Sedimentology of a fluvial-to-marine transition: The Chilhowee group (Lower Cambrian), southeast Tennessee

The Chilhowee Group (Lower Cambrian-Lower Cambrian (?)) of southeastern Tennessee consists of (in ascending order) the Cochran, Nichols, Nebo, Murray, and Hesse formations, which represent a fluvial-to-marine transitional sequence. Four fades were distinguished within the Chilhowee, and subfacies were defined based upon associations of sedimentary structures, bedding characteristics, stratigraphic position, and related fades. The four lithofacies comprising the Chilhowee fades sequence at Bean Mountain, southeast Tennessee include:

1. Conglomerate/conglomeratic sandstone fades: texturally and compositionally immature massive conglomerate, large-scale cross-stratified conglomeratic sandstone, and thin-bedded, cross-laminated sandstone.

2. Interlaminated mudstone-sandstone facies: alternating mudstone and sandstone laminae which are arranged in an overall planar fashion.

3. Heterolithic facies: interbedded conglomerate, sandstone, and mudstone exhibiting fairweather- and storm-wave produced sedimentary structures.

4. Quartz arenite facies: compositionally supermature quartz arenite lithologies with trough and planar-tabular cross-stratification.

Individual facies represent specific depositional environments in which subenvironments are designated as subfacies. The congloraerate/congloraeratic sandstone facies is interpreted as a braided stream system. Active braided stream subenvironments included longitudinal (massive conglomerate subfacies) and transverse bars (cross-stratified conglomeratic sandstone subfacies). Inactive braidplain sedimentation was dominated by overbank-floodplain and lacustrine subenvironments of the cross-laminated sandstone subfacies and interlaminated mudstone-sandstone facies respectively. The alluvialfluvial system interfaced with a low-energy marine environment (heterolithic facies) in which only a minor shoreface zone developed (Cochran-Nichols quartz arenite beds). Both subsidence and transgression at the craton-margin contributed to the abrupt nature of this fluvial-to-marine transition.

The heterolithic facies represents sedimentation in an offshore, storm-dominated shelf paleoenvironment. Storm-generated sandstone beds, debris-flows, and liquified sediment-gravity flows punctuated quiet-water, fairweather siItstone and mudstone sedimentation. Combined-flow currents comprised of both unidirectional and oscillatory components were responsible for storm-bed emplacement. Storm-currents on the Chilhowee shelf also initiated and maintained a shelf sand-ridge complex (lower quartz arenite subfacies), which was bounded onshore and offshore by fairweather siltstone-mudstone and storm-derived sandstone deposits.

Stable, passive-margin sedimentation commenced with upper quartz arenite subfacies deposition. Quartz arenite lithologies of this subfacies represent sedimentation in a high-energy shoreface paleoenvironment. This shoreface paleoenvironment was affected by interacting ebb-tidal, longshore, and offshore-directed storm currents. The progradation of the upper quartz arenite subfacies sedimentary environment onto the Chilhowee shelf (heterolithic fades) indicates a relative sea-level decrease, following the extensive basal Chilhowee (Cochran-Nichols) transgression.