Abstract

Linkage of paleontological and geological discoveries provides new opportunities to strengthen interpretations of paleogeographic evolution of the Rocky Mountains' deepest structural basin. We report discovery in the northeastern Hanna Basin (south-central Wyoming) of a lower first molar of Meniscoessus cf. M. robustus, an advanced form of multituberculate mammal known only from the North American Western Interior in Upper Cretaceous local faunas of the Lancian North American Land Mammal Age. It aids in dating patchy outcrops of the Ferris Formation, overlain and covered laterally by significantly younger, thrust-emplaced Hanna Formation. The specimen documents a member of the M. robustus species group, also recovered from more southwesterly strata of the Ferris Formation in the Hanna Basin. The fossil-bearing strata were deposited close to ancient sea level but are tectonically overturned and bounded above and below by what originally were north-vergent thrust faults. We present a new geologic map (scale 1:24,000) including two representative cross sections. Using an interpretive cross-sectional evolutionary model, we propose that the Hanna Basin, until late in Laramide orogenesis, had a markedly more extensive northern existence in the upland areas now occupied by the Freezeout Hills and southern Shirley Mountains. Local Laramide orogenic history in the mapped area is dominated to the north by development of at least 10 kilometers of Cretaceous–early Eocene structural relief across Archean granitic rocks. Those ancient rocks today form the NNE–SSW-oriented, axial core of the asymmetrical Shirley Mountains Anticline. Completion of the Shirley Mountains' uplift postdated deposition of almost the entire stratigraphic sequence now exposed along the northern Hanna Basin. North-vergent, out-of-the-basin thrust faults developed in response to crowding initiated by the much larger, south-vergent, basement-involved thrust complex known as the Shirley Fault. Those out-of-the-basin thrust faults had mostly bedding-parallel planes of displacement. But they commonly cut stratigraphically down-section during basin-margin deformation, thus placing younger strata of the hanging walls onto older strata of the footwalls. These thin-skinned, younger-on-older fault relationships today exhibit steeply basinward-dipping to overturned strata. The faulting led to greatly thinned stratigraphic sections when juxtaposed against basin-margin, mountainous uplifts expressing oppositely vergent, basement-involved thrust-fault systems. These kinds of down-section thrust faults probably will become recognized as common expressions of basin subdivision along steeply dipping, basin-margin strata throughout the Rocky Mountain province. Furthermore, several occurrences of this phenomenon appear to have been long-misinterpreted as depositional/erosional angular unconformities. Such recognition demands re-thinking of the areas' geologic histories. Complexities of erosional history constitute central parts of our evolutionary scenario. Locally derived clastic deposits within uppermost Cretaceous and Paleogene sequences of the northern Hanna Basin originated principally from north of the Shirley Mountains and other upland areas that today closely border the basin. Multiple source areas existed across deeply eroding, mountainous landscapes (the Granite Mountains) that existed during latest Cretaceous through Eocene time. Beginning in the latest Eocene and continuing late into the Miocene, dominantly airfall volcaniclastic materials from distant sources covered all but the high peaks of Wyoming. Late in the Miocene, however, the heavily eroded core of the Granite Mountains collapsed via extensional tectonics, allowing preservation of remnant volcaniclastic strata atop the Granite Mountains Graben.

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