MIDDLE MISSISSIPPIAN GREENHOUSE TO ICEHOUSE TRANSITION, ARROW CANYON RANGE, SOUTHEASTERN NEVADA

The Late Paleozoic Ice Age (LPIA) spanned ~100 Myr, during which time ice sheets waxed and waned across southern Gondwana. However, it is increasingly clear that the LPIA consisted of several discreet icehouse and greenhouse climate modes. We document the timing and dynamics of a transition between two climate modes: from the lower Visean greenhouse to an upper Visean-Serpukhovian icehouse. This transition is manifest in the stratigraphy, karsting and associated diagenesis of an equatorial ramp from the Arrow Canyon Range, Nevada.

The Visean Yellowpine and Battleship Wash formations consist of shallow echinoderm-rich subtidal to intertidal, locally fenestral carbonates. An initial exposure horizon developed on these subtidal limestones proximal to the Meramecian-Chesterian (mid-Visean) boundary. The contact between the upper Visean Battleship Wash and Serpukhovian Indian Springs Formation is unconformable. Multiple parallel outcrop sections reveal 3 exposure surfaces developed on subtidal limestones; in many localities, these horizons are amalgamated and associated with significant karsting. The overlying Indian Springs contains repeated siliciclastic influxes, common exposure surfaces developed on subtidal carbonates, and abundant paleosols—including vertisols, argillisols, spodosols, and protosols, suggesting a sub-arid to sub-humid climate. Subtidal exposure horizons continue well into the Bashkirian in the overlying Bird Spring Formation.

We suggest that the exposure and meteoric diagenetic history preserved in the Arrow Canyon succession records the onset of glacio-eustasy close to the Meramecian-Chesterian boundary. Initial glacio-eustasy (first 1-2 Myr) was low-amplitude (<15 m) followed in the uppermost Visean (upper bilineatus to naviculus zone), by higher amplitude sea-level changes that repeatedly exposed the shelf then flooded it to sub-wavebase. Relatively large sea level fluctuations continued throughout Serpukhovian and Bashkirian time. Similar records from the U.K. and U.S. Midcontinent also document the onset of glacio-eustasy during upper Visean time. However, the distribution of climate indicators suggests that during this transition, western equatorial Pangaea remained sub-humid to sub-arid, as the Mid-continent and U.K. grew marginally more humid.