LATE MIOCENE NORTHWARD DRAINAGE OF THE COLORADO PLATEAU; EVIDENCE FROM A NEWLY RECOGNIZED RIVER GRAVEL IN SOUTHWESTERN WYOMING

A river gravel of probable latest Miocene (~10-5.2Ma) age straddles the Wyoming – Colorado border 50km north of the east end of the Uinta uplift. Overlying moderately folded Eocene strata with slight angular unconformity in most areas, it also overlies the 24-7Ma Browns Park Formation at its proposed type locality on Powder Rim in southern Wyoming. The conglomerate is defined by a distinctive assemblage of rounded to very well-rounded, super-resistant (quartzite>argillite>chert>>volcanic) apparently far-traveled pebbles and cobbles that dominate in Wyoming (>90%), but whose abundance relative to a locally derived Uinta sub-assemblage of carbonate and red sandstone decreases dramatically to the south. That the more friable Uinta clasts decrease in size and abundance to the north is consistent with the proposed north-directed transport for the gravel. Sparse paleocurrents support this.

The basal unconformity of the gravel climbs gently to the north in Colorado where it arches over a gentle anticline corresponding to a slightly steeper-limbed fold in the Eocene bedrock (Nipple Rim anticline). The unconformity then dips gently to the north for several kilometers, but climbs again as it arches over the Powder Rim of Wyoming (a faulted anticline) where it jogs to the west and peters out in a series of scattered lag gravels in the northern Washakie basin.

The conglomerate of Powder Rim offers a viable explanation for how the southern Colorado Plateau evolved from a long-lived, high-standing Pacific Northwest headwater region to the deeply incised canyon country paying tribute to the Sea of Cortez it is today. Uplift of the Plateau started in the south in the late Oligocene and progressed gradually north through the Neogene, culminating in the late Miocene with uplift in southwestern Wyoming. This uplift eventually caused the major drainage reversal that lead to sudden incision of the Grand Canyon in the earliest Pliocene. The conglomerate of Powder Rim may have been deposited in the fore of the uplift as it passed into southern Wyoming in the late Miocene. Uplift may have been related to passage of the northern edge of the slab window as it migrated north throughout the Neogene uplifting the Plateau in its wake. Folding of the unconformity may be related to flexure of the crust in response to passage of the uplift front.