Abstract
Quartzite, conglomerate, and shale of the Mazatzal Group record the filling of a Prot erozoic intra-arc basin in central Arizona. U-Pb ages of zircons from rhyolite ash-flow tuff indicate that deposition began at 1701 ± 2 Ma. Basal deposits of the newly defined Pine Creek Conglomerate formed in an alluvial-fan setting, synchronous with the final phase of extrusive rhyolite volcanism and active faulting. After volcanic activity ceased, shallower-slope braided-stream environments developed in early Deadman Quartzite time. Subsequent marine transgression produced shallow subaqueous deposits of the upper part of the Deadman Quartzite. These were overlain by prodelta sediments of the Maverick Shale, indicating a phase of basin deepening. Finally, the Mazatzal Peak Quartzite was deposited as the basin shoaled again, first to shallow- marine and subsequently to fluvial conditions.
The quartzite is diagenetic quartz arenite, originally deposited as lithic arenite and lithic arkose. The provenance was initially restricted, and earliest sediment was derived mainly from the subjacent Red Rock Group. When tectonic activity ceased, how ever, the surrounding highlands were planed down by erosion, and detritus from a wider variety of source rocks was funneled into the basin. This included contributions from arc-related supracrustal rocks of the Payson Ophiolite and East Verde River Formation, and finally a granitic basement input. Detrital quartz in the lower part of the Mazatzal Group is largely monocrystalline, and volcanic in origin. Polycrystalline quartz, associated with detrital feldspar and of probable continental affinity, is concentrated in the uppermost parts of the sequence.
The life span of the intra-arc basin was on the order of 30 m.y., from the formation of the Payson Ophiolite at 1.73 Ga to the deposition of the upper Mazatzal Peak Quartzite sometime after 1.70 Ga. The pre-Mazatzal Red Rock Group represents the last stages of volcanic arc activity, and the Mazatzal Group records the transition from orogenic to nonorogenic sedimentation in central Arizona.
