PALEOFLOW PATTERNS AND MACROSCOPIC SEDIMENTARY FEATURES IN THELATE 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

ABSTRACTPreviously, paleocurrent data from the

Chattanooga Shale and its lateral equivalentssuggested deposition on a westward dippingpaleoslope (westward paleoflow). However,new paleocurrent data (measurements ofanisotropy of magnetic susceptibility, AMS) arenot in agreement that view. They indicateeastward flowing paleocurrents in easternTennessee, and irregular flow in centralTennessee.

Macroscopic sedimentary features alsopoint to significant differences in sedimentarysetting between eastern and central Tennessee.In eastern Tennessee, the Chattanooga Shaleintertongues with turbidites of the BrallierFormation, lacks shallow water features, andmay have been deposited in depths between 100-200m. In central Tennessee, HCS siltstones andsandstones, shale-on-shale erosion surfaces, lagdeposits (bone beds), and ripples, suggestrelatively shallow water (tens of metres) andreworking of the sea bed by waves and strongcurrents.

A shallow water platform in centralTennessee, and deeper water conditions ineastern Tennessee require an eastward dippingsea floor (paleoslope) between these two areas.Further east however, where the ChattanoogaShale interfingers with the Brallier Formation,the paleoslope was inclined in the oppositedirection (westward flowing paleocurrents).These two opposing slopes formed an elongatetrough along the eastern margin of theAppalachian Basin.

The portions of the Chattanooga Shalewhose AMS data suggest eastward paleoflow,occur between the shallow water Chattanooga ofplatform origin and the area where it interfingerswith the Brallier Formation. Thus, the eastwarddipping paleoslope suggested by sedimentaryfeatures is in good agreement with AMSpaleocurrent data.

INTRODUCTIONResearch on Upper Devonian black

shales in the Appalachian Basin (e.g. theChattanooga Shale) received substantial impetus

from the search for uranium and hydrocarbons,and many studies were carried out along multiplelines of inquiry (stratigraphy, petrology,sedimentology, geochemistry, paleontology etc.).The efforts of the last 30 years led to a wellestablished stratigraphic framework (Woodrowet al., 1988), and a large amount of data exists ongeochemistry (e.g. Leventhal, 1987), distributionof lithologies, and sedimentary features (e.g.Conant and Swanson, 1961; Broadhead et al.,1982; Lundegard et al., 1985; Ettensohn et al.,1988). In fact, the published record leaves onewith the impression that this is one of the mostthoroughly investigated shale sequences, and thatmajor questions concerning its origin have beensatisfactorily answered.

However, a variety of issues, such as forexample water depth and environment ofdeposition, remain a subject of debate. Both ashallow (e.g. Conant and Swanson, 1961) anddeep-water origin (e.g. Potter et al., 1982,Ettensohn, 1985) has been proposed, and areview of the issue by Woodrow and Isley(1983) showed that various methods led to depthestimates ranging from 39 to 375 metres. Thepopular idea that these shales may owe theirorigin to pycnocline development (Byers, 1977),led to suggestions of water depth of at least 100-200 metres (Potter et al., 1982). Lundegard et al.(1985) even arrived at a maximum estimate inexcess of 900 metres. Large water depth andstratification of the water column provide forstagnant bottom waters with little or only weakcurrent activity, commonly thought to be theconditions under which finely laminated blackshales (such as the Chattanooga Shale) are mostlikely to accumulate (Potter et al., 1980).

Conant and Swanson (1961) studied theChattanooga Shale in central Tennessee andfound that (1) it rests on a basal unconformity; 2)it contains silt and sandstone lenses and scourchannels; 3) it onlaps towards the NashvilleDome; 4) the region had low relief and shallowwater sedimentation since the Precambrian; 5)the Mississippian sea was shallow and moreextensive than the Devonian one; 6) it containslinguloid brachiopods. Although these

observations suggested shallow water depositionto Conant and Swanson (1961), most of theevidence is circumstantial in nature.

This paper was written to report newobservations that have a bearing on the origin ofthe Chattanooga Shale. Aside of paleocurrentmeasurements via AMS (anisotropy of magneticsusceptibility), other observations that arereported were exclusively made in outcrop.They suggest that contrary to commonly heldopinions, large portions of the ChattanoogaShale were deposited in comparatively shallowwater, and

that strong currents at times eroded andwinnowed the sea floor.

