MID-CENOZOIC PALEOSHORELINES, WESTERN TEHACHAPI-EASTERN SAN EMIGDIO MTNS CA

The late Eocene-early Miocene Tecuya Fm (Tc) is exposed for 30km in an EW cross-section on the southern edge of the San Joaquin Basin. Tc consists of 0.2-1.2km of polymict conglomerate, sandstone, and early Miocene bimodal volcanic rocks (Mv) that were deposited at or near sea level. Tc grades N and W into marine San Emigdio, Pleito, Vedder, and Temblor Fms. New detailed mapping and stratigraphy of Tc, 10 km north of the junction of the San Andreas and Garlock faults, provides a high resolution record of uplift, erosion, and subsidence relative to sea level during the transition to a transform margin.

In the late Eocene, 20-200m of WSW-flowing conglomerate and sandstone rich in exotic orthoquartzite clasts (Tc1) were deposited on a planar erosion surface cut across the early-mid Eocene marine Tejon Fm and pre-Cenozoic crystalline basement. Tc1 grades E to W from braided fluvial to nearshore facies.

In the Oligocene, Tc1 was overlain by ≈800m of exotic polymict conglomerate and locally-derived breccia (Tc2) that were deposited in N-flowing alluvial fans sourced to the S. Paleoshoreline moved N and W.

In the late Oligocene-early Miocene, 20-70 m of locally-derived nearshore conglomeratic sandstone (Tc3) was deposited on a transgressive erosion surface cut across Tc1-2 and basement. Beach, shoreface, and wave-reworked alluvial deposits rim eroded basement highs and Tc1-2 in the footwalls of NW-striking normal faults. Tc is up to 5 times thicker in the hanging walls.

In the far W, wave-reworked Tc3 fanglomerate is overlain by inner shelf deposits. In the E, Tc1-3 and basement are cut by high relief channels and overlain by 40-100m of nonmarine, locally-derived, NNE-flowing, boulder conglomerate and megabreccia (Tc4) with interbedded olivine basalt flows. Overlying basalt, a rhyodacitic vent complex fed by peperitic dikes formed a topographic high.

In the W, subaerial Mv flows are overlain by recycled nearshore conglomeratic sandstone (Tc5) that grades westward to shelf deposits. In the E, Mv contains erosional or fault-related NE-facing paleoscarps that deflected flows. Interbedded, NE-flowing, fluvial conglomerate recycled from eroded Tc is cut by peperitic dikes.

Tc was then tilted 20 degrees to the N and cut by a high relief erosion surface. Overlying Relizian and younger nonmarine conglomerates contain recycled Tc.