PETROGRAPHIC ANALYSIS OF SILURIAN ROCKWOOD STRATA NEAR CHATTANOOGA, TN

Four stratigraphic sections through parts or all of the Silurian Rockwood Formation were sampled to make thin sections. Three recently measured sections at Cameron Hill, Trenton and Wildwood are compared to a section measured at Tiftonia. Petrographic studies of beds from these sections allowed us to update and correct field observations. Many units previously identified in hand specimen as fine-grained sandstones and siltstones with calcareous cement are re interpreted as dolomitic wackestones. In many thin-sections there is a significant component of quartz silt as well as euhedral dolomite crystals. Most localities also contain a few beds of hematitic grainstones in the upper portion of the Rockwood, though they were not measured at Trenton or Wildwood, they were spotted above the measured section.

Beds identified in the field as crystalline limestones are actually wackestones to rudstones. Large fossil fragments and the presence of spar cement were the basis for the field determination, but more detailed petrographic investigation indicated that these, too, were wackestones. Many beds contain disseminated primary FeO₃ spots , though some are obviously diagenetic. Glauconite grains are present but are not a significant component. Many of these beds contain a coarse-grained lag deposit. Most are crinoid, coral (tabulate & solitary rugose), brachiopod and bryozoan fossil fragments. Vugs in Trenton beds are lined with micrite peppered with euhedral dolomite crystals, then filled with blocky calcite spar.

On a larger scale, this study also determined that many units in the field interpreted as shale intervals on the basis of their recessive nature, are actually composed partially or wholly of mudstones and wackestones and most are associated commonly with quartz silt and euhedral dolomites.