PRELIMINARY STRATIGRAPHY OF THE SAHUARIPA BASIN, SONORA, MEXICO

The Sahuaripa basin in east-central Sonora, Mexico, is a continental basin formed during Basin and Range and Gulf of California extension. A west-dipping normal fault system dominates the structure of the basin, which is roughly 30 km long and 10 km wide. Jurassic and Cretaceous sedimentary sequences underlie the eastern range footwall and western range. A thorough analysis of continental sedimentary basins in eastern Sonora such as this one will help us to better understand how these two deformation regimes relate to each other temporally and spacially. The Sahuaripa basin roughly coincides with the modern Sahuaripa river valley where recent stream incision clearly exposes the gently dipping sequence of conglomerate, pebbly sandstone, siltstone, and basaltic to andesitic flows and breccias. The basin sequence is underlain by at least one ignimbrite deposit. At the southern extent of the basin, one of these tuffs was determined to be 20 Ma, dated by ⁴⁰Ar/³⁹Ar method by Gans (1997). Additional tuffs lie above a thin sequence of conglomerate in the north-eastern part of the basin and are then overlain by local basalt flows and conglomerate indicating the basin formed just before or shortly after at least two different tuffs were deposited. Previous geologic mapping of the Arivechi and Sahuaripa 1:50,000 sheets neglected the presence of rhyolitic tuffs that crop out on each side of the basin and in spectacular highway cuts north of Sahuaripa. Basalt flows are also locally interbedded in the clastic sequence reflecting small volcanic centers usually along faults. The sequence of mafic lava flows and breccias interbedded with poorly indurated red clastic sediments was termed the "Baucarit Formation" by King (1939). Many of the basins in the region thus were mapped with the generic Baucarit Formation without regard to the deposits' ages or actual character. Preliminary mapping also shows that the conglomerate beneath the mafic flows generally has more angular clasts and is poorly bedded, while higher in the section bedding improves and the units are sandier. Samples have been collected for future geochronologic analysis which will give age data to constrain the timing of basin development and sedimentation so that this basin can be correlated with others.