GEOLOGIC SETTINGThe Chattanooga Shale and its lateral

equivalents form the distal part of a westwardthinning clastic wedge that accumulated in aforeland basin to the west of the AcadianMountains. It is known as Chattanooga Shaleprimarily in Alabama, Tennessee, and south-central Kentucky, towards the northeast as OhioShale, and towards the northwest (Illinois Basin)as New Albany Shale (de Witt, 1981).

Overall sediment transport was towardsthe west (Lundegard et al., 1985). Going east towest, major environments of deposition includealluvial plain, delta plain, shelf, slope withturbidites, and basin floor with black-shaleaccumulation (Woodrow et al., 1988). Estimatedpositions of the Chattanooga Sea range fromequatorial (e.g. Ettensohn and Barron, 1981) to30 degrees south latitude (Witzke and Heckel,1988). Climatic indicators suggest seasonalrainfall, tropical temperatures, andpredominantly perennial streams (Woodrow etal., 1988).

In central Tennessee the ChattanoogaShale is of Late Devonian (Frasnian-Famennian)age, and is a nearly flat-lying Formation thatreaches a thickness of slightly above 9m (Fig. 1).It has been subdivided into the Hardin (base),Dowelltown, and Gassaway (top) members (Fig.1), and is conformably overlain by the LowerMississippian Maury Formation (Conant andSwanson, 1961). The Hardin Sandstone Memberis only locally present at the base of theChattanooga Shale, and grades into the overlyingDowelltown Member. Uniform thickness andlow rates of lateral thickness change suggest that,aside of a few exceptions, the Chattanooga Shalewas deposited on a very smooth and evenpeneplain. The most notable of these exceptionsis known as the Hohenwald Platform (Fig. 1), an

island in the Chattanooga Sea SW of Nashville(Conant and Swanson, 1961).

In eastern Tennessee the ChattanoogaShale reaches a thickness of approximately 650m (Hasson, 1982), and is subdivided into thebasal Millboro Member, the middle BrallierMember, and the upper Big Stone Gap Member(Fig. 1). The Chattanooga Shale unconformablyoverlies the Lower Devonian Wildcat ValleySandstone, and is conformably overlain by theMississippian Grainger Formation.

OBSERVATIONSMacroscopic Sedimentary Features, WesternPortion of Basin

Hummocky cross-stratification (HCS),soft sediment deformation, erosion surfaces,bone beds, and ripples can readily be observed inoutcrop and provide information about waterdepth and energy conditions. In addition, awealth of small scale sedimentary features can beobserved in sawed slabs and thin sections(Schieber, this volume).

HCS BedsConant and Swanson (1961) described a

local marker bed ("varved bed") from outcrops inDeKalb County (Fig. 1), Tennessee. It is thin (1-10cm) and fine grained (silt to fine sand), has anerosional lower contact, and is overlain bygreenish-gray shale (Fig. 2). Thicknessvariations are regular (pinch and swell) and thesurface of the bed appears hummocky. Internallaminae conform to the basal erosion surface aswell as to gently undulose internal erosionsurfaces, and may show systematic thickeninginto hummocks and thinning into swales.

These characteristics fit the definitionfor hummocky cross-stratification as proposedby Harms et al. (1975). Thinner, but otherwisecomparable beds of laminated silt and sand wereobserved elsewhere in the Chattanooga Shalewhere it contains interbeds of greenish-grayshale. 200 km to the SW in Hardin County (Fig.1), Tennessee, HCS beds (medium sand) arecommon in the Dowelltown Member, andamalgamation of HCS beds has been observed aswell.

Soft Sediment Deformation

Ball-and-pillow structures wereobserved in association with a HCS sandstonebed (10-20 cm thick) in the DowelltownMember. This bed has in places completely

dissintegrated, forming isolated balls(10-20cm diameter; Fig. 3) in a shale matrix.

"Pockets" of chaotic bedded shale occurin the same outcrop. Shale beds in these"pockets" (several metres wide, up to 1 m thick)show convolution accompanied by disruption ofbeds. A matrix of homogenized black shalesurrounds randomly oriented bed fragments.There are no sharp boundaries between pocketsand surrounding, even bedded shales.

Large scale deformation of shale bedsoccurs below major submarine erosion surfaces(Fig. 4). It is probably best described asconvolute bedding.

Erosion SurfacesErosion surfaces in the Chattanooga

Shale are undulose to almost flat and horizontal,truncate underlying beds of black shale typicallyat angles of less than 10 degrees, and showsurface relief amounting to as much as onemetre. They are overlain by conformable layersof black shale, which themselves may betruncated by later erosion surfaces (Fig. 5).

Correlation of erosion surfaces between closelyspaced outcrops has so far

been unsuccessful, possibly suggestingthat they are phenomena of only local extent.

An exception may be the erosion ofDowelltown shales at the Dowelltown/Gassawaycontact in areas NE of Nashville (Fig. 6). Thiscontact is knife sharp all over central Tennessee(Conant and Swanson, 1961), suggesting thatpre-Gassaway erosion was widespread eventhough it is not always as obvious as in Fig. 6.

Bone BedsConkin and Conkin (1980) reported

four bone beds (phosphatic debris, glauconite)from the Chattanooga Shale, one each at baseand top of the formation, one within theDowelltown, and one at theDowelltown/Gassaway contact. Locally theycan be as thick as 30 cm, but typically they areonly a few cm thick and lenticular. In manyplaces they are only represented by a thin (a fewmm) layer of fine sand or silt.

RipplesThin silt lenses (up to 5mm thick, up to

50mm long) are the most common ripple type,

Figure 1: Location map. Shows localities mentioned in text and AMS paleocurrent directions in central andeastern Tennessee. Arrows represent vector means, numbers besides arrows indicate number of measurements perlocation. The two rose diagrams combine measurements for central and eastern Tennessee (N=number ofmeasurements; note that rose diagrams have different scale). Data indicate more or less uniform eastward flow ineastern Tennessee. In central Tennessee the pattern is random to possibly circular (only data from GassawayMember are shown). Overview stratigraphic columns are for the type locality in central Tennessee (left) asdescribed by Conant and Swanson (1961), and for eastern Tennessee after Hasson (1982, his section 1). Note thatthe scale of the two stratigraphic columns differs by two orders of magnitude. The hatched area marked with HPshows location and outline of the Hohenwald Platform (Conant and Swanson, 1961), an area of non-depositionduring the Late Devonian. County outlines have tick marks on the inside.

and occur single or as ripple trains onbedding planes. Silt grains are of the same sizeas present in the associated shales. Low anglecross-lamination has been observed.

Other ripples consist of a mixture (grainsize range 0.1-1.5 mm) of quartz grains, fishbones, glauconite granules, and dolomite, and areup to 10mm thick and up to 80mm long. Theyare typically found in the basal 30 cm of theChattanooga Shale, may be cross-laminated, andmay occur as single ripples or discontinuouswavy sandstone beds.

Ripples of the same composition, but ofsomewhat finer grain size, are common in theHardin/Dowelltown transition in Hardin County,

Tennessee. Depending on the sand/mudratio in the section, flaser bedding, wavy

bedding, and lenticular bedding occurs.These ripples show cross-lamination, chevronstructures, bundle-wise upbuilding, andintricately interwoven cross-lamination.

Macroscopic Sedimentary Features,Eastern Portion of Basin

In eastern Tennessee, the Chattanoogashale thickens and has a siltstone-rich middleinterval that is equivalent to the BrallierFormation farther north (Dennison, 1985).These siltstones were described by Lundegard et

Figure 3: Ball and Pillow structures related to HCSsandstone bed in the Dowelltown Member. Thesandstone bed (arrow S) has broken up into pillowsin the centre of the photograph (arrows P). Photoshows approximately four metres of exposure.

Figure 2: Varved bed (middle unit of GassawayMember) with hummocky appearance. Hummockybed highlighted inline drawing below. One of theinternal truncationsurfaces is clearly visible(dashedline in line drawing). Hummock is just below thehammer head, swale is located at arrow S. HCS bedis overlain by greenish-gray shale and overliesblack shale of the lower unit of the GassawayMember Hammer is 31 5 cm long

Figure 4: Contorted Bedding below theDowelltown/Gassaway contact. Shale beds of theDowelltown have been folded over to the left(arrow). Photo shows approximately two metres ofexposure.

al. (1985), and their observations aresummarized here.

Siltstones are typically thin bedded (1-30cm) and consist of even, persistent beds with

base truncated Bouma sequences(Tbcde, Tcde). Top portions of beds may showripple cross-lamination; sole marks are commonat the base of beds.

A small proportion of siltstone bedsoccurs as thickly bedded (2-150cm) bundles ofsharply defined beds with erosional bases (top-truncated Bouma sequences; Ta, Tab, Tabc).Beds may contain shale clasts and woodfragments, and may show amalgamation.

Thin siltstone beds (1-10cm) that arefound in shale-dominated portions of thesequence and in the black shales below andabove the Brallier Formation typically showbase-truncated Bouma sequences (Tcde, Tde).

New Paleocurrent Data from AMSMeasurementsMeasurement of anisotropy of magneticsusceptibility (AMS) has been an extensivelyapplied method in fabric studies of sedimentsand sedimentary rocks (e.g. Hrouda, 1982).AMS is commonly expressed as an ellipsoid,which in general terms reflects the orientation ofmagnetic elements in the sample. In sedimentsamples, the AMS ellipsoid reflects thepetrofabric orientation (Taira and Lienert, 1979),which results from any process orienting grainsin the sediment (current flow, compaction,diagenesis). In sand- and silt-sized terrigenousclastics the long axis of the AMS ellipsoidgenerally coincides with flowdirection (e.g. Tairaand Lienert, 1979), and the ellipsoid is inclinedin upcurrent direction.

In the past, extraction of paleocurrentinformation from shale sequences has beenfraught with difficulties. A variety ofapproaches have been applied, e.g. (1)measurement of flow indicators in interbeddedsandstones and carbonates, (2) alignment offossils and detrital quartz grains, and (3) lateraldistribution of detrital components. Drawbacksare that paleoflow indicators from interbeddedlithologies may not be representative of

Figure 6: Erosion surface with appearance of angularunconformity at base of Gassaway Member. The tilting of thebeds below the contact is, however, not due to tectonic forces, butrather to soft sediment deformation and convolution of beds in theDowelltown Member (contorted bedding shown in Fig. 6 wasobserved in the same outcrop). Sequence of events appears tohave been: 1) deposition of Dowelltown beds; 2) disturbance anddestabilization of beds e.g. by storm waves; 3) a prolonged periodof erosion, winnowing, and levelling; and 4) deposition ofGassaway beds. Photo shows approximately two metres ofexposure.

Figure 5: Pseudo-crossbedding in Gassaway Member,produced by repeated erosion alternating with shaledeposition. Line drawing highlights erosion surfaces(heavy lines) and bedding planes (thin lines). The"cross-bedded" interval highlighted in the line drawing isapproximately one metre thick.

Figure 7: Stratigraphic variability of AMSpaleocurrent data in the Hurricane Bridge section of theChattanooga Shale, DeKalb County (Kepferle and Roen,1981). Arrows indicate paleoflow direction relative to north.Center of arrow approximately at sample elevation abovebase. Lithologic symbols are the same as in Figure 1. Onlythe more consistent data for the Gassaway member are shownin Fig. 1.

paleoflow in the shale itself, and thatother methods either require fortuitouscircumstances (e.g. oriented fossils) or are verytime consuming because of extensive field work(detailed mapping) or laboratory work(microscopy).

Recent work by Schieber and Ellwood(1988, 1993) has shown that the same principlesthat are applied to deduce flow directions ofsandstones and siltstones, can also be applied toshales. In contrast to other methods, the AMSpaleoflow method is applicable to all kinds ofshales, and will probably become a widelyapplied tool in paleocurrent studies of shalebasins.

For application of the AMS method,oriented cores (24mm diameter) are drilled in

outcrop or from oriented hand specimens. TheAMS is then measured using a low-field torsionfiber magnetometer and an AMS ellipsoid iscalculated for each core. Vector means of thelong axis orientations of the ellipsoids are thencalculated for each outcrop.

In a study of magnetic fabrics of theChattanooga Shale, approximately 200 AMSmeasurements were made on oriented samplesfrom Tennessee. These samples were collectedalong and in the vicinity of Clinch Mountain(northeast of Knoxville, Fig. 1), and in outcropsin central Tennessee. This study is stillcontinuing, and at present the data coverage forcentral Tennessee is less dense as one woulddesire. However, preliminary data from centralTennessee are reported here because theycontrast so strongly with those from easternTennessee.

