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The Iron Budget and Sulfur-Iron-Organic Carbon Relationships in Condensed Sections of Mancos Shale

M. Dumitrescu
University of Nebraska at Lincoln, Dept. of Geosciences, Lincoln, NE

Although numerous studies have focused on the seismic, well log, or biostratigraphical expression of condensed sections, there are few detailed geochemical analyses of these strata. In particular, there are no analyses of iron budget (iron content and mineralogy) in a sequence stratigraphic framework. The purpose of this research is to asses the relative role played by dissolved sulfate, reactive iron, and labile organic matter in the formation of diagenetic iron minerals in condensed sections of the marine Cretaceous Mancos Shale.

It is reasonable to suspect that iron content and mineralogy will be dependent on position within a stratigraphic sequence. Condensed sections typically form distal to sediment sources, where finegrained sediments with greater affinity to adsorb colloidal hydrous ferric oxides and organic compounds accumulate. In addition, sealevel highstands are considered to be times of enhanced chemical weathering which, in turn, are expected to produce relatively greater amounts of ferric oxyhydroxides. Moreover, slower sedimentation rates, typical for condensed sections, lead to pyrite formation, while higher sedimentation rates enhance siderite formation. Based on all these observations, we expect to find more iron in condensed sections. Therefore, in this research we will attempt to identify the controls on the relative amounts of reactive iron, pyrite, and siderite as well as on the preservation of organic carbon. A better understanding of these controls will contribute to a better understanding of the geochemistry of the condensed sections.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid