The origin of the Upper Dolostone (Upper Knox Group, Middle Ordovician), Black Warrior Basin, subsurface Mississippi, United States of America

The origin of Ordovician Upper Knox Group dolostones from eastern North America has long been the subject of controversy; the major debate has focused on whether these dolostones formed early within the environment of deposition, or late under deep burial conditions at elevated temperature. In this investigation, petrographic and geochemical analyses of Upper Knox dolostones from the Black Warrior Basin, a relatively unstudied area, strongly supports the hypothesis that these finely-crystalline, peritidal carbonates were dolomitized early by seawater or evaporatively-concentrated seawater and were subsequently modified texturally and geochemically by stabilization-driven recrystallization under shallow burial conditions.; Petrographic and biostratigraphic analyses confirm the presence of a major unconformity at the top of Upper Knox dolostones in the Black Warrior Basin. The unconformity is interpreted to represent the well-known Sauk-Tippecanoe mega-sequence boundary which is found over much of North America. This boundary, known in eastern North America as the Knox-Beekmantown unconformity, was previously hypothesized to be absent in the Black Warrior Basin.; Comparison of regional thickness patterns of Middle Ordovician dolostones with known and suspected Neoproterozoic to Early Paleozoic graben structures in the Black Warrior Basin suggests that the accumulation of these dolostones was controlled by regional tectonics. Convergence of proto-North and South America during the Early and Middle Ordovician, based on recently published plate reconstructions, likely lead to the development of a restricted embayment in the Black Warrior Basin, which in turn may have promoted accumulation of peritidal dolostones of the Upper Knox Group.; A record of the strontium isotopic composition of seawater during the Ordovician was constructed using unaltered conodonts from continuous, well-dated sedimentary sections from the eastern United States. The strontium isotope record is confirms the age of Upper Knox strata in the Black Warrior Basin, and helps constrain the origin of diagenetic carbonates in these rocks. Furthermore, this new strontium isotope record can be used for stratigraphic and diagenetic studies of Ordovician strata throughout North America and, perhaps, globally.