THE KERN RIVER IS NOT THE SOURCE OF THE KERN RIVER FORMATION: EVIDENCE FROM FLUVIAL CONGLOMERATES

Over 70% of the oil and gas reservoirs in the San Joaquin Valley (SJB) are hosted in Miocene-age rocks. The largest is the Kern River Formation (Tkr) with a 2015 production of 25.7 million barrels. Once thought to be as young as Pleistocene, recent radiometric dating has shown Tkr to be primarily late Miocene. Previous studies have concluded that Tkr was deposited in a fluvial-deltaic environment with sediment derived from the Sierra Nevada batholith. Recent detrital zircon studies from well cores in Tkr suggest a Miocene extraregional source, but cores are possibly contaminated by drilling mud. In this study clast composition, paleocurrents, and heavy minerals in the Tkr were sampled in outcrop along Breckenridge Road, Caliente Creek, and the type section at Kern River Bluffs. Underlying non-marine Oligocene – late Miocoene conglomerates of the Walker Formation (Tw), Bealville Fanglomerate (Tbe), and Bena Gravels (Tba) were also sampled. More than 3000 clasts >2 cm were counted in 27 categories.

Conglomerate composition in the type section of the Tkr averages 56% volcanic lithic clasts, 23% plutonic, and 21% metamorphic. The high percentage of volcanics suggests that Tkr was sourced from 21-10 Ma volcanic rocks in the Tehachapi/Mojave region. Common clasts of friable pink ash in Tkr may have their source in Tw which unconformably overlies Cretaceous plutonic rocks and contains a thick layer of pink tuff dated at 24 Ma. Cobbles of severely weathered granite suggest erosion of a regionally extensive Eocene erosion surface developed on Sierra Nevada granitic rocks. Tba, usually described as locally-derived granitic breccia that was mostly deposited before Miocene volcanism, contained 31% volcanic clasts in one sample near the base of the section.

The sudden influx into the SJB of large volumes of volcanic lithic sediment similar in composition to volcanic rocks in the Cache Peak/Mojave region is interpreted to represent drainage integration along the southern basin margin following early-mid Miocene volcanism, extensional tectonism and drainage derangement. The presence of abundant volcaniclastic sediment in Tkr sourced from the SE has implications for reservoir porosity, chemistry of pore fluids, and channel geometry of reservoir sands.