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AAPG Bulletin, V.
Regional fluid flow as a factor in the thermal history of the Illinois basin: Constraints from fluid inclusions and the maturity of Pennsylvanian coals
1U.S. Geological Survey, 12201 Sunrise Valley Drive, Mail Stop 956, Reston, Virginia, 20192; email: [email protected]
2U.S. Geological Survey, P.O. Box 25046, M.S. 973, Denver, Colorado, 80225; email: [email protected]
3U.S. Geological Survey, P.O. Box 25046, M.S. 977, Denver, Colorado, 80225; email: [email protected]
Elisabeth Rowan's current research is focused on fluid and heat flow in deep sedimentary basins. Previous research interests and expertise include Mississippi Valley-type lead-zinc, fluorite vein deposits, and fluid inclusion geothermometry. She received an M.S. degree in geology from University of California, Berkeley in 1982 and joined the U.S. Geological Survey the same year. She received a doctorate in hydrogeology from the Pierre & Marie Curie University (University of Paris VI) in 1998.
Martin Goldhaber received a B.S. degree in chemistry and a Ph.D. in geochemistry from UCLA. He joined the U.S. Geological Survey in 1975 following a postdoctoral fellowship at Yale University. His areas of research include sedimentary geochemistry, uranium in sedimentary environments, Mississippi Valley-type lead-zinc deposits, and environmental geochemistry.
Joseph Hatch received an M.S. degree and a Ph.D. from the University of Illinois and joined the U.S. Geological Survey in 1974. His expertise includes organic, trace-element, and carbon isotope geochemistry. His research is focused on the geochemistry of Pennsylvanian coals and marine shales and Paleozoic oils of the United States mid-continent.
We would like to acknowledge many valuable discussions with colleagues. We particularly thank B. Bekins, G. Garven, D. Hayba, M. Lewan, G. de Marsily, and J. Pitman for comments and suggestions that improved the article. Reviews by A. Byrnes, D. Kolata, and M. Person are also very gratefully acknowledged.
Vitrinite reflectance measurements on Pennsylvanian coals in the Illinois basin indicate significantly higher thermal maturity than can be explained by present-day burial depths. An interval of additional sedimentary section, now removed by erosion, has been suggested to account for the discrepancy. Although burial could indeed account for the observed maturity levels of organic matter, fluid-inclusion temperatures provide a stringent additional constraint. In this article, we combine measurements of coal maturity with fluid-inclusion temperatures from three sites to constrain the basin's thermal and burial history: the Fluorspar district at the Illinois basin's southern margin, the Upper Mississippi Valley zinc district at the basin's northern margin, and a north-central location.
Two-dimensional numerical modeling of a north-south cross section through the basin tests scenarios both with and without regional fluid flow. Vitrinite reflectance values can be matched assuming burial by 1.8-2.8 km of southward-thickening additional, post-Pennsylvanian sedimentary section. In the central and northern Illinois basin, however, these burial depths and temperatures are not sufficient to account for the fluid-inclusion data. To account for both parameters with burial alone does not appear feasible. In contrast, our best hypothesis assumes a wedge of post-Pennsylvanian sediment-thickening southward to about 1.2 km and a brief period of magmatism in the Fluorspar district. Significant advective heat redistribution by northward regional fluid flow accounts for fluid-inclusion temperatures and coal maturities throughout the basin. The modeling results demonstrate the potential contribution of advective heat transport to the thermal history of the Illinois basin.(Begin page 258)
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