Influence of Pre-Existing Plate-Margin Structures on Foredeep Filling

Sedimentary Geology 191 (2006) 115–133 www.elsevier.com/locate/sedgeo

Influence of pre-existing plate-margin structures on foredeep filling: Insights from the Taconian (Blountian) clastic wedge, Southeastern USA ⁎ ⁎ Germán Bayona ,1, William A. Thomas

Department of Geological Sciences, University of Kentucky, Lexington KY 40506, United States Received 26 June 2005; received in revised form 3 February 2006; accepted 9 February 2006

Abstract

Break up of continents gives rise to conjugate margins with irregular shapes, and with thermal histories and sedimentary records that differ along strike. These elements need to be considered in the integrated study of a foreland-basin/orogenic-belt pair that formed during subsequent contractional events. This paper relates the promontory and embayment configuration of southeastern Laurentia (North American craton) to: (1) along-strike variations in width of the initial depositional profile of the Middle to Upper Ordovician Taconian (Blountian) foredeep; (2) northeastward (toward the embayment) along-strike thickening and migration of siliciclastic depocenters in the foredeep, that favored the resumption of carbonate-ramp deposition on the promontory during the Late Ordovician; and (3) variation in intensity of deformation of intrabasinal structures as recorded by reactivation of across-strike structures located on the transition zone between the promontory and embayment. Geometry and composition of the Blountian foreland strata indicate that the foredeep slope was narrower and steeper on the Alabama promontory than on the southwestern flank of the Tennessee embayment. Differential slope geometries were accommodated by reactivation of transverse basement faults, which controlled local conglomerate deposition in deep-water settings on the transition zone between the promontory and embayment. In contrast, conglomerate beds at the top of coarsening-upward successions on the southwestern flank of the Tennessee embayment record the cratonward progradation of deltaic depositional systems. The results presented here may have relevance to other peripheral foreland basins formed over rifted continental margins or retro-arc foreland basins formed adjacent to inverted intracratonic rifts. © 2006 Elsevier B.V. All rights reserved.

Keywords: Foredeep filling; Fault reactivation; Collision; Blountian orogeny; Ordovician

1. Introduction

Continent or arc collision along an irregularly shaped, rifted continental margin may incorporate ⁎ Corresponding authors. Bayona is to be contacted at fax: +1 57 1 significant along-strike variations in orogenic-belt 310 1736. Thomas, fax: +1 859 323 1938. deformation and foreland-basin evolution (e.g., Bradley, E-mail addresses: [email protected] (G. Bayona), [email protected] (W.A. Thomas). 1989). Geodynamic modeling of foreland basins has 1 Present address: Corporación Geológica ARES, Calle 57 N. 23-09 investigated complex spatial and temporal variations in Of. 202, Bogotá, Colombia. lithospheric strength in the evolution of foreland basins

(e.g., Beaumont, 1981; Stockmal et al., 1986; Patton and Llanos foreland of Colombia and Venezuela; Cooper et O'Connor, 1988; Waschbusch and Royden, 1992; al., 1995). Lorenzo et al., 1998; Tandon et al., 2000; Cardozo and Promontories and embayments (plan view), as well Jordan, 2001), but only a few studies have considered as upper-plate and lower-plate geometries (cross-section the effects of the inherited structural/stratigraphic along- view) of rifted continental margins (Fig. 1), imply strike variations of rifted continental margins during stratigraphic/structural/thermal irregularities, which collision (e.g., Whiting and Thomas, 1994). The lack of have been defined by three-dimensional models of along-strike analysis of foreland basins is, in part, an rifted continental margins (see Lister et al., 1986; Buck artifact of the construction of two-dimensional strati- et al., 1988; Thomas, 1993; Thomas and Astini, 1999, graphic and structural models across thrust belts and for discussion and a more complete list of references on adjacent foreland basins perpendicular to the mountain the three-dimensional geometry and models of conti- belt. In this paper, we establish how the irregular plan- nental rifts). A three-dimensional array of extensional view shape and varying cross-section configurations faults limits the irregular geometry of rifted continental along the southeastern rifted margin of Laurentia margins and defines upper and lower plates of affected the patterns of filling, stratal architecture and extensional margins. The extensional structures include sediment composition of a peripheral foreland basin that low-angle detachment faults, synthetic and/or antithetic contains the sedimentary record of the Blountian phase normal faults, steep transform (transverse) faults that of the Taconic orogeny (hereinafter called the Blountian offset the planes of normal faults with consistent dip orogeny) (Rodgers, 1953). The relationship between directions but different offsets, and steep transform foreland evolution and the structural/stratigraphic con- (transfer) faults that separate domains of opposite dip figuration of rifted margins needs to be established directions of detachment faults. Structural domains of a because foreland basins are commonly superimposed on rifted margin can also be recognized from the synrift and earlier rifted margins (e.g., Papua New Guinea; see post-rift stratigraphy (Thomas, 1991, 1993). This paper examples in Sinclair, 1997) or to intraplate grabens (e.g., examines the response of lower and upper plates to

