CORRELATION AND CAUSES OF FIFTH ORDER CYCLES WITHIN THE UPPER CRETACEOUS EAGLE FORMATION, BIGHORN BASIN OF WYOMING

High-frequency cycles are prominent in the sandstones outcropping along the western margin of the Cretaceous western interior basin. They are generally described as “parasequences,” and are seen as the building blocks of sequence stratigraphy. They also appear as alternations of shale and limestone along the basin's axis and eastern margin, where they are called “rhythms,” and are attributed to orbital forcing (Milankovitch cyclicity). In order to compare the cycles of the basin-margin sandstones with the better-studied cycles of the basin's axis, 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.