GEOCHEMISTRY OF MIDDLE AND UPPER ORDOVICIAN CARBONATE STRATA AND K-BENTONITES AT TIDWELL HOLLOW, ALABAMA APPALACHIANS: IMPLICATIONS FOR PALEOCEANOGRAPHY AND REGIONAL CORRELATIONS

We present new data for Middle and Upper Ordovician carbonate strata (Chickamauga Group and Sequatchie Formation) near Tidwell Hollow in Blount County, Alabama obtained from 46 stained thin sections, 52 XRF analyses (for Si, Zr, Ti, Al, Fe, P, S, Ca, Mg, Mn, Sr, and K), LA-ICP-MS analyses of 48 apatite phenocrysts from the Deicke and Millbrig K-bentonites, and 58 samples processed for conodonts. Measurable increases in phosphate deposition, and changes in conodont assemblages and in the vertical distribution of calcareous green algae, and Tetradium sp. are associated with the “Black River – Trenton” boundary, newly identified disconformities, and the M4/M5 sequence boundary, each now identified at this section more precisely. The Deicke and Millbrig K-bentonites allow for precise integration of the Tidwell Hollow section into the regional stratigraphic framework.

The most P-rich samples correspond to P₂O₅ contents of 1.3 - 1.5 wt %, but more typical are P₂O₅ contents from 0.3 - 0.6 wt % both below and above the probable M4/M5 boundary. Typical “Black River” lithologies (lime mudstones and wackestones) have measurably lower P₂O₅ contents than the overlying “Trenton” lithologies (bioclastic grainstones and packstones), as well as markedly different faunal abundances and diversities. Although typical “Trenton” lithologies occur in the ~10 m interval of strata immediately above the Millbrig K-bentonite upsection to the probable M4/M5 sequence boundary, there are also coralline framestones, and calcareous green algae fragments in that 10 m interval, which is also where the Phragmodus undatus Zone - Plectodina tenuis Zone boundary occurs at 8.5 m.

So whereas the first appearance of these grainstones, packstones, and framestones is evidence that the “Trenton transgression” occurred earlier in this region, the presence of the coral and calcareous algae in that interval suggests that the transgression was not initially accompanied in this region by widespread cooling of the waters of the Laurentian epicontinental sea. The increase in P deposition may have been forced primarily by rapid subsidence in the later Ordovician along the southeastern margin of Laurentia during the initial (Blountian) stages of the Taconic Orogeny, rather than by changes in paleo-oceanographic circulation patterns and associated upwellings.