SEDIMENTARY RESPONSES TO CHANGING TECTONIC PATTERNS, MESOPROTEROZOIC APACHE GROUP/TROY QUARTZITE, CENTRAL ARIZONA

Mesoproterozoic basins scattered throughout western North America developed largely in intracratonic and rifted settings. The Mesoproterozoic Apache Group and Troy Quartzite in central Arizona reflect a complex interplay of marine and continental depositional systems. Changing intra- and extrabasinal tectonic (and possibly climatic) events profoundly influenced depositional styles.

Deposition of the Apache Group was initiated by faulting along the northern basin margin as indicated by debris flow and braided stream deposits (Scanlan Conglomerate) of prograding alluvial fans. Fan progradation was followed by development of fine-grained braidplains that periodically received influxes of volcanic ash from nearby vents resulting in disruption of drainage patterns (Pioneer Formation). The Barnes Conglomerate disconformably overlies the Pioneer and records influx of coarse clastics from the north due either to renewed movement on basin-margin faults or to stream downcutting following blockage of drainage by ash falls. Marine incursion are indicated by siliciclastic shoreline and shelf facies of the Dripping Spring Quartzite and the overlying carbonates of the Mescal Formation. Following prolonged exposure these strata were folded, faulted, and eroded prior to deposition of the fluvial-eolian complex of the Troy Quartzite. Obviously this unconformity was of extended duration. The Troy was deposited within intermontane, geomorphically partitioned basins. Intrabasinal structural adjustments occurred along a zone coincident with the Jemez Lineament resulting in renewed alluvial fan and coarse braidplain sedimentation within the basin. Thick intervals of seismically induced soft-sediment deformation occur within the Troy. Similar features occur within the Shinumo Quartzite (Unkar Group) in Grand Canyon. Whether this seismic activity reflects a response to initial stresses that would ultimately result in the much younger breakup of Rodinia is uncertain. In any case it is evident that both the Apache and Troy basins experienced numerous episodes of both extension and compression, interrupted by periods of tectonic quiescence.