INSIGHTS INTO THE LATE ORDOVICIAN PALEOENVIRONMENT BASED ON STRATIGRAPHIC EVIDENCE RECORDED IN SANDBIAN-KATIAN ROCKS OF THE SOUTHEASTERN MARGIN OF LAURENTIA

Determining the relative control that climate or tectonic forces exert on the environment requires precise stratigraphic correlation of geologically significant events, especially when opposing interpretations of the stratigraphic record exist. However, correlating events over long distances with the resolution required to discern between differing explanations can often be difficult. To correlate and test the interregional synchronicity of important events across the Sandbian-Katian interval, including changes in bulk rock geochemistry, the first appearance of Plectodina tenuis, and the M4/M5 sequence boundary, we established isochronous tie-points for K-bentonites at five spatially distant sections across the southern Appalachians based on LA-ICP-MS geochemical analysis of original apatite phenocrysts.

Results show that beds identified as the Deicke and Millbrig K-bentonites at Red Mountain Expressway, AL, Tidwell Hollow, AL, Big Ridge, AL, Hagan, VA, and ARC Hollow, WV display trends similar to previous analyses of the Deicke and Millbrig, while trends from a bed at ARC Hollow, WV were similar to previous analyses of Dickeyville K-bentonite. In addition, an unidentified K-bentonite from Big Ridge, AL displayed similar trends to an unidentified K-bentonite from Hagan, VA. Bulk rock chemical data from all sections displayed similar trends for elemental Zr and S and P, Si, Al, Ti, Fe, Mg, and K oxides through the Sandbian-Katian interval and show increased concentrations near the base of the M5 sequence.

The stratigraphic position of bulk rock geochemical trends relative to K-bentonite tie-points and the M4/M5 suggests that depositional changes occurred simultaneously across Sandbian-Katian interval. In addition, the stratigraphic position of the first appearance of Plectodina tenuis at the Tidwell Hollow, AL, and Hagan, VA, sections suggests that these events were both synchronous and related in space and time. However, the timing of the M4/M5 at ARC Hollow, WV and geochemical trends near the base of the M5 sequence suggest that changes may have been time transgressive, and that local tectonic forces and increased terrigenous sediment input from tectonic highlands associated with the Taconic Orogeny significantly influenced early Katian environmental change.