Fig. 1. (A) Block diagram facing northwest showing the marginal and intraplate basement structural geometry of the southern Laurentian passive margin after Cambrian rifting and opening of the Iapetus Ocean (modified from Thomas, 1993). Curved lines show the shape of the upper-plate surface on the Alabama and Virginia promontories. The study area is shown as a shaded polygon southeast of the Birmingham graben (see Fig. 2A for location within the USA). (B, B′) Schematic structural profiles showing the contrasting configuration of basement structures and overlying rift and passive-margin sedimentary cover for a lower-plate margin in the Tennessee embayment and an upper-plate margin on the Alabama promontory, respectively, in Early Ordovician time. (C, C′) Schematic structural profiles across the foreland and orogenic belt in Late Ordovician time. Note the difference in deformation of the orogenic belt and thickness of foreland strata between the profile of the lower-plate margin (in the Tennessee embayment) and the profile of the upper-plate margin (on the Alabama promontory). G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 117 deformation related to compression and tectonic loading On the basis of palinspastic restoration and strati- of the rifted margin. graphic analyses of upper Precambrian to Cambrian The effects of reactivation of intraplate rift-related synrift and Cambrian to Lower Ordovician passive- structures have been considered in the evolution of margin deposits in the Appalachian and Ouachita foreland basins. Basement structures striking parallel to orogenic belts, Thomas (1977, 1991) proposed an the margin may be reactivated as reverse structures orthogonally zigzag geometry of the eastern Laurentian during early pulses of the orogeny (Gupta and Allen, rift margin (Fig. 1). The lower-plate configuration of the 2000; Bayona and Thomas, 2003) or as flexural normal Tennessee embayment and the upper-plate configuration faults (Bradley and Kidd, 1991; Lehmann et al., 1995). of the Alabama promontory resulted from the Blue Furthermore, offsets of foreland depocenters and Ridge rifting episode during late Precambrian time forebulges (Royden et al., 1987), and along-strike (Thomas, 1993). Although significant changes in synrift differences in subsidence and stratal architecture stratal architecture and basement structure are localized (Castle, 2001), are localized across reactivated basement in the transition between the embayment and promon- faults that strike nearly perpendicular to the mountain tory, the eastern margin of Laurentia was covered by a belt. The pre-existing structures that influence evolution shallow-water carbonate platform by Late Cambrian of a synorogenic foreland may be examined in the time (Thomas, 1991). Extension associated with the context of an earlier rifted margin. Ouachita rifting episode in Early Cambrian time reached This study makes along-strike comparisons of: (1) intracratonic areas of the Alabama promontory forming the initial geometry of the Blountian foredeep, as several graben structures, such as the Birmingham recorded by the termination of carbonate deposition basement graben (Figs. 1 and 2). The along-strike and correlation with basinal shale and sandstone configuration of the Birmingham graben changes across turbidite units (e.g., Sinclair, 1997); (2) stratal architec- several transverse basement faults (Bayona, 2003), ture of the synorogenic clastic wedge; and (3) the forming an accommodation zone that likely connects composition of conglomerate clasts and interbedded with the Georgia transform fault system (Fig. 1A) sandstones in the clastic wedge. These comparisons (Thomas, 1993). permit the establishment of the role of reactivation of rift-related continental-margin faults in subsidence/ 2.2. Blountian foreland uplift and sedimentary filling of a peripheral foreland basin. The Blountian phase of the Taconic orogeny caused rapid drowning of the Lower Ordovician carbonate 2. Geologic setting platform (upper Knox Group), diachronous deposition of deep-water graptolite-bearing black shales in 2.1. Palinspastic reconstruction, basement structures, proximal foreland settings, and accumulation of thin and previous extensional tectonic setting beds of volcanic ash (Bradley, 1989; Finney et al., 1996) along the southeastern margin of Laurentia. In The Appalachian thrust belt in Alabama and the Blountian depocenter, coincident with the Tennes- Georgia (Fig. 2A) consists of Cambrian to Pennsylva- see embayment (Thomas, 1977), black shales overlie nian (Upper Carboniferous) strata displaced in thin- Middle Ordovician carbonate-ramp deposits and grade skinned thrust sheets during the latest Paleozoic upward in a coarsening-upward turbidite succession Alleghanian orogeny. Consequently, palinspastic resto- (Shanmugam and Lash, 1982; Shanmugam and Walker, ration is necessary to determine the position of strata in 1978, 1980; Diecchio, 1991). The Blountian clastic relation to the trace of subsurface basement faults and wedge thins southwestward from the depocenter in the the structural configuration of the Laurentian (North Tennessee embayment onto the Alabama promontory American) rift margin. We have prepared a palinspastic (Thomas, 1977; Thomas et al., 2002). Conodont and map of the thrust belt combined with a map of graptolite ages in uppermost carbonate and basal basement structures (Fig. 2B), using 18 balanced cross siliciclastic deposits document the diachronous north- sections that were based on outcrop geology, seismic westward and northeastward drowning of the carbonate reflection profiles, and deep wells (Thomas and platform on the Alabama promontory during the late Bayona, 2005). The data and methods for construction Middle Ordovician to early Late Ordovician, somewhat of the palinspastic map and mapping of basement faults earlier than farther north in the Tennessee embayment are presented in Thomas and Bayona (2002, 2005) and (Fig. 3)(Hall et al., 1986; Bradley, 1989; Finney et al., Bayona (2003). 1996). 118 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133

Fig. 2. (A) Locations of study stratigraphic sections in the Appalachian thrust belt of Georgia and Alabama. (B) Palinspastic map showing palinspastically located stratigraphic sections in the Alabama promontory, transition zone, and southwestern flank of the Tennessee embayment, with respect to location of subsurface Birmingham basement graben. The leading trace of the Appalachian metamorphic thrust belt (Talladega slate belt) is shown by gray dashed line in present location for reference. Cross-strike, transverse basement faults in the transition zone are oriented parallel to the Georgia transform at the margin (Fig. 1A). The distance between the southeasternmost sections and the leading edge of the Blountian orogenic belt is unknown.

Geologic criteria for defining flexural deformation by flexural subsidence that subdued the inversion. To and fault reactivation in the Blountian foredeep are avoid the effects of basement-fault inversion in the controversial. Quinlan and Beaumont (1984), Beaumont along-strike comparison of Blountian foreland deposi- et al. (1988),andEttensohn (1991) interpreted a tion, most of the data for this article are from sections regional unconformity (post-Knox unconformity) un- restoring palinspastically southeast of the inverted derlying the Blountian carbonate and siliciclastic graben (Fig. 2B). succession as a criterion to identify the migration of Provenance analysis of the Blountian clastic wedge the forebulge. Alternatively, Diecchio (1993) and indicates supply from basement rocks and a sedimentary Roberson (1994) interpreted the forebulge to be cover with a Laurentian-margin stratigraphy (Bayona, recorded as a thin succession of distal mixed carbon- 2003). Clasts within discontinuous conglomerate beds, ate-siliciclastic strata separating a deeper and more distributed at various stratigraphic levels of proximal clastic-rich basin on the southeast (foredeep) from thick, foreland strata along strike, consist of carbonates with shallow-marine carbonates on the northwest. In the subordinate amounts of sandstones, siltstones, and study area, Bayona and Thomas (2003) documented quartzites (Kellberg and Grant, 1956; Cressler, 1970). inversion of the Birmingham graben (Fig. 2) followed Blountian sandstones in the Tennessee embayment are .Byn,WA hms/SdmnayGooy11(06 115 (2006) 191 Geology Sedimentary / Thomas W.A. Bayona, G. – 133