The samples from Clinch Mountainwere collected from the basal Millboro Member.AMS data are fairly consistent and suggestsediment transport in a easterly to northeasterlydirection (Fig. 1). In contrast, samples fromcentral Tennessee (only data from GassawayMember are presented in Fig. 1) are much morevariable. Although more data are still needed todefine clear paleocurrent patterns in centralTennessee, the present data suggest a random orpossibly rotary flow pattern (Fig. 1), and contraststrongly with the unidirectional pattern observedin eastern Tennessee.

In order to examine changes ofpaleoflow directions with time, a comparativelylarge number of samples was collected over theentire thickness of the Chattanooga Shale in onestratigraphic section in central Tennessee (Fig.7). The data suggest very irregular flowdirections for the Dowelltown Member, andfairly consistent paleoflow directions for theGassaway Member. For that reason onlyGassaway data are shown for the centralTennessee portion of Fig. 1.

DISCUSSION AND INTERPRETATIONAMS Paleocurrent Data

These new data contrast with earlierpaleoflow studies of the Appalachian Basin,where it was concluded that paleoflow wasuniformly to the west (Potter et al., 1982;Lundegard et al., 1985). However, the latterassumption was solely based on data from theeastern portion of the basin (flute-castmeasurements in Brallier Formation), wheredown-slope gravity-driven flow (westward) andstorm-induced coastal downwelling were

dominant (Potter et al., 1982; Lundegard et al.,1985).

AMS data from Clinch Mountain(Tennessee) overlap geographically with data byLundegard et al. (1985), but suggest flow in aneasterly rather than a westerly direction. Thereare two possible reasons for this discrepancy. (1)Inclinations of AMS ellipsoids from theChattanooga Shale are not indicative of flowdirection (AMS measurements only allowdetermination of flow azimuth). (2) The AMSdata indicate the correct flow direction, but thediscrepancy may have to do with the fact thatAMS data are from the Millboro Member,whereas flute-cast data are from the overlyingBrallier Member (Fig. 1). Possibility (1) seemsunlikely from prior applications of the AMSmethod to shales (Schieber and Ellwood, 1988,1993), and possibility (2) is discussed in thefollowing paragraphs.

Paleoflow in a sedimentary basin isstrongly influenced by basin configuration(location of depocenters, distribution of shallowand deep water environments etc.), and flowpatterns can be altered due to changes in basintectonics, sediment supply, and sea level. In arecent review of the sedimentary dynamics of theAppalachian basin, Ettensohn et al. (1988)discuss the possible interrelationships betweensedimentation patterns, tectonism, and sea-levelchanges at length, and envision two basic basinconfigurations (Fig. 8). A smooth, westwarddipping paleoslope, merging gradually into thebasin floor to the west (Fig. 8A), would beestablished during times of abundant sedimentinput and rapid regression (sedimentationexceeds subsidence). In contrast, during times ofsmall sediment supply and rapid transgression(subsidence exceeds sedimentation), a peripheraltrough is supposed to have formed that acted as asediment trap and prevented coarser sedimentsfrom reaching the western portion of the basin(Fig. 8B). According to Ettensohn et al. (1988),these two configurations are associated withdeposition of "regressive" (more clasticcomponents, less organic matter) as opposed to"transgressive" (organic-rich) black shalesrespectively.

Because the Millboro Shale (sampledfor AMS in eastern Tennessee) was probably a"transgressive" black shale during deposition ofwhich a peripheral trough may indeed haveexisted (Ettensohn et al., 1988), AMS data mayactually be correct and indicate deposition ofthese shales on the western slope of such atrough (Fig. 8B). The Brallier Member in

contrast marks a regressive phase in basinhistory, and the observed westward sedimenttransport (flute-marks) probably indicates fillingof the peripheral trough and establishment of asmooth westward dipping paleoslope (Fig. 8A).

Working from fossil orientations, Jonesand Dennison (1970) published westwardpaleoflow in the Chattanooga Shale of ClinchMountain from the same outcrops that providedsamples for AMS data presented here. Althoughtheir results are in conflict with AMS data, if oneexamines their work carefully one finds thatfossil alignment only provided flow azimuth.Paleoflow direction was actually inferred fromUpper Devonian isopachs that were supposed toreflect paleoslope. As Fig. 8B shows, the latterassumption is incorrect during presence of aperipheral trough.

