New Mexico Geological Society Annual Spring Meeting — Abstracts

To address questions about the distribution and timing of deformation and basin formation associated with the northern Mexican orogen in the southern United States, we investigated the American Flag Formation exposed near Oracle, Arizona, in the Catalina Mountains. The American Flag Formation has been interpreted as a synorogenic deposit related to Late Cretaceous to early Paleogene contractional deformation but remains relatively understudied.

We used field observations, measured stratigraphic sections, clast-count analyses, sandstone petrography, and LA-ICPMS zircon U-Pb geochronology to determine depositional environments, sedimentation patterns, depositional age, and sediment provenance. Lithofacies associations, including poorly sorted, matrix-supported conglomerate, interbedded volcaniclastic sandstone, and volcanic rocks, indicate deposition in proximal alluvial fan systems dominated by debris flows and episodic fluvial reworking near an active arc.

A prominent mega-breccia within the succession likely records earthquake-triggered debris flows and associated mass-wasting processes. Rapid vertical facies changes and up-section variations in conglomerate clast composition and detrital zircon U-Pb age populations suggest progressive unroofing of Paleozoic passive-margin rocks and crystalline basement. A rhyolitic tuff at the base yields a crystallization age of 76 Ma, and four volcaniclastic sandstones and one interbedded lithic tuff have maximum depositional ages of 73–71 Ma, constraining sedimentation.

We interpret the American Flag Formation as a syntectonic deposit formed in response to uplift, extension, and volcanism, which controlled sediment production, transport, and accumulation. In a regional context, these results suggest deformation becomes younger to the east, reflecting progressive growth of the orogen toward the interior of North America during Late Cretaceous to Paleogene time.