turbidite formations all have different names, but are essentially the same, and all probably related to both the Ancestral Rockies and the assembly of Pangea. These are not directly part of the Marathon – Ouachita system, but do similar characteristics to the Rara and Plomosas Formations.