Fig. 3. (A) Late Middle (Llanvirn) and Late Ordovician Epochs (from Webby, 1998) and correlation of conodont and graptolite zones and K-bentonite beds (modified from Kolata et al., 1996). Radiometric ages of Mohawkian K-bentonites from Kolata et al. (1996, 1998), and of older K-bentonites from correlations made by Finney et al. (1996). Also shown are the positions of the key stratigraphic correlation surfaces discussed in the text (numbers 1 to 8, see Table 1 for definition of each of these surfaces). (B, C) Across-strike chronostratigraphic correlations of strata above the post- Knox unconformity showing progressively later onset of Blountian deposition toward the craton, as well as later initial deposition on the southwestern flank of the Tennessee embayment than in the Alabama promontory (black vertical bars show age range of preserved post-Knox Ordovician strata with respect to the chronostratigraphic diagram in A). 119 120 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 dominated by monocrystalline quartz, plagioclase, and between sandstones of the Upper Ordovician Bays sedimentary rock fragments, including detrital carbon- Formation in the Tennessee embayment and Lower ate grains (Mack, 1985). In the study area, Blountian Cambrian and late Precambrian siliciclastic rocks subarkosic-arkosic sandstones show an upsection de- (Cummings, 1965) corroborate this interpretation of crease in the content of feldspar sand grains, and along- provenance. strike variations in siliciclastic and calcareous lithic clasts (Bayona, 2003). Compositions of gravel- and 3. Methods sand-size detritus indicate that sources contained low- grade metamorphic rocks, plagioclase-rich coarse- Stratigraphic and provenance data were collected grained plutonic or gneissic rocks, and a sedimentary from fieldwork and previous investigations at 14 cover of variable composition (Mack, 1985; Bayona, localities. A more complete documentation is given in 2003); some proximal source areas included the passive- Bayona (2003). Age control for each section is margin succession of the Laurentian margin (Cressler, documented by conodonts (Bergström, 1973, 1977; 1970). The similarities in heavy-mineral populations Hall, 1986; Hall et al., 1986; Shaw et al., 1990),

