Evidence for high-energy events and shallow-water deposition in the Chattanooga Shale, Devonian, central Tennessee, USA

The Upper Devonian Chattanooga Shale of central Tennessee, a classical black shale, was deposited in an epicontinental setting, west of the Appalachian foredeep. Its finely laminated and highly carbonaceous nature is commonly interpreted to indicate deposition in comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale, indicating oxygenated bottom waters, are commonly ascribed to pycnocline fluctuations. However, laminated fine sand and silt and hummocky cross-stratification (HCS) at the base of some of these beds indicates interaction of storm waves with the seabed, and suggests that greenish-gray shale beds are post-storm mud drapes. Other interesting features are inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies). These observations show that the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (particle alignment). The epicontinental sea setting and the presence of HCS and other storm-produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation. INTRODUCTION-statement of problem The Chattanooga Shale is part of a thin, epicontinental black shale sequence of Upper Devonian age that was deposited over vast areas of the North American craton (de Witt et al., 1993). The depth at which these shales were deposited has been a subject of debate, and both a shallow (e.g. Conant and Swanson, 1961) and deep water origin (e.g. Potter et al., 1982, Ettensohn, 1985) has been proposed. Various estimation methods (Woodrow and Isley, 1983) lead to depth estimates ranging from 39 to 375 meters. Deposition below the dysaerobic-anaerobic boundary of a stratified water column (Byers, 1977) has been a particularly popular idea, leading to suggestions of 100-200 meters water depth (Potter et al., 1982). A water depth in excess of 900 meters was proposed by Lundegard et al. (1980, 1985). In central Tennessee, the Chattanooga Shale overlies an unconformity and reaches a thickness of slightly more than 9 m. It has been subdivided into two members (Fig. 1) and is conformably overlain by the Lower Mississippian Maury Formation. Locally, a third member, the Hardin Sandstone, is present at the base of the Chattanooga Shale. The Hardin Member grades into the overlying Dowelltown Member. The upper portion of the Formation consists of the Gassaway Member. The latter contains in many places subspherical to discoidal phosphate nodules (up to 10 cm long) in the uppermost 50 cm. Conant and Swanson (1961) studied the Chattanooga Shale in central Tennessee and proposed a shallow water origin based on the following observations: 1) the fact that it overlies an unconformity; 2) presence of siltstone/sandstone lenses and scour channels; 3) onlap of black shales towards the Nashville Dome; 4) the area on which the Chattanooga Shale accumulated was of low relief and site of shallow water sedimentation since the Precambrian; 5) the succeeding Mississippian sea, generally considered shallow, was more widespread and presumably deeper than the Chattanooga Sea; 6) the presence of linguloid brachiopods. Conant and Swanson (1961) state with regard to water depth that they "....believe that it was only a few tens of feet, quite likely less than 100 feet" and that "....in some places near the shore, accumulation of mud must have begun when the water was only a few feet deep". However, most of their evidence is circumstantial, and a depth assessment that is more directly based on sedimentary features is desirable. Sedimentary features that strongly suggest a relatively shallow origin for the Chattanooga Shale of central Tennessee were observed during recent field work and are described below.