U-PB ZIRCON AND BIOSTRATIGRAPHIC AGE CONSTRAINTS FOR THE TRACE CREEK SHALE MEMBER OF THE ATOKA FORMATION: A STEP TOWARDS CALIBRATING CARBONIFEROUS CYCLICITY IN NORTH AMERICA

The Carboniferous Period was the apex of Earth’s penultimate icehouse, the late Paleozoic ice age (LPIA). Euramerican stratigraphic successions from the Carboniferous preserve large-magnitude (10’s of m) short duration (10⁴-10⁶ yr) base-level changes, which manifest as the repeat stacking of open marine and non-or shallow marine strata over a few to 10’s of meters of stratigraphy. These deposits, referred to as cyclothems in the Midcontinent region of the United States, are interpreted to form in response to dynamic high-latitude ice. Absolute age constraints on the timing and duration of cyclothem deposition in North America are limited, with relative ages defined by biostratigraphic zones, which have been calibrated by U-Pb zircon chemical abrasion-isotope dilution-thermal ionizing mass spectrometry (CA-ID-TIMS), in the Donets Basin of Ukraine and the Ural Mountain region of Russia. Here we present for the first time, new high-resolution U-Pb zircon CA-ID-TIMS age control for a volcanic ash deposit, which was identified in a newly recovered core from the Boston Mountains of Arkansas. The volcaniclastic unit is hosted within a thick (~20 m) regionally correlative black shale interval deposited above a flooding surface, referred to as the Trace Creek Shale. The new U-Pb age and conodont biostratigraphic control indicate a latest Bashkirian age for the Trace Creek Shale which is found in the lower Atoka Formation. Importantly, we observe multiple high frequency base-level changes throughout the 138-m-thick cored interval across the Atokan-Morrowan series, which manifest as the juxtaposition of near shore or intertidal facies with open marine mudstones. Base-level changes across the cored interval are accompanied by changes in trace element chemistry, reflecting alternating marine and brackish water conditions, and coupled with the stratigraphic observations, suggest a glacioeustatic influence for the lower Atoka Formation. The presented stratigraphic history for the Arkoma basin will serve as a tie point between the North American midcontinent base level record and other far-field LPIA eustatic records.