EVIDENCE FOR CENOZOIC LANDSCAPE EVOLUTION IN THE TEHACHAPI MTNS, CA FROM POLYCYCLIC FLUVIAL CONGLOMERATES

In the Tehachapi Mtns (TM), Cenozoic fluvial conglomerates were sampled for clast composition (45 locations, n = 12673), paleocurrents (32 locations, n = 716), and sedimentology (16 measured sections) to test models of uplift, provenance, and basin evolution.

The late Cretaceous?-Paleocene Goler Formation, Pg, is 4 km thick in the El Paso Mtns and in the subsurface of the northern Mojave. It was probably more extensive east of TM in the Paleogene. Three statistically distinct clast assemblages are restricted to dated stratigraphic intervals in Pg and were an important source for Cenozoic conglomerates in TM.

In the eastern TM (ETM), the Witnet formation, Tw, is ≤1km thick and was deposited by W-flowing rivers on an erosion surface developed on crystalline rocks. Clasts were eroded from southern Sierra-northern Mojave sources and recycled from upper Pg. The Tw paleovalley is preserved in buttress unconformities beneath Tw and Oligo-Miocene rocks in ETM and in the subsurface of the Tejon Embayment. Tw rivers may have supplied Pg clasts and Jurassic zircons to Eocene marine sediments in the Santa Cruz Mtns.

The western TM (WTM) were a drainage divide at this time. The Tejon formation, Tt, was deposited near base level during Eocene transgression and contains locally derived conglomerates. In the late Eocene, a Tw? braided river prograded over Tt and deposited a sheet of orthoquartzite conglomerates recycled from upper Pg.

In WTM, Oligocene N-flowing alluvial fans with exotic sources deposited 0.5-1km thick wedge-shaped conglomerate units that indicate high-relief, tectonism, and uplift to the S. In the early Miocene, an erosion surface cut near base level across a landscape of half grabens with ≈500m footwall erosion was covered with lava flows, then tilted 20° N. Overlying Miocene coarse clastic units are <200m thick, locally derived, bounded by buttress unconformities, and contain mega-breccias derived from the S.

In the mid-Miocene, W- and N-flowing drainages were sourced in the Cache Peak volcanics and northernmost Mojave. Late Miocene Chanac Formation conglomerates record the end of Cache Peak clastic input possibly due to uplift of the NE side of the Tejon Creek fault.

Quaternary terraces along the present drainages reflect Plio-Quaternary uplift, beheading, and capture along the Garlock fault.