We report and describe abundant, well-preserved, parallel-sided, U-shaped spreite burrows (Rhizocorallium) in the upper part of the latest Cretaceous into early Paleocene Ferris Formation of south-central Wyoming's Hanna Basin. Rhizocorallium typically is a component of the widely represented ‘Cruziana ichnofacies,’ principally involving benthic marine environments seaward of the intertidal zone in shallow to offshore settings. Traditionally, this Ferris section has been interpreted as coal-bearing, continental deposits formed after full withdrawal of the Western Interior Seaway from eastern Wyoming and adjacent areas. The burrowed strata overlie fossiliferous rocks diagnostic of parts of the Puercan Land Mammal Age, early Paleocene. At time of burrow formation, and for several million years thereafter, the vicinity of the future Hanna Basin remained as an undivided, eastern component of an enormous, greater Green River Basin that encompassed almost all of Wyoming's southern half. The Rhizocorallium-bearing marine strata represent westward expansion of a previously more restricted Western Interior Seaway that persisted through latest Cretaceous time in what is now the western Great Plains. Even though tidal influences may have affected rapidly aggrading fluvial systems far upstream to the west in Montana and Wyoming, we regard actual Paleocene marine inundations to have been uncommon and geologically ephemeral events as far west as the Hanna Basin.
Diverse fossil assemblages from strata of the Williston Basin, representing the first five million years of Paleocene time, have led to documentation of fully open, marine conditions as the Cannonball Formation was deposited. Stratigraphic distribution of fossils within the Cannonball shows persistence of the Western Interior Seaway in the northern Western Interior through the Cretaceous, followed by expansion (renamed the ‘Cannonball Sea’) during early Paleocene time. Connections of that seaway to the south, however, remain poorly understood because of later Cenozoic massive erosion of any Paleocene rock record that had existed south of the borderland between the Dakotas. No Paleocene localities in southern Wyoming or Colorado have yet yielded assemblages of marine invertebrate body fossils or microfossils as known from the Williston Basin. No verifiable means, therefore, have been recognized to characterize the Paleocene marine record of the Hanna Basin in terms of species uniquely shared with the Cannonball biota. Short-lived, Paleocene seaway excursions into the Hanna Basin may have been: (1) direct and exclusively from the Gulf Coast; (2) solely from the Cannonball Sea, with seaway contiguity east of the emerging Black Hills; or (3) initiated from a more extensive, midcontinental seaway connecting the Arctic Ocean to the Gulf of Mexico. Substantial structural uplift of the Laramie Mountains prior to mid-Paleocene time would have precluded even brief westward pulses of marine inundation into the vicinity of the presumptive Hanna Basin.