143-12 Late Middle Devonian Rise of the Ozark Dome Created Restriction of the Midcontinent Seaway

Abstract:

Upper Devonian to lower Mississippian black shales of the U.S. midcontinent contain elevated levels of critical minerals. While many metalliferous black shales in the sedimentary record are associated with discrete (<1 m.y.) oceanic anoxic events, the Upper Devonian New Albany-Chattanooga interval in the southern midcontinent spans >20 m.y. The source of this prolonged shallow to deep marine anoxia is debated but may primarily be a result of restricted exchange with the open ocean imposed by uplift of the Ozark Dome from far-field forces of the Acadian orogeny.

To test the “Ozark barrier” hypothesis, a transect of sections were studied in western Tennessee. The Hardin Sandstone and overlying Chattanooga Shale were sampled at 30 cm resolution in three sections from the Alabama to Kentucky borders. In the lab, cleaned and trimmed samples were described and analyzed using portable X-ray fluorescence. Results were plotted in cross sections with existing data. Two samples of the Hardin Sandstone were crushed, panned, and heavy liquid separated. Zircon grains were mounted and polished in epoxy pucks and analyzed with LA-ICP-MS at the Arizona Laserchron Center. Geochronologic results were converted to probability-density plots to evaluate sediment provenance.

The new Tennessee data, together with nearby subsurface data sets, demonstrate that uplift of the Ozark Dome during the Middle Devonian extended across the area of the Reelfoot embayment, closing the long-lived deep-water connection between the midcontinent seaway and the open ocean. Chronostratigraphic correlation and isopach maps of the Hardin Sandstone indicate formation of an incised valley following blockage of the embayment. Although reworked Middle Ordovician sandstones of the Ozark Dome dominate much of the midcontinent, detrital zircon analysis of the Hardin sandstone indicates that the valley fill is sourced from the Appalachian margin. These patterns indicate a shallow marine passage persisted between the uplifted Ozark and Nashville domes providing restricted exchange of open ocean surface water with the midcontinent seaway that contributed to the spread of anoxia, chemical stratification, and accumulation of critical minerals.

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