Table 1 Explanation of key stratigraphic surfaces Age Significance of the event in sections restoring close Significance of the event in sections restoring near to the graben (Fig. 4A) the plate margin (Fig. 4B) 8 ca. 454 Ma Millbrig and Deicke K-bentonite interval in red siltstones in section HM, at the base of quartzarenite deposits in sections HL, and toward the top of the quartzarenite interval in GS. 7 Early Late Ordovician Subaerial exposure followed by regional marine (middle Mohawkian, flooding below surface 8 in sections HM, HL and GS. ca. 456) Resurgence of limestone deposition in section RH. 6 Late Ordovician Coarsening-upward, shallowing, and termination of Regional marine flooding in southern section PF (early Mohawkian) carbonate deposition. Onset of fine-grained siliciclastic (N. gracilis to C. bicornis zones). Deposition of deposition in northern sections HM and RH fine-grained deposits over conglomerate beds in (P. gerdae zone). section CI. 5 Late Ordovician Shoaling, termination of carbonate deposition, and Regional marine flooding in southern section PF (early Mohawkian) onset of fine-grained siliciclastic deposition in section (N. gracilis to C. bicornis zones). GS (younger than P. sweeti Conodont zone). 4 Middle-late Ordovician Regional marine flooding in southern section PF (ca. 458) (N. gracilis to C. bicornis zones). Black shale deposition in section CI. 3 Middle-late Ordovician Drowning of the carbonate platform. Graptolitic, Drowning of the carbonate platform. Graptolitic, (ca. 458) black shale deposition in section AB black shale deposition in section CL; slight shoaling Onset of carbonate deposition inferred for section and drowning at PF (top of G. teretiusculus to GS (P. serra zone). N. gracilis zones; Finney et al., 1996). 2 Late-middle Ordovician Onset of carbonate deposition at RH (P. serra zone) Drowning of the carbonate platform in the (late Whiterockian, and uncertain for HM. Tennessee embayment. Graptolitic, calcareous ca. 463) Marine flooding events may have been recorded in shale deposition (D. teretiusculus zone) at CI the carbonate interval in section AB. (Finney et al., 1996). Marine flooding events may have been recorded in the carbonate interval in sections PF and CL. 1 Late-middle Ordovician Localized onset of carbonate deposition in section AB. Drowning of the carbonate platform on the (middle Whiterockian, transition zone and Alabama promontory. ca. 466–464 Ma) Graptolitic shale deposition (D. murchisoni zone) at RK, LM, FC, and HV (Finney et al., 1996). Localized onset of carbonate deposition in section PF (C. friendsvillensis zone), and uncertain in section CL. Post-Knox unconformity. This surface incorporates Post-Knox unconformity. This surface corresponds surfaces 1, 2, 3, or 4 because of the diachronous to surface 1 in section HV (Alabama promontory). onset of carbonate deposition in the distal foreland. In sections FC, FM, and RK (transition zone), peritidal carbonates < 38 m thick overlie the unconformity and underlie surface 1. G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 121 graptolites (Finney et al., 1996; graptolite identifications carbonate drowning and foredeep filling among the made by Stan Finney in sections AB, CL, HV, FM and Alabama promontory, transition zone, and southwestern PF are fully described in appendix B of Bayona, 2003), flank of the Tennessee embayment. For each area, we and identification of K-bentonite beds (Haynes, 1994; describe the stratigraphic and sedimentologic character- Bayona, 2003). Two lines of along-strike stratigraphic istics first of the sections restoring closer to the rifted correlation help to illustrate differences in stratal plate margin and then the sections closer to the architecture and composition among the Alabama Birmingham graben. promontory, the transition zone (which corresponds to the area with several transverse basement faults that 4.1. Alabama promontory likely connect to the Georgia transform fault system, Fig. 1A), and the southwestern flank of the depocenter Basal graptolite-bearing black shales of the Athens in the southern part of the Tennessee embayment. Shale overlie carbonate strata of different lithological Stratigraphic correlation is supported by the identifica- associations and age on the Alabama promontory (Fig. 4). tion of eight chronostratigraphic surfaces, such as Nearest the rifted margin of continental crust, in section unconformities, termination of carbonate deposition, HV, Middle Ordovician carbonates are not reported and marine-flooding surfaces, and K-bentonite beds (see the Athens Shale directly overlies the post-Knox Table 1 for criteria to identify each surface), that may be unconformity (Figs. 3B and 4B). Farther northwest, in correlated across different depositional systems. section CL, skeletal intraclastic wackestones grade Variations in composition of the coarse-grained abruptly upward to argillaceous bioclastic debris beds clastic wedge were determined from identification of and calcareous black shales. In section PF, the upper clast types in conglomerate beds and from sandstone Lenoir Limestone consists of well-sorted grainstones and petrography. Clast counts of section RK are from packstones with crinoid, bryozoan, brachiopod, sponge, Bayona (2003), and interpretations of possible sources mollusk, and trilobite fragments. Adjacent to the graben at are from Cressler (1970) and Bayona (2003). Clast section AB, the uppermost carbonates consist of argilla- count and interpretation of possible sources of section ceous, intraclastic, algal (Nuia, Girnavella,calcisphere, CI are from Kellberg and Grant (1956). Fourteen thin dasyclads), and skeletal wackestones to packstones (Fig. sections of fine- to medium-grained sandstones, selected 4A). Strata are slump-folded in upper carbonates and in from outcrops near or interbedded with the conglomer- black shales, in sections AB and PF, respectively (Ferrill, ate beds, were point-counted utilizing 300 framework 1989; Bayona, 2003). points per thin section and using the Gazzi-Dickinson The Athens Shale consists mostly of siliciclastic technique for counting (e.g., Ingersoll et al., 1984)to black shales in sections AB and CL on the Alabama eliminate compositional variation due to grain size. Thin promontory (Fig. 4A and B). In section HV, nearest to sections were stained for identification of plagioclase the rifted plate margin, siliciclastic black shales are and potassium feldspars. Detrital modes exclusive of overlain by a thick and highly deformed succession of carbonate grains were calculated from the point-count dark-colored silty shales and sandy siltstones with thin results following the technique of Dickinson (1985) and to medium beds of fine- to coarse-grained argillaceous were plotted in QFL and QmFLt ternary diagrams. sandstones. Sandstones are mostly massive and have Additional provenance data (sandstone petrography and sharp contacts with underlying and overlying beds. A Nd-isotopes) from the Blountian clastic wedge are in few sandstone beds in section HV have internal Bayona (2003). gradations and planar lamination. Farther southwest in section PF (Fig. 4B), the graptolite-bearing shales are 4. Drowning of the carbonate platform and calcareous and have isolated hummocky cross-beds; the architecture of the clastic wedge in the Blountian succession passes upsection to laminated, argillaceous foredeep calcareous mudstones with fine-grained bioclastic debris beds composed of bryozoans, brachiopods, and Blountian strata discussed in this paper include upper trilobites (Finney, 1977). beds of the Lenoir Limestone and the overlying succession in sections restoring palinspastically south- 4.2. Transition zone east of the Birmingham graben (Fig. 4), with the exception of sections HL and DG that restore inside the In sections closer to the rifted plate margin in the graben (Fig. 2B). General sedimentologic characteristics transition zone, the Rockmart Slate in section RK and are reviewed here to highlight the differences of the Athens Shale in sections FM, FC, and LM overlie 122 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 .Byn,WA hms/SdmnayGooy11(06 115 (2006) 191 Geology Sedimentary / Thomas W.A. Bayona, G. – 133 123 Fig. 4 (continued). 124 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 irregularly either the post-Knox unconformity or beds of carbonate lithofacies on the southwestern flank of the the Lenoir Limestone (Fig. 4B), which consists mostly Tennessee embayment. In section CI, the Lenoir of mud-dominant carbonate lithologies with flattened Limestone is 5 m thick and consists of peloidal to fenestral textures and scarce fragments of gastropods, intraclastic packstones and grainstones interbedded with brachiopods, cephalopods, and trilobites. The Rockmart fenestral peloidal mudstones. Farther northwest in Slate and Athens Shale include dominantly calcareous sections RH and HM, the uppermost carbonate beds and siliciclastic shales and silty shales. In section RK, are coarse-grained, mixed crinoid and bryozoan grain- the interval of sandstone and conglomerate interbeds is stones with quartz-rich laminae and red calcilutite in the more common and thickens to the southeast (Fig. 4B) matrix (Caldwell, 1992). Calcareous laminated mud- (Cressler, 1970; Sibley, 1983). Normal and inverse stones of the Ottosee Formation in section RH and green grading, and gradational and planar contacts were to red shales with thin interbeds of skeletal grainstones observed in some sandstone beds, despite common of the Greensport Formation in section HM overlie the slaty cleavage that indicates low P–T conditions of skeletal carbonate succession. metamorphism (Sibley, 1983). Conglomerates are Strata of the Athens Shale and the overlying Chota massive and matrix-supported in medium to very thick Formation in section CI record the most complete and lenticular beds with sharp contacts, and the matrix is thickest Blountian succession in the study area. The calcareous and subarkosic. Athens Shale in section CI (Fig. 4B), closer to the rifted In sections GS and HL, closer to the Birmingham plate margin on the southwestern flank of the Tennessee graben in the transition zone (Figs. 2B and 4A), the embayment, consists of (1) calcareous black shales and Greensport Formation, composed of red shales, silt- 15-to-60-m-thick coarsening-upward successions of stones, and thin interbeds of carbonates, overlies the planar-laminated shales and siltstones in the lower Lenoir Limestone. In section GS, the upper Lenoir part; (2) bioturbated siltstones and very fine-grained Limestone includes skeletal (Tetradium, mollusk, ostra- sandstones with wavy, lenticular, and ripple laminations cod, trilobite) limestones and grades upward to fenestral in the middle; and (3) medium- to thick-bedded, fine- to and algal-laminated dolomites. In section HL, the thin medium-grained calcareous sandstones in the upper Lenoir Limestone is very argillaceous and includes part. Sandstones are massive in the lower beds but intraclast, skeletal, and algal fragments. The Greensport ripples and cross-bedding structures, as well as wavy to Formation includes dominantly red siltstones and shales planar contacts, are identified near the top of the Athens with some local interbeds of fine- to coarse-grained Shale. sandstones. Bioturbation, horizontal lamination, wavy Coarsening-upward successions are also identified in ripples, and heterolithic lamination are present at the Chota Formation (Fig. 4B). Silty shales are different levels. The Colvin Mountain Sandstone over- laminated, and sedimentary structures of sandstones lies the Greensport Formation with a sharp contact and change upsection from ripple lamination in the lower is characterized by quartzarenites with trough and planar part, to hummocky and planar cross-beds in the middle. cross-beds and local bioturbation. Bryozoans and crinoid fragments were identified in lower sandstones of the Chota Formation, and in lenses 4.3. Southwestern flank of the Tennessee embayment in the middle part of the Chota Formation (Salisbury, 1961). Toward the top, planar and trough cross-bedded Calcareous black shales of the Athens Shale in the sandstones and horizontal-bedded, matrix-supported, southeast and red siliciclastic beds of the Ottosee and cobble- to pebble-size conglomerates (Fig. 4B) are Greensport units in the northwest overlie two different overlain by red and bioturbated sandy siltstones

