Date of Award

1985

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Erle G. Kauffman

Second Advisor

Lisa M. Pratt

Third Advisor

Don L. Eicher

Abstract

The upper Smoky Hill Member of the Niobrara Formation (late Middle Santonian - Lower Campanian) was deposited at water depths of 300 to 600 feet (100 to 200 meters) on the "hinge zone" of an asymmetrically-subsiding foreland basin during the second largest transgressive-regressive eustatic cycle in the Cretaceous Western Interior Seaway of North America. The interval consists predominantly of rhythmically-bedded calcareous shales and limestones of the middle chalk, upper chalky shale, and upper chalk units, and progressively less calcareous shales of the transition zone. These four units record fourth-order eustatic cycles which deposited limestones in the study area at peak transgression and shales at peak regression. Six highly detailed measured sections between Boulder and Owl Canyon, Colorado, resulted in a high-resolution event-stratigraphic correlation of clustered bentonite and clustered limestone-marlstone rhythm marker units. This correlation revealed a northward thinning upper chalk unit, significant thinning of the study interval at Lyons, and an abruptly thinned transition zone at Owl Canyon. Utilizing detailed organic geochemistry and presence/ absence biostratigraphy these features have been attributed to a northward facies change to less calcareous deposits, a paleostructural high at Lyons, and a Niobrara-Pierre disconformity, respectively. Limestone-marlstone couplets exhibit an inverse proportionality between organic carbon (C ) and carbonate carbon (C b), org car and clustered limestone-marlstone rhythm marker units exhibit a sub-parallel relationship to clustered bentonite marker units. This suggests laterally synchronous deposition of limestone beds, and thus possible climatic controls on rhythmic variations in terrestrial clay input, basin stratification, and carbonate and non-carbonate productivity. An average duration of 28- to 29-thousand years per couplet suggests these climatic cycles may have been affected by the earth's precessional cycle. Consistantly low benthic faunal abundances and consistantly high C values suggest generally dysaerobic, poorly circulated org bottom waters. Lateral variations in benthic faunal distribution and C percentages suggest that structural basin topography org may have created benthic microenvironments of well oxygenated, habitable conditions with low preserved organic material atop highs, and poorly oxygenated, less habitable, organic rich bottom conditions in the lows.

Comments

Please see bottom of the page for Figure 27.

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