Paleoflow Patterns and Macroscopic Sedimentary Features In
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PALEOFLOW PATTERNS AND MACROSCOPIC SEDIMENTARY FEATURES IN THE LATE DEVONIAN CHATTANOOGA SHALE OF TENNESSEE: DIFFERENCES BETWEEN THE WESTERN AND EASTERN APPALACHIAN BASIN Jürgen Schieber Department of Geology, University of Texas at Arlington, Arlington, Texas 76019 ABSTRACT Previously, paleocurrent data from the from the search for uranium and hydrocarbons, Chattanooga Shale and its lateral equivalents and many studies were carried out along multiple suggested deposition on a westward dipping lines of inquiry (stratigraphy, petrology, paleoslope (westward paleoflow). However, sedimentology, geochemistry, paleontology etc.). new paleocurrent data (measurements of The efforts of the last 30 years led to a well anisotropy of magnetic susceptibility, AMS) are established stratigraphic framework (Woodrow not in agreement that view. They indicate et al., 1988), and a large amount of data exists on eastward flowing paleocurrents in eastern geochemistry (e.g. Leventhal, 1987), distribution Tennessee, and irregular flow in central of lithologies, and sedimentary features (e.g. Tennessee. Conant and Swanson, 1961; Broadhead et al., Macroscopic sedimentary features also 1982; Lundegard et al., 1985; Ettensohn et al., point to significant differences in sedimentary 1988). In fact, the published record leaves one setting between eastern and central Tennessee. with the impression that this is one of the most In eastern Tennessee, the Chattanooga Shale thoroughly investigated shale sequences, and that intertongues with turbidites of the Brallier major questions concerning its origin have been Formation, lacks shallow water features, and satisfactorily answered. may have been deposited in depths between 100- However, a variety of issues, such as for 200m. In central Tennessee, HCS siltstones and example water depth and environment of sandstones, shale-on-shale erosion surfaces, lag deposition, remain a subject of debate. Both a deposits (bone beds), and ripples, suggest shallow (e.g. Conant and Swanson, 1961) and relatively shallow water (tens of metres) and deep-water origin (e.g. Potter et al., 1982, reworking of the sea bed by waves and strong Ettensohn, 1985) has been proposed, and a currents. review of the issue by Woodrow and Isley A shallow water platform in central (1983) showed that various methods led to depth Tennessee, and deeper water conditions in estimates ranging from 39 to 375 metres. The eastern Tennessee require an eastward dipping popular idea that these shales may owe their sea floor (paleoslope) between these two areas. origin to pycnocline development (Byers, 1977), Further east however, where the Chattanooga led to suggestions of water depth of at least 100- Shale interfingers with the Brallier Formation, 200 metres (Potter et al., 1982). Lundegard et al. the paleoslope was inclined in the opposite (1985) even arrived at a maximum estimate in direction (westward flowing paleocurrents). excess of 900 metres. Large water depth and These two opposing slopes formed an elongate stratification of the water column provide for trough along the eastern margin of the stagnant bottom waters with little or only weak Appalachian Basin. current activity, commonly thought to be the The portions of the Chattanooga Shale conditions under which finely laminated black whose AMS data suggest eastward paleoflow, shales (such as the Chattanooga Shale) are most occur between the shallow water Chattanooga of likely to accumulate (Potter et al., 1980). platform origin and the area where it interfingers Conant and Swanson (1961) studied the with the Brallier Formation. Thus, the eastward Chattanooga Shale in central Tennessee and dipping paleoslope suggested by sedimentary found that (1) it rests on a basal unconformity; 2) features is in good agreement with AMS it contains silt and sandstone lenses and scour paleocurrent data. channels; 3) it onlaps towards the Nashville Dome; 4) the region had low relief and shallow INTRODUCTION water sedimentation since the Precambrian; 5) Research on Upper Devonian black the Mississippian sea was shallow and more shales in the Appalachian Basin (e.g. the extensive than the Devonian one; 6) it contains Chattanooga Shale) received substantial impetus linguloid brachiopods. Although these observations suggested shallow water deposition island in the Chattanooga Sea SW of Nashville to Conant and Swanson (1961), most of the (Conant and Swanson, 1961). evidence is circumstantial in nature. In eastern Tennessee the Chattanooga This paper was written to report new Shale reaches a thickness of approximately 650 observations that have a bearing on the origin of m (Hasson, 1982), and is subdivided into the the Chattanooga Shale. Aside of paleocurrent