INITIAL MARINE FLOODING OF A RIFT BASIN RECOGNIZED BY ALLUVIAL FAN TO FAN-DELTA FACIES TRANSITION: THE MIO-PLIOCENE SYN-RIFT SUCCESSION OF THE FISH CREEK-VALLECITO BASIN IN THE SALTON TROUGH, GULF OF CALIFORNIA

The initial marine flooding of a rift basin has implications on the basin’s tectonic history. Yet, the recognition of such flooding is often difficult in coarse-grained depositional systems in proximal parts of the basin. This difficulty stems from the lack of marine fauna and the dominance of gravel-rich sediment gravity flows that only show subtle sedimentologic differences between their subaerial and subaqueous deposition. To understand the initial marine flooding and early depositional history of rift basins, we have measured the grain-size and bed thickness trends and performed process-oriented facies analysis of more than 700 meters of stratigraphic section spanning the subaerial to subaqueous transition in the Mio-Pliocene syn-rift succession (Elephant Tree Formation) of the Fish Creek-Vallecito Basin in the northern Gulf of California. Lithologically, we have found that the initial marine flooding is marked by an abrupt reduction in grain-size (from boulder to granule) and bed thickness (from meter-scale to centimeter-scale). Depositionally, there is a marked shift from subaerial boulder-rich debris flow facies to subaqueous sandy debris flow facies. Textural changes along several depositional units in outcrops close to the paleo-flow orientation shows that inversely graded boulder-rich conglomerate beds get thinner, become more loosely packed and develop normal grading in the downstream direction suggesting rapid flow transformations. Our results show that sedimentary processes and the resulting stratigraphic geometries in debris-flow fed fan-deltas vary considerably from classic Gilbertian fan-delta models. Fluvial-fed Gilbert deltas have a high sedimentation rate allowing the development of a classic tripartite topset-foreset-bottomset geometry. While debris-flow fed fan-deltas are characterized by episodic and low rates of sedimentation and a lack of well-defined foreset geometries.