Paper No. 87-22
Presentation Time: 9:00 AM-5:30 PM
PROVENANCE OF FLUVIAL AND SHALLOW MARINE CONGLOMERATES IN THE LATE EOCENE(?)-EARLY MIOCENE TECUYA FORMATION, SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA
The late Eocene(?)-early Miocene Tecuya Formation (Ttc) is exposed along the uplifted southern rim of the San Joaquin Valley. It overlies early-mid Eocene marine sedimentary rocks to the west and crystalline basement to the east and contains 100-700m of fluvial and shallow marine conglomerates that are time-equivalent to the Vedder hydrocarbon reservoir. In a 10 km cross-section between Tecuya and Tunis Creeks, over 6000 conglomerate clasts from 22 locations were sampled to test models of provenance, paleogeography, and basin formation. Composition, size, and paleocurrent data were collected and passed tests of reproducibility at the 95% confidence interval. Means and 95% confidence intervals were calculated in the compositional categories: quartzite, Q; low-grade metavolcanic, V; and plutonic/metamorphic, PM. In the stratigraphically lower part of Ttc, well-rounded, imbricated conglomerates contain three compositional assemblages. Eastern exposures from Pastoria to Tunis Creek are categorized by W-NW flowing paleocurrents and high Q (40-70%) assemblages rich in exotic orthoquartzite, Qo. Qo has low porosity, is not foliated, contains no megascopic neoblastic mica, has well-preserved sedimentary structures, and is formed primarily by diagenetic or low-temperature metamorphic processes. Similar orthoquartzites are presently exposed 150-500km to the east in the Cordilleran miogeocline and craton. Central exposures from Metralla to Live Oak Canyon are categorized by high V (40-50%) assemblages of light colored siliceous and felsic porphyries, and rare but ubiquitous trachyte porphyries similar to Jurassic metavolcanic rocks of the central Mojave. Qo is <30%. Western exposures in Tecuya Creek contain a mixed V and PM assemblage characterized by chert and greenstone clasts interpreted to be sourced in the Coast Ranges. Qo is <5%. The upper part of Ttc contains conglomerates with 50-80% locally derived plutonic and metamorphic clasts. These are overlain by and interbedded with 22-23±1.0 Ma volcanic rocks. Paleocurrents in upper conglomerates are N-flowing. Upper conglomerates are interpreted to overlie a previously unrecognized regional unconformity within Ttc that signals drainage beheading and uplift to the south at the onset of regional early Miocene extension.