basal Millboro Member, the middle Brallier measurements via AMS (anisotropy of magnetic Member, and the upper Big Stone Gap Member susceptibility), other observations that are (Fig. 1). The Chattanooga Shale unconformably reported were exclusively made in outcrop. overlies the Lower Devonian Wildcat Valley They suggest that contrary to commonly held Sandstone, and is conformably overlain by the opinions, large portions of the Chattanooga Mississippian Grainger Formation. Shale were deposited in comparatively shallow water, and OBSERVATIONS that strong currents at times eroded and Macroscopic Sedimentary Features, Western winnowed the sea floor. Portion of Basin Hummocky cross-stratification (HCS), GEOLOGIC SETTING soft sediment deformation, erosion surfaces, The Chattanooga Shale and its lateral bone beds, and ripples can readily be observed in equivalents form the distal part of a westward outcrop and provide information about water thinning clastic wedge that accumulated in a depth and energy conditions. In addition, a foreland basin to the west of the Acadian wealth of small scale sedimentary features can be Mountains. It is known as Chattanooga Shale observed in sawed slabs and thin sections primarily in Alabama, Tennessee, and south- (Schieber, this volume). central Kentucky, towards the northeast as Ohio Shale, and towards the northwest (Illinois Basin) HCS Beds as New Albany Shale (de Witt, 1981). Conant and Swanson (1961) described a Overall sediment transport was towards local marker bed ("varved bed") from outcrops in the west (Lundegard et al., 1985). Going east to DeKalb County (Fig. 1), Tennessee. It is thin (1- west, major environments of deposition include 10cm) and fine grained (silt to fine sand), has an alluvial plain, delta plain, shelf, slope with erosional lower contact, and is overlain by turbidites, and basin floor with black-shale greenish-gray shale (Fig. 2). Thickness accumulation (Woodrow et al., 1988). Estimated variations are regular (pinch and swell) and the positions of the Chattanooga Sea range from surface of the bed appears hummocky. Internal equatorial (e.g. Ettensohn and Barron, 1981) to laminae conform to the basal erosion surface as 30 degrees south latitude (Witzke and Heckel, well as to gently undulose internal erosion 1988). Climatic indicators suggest seasonal surfaces, and may show systematic thickening rainfall, tropical temperatures, and into hummocks and thinning into swales. predominantly perennial streams (Woodrow et These characteristics fit the definition al., 1988). for hummocky cross-stratification as proposed In central Tennessee the Chattanooga by Harms et al. (1975). Thinner, but otherwise Shale is of Late Devonian (Frasnian-Famennian) comparable beds of laminated silt and sand were age, and is a nearly flat-lying Formation that observed elsewhere in the Chattanooga Shale reaches a thickness of slightly above 9m (Fig. 1). where it contains interbeds of greenish-gray It has been subdivided into the Hardin (base), shale. 200 km to the SW in Hardin County (Fig. Dowelltown, and Gassaway (top) members (Fig. 1), Tennessee, HCS beds (medium sand) are 1), and is conformably overlain by the Lower common in the Dowelltown Member, and Mississippian Maury Formation (Conant and amalgamation of HCS beds has been observed as Swanson, 1961). The Hardin Sandstone Member well. is only locally present at the base of the Chattanooga Shale, and grades into the overlying Soft Sediment Deformation Dowelltown Member. Uniform thickness and low rates of lateral thickness change suggest that, Ball-and-pillow structures were aside of a few exceptions, the Chattanooga Shale observed in association with a HCS sandstone was deposited on a very smooth and even bed (10-20 cm thick) in the Dowelltown peneplain. The most notable of these exceptions Member. This bed has in places completely is known as the Hohenwald Platform (Fig. 1), an Correlation of erosion surfaces between closely spaced outcrops has so far Figure 1: Location map. Shows localities mentioned in text and AMS paleocurrent directions in central and eastern Tennessee. Arrows represent vector means, numbers besides arrows indicate number of measurements per location. The two rose diagrams combine measurements for central and eastern Tennessee (N=number of measurements; note that rose diagrams have different scale). Data indicate more or less uniform eastward flow in eastern Tennessee. In central Tennessee the pattern is random to possibly circular (only data from Gassaway Member are shown). Overview stratigraphic columns are for the type locality in central Tennessee (left) as described by Conant and Swanson (1961), and for eastern Tennessee after Hasson (1982, his section 1). Note that the scale of the two stratigraphic columns differs by two orders of magnitude. The hatched area