Geology of Wyoming and 4 Billion Years of Earth’s History
October 6 (Tuesday), 6 p.m., Teton Co. Library Auditorium – Open to Public. Presentation: ” Geology of Wyoming—nearly 4 billion years of Earth history”, Presented by Art Snoke, Professor Emeritus, University of Wyoming
The early geologic history of Wyoming involved complex accretion and subsequent rifting of an Archean craton (part of the Precambrian core of North America), commonly referred to as the Wyoming province. Late in the Early Proterozoic, an oceanic-arc terrane was accreted to the southern margin of the Wyoming province during an oblique collision. The Proterozoic suture, called the Cheyenne belt for exposures in southeastern Wyoming, is a zone of penetrative deformation. Middle Proterozoic anorthositic to granitic rocks intruded this tectonic zone.
After the mid-Proterozoic, a nearly billion-year gap exists in the geologic record of Wyoming. Lower and middle Paleozoic shelf-facies strata, commonly separated by disconformities, form a sedimentary veneer upon Precambrian igneous and metamorphic basement rocks. Uplifts of the late Paleozoic Ancestral Rocky Mountains extended northward into southeastern Wyoming and locally were eroded to Precambrian rocks. Late Paleozoic and early Mesozoic strata record diverse paleoenvironments, including coastal, eolian, and fluvial settings. Beginning in the mid-Cretaceous, a persistent interior seaway extended from the Arctic Ocean to the Gulf of Mexico. A nearly coeval, thin-skinned, fold-and-thrust belt (Sevier orogenic belt) evolved west of this seaway and remained active into the early Eocene. East of the fold-and-thrust belt, during the Late Cretaceous through early Eocene, deep-rooted reverse and thrust faults fractured the craton, forming basement-involved uplifts separated by deep, actively subsiding basins. These juxtaposed structures are the classic features of the Laramide orogeny of the eastern Rocky Mountains.
Locally important magmatism formed volcanic piles and subvolcanic intrusive complexes during the middle to late Eocene (e.g., Absaroka Range). Extensive lakes also developed during the middle Eocene in southwestern, central, and northern Wyoming. During the latest Eocene through middle Miocene, airborne volcaniclastic debris, chiefly derived from the Great Basin, fell over the entire Wyoming foreland. This aggradational cycle continued intermittently throughout much of the Neogene, gradually burying all but the highest parts of the Laramide tectonic landscape. During the last 5 million years, the uplifts have been exhumed, exposing the Laramide skeleton as the present mountainous topography.
The massive Yellowstone Plateau is the product of dominantly rhyolitic volcanic eruptions that occurred during the past 2.1 million years associated with uplift above an inferred mantle hotspot. Recent seismic activity is common near the western border of Wyoming (Intermountain seismic belt); historic seismic events have been comparatively infrequent elsewhere in the state.