GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 44-5
Presentation Time: 9:05 AM


JOHNSON, Elizabeth A.1, SMITH, M. Elliot2, UMHOEFER, Paul J.2 and GIANNINY, Gary L.3, (1)School of Earth Sciences and Environmental Sustainability, Northern Arizona University, 625 S Knoles Dr, Flagstaff, AZ 86011, (2)School of Earth and Sustainability, Northern Arizona University, 625 Knoles Drive, Box 4099, Flagstaff, AZ 86011, (3)Department of Geosciences, Fort Lewis College, 1000 Rim Drive, Durango, CO 81301

Pennsylvanian marine strata of the Paradox basin were deposited coincident with high frequency glacio-eustatic sea level changes. These cyclic, predominantly carbonate strata were an early example of the interpretation of high frequency sea level changes from stratal stacking patterns. Later works cite bioherm-generated lateral thickness variation to question coherency of the cyclostratigraphy in the Paradox basin, limiting its worth as a sea level indicator. To quantify the impact of bioherms on stratal geometries and cycle thickness on cyclostratigraphic lucidity, we exploit 3 dimensional outcrop using photogrammetry and total station surveys to visualize the lower Paradox Formation within the Goosenecks of the San Juan River, southeast Utah. These data are projected on an age framework provided by U-Pb ages for volcanic ashes in Moscovian strata of Ukraine and the Ural Mountains using conodont and fusilinid biostratigraphy.

Paradox Formation bioherms range in size from 3 to 18 meters tall and 22 to 192 meters long. Average bioherm height is greatest in thickest cycles, however, the differential height relative to the containing bed is greatest within thin cycles. The predominant mound building biota (Chataetes and stromatolites) are laterally consistent within particular sequences, with some intervals dominated by Chaetetes and others by stromatolites. Bioherms containing Chaetetes are thicker than those without and stromatolitic bioherms are often longer with a lower amplitude. Bioherm occurrence appears to be distributed across the paleo-seafloor. Bioherm-generated paleobathymetry impacts overlying cycles via: 1) paleobathymetric highs formed by bioherms may serve as a nucleation site for later bioherms; and 2) bioherms from underlying sequences were causing progradational geometries in overlying units.

Sequences in the lower Paradox Formation cycles thicken up section, corresponding to an increase in apparent depositional duration. Thinner Barker Creek interval sequences reflect apparent durations between 100 – 350 ky, while thicker, overlying Akah intervals reflect longer durations from 200 ky to 1 My. Absent tuning, U-Pb and biostratigraphy suggest that Pennsylvanian sea level change is coherent across multiple locations across Laurasia, though local signals are also present.