Paper No. 96-1
Presentation Time: 9:00 AM-1:00 PM
SYNOROGENIC DEPOSITION AND EVIDENCE FOR MULTIPLE PHASES OF CONTRACTIONAL DEFORMATION DURING THE LARAMIDE OROGENY IN THE NORTHERN CHIRICAHUA MOUNTAINS, ARIZONA
Components of the North American Cordilleran system can be traced for 1000s of kilometers along strike. However, due to structural and magmatic overprinting, the nature and existence of contractional deformation in the southern U.S. Cordillera is uncertain. To resolve the complex tectonic history of the region, we conducted detailed geologic mapping in the northern Chiricahua Mountains, Arizona. A 495 m thick, upper Cretaceous to Paleogene conglomerate and volcaniclastic succession, assigned to the Show Goat Formation, overlies lower Cretaceous rocks of the Bisbee Group as well as upper Paleozoic carbonate rocks in a prominent angular unconformity. The Show Goat Fm. is interpreted to have been deposited syn-kinematically. Sandstone and conglomerate clast compositions indicate increasing volcanic contributions up-section, with the uppermost members of the formation dominated by andesitic lava flows and lithic tuffs. In one location, the low-angle Hidalgo thrust fault places upper Paleozoic carbonate rocks structurally over the Bisbee Group and Show Goat Fm. In a second location, a high-angle reverse fault places the lower Cretaceous Bisbee Group and upper Paleozoic carbonate rocks above the Show Goat Fm. Dramatic changes in the thickness of the Bisbee Group across the high-angle fault suggests that it originated as a normal fault and was subsequently reactivated. The map relationships suggest multiple phases of contractional deformation during the Cretaceous to Paleogene including; 1) folding/tilting and erosion of the Bisbee Group followed by deposition of the Show Goat Fm., 2) low-angle slip on the Hidalgo Thrust Fault, and 3) reactivation and slip on a high-angle reverse fault. Detrital and igneous zircon U-Pb ages show that the Show Goat Fm. was deposited ca. 74-68 Ma, consistent with the timing of the Laramide orogeny. The Show Goat Fm. is likely equivalent with similar synorogenic rocks throughout the region, including the Salero, Bobcat Hill, Ringbone, Skunk Ranch and Hidalgo Formations.