Investigation Of The Potential For Isotopic And Chemostratigraphic Correlation Within The Miocene Monterey Formation And Linkages To The Global Climate Record

Abstract

Despite over a century of study, dating and long-distance correlation within the Monterey Formation remains a persistent problem. The 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 that has been little explored. The Miocene climate record is well-documented from deep-sea cores, and the major climatic events (such as the development of permanent ice sheets) have well-constrained ages. Thus chemostratigraphy, using paleoclimate proxies, has potential utility for correlation and age-constraints in the Monterey. Although the sediments of the Monterey Formation were deposited in different basins with differing tectonic histories, the major lithostratigraphic units are remarkably similar. Due to this fact, we believe there is a high potential for the major global climatic events of the Miocene to be recorded in the sedimentary record. However, commonly used chemostratigraphic methods such as Sr/Ca ratios and oxygen isotopes typically use carbonates, which are only partly continuous in the lower and middle portions of the formation, thus excluding the upper portion, which is crucial to petroleum geologists. Trace metal analysis is an alternative method with high potential utility throughout the Monterey, including non-carbonate-bearing intervals. Trace metal uptake into sediment is driven by redox conditions in the basin, which are related to surface productivity, which is in turn influenced by climate. Thus the concentrations of trace metals in a section should be a good proxy for paleoclimate analysis and stratigraphic correlation. Potentially useful trace metals include Mo, V, and U, among others. Recent studies suggest that diatoms can hold an oxygen isotope record of paleoclimate similar to that of carbonate. Although this is potentially useful for the shallowly buried portions of the formation, previous isotopic work in the Monterey has found a diagenetic signal in the oxygen isotopes of opal-CT and quartz-phase rocks. However, we believe that if the water/rock ratio was low enough, such as in a porcelanite-dominated succession, then the original climate signal may still be present, but with values, shifted a certain amount by the diagenetic conditions. If this signal is present, then it provides another potential climate proxy for chemostratigraphic analysis within the Monterey Formation.

AAPG Datapages/Search and Discovery Article #90215 © 2015 Pacific Section AAPG Convention, Oxnard, California, May 3-6, 2015