The considerable paleoflow variabilitythat is indicated by AMS data from centralTennessee (Fig. 1) is typical for shelf seas(Pettijohn et al., 1987; Selley, 1988). Thesuggestion of a rotary pattern in centralTennessee could indicate the presence of wind-driven circulation cells in the Chattanooga Sea.The generally eastward dipping paleoslope thatis indicated for this area by gradual westwardoverstepping of the Nashville Dome during theUpper Devonian (Conant and Swanson, 1961)must have been too small to noticeably influencesediment transport across the seafloor.

In vertical section, the change from veryirregular paleoflow directions in the DowelltownMember to fairly consistent flow directions inthe Gassaway Member (Fig. 7), probably reflectsgradual sea level rise. During initial flooding ofthe Middle Devonian/Upper Devonianunconformity, surface relief probably causedcompartmentalization into numerous sub-basins(Kepferle and Roen, 1981). As water levels rose,communication between sub-basins probablyimproved and also caused ongoingreorganization of circulation patterns. ByGassaway time, deepening of the ChattanoogaSea had progressed sufficiently to leave onlysmall islands still exposed (Conant and Swanson,1961). At that point in time, the relief of theunderlying unconformity should have hadminimal influence on water circulation in theChattanooga Sea, allowing establishment ofstable circulation over larger portions of thebasin.

AMS data suggest that conditions ofblack shale accumulation in the AppalachianBasin differed between east and west. In theeastern portion, gravity driven processes on the

slopes of a peripheral trough or awestward dipping clinoform dominatedpaleocurrent patterns. In the western portion,wind driven currents could dominatepaleocirculation because the slope of the seabedwas insignificant.

Macroscopic Sedimentary FeaturesWestern Portion of Basin

Identification of HCS beds in theChattanooga Shale suggests that the water was

shallow enough for storm waves to interact withthe seabed (Duke et al., 1991). For the "varvedbed" (DeKalb Cty., Fig. 1)), the compositionalsimilarities between its silt fraction and the siltfraction of underlying black shales suggest that itprobably originated from winnowing ofcarbonaceous muds. HCS beds found in HardinCty. (Fig. 1) were deposited considerably closerto the shoreline of the Chattanooga Sea (Conantand Swanson, 1961). Presence of amalgamated

Figure 8: Two scenarios of basin configuration. 8A) during regression, whensediment deposition exceeds subsidence, the peripheral trough gets filled, and anoverall westward dipping paleoslope develops. 8B) during transgressions, whensediment supply is small, subsidence exceeds sedimentation and a peripheraltrough develops. Metre marks at right serve only as indicators of sedimentthickness. Water depth and basin topography are exaggerated.

HCS beds indicates deposition in shallowerwater (Dott and Bourgeois, 1982).

HCS beds also allow water depthestimates following an approach outlined byClifton and Dingler (1984). Considering thatHCS probably forms close to the stabilityboundary between sheet flow and rippled bedconditions (Dott and Bourgeois, 1982), thresholdorbital velocities between 50-100 cm/s areinterpreted for Chattanooga HCS beds. Takinginto account approximate size of theAppalachian Basin (Conant and Swanson, 1961),and likely heights and periods of storm waves(e.g. Shore Protection Manual; Bialek, 1966), themethod of Clifton and Dingler (1984) yields awater depth between 20-50m for the "varvedbed", and of 15-40m for the coarser grained HCSbeds in Hardin County.

Smoothness of internal erosion surfaces(Fig. 5) suggests a firm substrate and erodingcurrents that possibly exceeded velocities of 1m/s (Sundborg, 1956). Currents of thatmagnitude generally occur in offshore regions ofepicontinental seas only in areas of strong tidalactivity and during exceptionally strong storms(Johnson and Baldwin, 1986). However, tidalranges in the Appalachian Basin were mostlikely too small to have affected offshoresedimentation (Woodrow, 1985). The effects ofstorms on the other hand are well documentedfor shallow offshore areas of the eastern basinmargin (Woodrow, 1985; Halperin and Bridge,1988). In view of above HCS beds, this mightsuggest that storms were involved in producingthe observed erosion surfaces.

