Re-Os Geochronology of the Monterey Formation, Union Leroy 51-18 Well, Santa Maria Basin, California

The Os isotopic composition of marine Miocene strata records chronological variations in the world’s oceans. These Os- isotope variations exhibit changes in global seawater geochemistry that result from tectonic, paleoclimate and extraterrestrial processes. The Os- isotopic record of seawater throughout the Miocene has studied elsewherebeen constrained from several previous studies, but there are no 187Re-187Os isotope data available for the Monterey Formation, specifically during Serravallian time. The Serravallian Monterey has two predominant lithofacies of siliceous shalesiliceous-shale and phosphatic carbonatephosphatic-carbonate shale. In this study, 187Re-187Os isotope data, Rock-Eval and total organic carbon (TOC) data were obtained on samples from Serravallian Monterey Formation. The samples were acquired from core extracted from the Union Leroy 51-18 well in the Santa Maria Basin. The goal of this study was to delineate stratigraphic variability between the two lithofacies including depositional differences that led to their distinct compositions, and to obtain the first radiometric age for the Monterey Formation from 187Re-187Os isotopes. New 187Re-187Os geochronology of marine, organic-rich mud rocks from the Miocene Monterey Formation, Santa Maria Basin, California yields a scatterchron Model 3 age of 11.5 ± 1.5 Ma (13% age uncertainty, 2σ, n=28, mean square of weighted deviates [MSWD]=112), which agrees within uncertainty with the expected age of this formation based on regional correlation of biostratigraphy and bentonite age data, and is the first direct, absolute 187Re-187Os age for the Monterey Fm. in the Santa Maria Basin. The initial 187Os/188Os (187Os/188Os)i) = 0.7879 ± 0.0437 obtained by isochron regression represents the seawater 187Os/188Os during the Serravallian for the Santa Maria Basin, and is substantially less radiogenic than the 187Os/188Os of modern-day seawater (~1.06). Both radiogenic Os from continental weathering (187Os/188Os = ~1.4) and non-radiogenic Os from mantle and extraterrestrial sources (187Os/188Os = ~0.13) cause variations in the 187Os/188Os of seawater that are recorded with marine ORM (organic-rich mud rock) deposition. SObservation of small-scale changes between the siliceous shalesiliceous-shale and phosphatic carbonatephosphatic-carbonate shale just based on calculated (187Os/188Osi ) and stratigraphic depth were not resolvable, but incorporating TOC and rock-eval data with the 187Re-187Os isotope data helps better understandconstrains depositional environment differences between the two lithofacies. Comparing the slopes of TOC plotted against Os abundances (ppt) suggests that the phosphatic carbonatephosphatic-carbonate shale was deposited during a period of relatively higher sea level in a less restricted basin than the conditions of the siliceous shalesiliceous-shale, which reinforces previously suggested depositional environments (Mackinnon, 1989).