Albian (Lower Cretaceous) Carbon Chemozones Texas Comanche Shelf and Mexican Chihuahua Trough
Description
The objectives are to 1) synthesize geochemical and biostratigraphic data of three Albian OAE chemozones in Texas and Mexico, and 2) to document carbon and oxygen isotopic excursions in basinal organic-rich deposits and in shallow-water deposits where organic-rich rocks are absent. Closely spaced limestone and shale samples from seven stratigraphic sections were integrated with biostratigraphy. Isotopic curves correlate the chemostratigraphic zones in Texas and Mexico with Albian zones globally. Deeper water organic-rich deposits are coeval with shallower carbonate facies, which leads to new understanding of oceanic conditions resulting in chemozones. Approximately 0.5 g-1 g of powdered carbonate or shale in 10 mL Exetainer vials were flushed with Helium for 10 minutes. Samples reacted with five drops of 105% H3PO4, at 25°C for 18 hours ensured complete reaction of CaCO3 and analyzed with a Finnigan DeltaPlus XP isotope ratio mass spectrometer with a Gasbench II continuous flow sample preparation analysis device. Carbon isotopes are related to the Vienna Pee Dee Belemnite (VPDB); oxygen data are related to the Vienna Standard Mean Ocean Water (VSMOW). Some samples were analyzed using the Delta V Advantage Isotope Ratio Mass Spectrometer (IRMS), University of Tulsa Geosciences Department. Samples were run in conjunction with international reference materials, including National Bureau of Standards-18 (NBS-18) and National Bureau of Standards-19 (NBS-19). Total inorganic carbon (TIC) analyses were performed by Humble Geochemical Services, Houston, using a Leco carbon-sulfur analyzer. The 0.5 g of powder for TIC data were performed by the University of Arkansas Stable Isotope Laboratory, Fayetteville, Arkansas. To determine the quantity of organically bound carbon, organic molecules were broken down to single carbon units and converted to single molecular form. Total organic carbon wt % (TOC) concentrations were obtained by subtracting TIC from total carbon (TC). Albian Chemozones OAE 1b, 1c, and 1d are documented on the Texas Comanche Shelf, in the Mexican Chihuahua Trough and on the Mexican Sonoran Shelf. These units in the proto-Gulf of Mexico supports the hypothesis that oceanic low-oxygen water masses were widespread and organic-rich mudstones were deposited in cratonic basins as well as in oceanic basins; also, carbon isotope excursions incurred in shallow-water environments. Diverse biostratigraphic zones show that each of the three isotopic excursions correlate with OAE 1b, 1c and 1d. The low-oxygen water masses were as deep as at least 300 m on the forereef slope and in intrashelf basins where organic-rich deposits accumulated. In contrast on the shallow Comanchean and Sonoran shelves above the depth of storm wave-base nutrient-rich waters fostered organic buildups that recorded positive carbon isotope excursions in the absence of organic-rich shales.