Application Of Argon-40/Argon-39 Chronostratigraphy To Geologic Problems In Yellowstone Caldera And Cook Inlet Basin

Abstract

The 40Ar/39Ar dating method is a useful tool in addressing problems concerning stratigraphic distribution of rock units. This tool is used where previously K-Ar-dated units are problematic, and where further chronostratigraphic control is needed for lavas of the Central Plateau Member (CPM) of the Plateau Rhyolite in Yellowstone National Park and volcanic ash dispersed in the sedimentary units of the Kenai Group in Cook Inlet Basin. Sanidine 40Ar/39Ar ages for CPM rhyolite lavas indicate that xenocrysts, excess argon, and incomplete degassing affected the K-Ar age interpretations. With the 40Ar/39Ar dating method these effects are removed, revealing four apparent eruptive episodes at approximately 135, 122, 107, and 90 ka. The 40Ar/ 39Ar eruption ages are 10's ky younger than U-Th ages from zircon indicating significant residence time of the CPM magma chamber prior to eruption. 40Ar/39Ar dating using multiple analytical runs and statistical assessment of the data produced satisfactory results from the potassium-poor, plagioclase-bearing, tephras of the Kenai Group. The results indicate that excess argon, argon loss, and xenocrystic contamination exist. The chronostratigraphic framework indicates that faulting offsets strata of widely different ages causing repetition of the stratigraphy. Age data indicate that the Sterling and Beluga Formations are time-equivalent strata representing lateral facies variations rather than distinct time-stratigraphic formations. Based on crosscutting relations and structural folding, the dated horizons suggest that structural deformation is no older than early Pliocene in age. Based on the structural complexities affecting the Kenai Group strata, the previously measured coal resource estimates for the Kenai Peninsula are overestimated by 23% in the Clam Gulch area and 50% in the Diamond Creek area. Desorbed gas contents, ranging between 66 and 123 scf/t (daf) from cuttings collected from the Beaver Creek and Kenai Gas Fields, are applied to coal volumes to estimate coalbed methane gas-in-place. CBM in-place estimates suggest values of 93 Tcf for the Kenai Peninsula and 272 Tcf for Cook Inlet Basin. Hypothetical recovery rates suggest 4.7 and 13.6 Tcf for the Kenai Peninsula and Cook Inlet Basin, respectively. The hypothetical CBM recovery rates could continue natural gas supplies to the Anchorage area for an additional 60 years beyond the estimated shortened supply date of 2011.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirement: Adobe Acrobat.