Alluvial fan and fan delta facies architecture recording initial marine flooding in the Mio‐Pliocene syn‐rift sequence of the Fish Creek‐Vallecito Basin, southern California.

The timing and character of the initial marine flooding of extensional basins has implications for their tectonic history. Yet, the recognition of such flooding is difficult along rift basin margins due to the dominance of coarse‐grained systems and the lack of marine fauna. This study conducts detailed facies and stratigraphic analysis of a Mio‐Pliocene alluvial fan and fan‐delta succession in the Fish Creek Vallecito Basin in southern California. Our goal is to characterize the marine flooding surface, determine the paleogeographic position of the shoreline and estimate the magnitude of relative base‐level rise that occurred during the marine incursion associated with the opening of the Gulf of California. Our results show that the flooding of the Elephant Trees alluvial fans is often marked by an abrupt lithologic and facies change from meter‐scale boulder‐rich subaerial debrites (proximal alluvial fan facies association) to centimetre‐scale granule‐rich subaqueous debrites, ripple‐laminated sandstones and mudstones (prodelta facies association). By delineating the zone of transition between the subaerial and subaqueous facies, we place the initial flooding paleo‐shoreline 4 km up the alluvial fan's paleo‐depositional slope. Considering alluvial fan slope gradients between 1° and 5°, this 4 km transgression would require an estimated 70–350 m of water depth during the initial marine incursion. Interfingering of fan‐delta deposits with subaqueous marine and planktonic‐rich evaporites suggests that the basin was below sea‐level after, and perhaps even before, the marine flooding. Subaerial subsea‐level basins exist in Death Valley and the Salton Trough today within similar extensional and transtentional tectonic regimes. This subaerial subsea‐level interpretation might explain the high magnitude and abrupt relative base‐level rise recorded by the facies transitions in the Fish Creek Vallecito Basin. These results suggest that the Fish Creek Vallecito Basin underwent significant extension during its early and nonmarine depositional phase, allowing it to reach subsea‐level elevations. The tectonic history of the Fish Creek Vallecito Basin maybe similar to other extensional basins where rapid subsidence allows the accumulation of nonmarine strata below sea‐level prior to the marine flooding, then restricted and deep marine strata immediately after flooding. [ABSTRACT FROM AUTHOR]