Absence of lag deposits (such as bonebeds) on all but one of these erosion surfacesimplies that the eroded material was carriedelsewhere in the basin, and that strongunidirectional currents rather than simply waveaction caused the erosion. Scouring of theseabed might conceivably have been caused bystorm-induced seaward-returning bottomcurrents (Johnson and Baldwin, 1986), butalternative processes, such as (1) erosion byinternal waves (Baird et al., 1988); (2) loweringof sealevel; and (3) migration of mud banks(Allersma, 1971) have to be explored.

Submarine erosion surfaces(discontinuities) have been reported from blackshales of the Devonian Genesee Formation inNew York, and from the Upper CretaceousMancos Shale of Utah. They can be traced formany miles, and are covered with a lag deposit.In the Genesee Formation, erosion by internalwaves migrating along the pycnocline has been

suggested (Baird et al., 1988). Erosion causedby wave reworking due to lowering of sealevelhas been suggested for the Mancos Shale (Swiftet al., 1986).

As pointed out above, HCS bedssuggest relatively shallow water for theChattanooga Shale. Therefore, the areallyextensive erosion surfaces that are capped withbone beds may indeed indicate lowering ofsealevel and shelf mud reworking by waves.

However, the most common erosionsurfaces in the Chattanooga Shale are thoseshown in Fig. 5. They seem to be a more localphenomenon, and their lack of lateral continuityand lag deposits suggests that they do not owetheir origin to erosion by internal or surfacewaves.

Wind-driven currents are anotherpossible cause for these erosion surfaces (Fig. 5).However, potential erosion by such currentsgenerally diminishes with increasing waterdepth. The overstepping of the Nashville Domeby Upper Devonian sediments (Conant andSwanson, 1961; Ettensohn and Barron, 1981), aswell as increasing paleocurrent consistencyupward in the section (Fig. 7), indicates agradual rise of sea level. Yet, despite increasingwater depth, these erosion surfaces occur in boththe Dowelltown and Gassaway members. Thissuggests that another cause for their formationhas to be sought.

Yet another process that might producethe observed features of these erosion surfaces ismud-bank migration. Mud-banks have beenreported from shelf as well as from deep seaenvironments (Allen, 1982; Allersma, 1971).However, a deep sea (contourite) origin can beexcluded from further consideration because acompelling case can be made for shallow waterdeposition of the Chattanooga Shale in centralTennessee. In mud-banks from the Guyanashelf, annual periods of above average wind andwave action cause intermittent mud transport in aslurry-like flow. This process leads to depositionof inclined mud layers on the downstream side ofmud banks (Rine and Ginsburg, 1985), whereaserosion and exposure of consolidated mudsoccurs in interbank areas. The resultingstratigraphic sequence should resemble outcropsof the Chattanooga Shale where submarinetruncation surfaces were observed.

The observation that magnetic lineationand aligned shale particles are typical for theChattanooga Shale of central Tennessee,suggests that current flow over the seabed andwater circulation in the Chattanooga Sea was the

norm rather than the exception. This conclusionis more compatible with the mud-bankhypothesis than with a scenario that relies onshort lived erosive events (e.g. storms).However, the exact cause for the erosionsurfaces discussed above needs to beinvestigated more thoroughly.

Ball and pillow structures generallyindicate deposition on soft, water-saturated mud.The fact that they are rare in the ChattanoogaShale suggests that in most cases the mudsubstrate was sufficiently firm to withstanddifferential loading by overlying sand/silt beds,or that there was insufficient density contrastbetween silts and muds.

The observed association of HCS,convolute and chaotic bedding, and majorsubmarine erosion surfaces in the ChattanoogaShale suggests a possible causal connection.Convolute bedding in association with erosionsurfaces results for example from current drag influvial, tidal, and storm-dominated environments(Reineck and Singh, 1980). Chaotic bedding andcomplete disruption of sediment fabric occurs forexample in collapse depressions on theMississippi delta (Coleman and Garrison, 1977).They form in shallow-water sediments whenimpact by storm waves triggers sudden escape ofmethane gas bubbles and causes fabric collapse.The Chattanooga Shale is highly carbonaceous,and if deposited at the shallow depth suggestedby HCS beds, its surface layers should havecontained abundant gas bubbles from decay oforganic matter (F"rstner et al., 1968).Destabilization by storm wave impact and/ordrag by strong currents could then have producedconvolute and chaotic bedding.

