LATE CRETACEOUS PIGGY-BACK BASINS IN NORTHERN SONORA (NW MEXICO): EVIDENCE OF MAJOR LARAMIDE THRUSTING

The Late Cretaceous orogeny is a major event in western USA and Mexico. Its age has been bracketed between 110 and 40 Ma. However, in Sonora this major compressional event is poorly understood and even unrecognized by several workers. Here I provide stratigraphic evidence that supports the presence of the Laramide Orogeny (sensu lato) in Sonora. In the Early Cretaceous most of Sonora and southern Arizona were covered by a sea where the Bisbee Group was deposited. By the end of the Albian, sedimentation ceased because of a regional uplift causing a major regression towards the east. In Late Cretaceous time sedimentation resumed in continental basins in a compressional intra-arc setting. If no major event had occurred, the Upper Cretaceous would have been deposited upon the Lower Cretaceous in erosional to angular unconformity. The presence of older rocks as basement to the Upper Cretaceous strongly suggests that a nappe structure of regional dimensions emplaced in the early Late Cretaceous. North of the metamorphic belt of Late Cretaceous age, extending from SW Arizona-NW Sonora into northcentral Sonora (most probably the main thrust zone), the Upper Cretaceous was deposited upon the Lower Cretaceous, the contact varying from angular in the eastern part to erosional in the western part. South of this belt, the Upper Cretaceous was deposited upon Paleozoic and Triassic-Jurassic rocks. The Upper Cretaceous is a complex sequence consisting of almost entirely andesitic successions (central Sonora) to mixed volcanic/sedimentary and almost entirely sedimentary (NE Sonora and SE Arizona). These variations may be related to the position of the basin (or basins) relative to the thrust front and the volcanic arc. Where deposited upon the nappe plate, the sequence appears not to be folded, but only tilted. The Paleozoic and the Proterozoic crystalline basement have been found to be structurally emplaced on Upper Cretaceous sediments and volcanics in several localities, as in Sierra La Víbora (Caborca area), Sierra Santa Teresa (E of Hermosillo), Cerro de Oro and in the Sahuaripa area (E of Arivechi and Sierra Chiltepín). The regional picture was disrupted by Tertiary low-angle normal faulting in such a way that in nearby localities (Cerro de Oro, for example) the Upper Cretaceous is found lying upon the Bisbee Group (the lower plate) and the Paleozoic (the upper plate).