Fig. 4. Two lines of along-strike correlation (see Fig. 2B for location) showing stratigraphic units, biostratigraphic data, dominant lithofacies, and stratigraphic correlation surfaces (numbered 1 to 8). Stratigraphic positions of conglomerate clast composition (Table 2) and sandstone groups (ss gr 1 to 3 in sections RK and CI; plots of modal composition in Fig. 7 and raw data in Table 4) are shown. Note the variation in the content of carbonate rock fragments (Rc) through the stratigraphic succession on the southwestern flank of the embayment (section CI). (A) In sections restoring inside to southeast of the Birmingham graben, carbonate strata dominate the succession on the promontory, whereas siliciclastic lithofacies thicken northeastward and are dominant in the southwestern flank of the embayment. The datum is defined by the Deicke and Millbrig K-bentonites. (B) In sections restoring closer to the rifted plate margin, deep-water carbonates to black shales dominate on the promontory after drowning of the shallow carbonate platform; in contrast, thin carbonate deposition on the transition zone and embayment occurred prior to the drowning, and the Blountian succession passes upward from deep-shelf turbiditic sandstones and siltstones to deltaic conglomerates, sandstones, and siltstones. No specific datum is used, but lines of correlation are shown between sections at the tops of biozones and K-bentonite levels. Note that CI is plotted at a slightly smaller scale than the other sections to accommodate space in the diagram. G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 125 interbedded with argillaceous sandstones with scoured sections RH and HM on the southwestern flank of the lower contacts and shale rip-up clasts. Massive and embayment occurred more because of the arrival of fine- planar cross-beds in sandstones, and a thin bed of grained, synorogenic, red siliciclastic detritus than quartzose conglomeratic sandstone are identified at the because of deepening of the foreland plate, as occurred top. Equivalent strata to the northwest include calcar- on the promontory. In sections restoring near the eous laminated mudstones, sandstones, and shales of the southeast border of the Birmingham graben and on the Ottosee Formation in section RH. promontory (sections PF and AB), calcareous black Near the Birmingham graben margin on the shales and bioclastic debris with slump structures have southwestern flank of the Tennessee embayment, red been interpreted to represent deposition in deep shales and siltstones of the Upper Ordovician Green- carbonate ramp environments; these deposits were sport Formation dominate the clastic wedge stratigra- subsequently covered by basinal black mud (Benson, phy (sections HM and DG; Fig. 4B). The lithologies 1986). In contrast, on the transition zone and south- of the Greensport Formation are similar to those western flank of the embayment (e.g., sections GS, HL described for sections on the transition zone, but in and HM), the upsection change from dolomitic lime- sections HM and DG, interbeds of sandstones and stones and skeletal-algal limestones to red fine-grained mudcracks are more common in red dolomitic siliciclastic sediment has been interpreted as the shift mudstones. from deposition in subtidal to peritidal environments associated with the inversion of the Birmingham graben 4.4. Comparison of depositional systems of the early (Bayona and Thomas, 2003) to deposition in a shallow, Blountian foredeep low-energy clastic shelf (Fig. 5B and C) (Bayona, 2003). In sections restoring closer to the rifted plate margin, little or no accumulation of shallow-water carbonate 4.5. Comparison of depositional systems in the clastic lithofacies of the Lenoir Limestone (Drahovzal and wedge Neathery, 1971), and subsequent deposition of graptolite-bearing black mud suggest rapid drowning of the Two siliciclastic successions of different ages and carbonate platform in late Middle Ordovician time. lithologies characterize the cratonward progradation of Along-strike and across-strike variation in composition the Blountian clastic wedge (Fig. 5), the thickness of of basal synorogenic siliciclastic beds, however, may which increases considerably northeastward (Fig. 4). indicate significant differences in the deposition of One siliciclastic succession is of late Middle Ordo- graptolite-bearing black shales between the promontory vician to Late Ordovician age, and by northwestward and and the southwestern flank of the embayment (Fig. 5A northeastward progradation, it diachronously covered and B). On the promontory and the transition zone, the former carbonate platform (Fig. 5). The most siliciclastic graptolite-bearing black shales extend to the complete record of this siliciclastic succession corre- northwest as far as to sections CL and LM (Fig. 5B), sponds to the coarsening- and shoaling-upward trend of suggesting that the foreland plate there reached water- siliciclastic strata closer to the rifted plate margin, as depth conditions somewhat similar to those conditions recorded in section CI. Because of post-Ordovician reached in sections restoring close to the rifted margin erosional truncation, only the lower part of this (e.g., section HV). On the southwestern flank of the succession, corresponding to deep-water shales, has embayment, calcareous black shales extend to the been preserved in the other sections. In sections restoring northwest as far as section CI (Fig. 5B). The on the promontory and the transition zone, massive to environment of siliciclastic black mud on the promon- isolated planar lamination, internal gradations, sharp tory has been interpreted as basinal (Benson, 1986). In lower contacts, and lack of bioturbation in sandstone and contrast, the calcareous black mud on the southwestern conglomerate interbeds indicate that submarine turbidity flank of the embayment represents deposition in lesser current deposition intermittently interrupted pelagic water depths on a carbonate ramp that dipped toward the black mud deposition, as suggested by Shanmugam orogen. and Walker (1978) for similar deposits in the depocenter Lithofacies associations of upper Lenoir beds and in the center of the Tennessee embayment. In contrast, overlying siliciclastic beds indicate that depositional bioturbation and low-energy regime, wave-influenced systems were shallower on the southwestern flank of the sedimentary structures in sandstone of the Athens Shale embayment than on the promontory. The termination of on the southwestern flank of the embayment support the shallow-water, high-energy platform deposition in interpretation of deposition at shallower depths there 126 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133

