THE SUBSURFACE EXTENT OF THE BOUSE FORMATION IN THE LOWER COLORADO RIVER CORRIDOR AND POSSIBLE TECTONIC IMPLICATIONS

The Bouse Formation, a Miocene-Pliocene sedimentary unit in the lower Colorado River corridor, records the inception of the Colorado River system. It consists of a basal carbonate and overlying siliciclastic deposits that represent filling of preexisting basins as the Colorado River advanced southward towards the evolving Gulf of California. This work focuses on the compilation of more than 40 years of subsurface data, which has not been collectively analyzed in the context of new tectonic or river evolution models. We have standardized 296 well logs and 44 measured sections, totaling 35 km of stratigraphic section. The subsurface data and available surface constraints are interpolated to create isopach and structure contour maps to provide a comprehensive look at the subsurface distribution of this formation. The data reveal four depocenters with the thickness and depth of Bouse sediments increasing southward to the Blythe basin. The Blythe basin likely contains the most complex record of Bouse sedimentation, has a controversial depositional origin and is a primary focus of our efforts.

Pre-integration basin geometry was controlled by Miocene transtension, but the northward extent of deformation and the degree to which it continued into the Pliocene are poorly understood. The thickest Bouse deposits and deepest isostatic gravity in the Blythe basin generally trend west-northwest, whereas the modern valley is northeast-trending. A compilation of mapped Cenozoic faults shows several northwest-striking, dextral faults bounding the basin. Profiles of the overlying Pliocene Bullhead Alluvium tread, from the basin margins, suggest a lack of regional deformation. However, subsurface data in the basin center show that the base of the Bullhead Alluvium is locally below sea level and potentially caused by faulting. We interpret the deepest Bouse deposits in the Blythe basin to be the result of transtensional basin formation between these right-stepping, dextral faults. Future work will focus on: additional surface and subsurface constraints, geophysical datasets, down-hole geophysical logs, more complex interpolation techniques, and fence diagrams. This work will continue to reveal insights into the tectonic and fluvial evolution of the region and provide promising targets for future scientific drilling.