HR: 1340h
AN: PP43B-1268    [Abstracts]
TI: Organic Matter Production And Preservation During Transgression And Highstand Of The Niobrara Cyclothem, Cretaceous Western Interior Seaway
AU: * Salacup, J M
EM: jsalacup@geo.umass.edu
AF: Dept. of Geosciences, University of Massachusetts Amherst, 611 N. Pleasant St., Amherst, MA 01003, United States
AU: Petsch, S T
EM: spetsch@geo.umass.edu
AF: Dept. of Geosciences, University of Massachusetts Amherst, 611 N. Pleasant St., Amherst, MA 01003, United States
AU: Leckie, R
EM: mleckie@geo.umass.edu
AF: Dept. of Geosciences, University of Massachusetts Amherst, 611 N. Pleasant St., Amherst, MA 01003, United States
AB: The Upper Cretaceous Niobrara cyclothem (upper Turonian-lower Campanian) is a second-order transgressive- regressive cycle of the Cretaceous Western Interior Seaway reflecting interactions among eustatic sea level change, regional tectonic events, and sediment supply. These strata provide a unique window into Late Cretaceous sediment deposition and paleoceaonographic conditions in an epicontinental seaway. However, the response of organic matter production and burial to these forcings remains less than fully resolved. Geochemical analyses of the Montezuma Valley and Smoky Hill Members of the Mancos Shale at its principal reference section at Mesa Verde, Colorado, reveal potential relationships among organic matter abundance and composition, paleoceanographic conditions inferred from microfossils, and sea level change. These rock units represent transgression and early highstand of the Niobrara cyclothem. At Mesa Verde, the upper Smoky Hill coincides with the spatially-restricted but temporally-extended Oceanic Anoxic Event 3 of middle Coniacian to early Santonian age (~88.5-86.5 Ma). It is broadly characterized as dark-gray, foraminifer-rich calcareous shale, mudstone, and marlstone. Bulk geochemical properties, including %TOC, %CaCO3, and C/N, reflect changes in organic matter delivery and preservation, and are closely correlated to inferred water-depth and/or distance from shore. Proximity to the western paleo-shore appears to exercise a primary control over the composition of the identified biomarkers with secondary influence from redox-sensitive diagenetic processes and autochthonous microbial production, which in turn may reflect higher-order sea-level fluctuations. Changes in n-alkane, hopane, and sterane distributions are coincident with the second-order transgression of the seaway. Additionally, the presence in some samples of long-chain alkylcycloalkanes and alkylbenzenes may reflect the direct cyclization and aromatization of precursor algal and bacterial fatty acids in a reducing environment. Branched alkanes with quaternary substituted carbon atoms, thought to reflect input from chemosynthetic bacteria living near redox boundaries, are also detected. Relationships existing between sequence stratigraphy and the bulk and detailed molecular geochemistry of the Montezuma Valley and Smoky Hill members shed light on the factors, both global and regional, responsible for the composition of biogeochemical signals expressed in the sedimentary record. A better understanding of the processes affecting such signals is crucial in their use and application in paleoenvironmental and paleoceanographic reconstructions.
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1055 Organic and biogenic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting