Structure, stratigraphy, and tectonics preserved of the northwestern Bays Mountain synclinorium, Greene and Hawkins Counties, northeast Tennessee

by 1977- Bultman, John Garrett

The Bays Mountain synclinorium and surrounding area provide critical facies data to address Sevier-Bountian basin development. This area preserves the Middle Ordovician post-Knox unconformity, the Sevier-Blountian basin depositional sequence, overlying Bays Formation quartz arenite-redbed sequence, the Bays/Martinsburg unconformity, and the base of the overlying Caradocian Martinsburg clastic wedge. Detailed geologic mapping in the Northwestern half of the Sevier-Blountian basin has divided the Sevier Shale into three members: a lower thin-bedded, dark gray-black, graptolite-bearing shale overlain by dark gray-black micritic limestone; a middle siltysandy limestone interbedded with light gray silty shale and calcareous sandstone; and an upper thick-bedded light gray shale grading upward into massive fossil-bearing sandy limestone. Detailed geologic mapping has also delineated previously unrecognized small and large displacement faults in part of the Bays Mountain synclinorium. Total thickness of the Sevier Shale increases from 1006 m (3,300 ft) in the northernmost part of the study area, where entire unit thickness is measurable, to an estimated 1830 to 1981 m (6,000 to 6,500 ft) in the southern part (5,000 ft preserved and 1,500 ft estimated removed). These thickness changes are important not only for basin analysis but also should be considered when constructing geologic cross sections elsewhere in the Valley and Ridge. iv Graptolites collected from the lower Sevier Shale lie in the Nemagraptus gracilis zone. This is slightly older than the Athens Shale. The Sevier Shale and lower portion of the Bays Formation have been given an age estimate of 461-454Ma (Llanvirnian- LLandeilian) based on the age of the conodont and graptolite zones, and the age of the Milbrig bentonite that occurs in the upper Bays Formation. The Bays/Martinsburg contact in this area likely represents an unconformity of ~ 9 m.y. duration and marks the transition from the Sevier-Blountian to Martinsburg basin deposition in the study area. The study area is part of the Saltville-Carter Valley thrust sheet and is overlain by the Pulaski thrust sheet to the southeast. Detailed geologic mapping has delineated previously unrecognized faults in part of the Bays Mountain synclinorium. The Sevier Shale exhibits disharmonic folding between the stronger structural lithic units of the Knox Group and Bays Formation. Evidence for the recognized faults in the Sevier Shale included: noted deformation zones (small faults, dismembered slickensided calcite veins, slickensides, abundant cleavage), stratigraphic discontinuities, and seismic reflection profiles. The stratigraphic discontinuity and deformation zone noted between the lower Jonesboro Limestone and Sevier Shale mark the Pulaski fault along the northwestern edge of the Babbs Knobs flap. A new model for basin dynamics was constructed to better conceptualize basin deposition during the Taconic orogeny. The model depicts the transition from a remnant ocean basin to the Sevier-Blountian phase of the Taconic orogeny due to a tectonic load emplaced onto the Laurentian margin. The prograding overthrust belt, zone of subsidence, basin axes, and flexural foreland bulge migrated west toward the foreland. v Sediments evolved from shallow marine to intertidal environments and prograded over the shelf sequence of the former passive margin. The Sevier-Blountian phase represents the rapid erosion of the highly elevated fold-thrust belt west of the approaching arc. Isostatic uplift (Bays Formation) was initiated in the inner parts of the foreland basin. This was then followed by the “Taconic” (Martinsburg) pulse and basin formation. vi