Paper No. 87-17
Presentation Time: 9:00 AM-5:30 PM
EVIDENCE FOR EARLY MIOCENE SYNEXTENSIONAL CONGLOMERATE SEDIMENTATION IN THE TUNIS HALF-GRABEN, SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA
Early Miocene extensional deformation and hydrocarbon basin formation in the southern San Joaquin Valley is dated by a 300+m sequence of 22-23±1.0 Ma synextensional volcanic rocks (Tv) in the km-scale Tunis half-graben in the Tejon Embayment. The NW-trending, down to the east, Tunis fault bounds the half-graben on the west and is overlain by Relizian and younger sediments. Underlying Tv is a 100-230m thick sequence of conglomerates and sandstones of the Tecuya Formation (Ttc). Ttc is considered age equivalent to the Vedder hydrocarbon reservoir. Conglomerates in Ttc are reported to have a local source. To better understand early Miocene extensional deformation and basin formation we collected regional mesostructural and sedimentologic data and mapped outcrops at 1:2000 scale. Ttc unconformably overlies gneissic basement in the hangingwall of the half-graben. From base to top it consists of 1) ≤25m matrix-supported conglomerate and arkosic sandstone with well rounded pebbles and cobbles of gneiss, schist, and granitoids (Ttcg), 2) 60-150m of clast supported, well-rounded cobble-boulder conglomerate with 40-70% exotic orthoquartzite clasts (Ttcq) and 3) 3-60m of channelized, clast supported, angular to rounded boulder conglomerates interbedded with lens and sheets of thick bedded, poorly sorted, arkosic sandstone and pebbly sandstone (Ttcpm). Clasts in Ttcpm boulder conglomerates are 70-80% local plutonic and metamorphic rocks including >1m size boulders of underlying arkosic sandstones and footwall-derived mylonitic gneiss. Lenses of Ttcpm are interbedded with Tv. On the footwall, Ttcg and Ttcq are absent and Ttcpm lies directly on basement. Within the half-graben, stratal dip decreases from 45-50° at the base to 20-25° at the top of the sequence. Small faults with offsets of 10-100cm are overlain by undeformed conglomerate and sandstone. Unit Ttcpm thickens to the west towards the Tunis fault. The contact between Ttcq and Ttcpm is erosional, channelized and shows 10-15° angular discordance. Paleocurrents change from W-NW flowing in Ttcg and Ttcq to N flowing in Ttcpm. We interpret the contact between Ttcq and Ttcpm to be a previously unrecognized intraformational unconformity that signals uplift to the south at the beginning of NE-SW extension in the late Oligocene?-early Miocene.