ABSTRACT: Early Diagenetic, Aerobic Degradation of Organic Matter and Sulfides in Some Middle and Upper Pennsylvanian Marine Shales, Mid-Continent Region, United States

J. R. Hatch, J. S. Leventhal, G. A. Desborough

Organic and elemental analyses of 127 core samples of Middle and Upper Pennsylvanian organic-matter-rich marine shales (offshore shale lithofacies) from the Mid-Continent region of the United States show that organic matter and sulfides in some shales have been extensively altered. Altered and unaltered shales are similar in that they are laminated, phosphatic, organic-matter rich, and metalliferous with V contents 500-2500 ppm; Cr, 200-800 ppm; Ni and Mo, 100-500 ppm; and U, 30-100 ppm. Other compositional parameters, however, are significantly different. For example, the Desmoinesian Excello Shale Member of the Mouse Creek Formation in southern Iowa and northern Missouri is altered and has (1) relatively low organic carbon contents (3.8-9.2%

compared to 8.2-30% for unaltered shales from the same area); (2) very low hydrogen indices (3-58 mg/g compared to 230-410 mg/g); (3) isotopically heavy organic matter (^dgr¹³C = -22.0 to -24.3^pmil compared to -25.2 to -27.2^pmil); and (4) low contents of sulfur (<0.1 to 1.0% compared to 1.2 to 2.6%). These differences suggest that hydrogen-rich, isotopically light organic matter and most sulfides have been lost from the Excello Shale.

Alteration apparently took place during subaerial exposure following marine regression. Oxygenated rainwater circulated down to the level of the Excello mud and allowed metabolism of organic matter and sulfides by aerobic microorganisms. This hypothesis is supported by previous researchers, who have identified freshwater carbonate cements within, and soil horizons at the top of the limestone overlying the Excello Shale (Blackjack Creek Limestone Member of the Fort Scott Limestone). The period of alteration is constrained in that organic matter and sulfides appear unaltered in the Little Osage Shale Member of the Fort Scott Limestone, which immediately overlies the Blackjack Creek Limestone, 20-25 ft (6.1-7.6 m) above the Excello Shale. Alteration similar to that in the Excello Shale h s also occurred in the Missourian Husllpuckney Shale Member of the Swope Limestone in north-central Kansas.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990