TRACE-ELEMENT ANALYSIS OF FOUR NEOGENE VOLCANIC ASHES: IMPLICATIONS FOR THE STRATIGRAPHY OF PETROLEUM-BEARING FORMATIONS IN THE SAN JOAQUIN VALLEY, CA

The Kern River Ash is a prominent layer of volcanic ash separating the K and G sands of the Kern River Formation, a major petroleum-bearing formation in the Southern San Joaquin Valley of California. The upper age of the Kern River Formation has long been based on the major-element geochemical correlation with the Bishop Tuff, a volcanic tephra layer erupted from the Long Valley Caldera at 0.774±0.002 Myr.

Recent radiometric dating of sanidine separates from the Kern River ash at 6.12±0.05 Myr has prompted a review of the geochemical correlations. Based on eight major elements determined by microprobe analysis, two additional possible sources for the Kern River Ash have been identified. These possible sources are a 6.0±0.2 Myr tephra from the Fish Lake Valley erupted from the Volcano Hills/Silver Peak range eruptive center in Nevada and the 0.639 Myr Lava Creek B tephra from the Yellowstone caldera complex in Wyoming.

We measured the trace element composition of two samples of the Kern River Ash, of a Bishop Tuff sample, of a Fish Lake Valley Ash sample, and of four samples of the Lava Creek B tephra. Glass separates of the ash samples were dissolved by microwave digestion and analyzed by ICP/MS. In addition, we directly analyzed the glass separates by Laser Ablation ICP/MS.

Similarity coefficients were calculated from both the ICP/MS and the Laser Ablation ICP/MS results. In addition, samples were correlated using hierarchical clustering (dendrogram) analysis and cluster diagrams of selected elements. With all three methods, the Kern River Ash consistently correlates to the Fish Lake Valley tephra from the 6.0±0.2 Myr Volcano Hills eruption.

The present correlation and the previous dating of sanidine separates suggest a much older age of around 6.1 Myr for the Kern River Formation than previously thought. This new age dramatically alters our understanding of the stratigraphic relationship of the Kern River Formation with other formations in the San Joaquin Valley. Re-interpretation of the stratigraphy of the San Joaquin Valley may now allow correlating sands that had previously been correlated to non-productive formations to be correlated to the Kern River Formation, providing additional opportunities for petroleum exploration in the San Joaquin Valley.