DEPOSITIONAL HISTORY OF DEVONIAN TO LOWER MISSISSIPPIAN STRATA IN NORTHERN WYOMING AND SOUTHERN MONTANA, USA

Devonian to Lower Mississippian strata record a major transition in Earth history that includes a shift from greenhouse to icehouse conditions and the rise of vascular plants. The lower Frasnian (Upper Devonian) Maywood Formation records incision of valleys into Paleozoic bedrock in fluvial to estuarine settings in northern Wyoming, and deposition in estuarine to marine environments in southern Montana. A distinctive fossil assemblage of microconchids, fish, plant compression, and microspores represent fauna and flora that lived in, and adjacent to, salinity-stressed ecological niches in the upper reaches of the Maywood valley. A similar fossil assemblage is recorded in older Devonian valley-fill deposits of the Early to Middle Devonian Beartooth Butte Formation, indicating that valley incision and subsequent transgression occurred repeatedly over a span of nearly 30 million years, with organisms tracking the marine incursions into the valleys. Fossil charcoal in the Maywood Formation records fire in adjacent terrestrial ecosystems and the presence of near modern levels of O₂ in the earliest Late Devonian atmosphere. The Maywood is overlain by extensive shallow shelf carbonate of the Jefferson Formation indicating a major transgression in the earliest Frasnian. Paired positive and negative δ¹³C isotopic excursions in the Jefferson with a range of >6‰ are a signal of the globally recognized ‘punctata’ Event. The unconformably overlying Mississippian Madison Limestone contains two positive δ¹³C excursions (up to ~ 7.5‰) that make up the mid-Tournaisian carbon isotope excursion (TICE), which records carbon cycle perturbations and the onset of Late Paleozoic icehouse climate. These isotope data provide a framework for regional and global correlation of northern Rocky Mountain strata and an archive of environmental and evolutionary change during the middle–late Paleozoic transition.