FIELD EVIDENCE FOR PALEOGEOGRAPHY AND PROVENANCE OF OLIGOCENE-EARLY MIOCENE POLYMICT NON-MARINE CONGLOMERATES, SAN EMIGDIO AND TEHACHAPI MTNS AND OFFSET EQUIVALENTS IN THE GABILAN RANGE, CA

The Tecuya Fm. is a 0.2-1.2km thick sequence of Oligocene-early Miocene non-marine coarse clastic rocks (Ttc) and 21-25Ma bimodal volcanic rocks (Tv) exposed in a 30km EW-trending outcrop belt, 10km north of the junction of the San Andreas and Garlock faults. Coarse-clastic rocks can be divided into three informal members 1) lower (Ttcl ); 2) middle (Ttcpm); and 3) upper (Ttcu) that are respectively pre-, syn-, and post-Tv.

Based on detailed mapping and sedimentology, Ttcl conglomerates contain 3 statistically distinct compositional assemblages that, from N to S, contain >50%, 1) orthoquartzite (Ttcoq,), 2) siliceous-felsic metavolcanic rock (Ttcmv), or 3) mixed carbonate rocks (Ttcc) that are areally restricted to 3 separate coarsening-upward, alluvial fan-braided river systems with sources to the west and south. Resedimented late Paleocene Goler Fm. and/or late Cretaceous-early Miocene Witnet Fm. are possible sources, but Ttcmv assemblages are unique and a regional petrographic survey of quartzites found no source for supermature Ttcoq orthoquartzites.

Ttcpm overlies a regional high-relief erosion surface with local angular discordance. It is thickest adjacent to high angle faults interpreted to be early Miocene domino-style, extensional faults. Composition is >50% local plutonic and metamorphic rocks with paleocurrents that flowed NE in the east and NW in the west.

Ttcu overlies a second high-relief erosion surface cut into Tv and contains up to 5% Tv clasts, 5 distinct conglomerate assemblages, and NE flowing paleocurrents. Ttcpm and Ttcu grade to the west and up section into marine sandstone of the Temblor Fm. The poorly exposed mid-Miocene Nameless Conglomerate overlies all Oligocene-early Miocene rocks with angular discordance. Oligocene redbeds in the Gabilan Range contain similar assemblages and stratigraphic relationships. Detailed paleogeographic reconstruction of Oligo-Miocene alluvial fans may identify additional provenance relationships and piercing points that will be useful in evaluating complex fault kinematics at the Big Bend.