PALEOHYDRAULIC APPROACH TO CLIMATE RECONSTRUCTION: EXAMPLE FROM THE LOWER JURASSIC KAYENTA FORMATION

Widespread fluvial deposits of the Kayenta Formation (Lower Jurassic), of eastern Utah and western Colorado interfinger with thick eolian sand sea deposits of the Wingate Formation, below, and the Navajo Formation, above. This stratigraphy (the Glen Canyon Group) records a major oscillation in amount of surface water through time. To more thoroughly quantify how large a change in water supply is represented by these deposits, and therefore estimate paleoclimatic fluctuation, we have developed an approach that utilizes various paleohydraulic parameters to calculate flow conditions in the past. The approach estimates water depths, local water discharges, and flood durations using a suite of sedimentary structures and textures. Specifically, grain size data from gravel lags, dune foresets, and climbing ripples yield skin-friction shear stress values (the component of total stress responsible for sediment transport) associated with initial grain motion, the transition from bedload to suspended load, and full suspension, respectively. Internally consistent values for skin friction obtained using the three different transport conditions increase our confidence in the environmental reconstruction. Reconstructed values of skin-friction shear stress are combined with measures of water depths, from bar heights and channel fills, and estimates of channel roughness to calculate average current velocities, water discharges, and river slopes. These preliminary quantitative estimates of surface water flow provide us with prerequisite data to evaluate climatic changes recorded by the progradation of the Kayenta fluvial system into eolian sand seas during Jurassic time.