Date of Award

Spring 2005

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ocean/Earth/Atmos Sciences

Committee Director

Donald J. P. Swift

Committee Member

Dennis Darby

Committee Member

James Castle

Committee Member

Chester Grosch


Cyclic stratification was examined in the Upper Cretaceous (Santonian-Campanian) section (Eagle Formation) within the Bighorn Basin of Wyoming. Of particular concern was the 103 to 104 year band, which in marine settings has been attributed to orbital forcing (Milankovitch cyclicity). A series of 19 sections were measured through the Virgelle Member of the Eagle Formation. Most were measured on the “J”-shaped escarpment that constitutes the nose and north flank of the Thermopolis anticline. Several others were measured at sites up to 30 km to the northwest along paleo-shoreline. In the study, detailed thickness data of all sections and grain size data of three sections were used to test the hypothesis that fifth-order cycles are of eustatic origin. Spectral analysis of limited grain size data clearly indicated a strong 41,000-year Milankovitch cycle (the obliquity cycle) along with a well-defined 20,000-year Milankovitch cycle (the precession cycle). The Sherman statistic, a statistic which tests whether events are periodic or random, was used on the more extensive thickness data. Results indicated the presence of Milankovitch events at periodicities of 41,000 and 20,000-years, and these events have been correlated along paleo-shoreline as well as into the basin. This data has been interpreted as a result of varying monsoonal strength often ascribed to precessional control. Variation in monsoonal strength may have caused the rate of sediment input to vary, or may have caused sea level to vary, or both.





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