Climatic influences on continental deposition during late-stage filling of an extensional basin, southeastern Arizona Available to Purchase

Climatic, rather than tectonic, influences on continental deposition are recorded in the late-stage fill of a Neogene extensional basin in south-eastern Arizona. Regional geomorphic relations, low sedimentation rates, and stratigraphic overlap of the principal basin-bounding structures identified in a gravity anomaly profile, indicate that the Plio-Pleistocene St. David Formation accumulated in the San Pedro Valley during a period of tectonic quiescence. Stable isotopic composition of paleosol 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 integration at ca. 3.4 Ma corresponds with a transition to wetter conditions, less seasonally variable precipitation, and a rising water table indicated by hydromorphic paleosols and pond deposits. Gradually increasing 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 coincides with abrupt appearance of sheet-flood-dominated fan gravels above earlier vertically aggraded fluvial-channel and interfluve-flood-plain deposits. This change in facies is interpreted as the result of climatic change rather than an example of subsidence-driven gravel progradation. Sedimentation rates were greatest when climate was relatively wet, with dominant winter rainfall, and about equally low for relatively dry, with dominant summer rain, and relatively wet, but nonseasonal, conditions. Contrary to past model simulations, channel: flood-plain-facies ratios and sedimentation rate varied directly. Ribbon, as opposed 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 restricted interpretation of paleosolisotope paleoclimate data and the poor understanding of sedimentological response to climate change during 10⁵- to 10⁶-yr. time intervals. The results of this study suggest, however, that caution be applied to a priori interpretations of stratigraphic variations within thick syntectonic basin fills as responses to varying subsidence rates only.