Abstract

The San Pedro trough is an elongate structural depression of the Basin and Range Province extending NNW-SSE for 125 km subparallel to the Catalina core complex and associated uplands in southern Arizona. Intricate Pliocene-Quaternary dissection by the axial San Pedro River, tributary to the down-cutting Gila River, and by multiple (n~75) transverse piedmont tributaries has exposed to detailed view the depositional systems within upper tiers of the Miocene-Pliocene basin fill (Quiburis Formation). Quiburis strata were deposited within an internally drained structural valley with a lacustrine deposystem occupying a central segment of the trough. Although locally faulted against bedrock in bounding ranges, the largely undeformed Quiburis Formation was deposited after the overall morphology of the San Pedro trough had been delineated by extensional faulting. The areal distribution of Quiburis facies illustrates the relationships of facies patterns in nonmarine extensional basins to basin flanks, and to intrabasinal tiltblocks that influence drainage patterns and the lateral extent of depositional subsystems.

Conglomeratic alluvial fan and bajada braidplain deposits form elongate facies belts along both flanks of the San Pedro trough, representing proximal clastic facies derived from immediately adjacent highlands. A basin-floor facies belt along the axis of the trough includes lacustrine deposits, confined to the central segment of the trough, which grade to the northwest and southeast into fluvial deposits of axial streams that drained centripetally into the central lake. The lacustrine facies tract locally extends continuously to the basin flank where a syndepositional fault scarp along one segment of the trough delimited the edge of the lacustrine deposystem. Lacustrine deposits are mainly laminated clastic strata composed of clay, silt, and fine sand, but also include carbonate, gypsum, and diatomite phases. Sandflat facies, including both marginal-lacustrine and fan-toe deposits, form narrow paired NNW-SSE facies belts intervening between basin-flank conglomeratic and central lacustrine facies tracts. Near the northwest limit of the lacustrine facies tract, a distinctive deltaic complex that includes sandy delta-front deposits grading to prodelta mudstone was probably fed by the ancestral Gila River, but at the southeast limit of the lacustrine facies tract, sandy fluvial facies grade more imperceptibly into lacustrine strata.

Quiburis strata commonly bank depositionally, in pronounced buttress unconformity, against bedrock exposures in multiple tiltblocks located within the interior of the trough. Alluvial fans and fluvial drainages were partly diverted around basin-interior tiltblocks, giving rise to local shadow zones that distorted the trends of facies belts, and some tiltblocks partitioned the trough into parallel sub-basins where contrasting facies were deposited. Even though the overall pattern of facies belts is regular and systematic on the scale of the San Pedro trough as a whole, paleotopographic irregularities in the basin floor controlled by structural features thus led to locally intricate facies relationships.