--> Fluvial Architecture and Sediment Partitioning in Response to Early Laramide Structures: The Canyon Creek Member, Southern Wyoming, USA

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Fluvial Architecture and Sediment Partitioning in Response to Early Laramide Structures: The Canyon Creek Member, Southern Wyoming, USA

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. These trends are caused by 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 within individual Canyon Creek “sub-basins” 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. This work illustrates how important it is to recognize the effect of structural patterns in fluvial systems before attempting to ascribe them to eustatic or climatic variations in a sequence stratigraphic framework, and to make accurate predictions of sandstone geometry, net-to-gross distribution and reservoir characteristics in the subsurface.