| Title: | Middle Turonian black shale deposition within the Western Interior Basin (U.S.A.) - an integrated model |
| Author: | Astrid Forster |
| Date Submitted: | 04/29/2002 |
| Address: | P.O. Box 59
Department of Biogeochemistry and Toxicology
Den Burg
Texel
The Netherlands
1790 AB
|
| Phone: | +31/222/367 580 |
| Email: | forster@nioz.nl |
| Co-Authors: | Dean, Walter E., U.S. Geol. Survey; dean@usgs.gov; Schwark, Lorenz, Univ. Koeln, lorenz.schwark@uni-koeln.de; Thein, Jean, Univ. Bonn, jthein@uni-bonn.de; Trappe, Joerg, Univ. Bonn, trappe@uni-bonn.de; Vondra, Carl, F., Iowa State Univ., cfvondra@iastate |
| Affiliation: | Royal Netherlands Institute for Sea Research (NIOZ) |
| | |
| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725506 |
| Author Homepage: | http://www.geology.de/forster |
| Keywords: | Cretaceous; Western Interior Basin; paleoceanography black shale sedimentation sea-level control cyclicity biomarker |
| Abstract: | A basin-wide study on the paleoenvironmental controls on the formation of the Middle Turonian Carlile Shale and time-equivalent black shales was carried out utilizing an east-west transect through the Western Interior Basin (WIB). In the central part of the basin, three major black shale bearing-sequences are recorded during Albian to Santonian time related to long-term sea-level fluctuations. During transgressions, an epicontinental seaway extended meridional from northern Canada to the Gulf of Mexico. Perpendicular to the elongated axis of the seaway, our transect extends over approximately 1800 kilometers from Iowa to Utah and is based on seven cores. The Amoco Rebecca Bounds core located in the central WIB (Kansas) and the U.S.G.S. Portland core at a more proximal setting (Colorado) were chosen for high-resolution investigations of biomarkers and stable isotopes (bulk kerogen and carbonate). The investigation of sedimentary facies was carried out by screening samples from all sites for the amount and maturity of organic matter, analysis of the element-spectrum, and the main mineral compounds.
At the Bounds core locality, the reference section for the transect, the lowermost Fairport Member of the Carlile Shale consists mainly of laminated marlstones, deposited under oxygen-depleted conditions. The organic matter is immature and predominantly of marine origin, but a strong freshwater influence is indicated by biomarkers and 18O-data. The silty shales of the overlying Blue Hill Shale Member represent a transition to shallower water in a still oxygen-deficient setting, but with an increased influx of terrigenous organic matter. The long-term development of the Carlile Shale is linked to a continuous third-order regression modulated by sea-level fluctuations of fourth- and higher order, enabling subdivision into sub-members and small-scale cycles. Those are developed in the Fairport and Blue Hill Members and are expressed by variations in lithology and geochemistry. These cycles can be correlated to the Portland site, where the Carlile Shale interval is more condensed and incomplete. The small-scale cycles observed at both locations are best explained by high-order trans- and regressions.
A paleoceanographic model for black shale deposition in the WIB is proposed, combining the sea-level history with constraints from the basin's morphology and salinity-proxies. It is primarily based on the different contributions of the two main water-masses in the WIB controlling spatial variations in bioproductivity and stratification patterns. Each transgression caused a renewed incursion of both water-masses into the basin, but to a different extent depending on the transgression's magnitude. At Fairport-time, the Portland location was more influenced by the fresher, cooler waters entering the WIB from the north than the Bounds site, which was dominated by a more saline, southern-derived water body. During the deposition of the Blue Hill Member, the morphological sill at the basin's southern aperture blocked these waters more or less completely from entering, resulting in dominance of the cooler, northern water-mass. |
|