Organic and Inorganic Constituents of the Niobrara Formation in Weld County, Colorado
Published:January 01, 1985
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Wi11iam F. Precht, Richard M. Pollastro, 1985. "Organic and Inorganic Constituents of the Niobrara Formation in Weld County, Colorado", Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes, Lisa M. Pratt, Erle G. Kauffman, Frederick B. Zelt
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Detailed analyses of deeply-buried chalks from cores of both the Smoky Hill Chalk and Fort Hays Limestone Members of the Niobrara Formation were performed. The identified inorganic and organic constituents, and sedimentary structures helped to characterize the lithofacies, biofacies, burial diagenesis, bulk rock and clay mineralogies, and organic geochemistry of the Niobrara Formation in Weld County, Colorado.
Many depositional and diagenetic relationships are observed: 1) the chalks are comprised primarily of whole and fragmented coccoliths with subordinate planktonic foraminifera, inoceramid prisms, oysters, and other skeletal grains; 2) deposition was predominantly below wave-base in tens-to-hundreds of meters of water; 3) petrographic evidence supports the idea that subsequent burial, mechanical compaction, and pressure-solution (chemical solution-transfer) are the primary processes responsible for porosity reduction; 4) porosity of the chalk is inversely proportional to maximum depth of burial and to the amount of non-carbonate (acid-insoluble residue content) in the chalk; 5) total organic carbon content in the chalk is proportional to the amount of acid-insoluble residue and relative abundance of pyrite in the acid-insoluble fraction; and 6) mixed-layer illite/smectic clay as a geo-thermometer in bentonite beds from the Niobrara Formation indicate that these rocks are thermally mature with respect to oil generation.
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Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes
Fine-Grained Deposits and Biofacies of the Cretaceous Western Seaway: Evidence of Cyclic Sedimentary Processes - This volume emphasizes the influence of cyclic sedimentary processes on the distribution of rock types and faunas in the Cretaceous strata in Colorado. As a whole, the volume provides an interdisciplinary view of the causes and consequences of gradual cyclic, periodic, and catastrophic changes in environmental conditions recorded by strata in the Kiowa-Skull Creek, Greenhorn, and Niobrara Cyclothems.