EVOLUTION OF SEDIMENTARY ENVIRONMENTS ON THE DELTA OF THE COLORADO RIVER

The Colorado River Delta, at the north end of the Gulf of California in the Salton Trough, is chiefly formed of detritus eroded from the Colorado Plateau and is cut by the trans-tensional regime of the southern San Andreas Fault System. We are using data from offshore and onshore wells and seismic reflection profiles from PEMEX, and from extensive drilling in the geothermal fields on the Delta, to gain insight into the sedimentation associated with the transform fault plate margin and its relationship to the geological history of the Grand Canyon and Colorado River. This is a progress report of the early stages of a multiyear study.

Great thicknesses of sediments have been rapidly deposited in tectonically active depocenters such as the Cerro Prieto and Salton Sea pull-apart basins. For example, a 4 km deep well in the Brawley pull-apart basin, near the center of the Imperial Valley, and 3 to 4 km deep wells in the Cerro Prieto area all bottomed in Plio-Pleistocene strata. Deep wells drilled in what are now inactive depocenters in NW Sonora, east of the modern plate boundary, also show up to 4 km of Plio-Pleistocene deltaic sediments.

The different geothermal fields in the Salton Trough exhibit a variety of sedimentary environments. The sediments of the Dunes Geothermal Field are a record of distributary channels, braided stream-dunes, and eolian sands on the northern flanks of the delta. However, in the Salton Sea Geothermal Field we see transitions from fluvial deltaic conditions to alternating lacustrine and evaporitic environments that persisted through the Pleistocene. At the Cerro Prieto Geothermal Field the sediments record transitions from a marine environment as the prograding delta front isolated the closed basin to the north now partially occupied by the Salton Sea. Seismic reflection data east of the Cerro Prieto Fault Zone along the flank of the delta, allow tracing the migration of this delta front southwards into the Gulf. Our long-term aim is to make a comprehensive three-dimensional model of the evolution of the different sedimentary environments, in order to understand how the Delta formed and was deformed. The Delta is a repository of information on its responses to tectonism, long-term climate changes, sea level changes, and transient events such as catastrophic flooding.