An alternative cause of sedimentdestabilization might be seismic shock.However, because of general tectonic quiescencein central Tennessee during the Late Devonian(Conant and Swanson, 1961), and because oflocalized occurrence and association witherosion surfaces, a seismic origin for theconvolute and chaotic beds of the ChattanoogaShale is considered unlikely.

Absence of soft-sediment deformationbelow most HCS beds suggests that typically theassociated storms did not impart sufficientenergy to the seabed for sediment destabilization.Favourable conditions might have been broughtabout by excessively strong storms, lowering ofsealevel, strong superimposed bottom currents,and combinations thereof.

Silt ripples in the Chattanooga Shaleattest to bottom currents and traction transport of

silt. Their presence and preservation alsoindicates that a firm substrate rather than a soupyorganic muck was the norm during most ofChattanooga deposition. Flaser, wavy, andlenticular bedding in the lower DowelltownMember of Hardin County, Tennessee, havemany of the attributes of wave-generated ripples(De Raaf et al., 1977), and suggest depositionabove wave base.

Eastern Portion of BasinOn the basis of Bouma sequences,

siltstone beds in the Brallier Formation wereinterpreted as turbidites by Lundegard et al.(1985). Silt beds with base-truncated Boumasequences were interpreted to reflect small-scalelow-velocity turbidity currents, whereas thickerbedded siltstone bundles were interpreted toresult from larger, high-flow-intensity turbiditycurrents, deposited in shallow channels or aslobes of higher energy turbidites. Uniformwestward directed paleocurrents suggest multiplepoint sources (migrating delta distributaries) forturbidites along the eastern basin margin.

Preservation of turbidites in the BrallierFormation implies deposition below storm wavebase, but the question of actual depth remains.Lundegard et al., (1985) suggested deposition onslopes with gradients comparable to those ofmodern lower-fans. Assuming a gradient of 0.06degrees (Nelson et al., 1970), and a slope ofapproximately 100km width (Woodrow et al.,1988), one ends up with a depth of about 130mfor the base of the clinoform. This suggests thatthe 100-200m depth estimate of Potter et al.(1982) is probably quite reasonable.

CONCLUSIONFor the area between Hardin and

DeKalb Counties (approx. 200km; Fig. 1), theChattanooga Shale of central Tennessee ischaracterized by sedimentary features (HCSbeds, lag deposits, erosion surfaces) that indicatedeposition in comparatively shallow water (tensof metres). Paleoslope must have beennegligible, and deposition on a shallow cratonicplatform is indicated. Random paleocurrentpatterns as observed for central Tennessee (Fig.1) are consistent with that conclusion, becausethey indicate a lack of slope control on paleoflow(typical for shelf and epeiric seas).

Combining this conclusion with a 100-200m depth estimate for the eastern AppalachianBasin and with paleocurrent measurements fromClinch Mountain, leads to recognition of aperipheral trough that acted as a sediment trap

(Fig. 8B). Periods of abundant sediment supplymay have led to a configuration as shown in Fig.8A. However, further AMS work on the BrallierMember and its black-shale equivalent will berequired to determine, if, as predicted, Bralliersedimentation filled this trough completely andestablished a smooth westward dippingpaleoslope.

The suggestion that the laminated blackshales of the Appalachian Basin accumulated inthe deepest portion of a stratified basin has beena popular hypothesis for a number of years now(e.g. Byers, 1977; Potter et al., 1982; Ettensohnet al., 1988). However, for central Tennessee atleast, observations reported in this paper areincompatible with their deposition in quietstagnant water. Abundant evidence for currentactivity makes a pycnocline origin for theseblack shales appear doubtful.

In central Tennessee, prolongedoperation of strong currents (on the order of 1m/s) is suggested by the possibility of mud-bankmigration. However, the question whethererosion surfaces and inclined shale beds are dueto mud-bank migration will have to beinvestigated further.

ACKNOWLEDGMENTSResearch on the Chattanooga Shale was

supported by the Advanced Research Program ofthe State of Texas (grant# 003656-017), and by agrant from the National Science Foundation(grant# EAR-9117701). This support isgratefully acknowledged. Constructive criticismand editorial suggestions by Frank Ettensohn andone anonymous reviewer helped to improveclarity and organization of the manuscript.

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