PALEOGENE MARINE ROCKS IN THE MOJAVE DESERT - REINTERPRETING THE DEPOSITIONAL ENVIRONMENT OF THE GOLER FORMATION, SOUTHERN CALIFORNIA

The Goler Formation comprises an early Cenozoic basin straddling the junction of the Basin and Range, Mojave, and southern Sierra Nevada Provinces in southern California. Previous studies of the Goler Formation have interpreted it to be a >3000 m-thick, dominantly terrestrial and fluvial basin with a <85 m-thick unit of marine strata near the top. Our re-evaluation of strata from the upper part of the Goler Formation indicates that marine deposition may be pervasive in the upper half of the section, representing significant change in the overall facies model. In this study, we remapped the upper 2000 m using hybrid field-based and remote sensing techniques, and measured four representative sections to understand the depositional environments. We divide the upper half of the Goler Formation into 3 map units, the lowest of which, unit Tg4a, is ~1300 m thick, contains fluvial sandstones with large (1 m) trough cross beds, soft sediment deformation, and petrified wood as well as imbricated pebble conglomerates. We interpret Tg4a as channel and floodplain deposits in a low-gradient, meandering river system. Unit Tg4b overlies Tg4a and consists of ~ 420 m of poorly sorted, laterally continuous massive red-green siltstones and sandstones. We interpret Tg4b as lagoon and tidal flat deposits as a marine transgression pushed inland. Unit Tg4b is overlain by unit Tg4m, which we rename as a concatenation of units Tg4c and Tg4d from previous studies. Tg4m is composed of ~1500 m of primarily cobble conglomerate with minor pebbly sandstone and shale interbeds. The conglomerate contains very well-rounded, well-sorted cobbles, and oyster fossils as well as gastropods, mollusks, infilled burrows, and petrified logs. Continuity of beds, grain-sorting, fossil assemblages, and sedimentary structures indicate shallow marine deposition. We therefore interpret this unit to represent prograding delta and shoreline depositional environments. The well-rounded cobble conglomerate suggests a high-energy fluvial system was inundated by a marine transgression, converting it to an estuary setting. Our observations indicate the Goler Formation may be more than 1/3 marginal marine, suggesting that marine transgression may have occurred earlier (possibly Paleocene or Late Cretaceous) and for a longer duration of time than reported previously.