Pyrite formation in the Lower Cretaceous Mowry Shale: Effect of organic matter type and reactive iron content

American Journal of Science
By: , and 

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Abstract

Geochemical analysis of the Lower Cretaceous Mowry Shale in Wyoming and Montana shows that organic matter type and reactive iron content limited pyrite formation during early diagenesis. Consequently, direct interpretation of paleosalinity cannot be made from a carbon/sulfur plot. Mowry Shale samples with hydrogen index (HI) less than 150 (mg hydrocarbon/ organic carbon) contain low amounts of sulfur. In these samples, bacterial reduction of sulfate may have been inhabited by low nutritive content of the organic matter. For Mowry samples with HI greater than 150 there is a range of sulfur values and a positive correlation between iron and sulfur. The iron sulfur regression is approximately parallel to the stoichiometric pyrite line and intersects the iron axis at 0.5 percent. On average, 60 percent of the iron present was nonreactive; the remaining iron reacted with sulfide to form pyrite. For samples with HI greater than 150, pyrite formation was limited by sulfide or organic carbon content but by reactive iron content. In one modern setting, where salinity is greater than one-half normal seawater, sulfate content does not limit pyrite formation (Berner and Raiswell, 1984). From this analysis then, it can be estimated that the Mowry Sea was not a fresh water body but was either of normal marine salinity or contained brackish water with greater than one-half normal salinity. 

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Pyrite formation in the Lower Cretaceous Mowry Shale: Effect of organic matter type and reactive iron content
Series title American Journal of Science
DOI 10.2475/ajs.288.9.873
Volume 288
Year Published 1988
Language English
Publisher Kline Geology Laboratory
Contributing office(s) Geosciences and Environmental Change Science Center
Description 18 p.
First page 873
Last page 890
Country United States
State Wyoming
Other Geospatial Mowry Sea
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