Tertiary sedimentary rocks associated with the harcuvar core complex in Arizona (U.S.A.): insights into paleogeographic evolution during displacement along a major detachment fault system

Abstract

Geologic mapping and analysis of Oligocene-Miocene sedimentary rocks in the upper plate of the Buckskin-Rawhide detachment fault system (west-central Arizona) reveals a complex paleogeographic history during fault displacement, involving shifting sediment-source areas and multiple drainage reversals. Within the study area, four upper-plate fault blocks are capped by homoclinal sedimentary sections that display fanning dip relationships indicating concurrent tilting and sedimentation. During deposition of these strata, upper-plate sedimentation was intermittently confined to separate half-graben, while at other times rapid rates of aggradation relative to fault-displacement resulted in burial of ridges separating sub-basins. Four sedimentary assemblages recognized within the study area can be correlated between fault blocks on the basis of lithologic similarity, stratal position, provenance, paleocurrent data, and sparse geochronologic constraints. The basal assemblage consists of lacustrine rocks and interfingering fluvial strata composed of detritus derived from the granitic terrane surrounding the northern part of the study area. These sediments were deposited during the earliest stages of tilting of upper-plate fault blocks, and inferred displacement along the Buckskin—Rawhide detachment fault system. The lower assemblage, which directly overlies the basal assemblage, consists of fine-grained lacustrine deposits, sandy conglomerate and breccia. During lower-assemblage deposition, mass-flow-dominated alluvial fans were derived from source areas consisting mainly of Mesozoic and Paleozoic rocks exposed to the south of the study area. These source areas may have been associated with the breakaway fault of the detachment system, or with fault blocks stranded above structurally elevated parts of the footwall. Tilting of fault blocks continued during deposition of the lower assemblage and strongly affected dispersal patterns and lithofacies distributions; lacustrine sediments were deposited mainly within large, deep sub-basins and, with few exceptions, lower-assemblage streams followed half-graben axes. The middle assemblage consists of conglomerate and sandstone deposited by an extensive south-directed stream system that probably flowed off undistended parts of the hanging wall, across extended parts, and locally onto the footwall. Tilting of upper-plate fault blocks continued during middle-assemblage deposition, but rapid burial of escarpments by sediment permitted relatively unobstructed southward drainage. The upper assemblage consists of sandy conglomerate deposited by a northeast-directed system of broad, shallow streams. These deposits display a variety of clast types, including Tertiary mylonitic, sedimentary, and volcanic rocks that were eroded from the upwarped footwall of the core complex and overlying klippen. Upper-assemblage streams maintained relatively consistent northeasterly flow, even during the early stages of deposition when some fault blocks were still rotating.