CHARACTERIZATION OF THE ELEPHANT TREES FORMATION, ANZA BORREGO WITH IMPLICATIONS FOR DETERMINING THE DEPOSITIONAL ENVIRONMENTS OF CONGLOMERATIC DEBRIS FLOWS

The stratigraphy of Split Mountain Gorge in Southern California records the regional tectonics, depositional environments, and evolution of the lower Colorado River since the Miocene. The onset of extension in the Salton Trough at ~ 8 Ma is constrained by the base of the Elephant Trees Formation within the Split Mountain Group. Overlying the Elephant Trees Formation are the sandy turbidites of the Latrania Formation, marking the onset of marine transgression at ~6.3 Ma. The Elephant Trees Formation is an alluvial fan succession that is stratigraphically bounded by continental deposits at the base and marine deposits at the top.

Here, we present 191 meters of measured section from the Elephant Trees Formation. The succession is primarily comprised of braided stream deposits and matrix- and clast-supported debrites. We compiled maximum clast size, bed thickness, degree of sorting, clast roundness, percent matrix, and grading for each individual bed in order to test two hypotheses: 1) debrites within the Elephant Trees Formation record the transition from subaerial to subaqueous deposition in the Salton Trough, and 2) subaerial and subaqueous debrites display unique relationships between maximum clast size and bed thickness as a result of variations in flow competence.

The Elephant Trees Formation contains terrestrial indicators such as root casts and desiccation cracks in several locations. However, many debrites show evidence of modification by turbulence that is most likely the result of deposition in standing water. It is therefore likely that the Elephant Trees Formation was deposited in a terrestrial environment with ephemeral lakes before the marine incursion that led to the deposition of the Latrania Formation. We aim to identify diagnostic features of debrites deposited in subaqueous versus subaerial settings in order to better understand the evolution of similar transitional environments.