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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 975

Last Page: 975

Title: Terrestrial Influence and Bioturbation Effects on Composition of Organic Matter in Middle Cretaceous Shale and Limestone Sequence near Pueblo, Colorado: ABSTRACT

Author(s): Lisa M. Pratt

Article Type: Meeting abstract


Soluble organic matter (chloroform), pyrolytic hydrocarbon yield (Rock-Eval), and carbon isotope ratios of insoluble organic matter were determined on core samples of the Hartland Shale and overlying Bridgecreek Limestone of the Greenhorn Formation. Trends in the organic geochemistry correlate with clay mineral content and sedimentary structures of the sediment, suggesting that (1) terrestrial input and (2) bioturbation are dominant influences on the composition of preserved organic matter.

(1) The Hartland clay shales are organic matter-rich (organic carbon contents 2.3 to 4.5%) but have lower ratios of pyrolytic hydrocarbon yield- and extractable hydrocarbons-to-organic carbon, and lower kerogen ^dgr13C values (from -27 to -28 ppt) than the Bridgecreek calcareous shales (organic carbon contents 1.8 to 5.4% and ^dgr13C values from -24.5 to -26.2 ppt. The laminated Hartland shales contain abundant terrestrial detritus (up to 70% quartz, illite, and mixed-layer clays). The laminated to partly bioturbated Bridgecreek shales contain minor terrestrial detritus (less than 20%), indicating a reduction in terrestrial influence on the basin after deposition of the Hartland clay shales.

(2) Within the Bridgecreek member, the types of organic matter preserved in the laminated calcareous shales, partly bioturbated calcareous shales and bioturbated limestones are different, and are though to result from increasing bioturbation and availability of oxygen in the bottom water. As bioturbation increases, the organic matter remains isotopically constant, but decreases (from about 5 to less than 1%) and is altered chemically as shown by decreasing ratios of pyrolytic hydrocarbon yield- and extractable hydrocarbons-to-organic carbon, and increasing ratios of pyrolytic carbon dioxide to organic carbon.

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Copyright 1997 American Association of Petroleum Geologists