Abstract

Laramide depositional basins in Wyoming were marine-dominated throughout most of the Campanian (80-72 Ma), with newly established fluvial drainage systems providing sediment to growing delta and coastal plains, as well as barrier islands, channel mouth bars, and shorefaces. During the latest Cretaceous (Maastrichtian: 68-65) and Paleocene (Danian-Selandian: 65-56 Ma), the river systems had much higher sediment yields and fed large, rapidly prograding fluvial deltas. The resulting deposits (Lance, Fort Union, Medicine Bow, Ferris, Hanna, Hell Creek, and Tullock formations) comprise a mosaic of flood plain, coastal and delta plain, bay, and estuary deposits that were influenced by local fluvial adjustments, as well as tectonic, climatic, and eustatic activity. The sheer volume of delta and coastal plain deposits has led most paleontologists and many geologists to wrongly assume that these environments were not linked to the Western Interior Sea (WIS), but rather accumulated in discrete, isolated depositional basins.

An abundance of newly-emerging paleontological and sedimentary data reveals, however, that many of these sedimentary successions contain a greater degree of marine influence than has previously been appreciated. The Hanna Basin acted as a sediment trap, sequestering sediment transported from the Greater Green River Basin (including the Washakie, Great Divide, Hanna, and Carbon basins) due to its rapid subsidence rate. This prohibited the rapid delta progradation that characterized other Laramide basins and led to the original misinterpretation of a continent-scale marine regression. The recognition of brackish water estuarine, distributary channel, and interdistributary bay deposits in the Ferris Fm. (66-63 Ma) and marine-influenced, paleovalley fills in the Hanna Fm. (?62-55 Ma) of southern Wyoming’s Hanna Basin area was a major step in recognizing the continued importance of marine influence throughout the latest Cretaceous and Paleocene.

Far from being the simplistic, 2-D pattern of “continental” vs. “marine,” that is often depicted, Wyoming’s latest Cretaceous and Paleocene paleogeography was similar in both scale and depositional environment to the Recent Texas Gulf Coast. Drainage systems of the Trinity, Sabine, and Brazos rivers are good analogs for the paleodrainage systems of the Greater Green River, Wind River, and Bighorn / Powder River basins.