Fig. 5. Paleogeographic maps showing distribution of depositional systems at the time of deposition of stratigraphic correlation surfaces 1, 3, and 7. Locations of source areas and directions of dispersal for synorogenic detritus are shown tentatively on the east side of each diagram. (A, B) Siliciclastic depocenters advanced northeastward and northwestward. (C) Dominance of carbonate deposition on the southwest on the promontory, and siliciclastic filling of the proximal foreland on the southwestern flank of the embayment. See Fig. 2 for identification of sections. than on the promontory. The upsection changes (1) Formation; (3) and to thick-bedded conglomerates and from ripple and wavy lamination structures in sand- red siliciclastic beds toward the top together indicate stones of the Athens Shale; (2) to hummocky, planar, shallowing of depositional environments from wave- and trough cross-beds in sandstones of the Chota dominated environments to tide-dominated deltas G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 127

(Bayona, 2003) and increasing influx of synorogenic Table 2 detritus with time (Fig. 5C). Conglomerate clast types and possible sources in sections closer to the rifted plate margin The other siliciclastic succession consists of lower Upper Ordovician Greensport and Colvin Mountain Clast type RK (% from CI (% from Possible source strata in sections restoring near the southeast fault 50 clasts) 848 clasts) formation Limestone (calcite) boundary of the Birmingham graben. Southwestward ⁎⁎ deepening of depositional conditions is interpreted from Black micrite 30 Athens Sh. , Lenoir Lst. ⁎⁎ strata underlying stratigraphic surface of correlation 8. Light-colored 2.5 Athens Sh. ⁎⁎, Mudcracks and wavy and heterolithic laminations in micrite Lenoir Lst. ⁎⁎ bioturbated red siltstone suggest deposition in intertidal Aphanitic light 77.7 Knox Gr. ⁎, ⁎ to supratidal conditions in sections on the southwestern gray, oolitic, Conasauga Fm. flank of the embayment. Cross-bedded quartzarenites in Undifferentiated Limestone (dolomite) the upper Greensport Formation and Colvin Mountain Sandy dolomite 28 Lenoir Lst. ⁎⁎, Sandstone record the influx of coarse-grained synoro- Knox Gr. ⁎ genic detritus and deposition in subtidal environments in Undifferentiated 1.5 Knox Gr. ⁎ Mixed lithologies sections restoring on the transition zone (Fig. 4B). This ⁎⁎ succession grades abruptly to carbonate strata southwest Dolomitized 15 Lenoir Lst. , siltstone Knox Gr. ⁎ of the transition zone (Fig. 5C). Calcarenite 2.5 Lenoir Lst. ⁎⁎ Siliciclastic 5. Provenance analysis of the coarse-grained clastic Sandstone 10.2 Athens Sh. ⁎⁎, ⁎ wedge Rome Fm. Siltstone 2.2 Athens Sh. ⁎⁎, Rome Fm. ⁎ 5.1. Clast composition Black calcareous 20 Athens Sh. ⁎⁎, shale Lenoir Lst. ⁎⁎ Conglomerates within the Blountian clastic wedge Chert 2.8 Knox Gr. ⁎, Shady Dst. ⁎ have clast populations and processes of deposition that ⁎ varied through time (Fig. 4B and Table 2). In section Quartzite 3.8 Chilhowee Gr. , Ocoee SuperGr ⁎. RK, massive conglomerates were deposited by deep- Vein quartz 2 water gravity flows. The clast composition is dominated Others 2 by dolomite and subordinate calcareous black shales in a Abbreviations: Gr.=group; Fm.=formation; Sh.=shale; Lst.=lime- subarkosic matrix (Fig. 6A, B); however, many pits stone; Dst.=dolostone. remain where clasts of indeterminate composition have * Units of the synrift and passive-margin succession of the southern been dissolved. In section CI, horizontal-bedded con- Laurentian margin (Thomas, 1991). ** Units of the basal Blountian foreland succession of the southern glomerates accumulated in progradational tide-domi- Laurentian margin (this paper). nated deltas; the clast population is dominated by limestone and includes subordinate clasts of sandstones and siltstones in a sublitharenite matrix (Fig. 6C, D). affected areas around section RK (Sibley, 1983). On the Composition of the clasts in both sections matches the southwestern flank of the embayment, Athens and lithologies of the Lenoir Limestone and older Paleozoic Chota sandstones are calcareous, and the composition units of the Laurentian margin (Table 2; Kellberg and changes upsection from arkose–subarkose in the Athens Grant, 1956; Cressler, 1970). Shale to sublitharenite and quartzarenite in sandstones Similar to the conglomerates within the Blountian of the Chota Formation (Bayona, 2003). Calcareous clastic wedge, interbedded sandstones have clast cement may be the result of dissolution of carbonate populations and processes of deposition that differ in grains and later precipitation. Sandstones of the upper time (see Table 3 for definition of point-counting Chota Formation show significant increases in non- parameters and Table 4 for point-counting data). Even metamorphic quartz, carbonate rock and sedimentary though the sandstone composition of the Athens Shale lithic fragments (Fig. 7). on the Alabama promontory is arkosic to subarkosic (Bayona, 2003), the high content of quartzose grains in 5.2. Interpretation of provenance samples of the Rockmart Slate (Table 4, Fig. 7) may have been enhanced by destruction of feldspar and lithic Integration of compositional data of different clast fragments during low-grade metamorphic events that sizes and at different spatial-temporal settings of the 128 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133

Fig. 6. Characteristic framework components of Blountian conglomerates and interbedded sandstones in different spatial and temporal settings. (A) Matrix-supported conglomerate of the Rockmart Slate with clasts of sandy dolomite (SD), micrite, calcarenite, and calcareous black shale (BS) in a calcareous, subarkosic matrix. (B) Photomicrograph (crossed nicols) of the matrix of the conglomerates of the Rockmart Slate. Note the presence of micritic and quartz-bearing micrite limestone clasts (Rc). (C) Thick trough cross-sets in sandstones interbedded with conglomerates of the Chota Formation. Jacob's staff=1.5 m. (D) Photomicrograph (crossed nicols) showing micritic (I) and calcareous sandstone (Ls) rock fragments, and chert (Ch) grains in calcareous sublitharenite sandstones of the Chota Formation.

