Earliest turbidite emplacement ~(6.3 -5.6 Ma) in the newly opening Gulf of California (Fish Creek-Vallecito Basin)

The Fish Creek-Vallecito Basin chronicles late Miocene through Pleistocene sedimentation into the newly rifting Gulf of California. Initial rifting in the basin began around 8.0 Ma, subsequently followed by sedimentation of locally derived alluvial fan conglomerates. Subsequent deepening of the basin created conditions for deposition of subaqueous gravity flow deposits. The deposition of the initial marine sediments is known lithostratigraphically as the Lycium Member turbidites (~6.3 to 5.6 Ma) of the Latrania Formation, comprising a thick succession of coarse-grained sediment up to 115m in thickness. Sedimentation and facies distribution throughout the system is a direct result of the tectonically derived topography in this rift basin environment. Detailed study of the facies distribution and nature of the sediment fill allows for realization of models of facies variability in rift basin environments. Deposition of this turbidite succession is somewhat problematic. A significant run-out distance as well as water depth is needed to deposit a turbidite succession of this magnitude and although there is currently no field evidence of a transitional shoreline or delta system previous detrital zircon analysis suggests a possible more northerly source. This study aims to seek out the processes which led to a thick turbidite succession in an early rift basin sequence and provide a complete picture of source-to-sink deposition. In depth analysis of turbidite architecture variability was conducted using over 500 m of outcrop logged section taken laterally throughout the 7 km outcrop belt. Lithofacies and architectural variability, reconstructed and imaged using correlation of high resolution sections and photo panel interpretations, show a transition from a proximal to distal basin floor fan environment. Provenance data, constrained through petrographic inspection of samples taken systematically throughout the system, indicate that the initial marine sediments deposited in the Miocene-Gulf of California are derived from the neighboring Peninsular Range Batholith. In depth stratigraphic analysis coupled with provenance study allows us to decipher the depositional history and basin paleogeography during initial marine inundation into the newly formed Gulf of California. A paleogeographic model of the Late Miocene (~6.3 to 5.6 Ma) is proposed. Sediment was transported from the northwest to the southeast along the West Salton Detachment Fault, depositing in a basin floor fan system.