Distribution and origin of organic carbon in the Upper Cretaceous Niobrara Formation and Sharon Springs Member of the Pierre Shale, Western Interior, United States

Persistent Link:
http://hdl.handle.net/10150/282423
Title:
Distribution and origin of organic carbon in the Upper Cretaceous Niobrara Formation and Sharon Springs Member of the Pierre Shale, Western Interior, United States
Author:
Tanck, Glen S., 1951-
Issue Date:
1997
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
The Upper Cretaceous Niobrara Formation and the overlying Sharon Springs Member of the Pierre Shale are two of several organic-carbon (OC) rich units deposited in the Western interior seaway. A key to assessing the validity of models proposed to account for OC enrichment in these units is understanding the three-dimensional (3D) distribution of OC within these units. Within the study area abundant subsurface data in the form of geophysical well logs are available. These logs were used to divide the Niobrara and Sharon Springs into regionally correlatable chronostratigraphic horizons and to estimate OC-content. By combining these elements a 3D picture of the OC-distribution was obtained. This distribution was compared to changes in lithology, sedimentation rate, and tectonic activity. The Niobrara and Sharon Springs contain local and regional disconformities which indicate water depths were near an estimated storm-wave base depth of 100 m. Local disconformities in the Niobrara are indicative of basinal tectonic activity that is linked to Sevier thrusting to the west. Large-scale Niobrara chalk/marl cycles also appear to be linked to Sevier thrusting. There is a regular vertical pattern of OC enrichment in the Niobrara, but no pronounced regional patterns are evident. Estimated paleoproductivities are moderate, except to the southeast where higher productivities may have been a consequence of upwelling. Small-scale chalk/marl cycles result from alternation of high productivity during periods of fairly vigorous circulation (chalks) with low productivity during periods of more sluggish circulation (marls). The regional diachrony of the Sharon Springs facies was a result of clastic dilution associated with progradation from the west and the paleobathymetry. Paleoproductivities were moderately high during Sharon Springs deposition, but there is no conclusive evidence of upwelling. Both units were deposited beneath bottom waters that were on average dysoxic, but oxygenation levels varied intermittently from fully oxic to anoxic. These changes were climatically modulated. At short time scales they may have been seasonal and at longer time scales they may have been driven by Milankovitch cyclicity.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Geology.; Paleoecology.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Geosciences
Degree Grantor:
University of Arizona
Advisor:
Parrish, Judith T.

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleDistribution and origin of organic carbon in the Upper Cretaceous Niobrara Formation and Sharon Springs Member of the Pierre Shale, Western Interior, United Statesen_US
dc.creatorTanck, Glen S., 1951-en_US
dc.contributor.authorTanck, Glen S., 1951-en_US
dc.date.issued1997en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractThe Upper Cretaceous Niobrara Formation and the overlying Sharon Springs Member of the Pierre Shale are two of several organic-carbon (OC) rich units deposited in the Western interior seaway. A key to assessing the validity of models proposed to account for OC enrichment in these units is understanding the three-dimensional (3D) distribution of OC within these units. Within the study area abundant subsurface data in the form of geophysical well logs are available. These logs were used to divide the Niobrara and Sharon Springs into regionally correlatable chronostratigraphic horizons and to estimate OC-content. By combining these elements a 3D picture of the OC-distribution was obtained. This distribution was compared to changes in lithology, sedimentation rate, and tectonic activity. The Niobrara and Sharon Springs contain local and regional disconformities which indicate water depths were near an estimated storm-wave base depth of 100 m. Local disconformities in the Niobrara are indicative of basinal tectonic activity that is linked to Sevier thrusting to the west. Large-scale Niobrara chalk/marl cycles also appear to be linked to Sevier thrusting. There is a regular vertical pattern of OC enrichment in the Niobrara, but no pronounced regional patterns are evident. Estimated paleoproductivities are moderate, except to the southeast where higher productivities may have been a consequence of upwelling. Small-scale chalk/marl cycles result from alternation of high productivity during periods of fairly vigorous circulation (chalks) with low productivity during periods of more sluggish circulation (marls). The regional diachrony of the Sharon Springs facies was a result of clastic dilution associated with progradation from the west and the paleobathymetry. Paleoproductivities were moderately high during Sharon Springs deposition, but there is no conclusive evidence of upwelling. Both units were deposited beneath bottom waters that were on average dysoxic, but oxygenation levels varied intermittently from fully oxic to anoxic. These changes were climatically modulated. At short time scales they may have been seasonal and at longer time scales they may have been driven by Milankovitch cyclicity.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectGeology.en_US
dc.subjectPaleoecology.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineGeosciencesen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorParrish, Judith T.en_US
dc.identifier.proquest9806807en_US
dc.identifier.bibrecord.b37541596en_US
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