Blountian foreland basin indicates that both intrabasinal conglomerates of the upper Chota Formation document and extrabasinal sources supplied sediments to the erosion from new exposures of sedimentary rocks in foredeep. Fabric and composition analyses of conglom- source areas adjacent to the southwestern flank of the eratic gravity-flow deposits in section RK suggest a embayment. Cratonward advance of the orogenic belt relatively short distance of transport and proximity to and increasing influx of synorogenic calcareous and uplifted areas in a deep-water system. This interpreta- siliciclastic detritus produced the upsection shallowing tion is further supported by the identification of a 15-m- trend of depositional environments on the southwestern diameter carbonate olistolith in section RK (Sibley, flank of the embayment (Bayona, 2003). 1983). Well-preserved bryozoan fragments and bioclastic debris beds in sections CI, PF, and RH (Salisbury, 6. Discussion 1961; Finney, 1977; Caldwell, 1992), along with other carbonate rock fragments, indicate a supply from Along- and across-strike correlations of synorogenic intrabasinal shallow-water carbonates. carbonate ramp, basinal shale, and sandstone turbidite In contrast, the upward change from subarkose to units provide insight into along-strike variations in the sublitharenite sandstones and the upward increase in initial geometry of the foredeep (Dorobek, 1995; G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 129

Table 3 western flank of the embayment. The initial Blountian Parameters for sandstone point counts foredeep had a steep slope dipping toward the orogen on Symbol Grain category Recalculated parameters the Alabama promontory, in contrast to a very gentle, Qm Monocrystalline quartz Q+F+L southeast-dipping carbonate platform on the transition Qpf Foliated polycrystalline quartz Q=Qm+Qpf+Qpo+Ch zone and on the southwestern flank of the embayment. Qpo Non-foliated polycrystalline F=P+K+Fu Therefore, along-strike variations in foredeep slope quartz L=Ls+Lv+Lm+Lu patterns may be linked to the structural configuration of Ch Chert P Plagioclase feldspar the rifted continental margin (Fig. 1). K Potassium feldspar Although flexural deformation advanced northeast- (orthoclase, microcline) ward to the embayment (Fig. 5B; Finney et al., 1996), Fu Unidentified feldspar, albitized Qm+F+Lt the slope of the foredeep continued to be steeper on the feldspar Qm=Qm promontory than on the southwestern flank of the Ls Sedimentary lithic fragments F=P+K+Fu Lm Metamorphic lithic fragments Lt=Ls+Lv+Lm+ embayment (Fig. 8). The relatively steep slope of the Lu+Qpf+Qpo+Ch foredeep is documented by coeval deposition of coarse- Lv Volcanic (devitrified) lithic grained submarine fan deposits of the Athens Shale in fragments the deepest part of the basin (section HV), the drowning Lu Unidentified lithic fragments of the middle foreland platform (e.g., sections CL, LM, Lp Plutonic rock fragments Rc Non-skeletal carbonate rock FC) to deep-water basinal settings, and deposition of fragments carbonate subtidal deposits in sections near the Inters.=interstitial (matrix-cement) points (n)/framework points Birmingham graben (sections PF and AB). In contrast, (300)+n. the relatively gentle slope of the foredeep on the southwestern flank of the embayment is documented by the lateral gradation to the northwest of deep-ramp Sinclair, 1997). The earliest response to flexure is calcareous shales in section CI to shallow-ramp recorded in section HV on the Alabama promontory, calcareous mudstones and skeletal limestones in sec- where deep-water graptolite-bearing shales rest on the tions RH and HM (Fig. 5B). youngest Knox strata (Fig. 5A). Farther northwest and Northeastward along-strike migration of flexural northeast, coeval peritidal to very shallow carbonate subsidence and synorogenic siliciclastic depocenters, as deposition was occurring in wider areas southeast of the well as slopes within the basin, exerted the primary graben on the promontory and the transition zone, controls on sedimentary filling of the Blountian foreland whereas subaerial exposure dominated on the south- basin. Coarsening- and shoaling-upward siliciclastic

Table 4 Point-count data and modal statistics of Blountian sandstones interbedded with conglomerate beds Sandstone Sample Inters. Quartz Feldspar Lithic QFL QmFLt group Qm Qpf Qpo Ch P K Fu Ls Lm Lv Lu Lp Rc Q F L Qm F Lt Group 1. H3-1007 45.8 73.0 2.0 4.3 0.3 8.6 0.0 8.6 0.3 0.6 0.0 2.0 81.5 17.6 0.9 74.7 17.6 7.7 Rockmart H3-1008 47.2 73.5 3.5 4.5 0.0 13.5 0.0 4.0 1.0 81.5 17.5 1.0 73.5 17.5 9.0 Slate, section H3-1009 39.0 75.0 1.6 6.0 4.6 4.0 0.0 7.6 0.0 1.0 87.4 11.6 1.0 75.2 11.6 13.2 RK H3-1012 45.4 76.0 1.6 5.3 1.6 9.6 0.0 4.0 0.6 1.0 84.8 13.6 1.6 76.2 13.6 10.1 H3-1013 38.8 82.3 1.0 6.0 0.6 2.6 0.0 7.3 90.1 9.9 0.0 82.5 9.9 7.6 H3-1301 49.4 81.0 0.5 10.0 0.0 3.5 0.0 5.0 91.5 8.5 0.0 81.0 8.5 10.5 H3-1303 45.7 87.0 1.5 3.0 2.0 3.5 0.0 2.0 1.0 93.5 5.5 1.0 87.0 5.5 7.5 Group 2. E3-2204 29.7 67.6 0.6 2.0 3.0 0.6 10.0 0.3 15.6 87.0 0.7 12.2 80.4 0.7 18.9 Chota E3-2205 31.5 61.6 1.0 2.6 0.6 0.3 16.0 1.0 16.6 79.2 0.4 20.5 74.1 0.4 25.5 Formation, E4-2901 28.4 77.0 0.6 4.0 1.3 0.3 15.6 1.0 83.1 0.3 16.6 77.2 0.3 22.5 section CI E4-2902 33.3 79.3 0.3 4.0 0.6 2.6 12.3 0.3 0.3 84.5 2.6 12.9 79.5 2.6 17.9 E4-2903 42.9 72.0 0.0 2.5 3.0 3.0 19.1 0.5 77.4 3.0 19.6 71.9 3.0 25.1 Group 3. E3-2207 32.0 78.0 3.5 5.5 3.0 1.0 9.0 90.0 1.0 9.0 78.0 1.0 21.0 Chota E3-2206 28.7 88.3 2.6 1.0 6.0 0.6 1.3 98.1 0.6 1.3 88.5 0.6 10.9 Formation, section CI See Table 3 for explanation of abbreviations; 0.0 denotes trace amounts. Ternary diagrams are in Fig. 7. 130 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133

