Genetic Stratigraphy of the Williams Fork Formation, Sand Wash Basin, Colorado and Wyoming

Abstract

The Williams Fork Formation forms the upper part of the Upper Cretaceous Mesaverde Group, Sand Wash basin. The formation can be divided into four genetic depositional sequences each bounded by regionally extensive, low-resistivity shale markers. The markers are continuous across the basin, extending from the southeastern margin to the southern Flank of the Rock Springs uplift. Recognizing these bounding surfaces was relatively straightforward in the eastern half of the basin, where they are interpreted to be maximum marine-flooding surfaces. Recognizing them in the continental facies to the northwest, however, was more difficult, but still achievable with detailed well-log correlation. Presence of the markers to the northwest indicates that either the marine flooding extended farther west than is generally recognized, or that the controls on the flooding (such as shutting off sediment supply) leave a record in the non-marine environment as surfaces of sediment starvation or non-deposition. The four genetic depositional sequences represent progradational clastic wedges of variable areal extend that were deposited during discrete episodes of basin filling. Geometry of the framework sandstones and log-facies character of each of the genetic units indicate a similar depositional style for genetic units 1-3. These units are characterized by upward-coarsening, sandstone-rich linear shorelinemore » systems in the southeast that are bounded updip by aggradational coals and interbedded mudrocks of the coastal plain that, in turn, pass landward into aggradational log motifs of thick, stacked sandstone units and interbedded mudstones on the alluvial plain. Unit 4 is characterized throughout by mudstone-rich coal-bearing facies interpreted as alluvial-plain deposits with lacustrine influence.« less