CLIMATIC INFLUENCES ON CONTINENTAL DEPOSITION DURING LATE-STAGE FILLING OF AN EXTENSIONAL BASIN, SOUTHEASTERN ARIZONA

Climatic, rather than tectonic, influences on continental deposition a re recorded in the late-stage fill of a Neogene extensional basin in s outheastern Arizona. Regional geomorphic relations, low sedimentation rates, and stratigraphic overlap of the principal basin-bounding struc tures identified in a gravity anomaly profile, indicate that the Plio- Pleistocene St. David Formation accumulated in the San Pedro Valley du ring a period of tectonic quiescence. Stable isotopic composition of p aleosol calcite is placed within a magnetostratigraphic framework that provides an interpretive record of variable arid to semiarid climate in the valley. Before 3.4 Ma the valley was hydrologically closed and experienced an arid climate with seasonal precipitation. Drainage inte gration at ca. 3.4 Ma corresponds with a transition to wetter conditio ns, less seasonally variable precipitation, and a rising water table i ndicated by hydromorphic paleosols and pond deposits. Gradually increa sing seasonality of precipitation after 2.8 Ma and further decrease in winter rainfall after 2.2 Ma led to establishment of a dry, monsoonal climate between 1.6 and 0.6 Ma. The change in climate at ca. 1.6 Ma c oincides with abrupt appearance of sheet-hood-dominated fan gravels ab ove earlier vertically aggraded fluvial-channel and interfluve-flood-p lain deposits. This change in facies is interpreted as the result of c limatic change rather than an example of subsidence-driven gravel prog radation. Sedimentation rates were greatest when climate was relativel y wet, with dominant winter rainfall, and about equally low for relati vely dry, with dominant summer rain, and relatively wet, but nonseason al, conditions. Contrary to past model simulations, channel:flood-plai n-facies ratios and sedimentation rate varied directly. Ribbon, as opp osed to sheet, channel bodies were favored by relatively wet climatic conditions and were less commonly formed during dry, strongly seasonal times. Precise process-response interpretation of this correlation of sedimentological variability to changing climate is limited by the re stricted interpretation of paleosolisotope paleoclimate data and the p oor understanding of sedimentological response to climate change durin g 10(5)- to 10(6)-yr. time intervals. The results of this study sugges t, however, that caution be applied to a priori interpretations of str atigraphic variations within thick syntectonic basin fills as response s to varying subsidence rates only.