--> Chemostratigraphy of the Miocene Monterey Formation within the San Joaquin Basin, California

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Chemostratigraphy of the Miocene Monterey Formation within the San Joaquin Basin, California

Abstract

Despite over a century of study, precise dating and long-distance correlation within the Monterey Formation remains a consistent problem. Widely-used benthic foraminiferal stages are time-transgressive and of long duration, yet are commonly used as there are few alternatives available. Chemostratigraphy is one such alternative method that has not been thoroughly developed. Our goal is to characterize the broad geochemical characteristics of the Monterey Formation in order to take the first steps towards a reproducible and robust method of chemostratigraphic correlation. Unfortunately, common chemostratigraphic methods such as Sr/Ca ratios and oxygen isotopes typically utilize carbonates, which are only partly continuous throughout the lower and middle sections of the Monterey Formation. Consequently these methods cannot be applied to the upper part of the formation that is often of particular interest to petroleum geologists. It is therefore necessary to use chemostratigraphic methods that are applicable to a wider range of lithologies. To this end, we have chosen to analyze redox and productivity-sensitive trace element concentrations, TOC, and carbon isotopes of Monterey Formation sediments that have the potential for widespread synchronous change across and even between sedimentary basins. Geochemical analysis was performed on 240 samples of cleaned and refined cuttings from 3 different wells in the southern San Joaquin Basin. These wells represent a west-side to central basinal transect and contain continuous and complete sections of the upper and middle parts of the Monterey Formation and we have focused on the organic-rich McDonald and Antelope members, as these units have potential for dramatic, correlative events. Additional geochemical data available for these wells, such as Lithoscanner and Electron Capture Spectroscopy well-logs, allow for expanded correlative potential beyond time-consuming wet-chemical analysis of cuttings.