Conodont Thermochronology: Expanding the Utility of the (U-Th)/He Method to Marine Carbonates and Shales

Abstract

(U-Th)/He thermochronology is a powerful hydrocarbon exploration tool that can provide important constraints on the burial and exhumation histories of source rocks, reservoir rocks, and structural traps and seals. Although the technique can be applied to a wide range of of lithologies, marine carbonates and shales present a major challenge because they lack the required accessory minerals, limiting the opportunity for sampling and for acquiring complete thermal histories. Conodonts have emerged as a novel solution for the application of the method in carbonate-dominated successions. These teeth-like fossils are made of hydroxyapatite with U and Th concentrations and a closure temperature (60-70 deg. C) that are comparable to magmatic sources of apatite. These fossils are already routinely used in basin studies for biostratigraphic control and as a geothermometer via the conodont color alteration index. This study explores the viability this biogenic source of apatite as a (U-Th)/He thermochronometer at two different calibration sites. Site 1 focuses on conodonts extracted from surface outcrops located in footwalls of two major Miocene low-angle normal faults in the Beaver Dam and Mormon Mountains, eastern Nevada and western Utah. New zircon and apatite (U-Th)/He data from the study area provide relatively tight constraints on the thermal histories of these faults, allowing for direct comparisons with conodont (U-Th)/He results. Site 2 focuses on two core-holes in western Kansas (Rebecca K. Bounds and Berexco Cutter KGS #1 core-holes) that penetrate relatively continuous sections of Paleozoic carbonates that are well studied in terms of their stratigraphy, paleontology, geochemistry, and reservoir properties, but lack needed thermochronometric constraints. The study pairs traditional (U-Th)/He analyses of the conodonts with x-ray computed microtomography and laser ablation inductively coupled plasma mass spectrometry. Major goals of the study are to evaluate the effects of complex morphologies, microstructural heterogeneities, parent isotope distributions and mobility, radiation damage effects, and color alteration index on conodont (U-Th)/He ages.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016