EARLY TERTIARY FLUVIAL SEDIMENTS IN EAST-CENTRAL SONORA: RELATIVES OF THE RIM GRAVELS?

Paleogene gravel sequences, termed “rim gravels”, have been identified and described in Arizona and New Mexico near the edge of the Colorado Plateau (e.g. Young, 1993; Elston and Young, 1991; Drewes, 1975; Potochnik, 1996). These gravels are believed to have been deposited by rivers draining generally north- and eastward from the uplifts of the Laramide orogeny resulting in assemblages of Proterozoic to early Paleozoic clasts deposited across a broad erosional surface. The age of the rim gravels is determined by volcanic deposits locally overlying them, and is generally agreed to be Paleocene to Oligocene(?) (Holm, 2001). A previously undocumented section of similar description and age to the rim gravels has been identified in the area of east-central Sonora, Mexico, just north of Sahuaripa. This section of moderately sorted fluvial gravels and sand is a maximum of 800 m thick at Cerro Ceja Bajio and rests in angular unconformity above gently to moderately north-east dipping Late Cretaceous tuffaceous sandstones and shales. The gravels are unconformably(?) capped by a sequence of ignimbrites related to the Sierra Madre Occidental, the oldest of which is dated to 37 Ma. Large angular blocks of the Late Cretaceous bedrock are found along the edge of the east-northeast trending gravel exposure, possibly indicating the section was deposited in a canyon. Reconnaissance paleocurrent data on clast imbrication and bedforms show flow was to the northeast. Clasts in the gravels are moderately to well-rounded and the assemblage includes white quartzite, black and red chert, and metavolcanic lithologies possibly representing Proterozoic formations not present locally. These early Tertiary gravels differ in clast assemblage, texture, sorting, and stratigraphic sequence from younger Oligocene to Early Miocene alluvial conglomerates and breccias deposited in the extensional Sahuaripa basin described by Blair and Gans (2002; 2003) and may be related to the rim gravels being that they were deposited in a paleodrainage flowing northeast out of Laramide age uplifts in what is today central Sonora.