Abstract

During Early Cretaceous time, (1) differential vertical movements along northwest and west-northwest-trending normal faults, local volcanic activity, extensive erosion and alluvial fan sedimentation followed by (2) gradual subsidence and transgression of a shallow sea from the southeast and (3) later regional uplift and regression of the sea to the east affected southern Arizona, southwestern New Mexico and northern Sonora, Mexico. This sequence of events is best illustrated in southeastern Arizona where the Bisbee Group of late Early Cretaceous age is well exposed. Correlation of this Group with lesser known Lower Cretaceous rocks to the west, south and east suggests a consistent paleotcctonic and paleogeographic scenario.

A widespread unconformity that shows locally strong relief separates Lower Cretaceous strata from all older rocks. In southeastern Arizona, this unconformity places the Glance Conglomerate, the basal formation of the Lower Cretaceous Bisbee Group, on rocks ranging in age from Jurassic to Precambrian. The Glance Conglomerate represents the proximal facies of alluvial fan systems that rimmed local fault-block mountain ranges and basins bounded on at least one side by northwest or west-northwest-trending normal faults.

Overlying and intertonguing laterally with the Glance is the Morita Formation, a thick sequence of nonmarine clastics, predominantly fluvial mid-to distal-fan and basin fill sediments. This grades vertically into the shallow marine Mural Limestone of Aptian-Albian age which contains carbonate banks and patch reefs and represents the northwesternmost incursion of marine waters which transgressed northwestward from the Chihuahua trough. These marine rocks are in turn buried by a regressive facies composed of deltaic and fluvial sandstones and siltstones of the Cintura Formation.

Equivalent strata in central and southwestern Arizona are nonmarine clastic rocks which suggest that the sea did not transgress those areas. Correlative rocks in southwestern New Mexico and northern Mexico suggest that the marine basin deepened to the east and southeast where marine sedimentation continued in the Chihuahua trough.

During Bisbee Group deposition, east dipping subduction with associated arc volcanism existed 200-300 km to the southwest along the west coast of North America. This places the report region in a backarc tectonic setting characterized by northeast-southwest oriented extension. Volcanic rocks of arc association become increasingly abundant in the Lower Cretaceous section to the west and southwest. By Late Cretaceous time, the locus of arc magmatism had migrated northeastward into the region, causing the Laramide orogeny with a new tectonic setting of northeast-southwest oriented compression which uplifted and deformed the Bisbee Basin strata.