Fluvial Architecture of the Upper Campanian Canyon Creek Member of the Ericson Sandstone: Signals of the Looming Laramide Orogeny, Greater Green River Basin, Wyoming

The Canyon Creek Member represents the fluvial feeder of the last and largest prograding clastic wedge that infilled the Cretaceous Western Interior Seaway. This complex fluvial system shows striking variability in thickness, architecture and net-to-gross-sand ratio in both sedimentary dip and strike directions, as seen within a combined dataset of more than 300 well-logs, more than 1200 meters of measured sections and interpreted photomosaics in the Greater Green River Basin.

The Canyon Creek Member overlies a strongly erosive and slightly angular unconformity, which separates it from the rest of the Ericson Sandstone Formation. We also recognize at least two other unconformities inside the Canyon Creek Member, and we interpret all three unconformities as the result of intermittent uplift pulses along incipient Laramide structures that later dominated the area during the Paleogene. The eroded and bypassed sediments of these unconformities fed the prograding shoreline tongues of the deltaic Twenty-Mile and Trout Creek Sandstones of N Colorado.

In the proximal part of the system, we have identified an E-W oriented paleo-valley on the basal Canyon Creek unconformity. We interpret this as the result of fluvial incision into the uplifting Moxa Arch. Since the fluvial system maintained its position while carving the valley and was not deflected by the uplifting arch, we are able to estimate the maximum rates of uplift, and to date this uplift to about 74Ma.

The large variability of fluvial architecture and thickness of Canyon Creek Member in the depositional strike direction is a result of the incipient compartmentalization of the basin by laramide structures. Very amalgamated, coarse-grained and cross-stratified fluvial deposits of braided to low-sinuosity rivers are encountered in the west of the basin, especially atop of the Laramide Rock Spring Uplift. Some 150-200 km to the E, in Wyoming, and to the SE, in Colorado, the Canyon Creek rivers became sinuous with less amalgamation and flood plain deposit preservation. This difference is the result of the increased accommodation created in the east by the laramide structures.

The Campanian Laramide uplifts, signaled during deposition of Canyon Creek, reached spectacular proportions during the Maastrichtian when there was up to 3km of uplift on the Wind River Range and deepwater subsidence, with transgression of the Lewis Sea, in the adjacent Washakie and Great Divide basins.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California