Abstract

The Upper Campanian Canyon Creek Member of the Ericson Formation in the Greater Green River Basin of southern Wyoming, USA, is a regionally extensive sandstone sheet of fluvial origin. A well-log and outcrop dataset allows this ancient fluvial system to be examined basin-wide, covering an area of approximately 45,000 km². Changes in fluvial style and architecture, and in thickness and net-to-gross occur both down-dip and along strike, across the whole fluvial system, allowing insight on both local and regional structural styles in the basin.

From the proximal reaches of the system in the west and northwest to the distal areas in the east and southeast, the Canyon Creek Member (and its equivalents) shows a first-order trend of thickening and of net-to-gross reduction, as well as a change of planform style from braided to meandering. The reason for these trends is an eastward increase of subsidence away from the orogenic belt, probably resulting from reduced activity in the Sevier fold–thrust belt, which decreased the importance of the tectonic loading and flexural subsidence and enhanced the signal of the dynamic subsidence.

The Laramide orogeny is typically thought to have been initiated during the Maastrichtian or Paleocene. However, study of the Canyon Creek Member strongly indicates earlier initiation during the Campanian. The above general proximal-to-distal trends in the Canyon Creek are locally modified by early differential movement across the Rock Springs Uplift, the Uinta Mountains, the Rawlins Uplift, and the Sierra Madre Uplift, all of which are Laramide-style structures. This early movement is shown by reduced thickness and increased net-to-gross over the crest of these structures and by the opposite trend down-flank on the structure into the footwall of the adjacent structure. This asymmetric pattern of thickness and net-to-gross development within individual Canyon Creek “subbasins” is the typical and distinctive Laramide basin signature.

The Canyon Creek Member of the Ericson Formation is a clear example of how varied a fluvial system can be laterally in response to minute changes in accommodation, slope, and other parameters, and illustrates how important it is to recognize that variability before embarking in the more complicated task of recognizing these changes through time in a sequence stratigraphic framework.