On Saturday, Nov. 3rd, students in the Sedimentation and Stratigraphy class loaded up into vans to visit the Sevier Basin in east Tennessee.
Field trip route to the Sevier Basin from Cookeville, TN
Deposits in the Sevier Basin are fascinating because they record the sedimentary response to one of the earliest episodes of mountain building in the eastern US. This mountain-building event is called the Taconic Orogeny and began approximately 472 million years ago. To get an idea of Tennessee paleogeography at this time, check out these amazing maps by Ron Blakey.
The Taconic Orogeny resulted from a series of small islands colliding with the eastern margin of Laurentia. As the islands accreted, they loaded the crust and caused concave-up flexure of the lithosphere. This resulted in a foreland basin – - the Sevier Basin.
The first stop of the day was at the Lighthouse Pointe Marina, where rocks of the Mosheim Member (Lenoir Formation) are exposed on the north side of Douglas Lake. In general, the thick limestone bedding strikes east-west and dips steeply (55-65) to the south. The rocks are dominantly carbonate – - micrites and grainstones with veins of sparry calcite. Dolomite is also common. To the north, an unconformity between the Knox Dolomite and the overlying Lenoir Formation crops out. Ages based on conodonts and graptolites put deposition of the Lenoir Formation beginning around 472 Ma.
Breaking in a new rock hammer on micrite.
Outcrops of the Lenoir Formation on the north shore of Douglas Lake, TN.
From the Marina, students could see the difference between the Lenoir Formation on the north shores of Douglas Lake and the Blockhouse Formation on the southern shores. The second stop was in Crescent Bend, a new subdivision with fantastic (and highly deformed) exposures of the Blockhouse shale. Students ate lunch in the shade of thinly-bedded siltstones and lime mudstones of the Blockhouse. The abrupt lithology change across Douglas Lake is due to the rapid subsidence of the Sevier Basin as it was tectonically loaded.
This diagram, from Ettensohn (1994) and Ettensohn (2008) shows the typical sequence of rocks that fill a foreland basin. In the case of the Sevier Basin: (1) the unconformity at the base would be the top of the Knox Dolomite; (2) the limestone above the unconformity would be the Lenoir Formation; and (3) the shales and siltstones would be the Blockhouse and Sevier Formations.
After lunch, the group drove south and west to Sevierville, with a brief stop at the Bush’s Beans Factory at Chestnut Hills, TN.
Mmmm… geologists love beans!
The final stop of the day was an exercise measuring stratigraphic sections in the Blockhouse Formation on Highway 66. Students fanned out across a 100m outcrop and worked in groups to characterize the interbedded lime muds, siltstones and fine sandstones. Each group measured stratigraphic sections, and the completed column showed a general coarsening-upward trend characterized by increasing silt and sandstone beds. Sedimentary structures included horizontal, silt-filled burrows found by Audrey and clay injection features identified by Bryant.
Measuring section in the Blockhouse Formation of the Sevier Basin, TN.
In addition to checking out foreland basin deposits, students worked their way through a vertical series of changing depositional environments, predicting the overlying lithologies using Walther’s Law. It was a productive and exciting day in the field.
Stay tuned for future sedimentary adventures in the Sevier Basin of east Tennessee.
Ettensohn, F.R., 2008. The Appalachian Foreland Basin in Eastern United States. In: Sedimentary Basins of the World, vol. 5. Elsevier, The Netherlands. P. 106-162.
Shanmugam, G. and Walker, K.R., 1980. Sedimentation, subsidence, and evolution of a foredeep basin in the Middle Ordovician, Southeastern Appalachians. American Journal of Science, v. 280, p. 479-496.