THE GOLER FORMATION: EOCENE FLUVIAL DEPOSITS, PALEOGEOGRAPHY, AND BASIN GEOMETRY AT THE SOUTHERN END OF THE SIERRA NEVADA

The Goler Formation consists of at least 2500m of continuous fluvial strata deposited just southeast of the southern margin of the Sierra Nevada. We re-evaluated the lower half (1254m) of the Goler Formation from the basal unconformity in the central part of the basin. Detailed sedimentologic observations are combined with new paleocurrent data (imbricated clasts, epsilon crossbeds, n=64) to document the vertical changes in stratigraphic architecture. Five lithofacies define 3 map units (1-3) based on the relative percentage of each lithofacies and architectural elements. Unit 1 spans from 0-175m and is dominated by 0.5-2.0m thick, laterally continuous, immature sandstone beds with minor siltstone interbeds deposited unconformably on the El Paso Mountains basement rocks. These strata are interpreted sand flat and sheet-flow deposits, with N-directed epsilon crossbeds (018°, 003°; n=3, 5). Unit 2 spans from 175-629m and is dominated by siltstone with occasional, laterally discontinuous 1-2m thick sandstone beds. These strata are interpreted as sand and mud flat deposits with minor meandering channel systems prograding into the mud flat. Unit 3 spans from 629-1254m and contains 10-20m thick sections of interbedded sandstone and conglomerate alternating with 10-20m thick sections of siltstone and shale. Petrified logs and laminated shale are common. Conglomerates provide NNE-directed paleocurrents from imbricated clasts (036°, 354°; n=23, 13). This section is interpreted as a northerly meandering river system with a main trunk channel and overbank components. In summary, early deposition of the lower Goler Formation in our section was locally derived in low-energy fluvial systems confined to an isolated valley floor. The distinct change to a larger meandering river system at 629m may reflect a Paleocene or Eocene drainage capture and basin-amalgamation event that linked the Goler basin to surrounding basins. This large new river must have linked the ancient Goler Basin with the Pacific Ocean to the west, and provided a conduit for the marine incursion that is observed at the top of the Goler Fm. Although the cause of the drainage capture event remains speculative, we suspect it may have been related to regional subsidence that occurred after subduction stopped along this portion of the Cordilleran margin.