LATE SILURIAN SUBAERIAL EXPOSURE AND INCIPIENT PEDOGENESIS: EVIDENCE FROM THE BROWNSPORT FORMATION (BOB MEMBER), TENNESSEE

Upper Silurian (Ludlow-Pridoli) marine carbonates outcrop in west-central Tennessee as part of an extensive carbonate ramp that was bound structurally by the Illinois Basin to the north, the Nashville Dome to the east, the edge of the Laurentian continent to the south, and the Reelfoot rift to the west. In the study area near Clifton, Tennessee, these marine carbonate deposits are part of the Upper Silurian Brownsport Formation; its three formal members include the basal Beech River, Bob, and Lobelville Members. The focus of this study is on deposits of the Bob Member near Bob’s Landing (northwest of Clifton), Tennessee, which consist of echinoderm packstone to grainstone, minor shale and clastic mudstone, and argillaceous limestone. These predominantly carbonate deposits also contain laterally continuous 5-15 cm thick reddened intervals/bands and/or strongly gleyed zones within the carbonates that allow the Bob Member to be subdivided locally into 3 distinct units. At the base of the Bob’s Landing section, Unit 1 is ~ 6 m thick and it is characterized by skeletal packstone composed of pelmetazoans, various brachiopods including Rhipidium, and corals. Unit 2 is ~ 3 m thick and it is a highly oxidized limestone interval with a central silty calcareous mudstone-wackestone interval (~ 60 cm thick) characterized by oxidized burrows, intraclasts, and strongly gleyed portions. Unit 3 is ~ 4 m thick and it consists of pelmetazoan packstone to grainstone as well as 5-15 cm thick bands of oxidized limestone. The depositional facies observed at this locality are consistent with previous carbonate ramp depositional interpretations; however, little attention has been given to the gleyed and/or oxidized bands and zones. These gleyed, bioturbated, and oxidized intervals, particularly common in Unit 2, have characteristics similar to intervals that have experienced subaerial exposure and incipient pedogenesis. This potential interruption in normal marine Late Silurian deposition in west-central Tennessee may explain differences between observed lithofacies, isotope excursions, and paleontological characteristics associated with the Lau event elsewhere in Laurentia.