Date of Award

Summer 2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Committee Director

Donald J. P. Swift

Committee Member

G. Richard Whittecar

Committee Member

Dennis A. Darby

Abstract

The stratigraphy of the Cretaceous Western Interior basin indicates that sea level oscillated during the late Cretaceous. The oscillations were polyharmonic, with periods ranging from millions of years to hundred thousands and ten thousands of years. However, there is disagreement over the extent to which sea level change was global in nature and the extent to which it was driven by local uplift or subsidence (tectonism). In orogenic regions where subsidence, sediment supply, and eustatic variation are all affecting sea level, comparisons of high and low frequency cycles can help to sort out forcing mechanisms. Low frequency eustatic sea level rise can be amplified by regional subsidence so that high frequency sea level falls are suppressed, and the high frequency oscillations appear as modulations of a steady rise. In such cases, the high frequency signal can be said to have been "rectified". This thesis focuses on two members of the Eagle Formation in the Bighorn basin of Wyoming, USA, the Virgelle and the Gebo, whose contrasting architectures appear to reflect oscillatory sea level fall (the Virgelle) versus pulsed sea level rise (the Gebo). I hypothesize that Gebo deposition was characterized by intensified sediment input during a variable-rate rise of sea level (pulsed rise) that was rectified by tectonics, resulting in the loss of the 3rd and 4th order sea level falls; whereas in the Virgelle, tectonic rectification was absent during a high frequency oscillatory sea level fall.

This hypothesis has been tested by comparing several architectural characteristics of the two members; unit thickness, the presence of sequences verses parasequences, the presence of sharp-based shorefaces and gutter casts, the inferred paleo-gradient, sequence of stacking patterns, and the presence of incised valleys and unconformities. The evidence shows that the evolution of these members was coupled closely to eustatic and tectonic changes. At the time of Virgelle deposition, the subsidence rate was too low to reverse eustatic sea level fall, and tectonic rectification did not occur. During Gebo deposition, the changing geodynamics expanded the zone of rapid subsidence and at the same time generated uplifts that shed sediment during a variable-rate rise of sea level (pulsed rise). This then resulted in the loss of the high order sea level falls. The Bighorn Basin appears to be characteristic of the proximal portion of Posamentier and Allen's zone B, where eustatic fall only periodically exceeds subsidence.

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DOI

10.25777/hnjy-jx36

ISBN

9780549255468

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