Johnson, Kimberly A.1, Donald J.P. Swift1
(1) Old Dominion University, Norfolk, VA
ABSTRACT: High-Frequency Cyclicity Modified by Coastal Facies Architecture, Campanian Eagle Formation, Bighorn Basin, Wyoming
The clastic wedges of the Cretaceous Western Interior Basin pushed high-sediment-input shorelines westward across the rapidly subsiding foredeep, and the resulting expanded sections preserve detailed records of the interaction of allogenic cyclicity (eustasy) with shallow marine facies architecture. In the Campanian Virgelle member of the Eagle Formation (Mesaverde equivalent), a high-frequency (“fifth order”) cyclicity of probable astronomical-climatic origin (orbital forcing) is appears in the heterolithic lowstand systems tracts of lower-frequency (“fourth order”) sequences. The cycles are overprinted in the proximal highstand sandstones by autocyclic repetition due to avulsion and beach ridge accretion, but reappear landward of the shoreline in overbank deposits.
In the lowstand beds, fifth-order cyclicity is generally overprinted onto otherwise random storm stratification patterns. Sandstone-shale cycles may occur as a wide but regular spacing of storm beds below and above the fourth order sequence. Storm beds in this distal setting can only be deposited at fifth-order lowstand when the shoreline is at its closest. Sandstone-shale cycles also occur through the fourth-order lowstand systems tracts. In these settings, storm beds decrease in spacing and increase in thickness up to the horizon of closest shoreline approach, followed by a repeat of the cycle. Despite the inference of a eustatic origin with its implication of continuity, the fifth-order cycles are surprisingly difficult to correlate along the trend of the paleoshoreline. Attempts to map these cycles across the southern Bighorn Basin suggest that they convey as much information concerning the location of coastal depocenters as they do concerning long-distance correlations.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.