Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Detrital zircon U/Pb ages of upper Miocene to Pleistocene strata of the SE San Joaquin Basin in comparison to zircon age patterns of the southern Sierra Nevada batholith-implications for late Cenozoic sediment provenance and dispersal patterns

Abstract

Detrital zircon (dz) ages from upper Cenozoic strata of the SE SJB are compared to a large database on southern SNB zircon ages as a means of better resolving sediment provenance and dispersal patterns as well as the incision history of the lower Kern River basement gorge. Recent stratigraphic and depth of burial studies show that ∼1500 m of SJB strata sat nonconformably above the low relief basement surface that the lower Kern gorge incised through over the past ∼1 m.y., calling into question the Kern River as the principal source channel for the Kern River Fm. We focused our studies on the Kern River Fm., and adjacent units such as the Bena and Chanac, which are partly equivalent to the Kern River Fm. These strata all constitute a distinct plutonoclastic lithosome that based on dz data and distinct clasts of Neogene andesite/dacite, was derived from the extreme SE Sierra Nevada. The Neogene clasts were derived from the Cache Peak volcanic center, whose basal ignimbrites contributed distinct ca. 21 Ma volcanic zircon to the SE SNB-dominated dz population. This lithosome was delivered to the SE SJB through a major river trunk that we call the Caliente River, whose terminal channel is clearly expressed in DEMs, issuing westwards into the SJB ∼10 km south of the lower Kern gorge. The Pleistocene channel walls of the lower Caliente River consist of the same lithosome, which we further trace into the subsurface as the principal lobes of the Stevens submarine fan. The Kern River-Bena-Chanac lithosome represents an upper middle Miocene through Pliocene fluvial-deltaic system that was delivered into the SE SJB through a structural trough that we call the Edison graben. The principal lobes of the Stevens submarine fan formed directly off the delta front. Greater Kern River drainage basement ages and matching dz ages from terrace sands within the lower Kern River are distinct from those of the Caliente lithosome. The Kern drainage dz signal is present in green mudstones and siltstones of latest Miocene-early Pleistocene age, and which replace the Caliente lithosome north of the lower Kern River. These strata are more akin to the Etchegoin and San Joaquin Fms. In parallel, the northern (Rosedale) lobe of the Stevens fan system contains the Kern drainage dz, signaling the northward deflection of the Kern drainage lithosome by the more voluminous Caliente River lithosome.