Fig. 7. Compositional and provenance ternary diagrams for Blountian foreland sandstones interbedded with conglomerates. Point-counted sandstones were grouped into three groups following these criteria: temporal position, location, and composition. Point count parameters are in Table 3; raw point-count data and modal statistics are in Table 4. Stratigraphic positions of the three groups are shown in Fig. 4B. facies in section CI document filling of the gently dipping embayment, as inferred from stratigraphic modeling of foreland basin on the southwestern flank of the Tennessee foreland basins (Flemings and Jordan, 1990). This embayment. At this stage, sediment load contributed to inference is further supported by thickening of the foreland widening and foreland subsidence in the foredeep succession to more than 2500 m closer to the

Fig. 8. Block diagram facing southwest (note difference in horizontal and vertical scales), illustrating the role of different rift-related structures on the early geometry of the Blountian foredeep (paleogeographic map in Fig. 5B), and distribution of depositional environments at different settings of the foreland (see Bayona and Thomas, 2003, for documentation of graben inversion). Evolution of the Blountian foreland basin was strongly controlled by the pre-existing configuration of the Laurentian rifted margin and reactivation of strike-parallel (inversion and flexural extension) and strike- perpendicular (differential flexural subsidence) intraplate basement faults. A narrower foredeep formed on the promontory, whereas the foredeep had a gentler slope on the embayment. G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 131 orogen farther northeast in the center of the Tennessee (Fig. 8). Intrabasinal uplifts, striking perpendicular to embayment (Thomas, 1977; Walker et al., 1983). In the foreland, supplied coarse-grained conglomerates contrast, the narrow foredeep on the promontory was that are interbedded with deep-water sandstones and starved of siliciclastic detritus, favoring resumption of shales. The deep-water conglomerates differ from those deep carbonate-ramp deposition. derived from the approaching orogenic belt, which are Migration of flexural deformation also reactivated distinguished on the basis of stratigraphic position, basement faults striking both parallel and perpendicular association with overlying and underlying lithofacies, to the rifted plate margin (Fig. 8). Flexural normal shallow-water sedimentary structures, and composition reactivation of basement faults striking parallel to the of clast population and matrix. rifted plate margin focused abrupt lateral changes from Northeastward along-strike migration of deposition- limestone to basinal shales on the promontory and al loads, as indicated by diachronous northeastward transition zone, and foreland-plate instability indicated (1) drowning of the Blountian carbonate platform in by slump structures in both carbonate and black shale sections near the rifted plate margin or (2) termination deposits. The transition from a narrow and steep of shallow-water carbonate deposition in sections near foredeep on the Alabama promontory to a wider and the Birmingham graben, controlled the migration of gentler foredeep on the southwestern flank of the the siliciclastic depocenter toward the southwestern Tennessee embayment apparently caused reactivation flank of the embayment and favored carbonate of cross-strike structures in the transition zone. Normal deposition on the promontory. Upsection coarsening flexural extension and cross-strike reactivation probably and shoaling of foredeep strata in sections restoring on created intrabasinal uplifts, which became the source the southwestern flank of the Tennessee embayment areas of olistoliths and gravity flows that are interbedded document: (1) upsection increase in influx of terrig- with turbiditic sandstones and black shales in section enous detritus throughout the section, (2) an abrupt RK. In contrast, conglomerate beds at the top of the increase of quartz and sedimentary lithic fragments in Chota Formation on the southwestern flank of the coarse deltaic deposits toward the top, and (3) Tennessee embayment record an increased influx of cratonward progradation of the transition from shallow sedimentary lithic, carbonate rock, and quartzose clastic platform to deltaic environments. In contrast, fragments, suggesting the approach of the Blountian the low influx of terrigenous detritus to the narrow thrust belt rather than uplift of intrabasinal blocks. foredeep on the Alabama promontory favored the resumption of deposition on a deep-water carbonate 7. Conclusions ramp, whereas shallow-water carbonate and siliciclastic deposition interfingered in the foredeep on the Foreland-plate slope, foredeep stratigraphy, and transition zone. sediment composition patterns of coarse-grained Blountian foreland strata document along-strike varia- Acknowledgements tions genetically related to the rifted-margin configuration of southern Laurentia. The foredeep slope, as Graptolite determinations by Stan Finney (Depart- recorded by a change in depositional depth of coeval ment of Geological Sciences, California State Univer- platform carbonates, basinal shales, and sandstone sity at Long Beach) for collections from sections AB, turbidites was narrower and steeper on the Alabama CL, HV, FM and PF allowed a better age constraint for promontory than on the southwestern flank of the the progradation of the Blountian clastic wedge. Blue Tennessee embayment. Flexural reactivation of base- Circle Cement and Vulcan Materials Company allowed ment normal faults in the foredeep on the promontory access to different quarries in Georgia and Alabama. contributed to foreland-plate instability and the rapid This research was supported by grants from National drowning of the carbonate platform. Foredeep widen- Science Foundation (EAR-9706735) and the Petroleum ing and thickening of synorogenic strata on the Research Fund (33390) to W. A. Thomas, and grants southwestern flank of the embayment was controlled from the Geological Society of America and the primarily by the distal effects of flexural subsidence Graduate School of the University of Kentucky to G. on the Alabama promontory and by sediment-load Bayona. Comments made by R. Robinson, J. Castle and subsidence as siliciclastic depocenters migrated north- M. Underwood on an earlier version of this manuscript, eastward. Differential slope geometries along the and by D. Bradley, C. Fielding and anonymous Blountian foredeep were accommodated by reactiva- reviewer for Sedimentary Geology contributed to tion of transverse basement faults in the transition zone improving the content and edition of this paper. 132 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133

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