Depositional history of Devonian to lower Carboniferous (Tournaisian) strata, northern Wyoming and southern Montana, USA

Abstract

The lower Frasnian (Upper Devonian) Maywood Formation records incision of valleys into lower Paleozoic bedrock in fluvial to estuarine settings in northern Wyoming and deposition in estuarine to marine environments in southern Montana (USA). A distinctive fossil assemblage of microconchids, plant compression fossils, fish fossils, and microspores represent fauna and flora that lived in, and adjacent to, salinity-stressed ecological niches in the upper reaches of the Maywood valleys. A similar fossil assemblage is recorded in older Devonian valley-fill deposits of the Lower 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. The fossil charcoal in the Maywood Formation captures a record of fire in adjacent terrestrial ecosystems. The amount of dioxygen (O₂) was thus above the fire window level (16% by volume) and might have been near modern levels in the earliest Late Devonian atmosphere.

The nearshore deposits of the Maywood Formation are overlain by extensive shallow carbonate shelf strata of the Jefferson Formation, likely resulting from a global transgression in the earliest Frasnian. A paired positive and negative δ¹³C_{carbonate [carb]} isotopic excursion in the Jefferson with a range of >6‰ is a signal of the globally recognized “punctata” Event. The unconformably overlying Madison Limestone is lower Carboniferous, except for a thin basal Upper Devonian unit with marine palynomorphs. The Madison regionally records eastward transgression and establishment of widespread marine deposition. It also contains two positive δ¹³C_carb excursions (up to ∼7.5‰) that make up the mid-Tournaisian (= Kinderhookian−Osagean boundary) carbon isotope excursion (TICE/KOBE). 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.

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