ALBERT

Description: 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.

Description: This geologic study is focused on a less than 5 square-mile (ca. 13 km2) tract of public land in northwestern Wyoming, 8 miles (12.9 km) south-southwest of the small town of Clark in Park County. The study area is south of Clarks Fork of Yellowstone River along the eastern base of the topographic feature called Bald Ridge, also known structurally as Dead Indian monocline. Since the Middle Eocene, the study area has been along the northwestern margin of the Bighorn Basin. Prior to that time, the study area existed near the west–east center of the basin. Bald Ridge became elevated late in the Laramide orogeny (no older than the Early Eocene) through east-directed faulting of basement rocks via the extensive Line Creek–Oregon Basin thrust system. As that active faulting occurred, the overlying Phanerozoic strata (Lower Cambrian through Lower Eocene) responded with numerous west-directed, out-of-the-basin thrusts as a new western-basin margin developed along the eastern realm of the newly born Absaroka volcanic field. Most of that deformation occurred after deposition of uppermost levels of the Lower Eocene Willwood Formation. The key purpose of the present paper was to improve the accuracy of mapping of the Jurassic into Eocene stratigraphy along the newly restricted, northwestern edge of Wyoming’s Bighorn Basin. The stratigraphic column in a north–south band along the eastern flank of the Beartooth Mountains and continuing southward into the present study area was markedly deformed and deeply eroded late during the Laramide orogeny. The present small, more southerly study area is structurally and erosionally simpler than its more northerly equivalent. Thus, its study adds important geological information to the history of the northern Cody Arch, a convex-westward string of related basement-involved uplifts extending southward to southwest of the city of Cody. Progressively steepening eastward dips of strata characterize a west-to-east transect from the summit of Bald Ridge (capped by the shallowly dipping, Mississippian Madison Limestone) to the western edge of strongly overturned outcrops of the Eocene Willwood Formation. The Upper Cretaceous Meeteetse Formation is the stratigraphic horizon at which the dips attain vertical or slightly overturned orientations. All consequential faults within the newly mapped area are thrusts, and they show generally westward (out-of-the-basin) displacements. Despite those west-directed displacements, their primary cause was tectonic shortening at depth below Bald Ridge that was directed to the northeast or east-northeast. During the Laramide orogeny, certain thrust planes within the east-dipping Phanerozoic rock column cut down-section stratigraphically (but uphill relative to Earth’s surface) and thereby placed younger strata upon older. The cumulative result, as recognized at several levels within the present area of study, was marked thinning of the total section. For example, surface exposures of the mostly Paleocene Fort Union Formation, 4,000 feet (1,219 m) thick only 7 miles (11.3 km) to the east, was completely eliminated from the local surface stratigraphy by that means. The northern end of Bald Ridge is formed by the highly asymmetric Canyon Mouth anticline. That structure differs strongly in the attitude of its hinge line from the general east-northeast dip of strata cloaking Bald Ridge. The Canyon Mouth anticline’s hinge line plunges steeply to the southeast, and dips on its northeastern flanks are vertical to partly overturned. Surprisingly, hinge lines and flanks of all other anticlinal/synclinal structures recognized within the present map area share those same orientations with Canyon Mouth anticline. These consistent but unexpected differences in orientation from unfolded strata may represent very late events in the history of Laramide strain vectors across the study area. Working in northern parts of this study area, an independent group determining radiometric ages of detrital-zircon grains reported close agreements in age with their host localities in the Early Cretaceous Mowry Shale and Frontier Formation. However, under the present paper’s interpretation of the local stratigraphy, the other workers misidentified formational hosts for all three samplings. That resulted in age-determination errors of depositional history within the Upper Cretaceous section of as much as 28.8 million years.

Description: We summarize faunal changes through the thickest and one of the most complete records of terrestrial vertebrates spanning Lancian (∼latest Cretaceous) and Puercan (∼earliest Paleocene) ages, the type Ferris Formation in the Hanna Basin, southern Wyoming. Observed faunal changes predate tectonic definition of local Laramide basins. Nonmammalian vertebrates exhibit no major changes in taxonomic composition below the Lancian-Puercan boundary; diversity of non-avian dinosaurs remains high within uppermost levels of the Lancian section. Nevertheless, dinosaurian extinction was not necessarily “catastrophic” within a biologically relevant interval. Primitive condylarths appear locally above the highest known dinosaurs, probably as immigrants. At least in this part of the North American western interior, the first evolutionary radiation of condylarths was subsequent to the last appearance of dinosaurs, not synchronous with or prior to it. Niche-partitioning among condylarths is first recorded near the boundary between Puercan Interval-zones Pu1 and Pu2 (early and middle Puercan time, respectively), by which time the first great mammalian diversification of the Cenozoic had begun. Major experimentations in dental morphology and increasing ranges of body sizes had developed within 400,000 years of the Lancian-Puercan boundary. We recognize no evidence suggesting that placental mammals were “recovering” from events that led to demise of the dinosaurs. The true diversity of marsupials and condylarths precisely at the Cretaceous-Tertiary boundary, throughout the western interior, remains unknown. We cannot, therefore, evaluate extensiveness of competition, if any, at that time among members of the two groups.