47277-AC8
Vitrinite Reflectance, Basin Thermal History, and a Link to Seafloor Methane Seeps, Cretaceous Western Interior Basin, United States
The 2008 project year was a success. All samples collected in 2007 were processed to polished particle pellets for coals and some organic-rich rocks. The lowest organic-carbon-content samples were centrifuged first to separate organic matter from mineral matter for more efficient organic petrologic study, while some organic-rich samples were mounted and polished as whole rock billets. Vitrinite reflectance measurements are ongoing. Fieldwork in 2008 began in the southern Front Range region near Pueblo, CO. Here samples were obtained from the Dakota Sandstone and Mancos Shale. As well, a suite of samples were obtained representing 3 facies from 5 Teepee Buttes - these samples are being processed for pyrite for Re/Os dating associated with the supplemental award. The samples were not as pyritic as indicated in the literature so we are unsure whether they can yield useful dates, but we remain hopeful. The fieldwork continued to the west toward Durango, with sampling of the Mancos Shale en route and from the Mesa Verde Group near Durango. Samples from both rock units were obtained further west in the vicinity of Mancos, CO. Fieldwork in Utah led to sampling from the thick Mancos Shale and coeval Tropic Shale in the western part of the state. The trip culminated in a three-day core sampling "intensive" at the USGS Core Repository in Lakewood, CO (Denver). A total of 52 samples for vitinite reflectance analysis were sampled including Mancos Shale, Mesa Verde Group, Niobrara Formation, Dakota Formation and Pierre Shale. About half of these samples have been processed and await reflectance analysis; sample processing and microscopy is ongoing. The combination of samples from two field trips provides us with excellent coverage to assess thermal evolution of the Western Interior Basin through the originally proposed three time slices: Albian-Cenomanian, Turonian-Santonian, Campanian-Maastrichtian (see figures at: zeemaps.com/map?group=93562, zeemaps.com/map?group=101952, and zeemaps.com/map?group=101954, respectively). Obviously some geographic gaps appear in the data sets some of which may be filled by further sampling - however we are generally content with the broad spread of samples obtained, well beyond the number of samples described in the proposal. All study samples will be processed to polished pellets or billets by the end of our spring semester 2009. At that point a graduate student will complete all microscopic analyses of vitrinite reflectance - this probably will require a no-cost extension for the project into Fall 2009, perhaps Spring 2010. Though we are somewhat behind schedule, our sampling and processing has greatly exceeded our proposed effort, and we anticipate a greater return in scientific understanding of basin thermal evolution, the nature of the inverted reflectance gradient described in our proposal, and the link between thermal history and the ancient Teepee Buttes methane seeps.
