PALEOLANDSCAPES PRESERVED BENEATH THE BOUSE FORMATION PROVIDE INSIGHTS INTO THE DEVELOPMENT OF THE LOWER COLORADO RIVER AND LATEST MIOCENE AND YOUNGER DEFORMATION

The character and form of the varied substrates beneath the Bouse Formation – the first evidence for the arrival of Colorado River (CR) water and sediment into this region – provide abundant evidence of the landscape that existed prior to Bouse deposition, which in turn provides important insights into the nature of the water bodies associated with Bouse deposition and some constraints on patterns of syn- and post-Bouse deformation.

General relationships are similar in all of the basins of the lower CR corridor. Along bedrock margins and upper piedmont areas, remnants of basal carbonate (including tufa/travertine and platy limestone) encrust paleolandscapes of bedrock and colluvium, with varied slopes and aspects. These deposits record progressive inundation of complex bedrock topography, and sub-Bouse slopes do not provide clear evidence of tectonism unless displaced by faulting. Basal Bouse exposures on piedmont tributary deposits are more limited because of post-Bouse erosion or deposition. Basal limestone, calcareous sandstone, and minor tufa were deposited over landscapes including active fluvial systems and moderately incised alluvial landforms. Concentrations of rounded local gravel or tufa clasts are common but not ubiquitous; interbedding of tributary gravel with Bouse deposits is less common. We suggest that minimally disturbed fan deposits and a paucity of interbedding reflect rapid rises in the water surface due to high CR influx rates (floods), with near-shore reworking of underlying deposits and intercalation of tributary deposits during times of water-surface stability. Slopes beneath Bouse deposits commonly appear to be slightly steeper than modern washes, but exposures provide limited perspectives on 3D piedmont paleotopography. Where outcrops are extensive we are experimenting with interpolating between basal Bouse points to develop paleo-piedmont profiles. Initial results indicate that piedmonts were steeper near the mountains and less steep downslope; concave-up profiles are characteristic of fluvial systems and do not require or preclude deformation. Preliminary downslope projections of paleo-piedmont profiles toward or across valley axes indicate that some subsurface basal Bouse elevations plot well below the profiles, consistent with syn- or post-Bouse subsidence.