The Appalachian-Ouachita rifted margin of southeastern

WILLIAM A. THOMAS* Department of Geology, University of , Tuscaloosa, Alabama 35487

ABSTRACT component of extension propagated north- rocks of Early and Middle Cambrian age along eastward to form the intracratonic the Southern Oklahoma fault system are over- Promontories and embayments along the systems northeast of the transform fault, but stepped by post-rift strata of Late Cambrian age late Precambrian-early Paleozoic Appala- most of the extension of the Ouachita rift was (Ham and others, 1964). The purposes of this chian-Ouachita continental margin of south- transformed along the Alabama-Oklahoma article are to synthesize available data into an eastern North America are framed by a transform fault to the Mid-Iapetus Ridge interpretation of the mechanisms controlling the northeast-striking rift system offset by outboard from the Blue Ridge passive shape of the rifted margin and to consider the northwest-striking transform faults. Inboard margin. implications of differences in age of rifting. from the continental margin, fault INTRODUCTION systems have two sets of orientation; one is RIFT-RELATED ROCKS AND northeast parallel with rift segments, and the Late Precambrian-early Paleozoic rifting and STRUCTURES other is northwest parallel with transform opening of the Iapetus (proto-Atlantic) Ocean faults. produced a North American continental margin Blue Ridge Late Precambrian clastic and volcanic syn- along which the late Paleozoic Appalachian- rift rocks overlie Precambrian basement Ouachita orogenic belt subsequently formed General Setting. The Blue Ridge is an elon- rocks along the Appalachian Blue Ridge. (Figs. 1, 2). Several interpretations have con- gate external basement massif (Fig. 1) along Lower Cambrian at the base of a verged on the conclusion that a zigzag trace of which late Precambrian syn-rift sedimentary and transgressive passive-margin succession over- the Appalachian-Ouachita rifted margin out- volcanic rocks, as well as older basement rocks, steps the rift-fill successions and basement lines large-scale promontories and embayments have been translated and deformed by younger rocks, defining the time of transition from an in the edge of North American continental crust Appalachian compressional structures, espe- active rift to a passive margin along the Blue (for example, Hoffman and others, 1974; Cebull cially large-scale Alleghanian (late Paleozoic) Ridge. Locally thick Early Late Cambrian and others, 1976; Rankin, 1976; Thomas, 1976, thrust faults. Westward-directed thrust faults of and older sedimentary rocks fill downthrown 1977, 1985a; Lowe, 1985); however, these in- large displacement characterize the southern blocks of the intracratonic Mississippi Val- terpretations differ in detail and in mechanisms part of the Blue Ridge. Toward the northeast ley-Rough Creek-Rome graben system and of rifting. Among the more significant differ- along strike, the surface structure is a northeast- Birmingham basement fault system. These ences, each of the large-scale embayments in the plunging anticlinorium above a blind detach- basement fault systems, which indicate north- continental margin is interpreted (1) as framed ment. Although rift-related rocks are in several west-southeast extension like the Blue Ridge by an intersection of the rift with a transform separate thrust sheets, along-strike distribution is rift, are overstepped by Upper Cambrian fault (Thomas, 1976, 1977) or (2) as formed at defined by mapping, and across-strike distribu- strata. The northwest-striking Southern Ok- the intersection of two "successful" arms of a tion can be inferred from restored cross sections lahoma fault system is interpreted to be a three-armed radial rift (rift-rift-rift triple junction) (for example, see Rast and Kohles, 1986). transform fault that propagated into the con- (Burke and Dewey, 1973; Hoffman and others, The tectonic framework of accumulation of tinent from the Ouachita rift. Early and Mid- 1974; Rankin, 1976). The trace and nature of late Precambrian sedimentary and volcanic dle Cambrian rift-related igneous rocks along intracratonic fault systems that extend from the rocks along the Blue Ridge is generally inter- the fault system and adjacent Precambrian Appalachian-Ouachita orogen into the preted in the context of fault-bounded basins basement are overstepped by Upper Cam- ("aulacogens" as defined by N. S. Shatski; see along an Atlantic-type rifted continental margin brian sandstone. discussion in Hoffman and others, 1974) are (for example, Hatcher, 1972, 1978; Rankin, The differences in age of rift-related rocks critical to discrimination between these alterna- 1975, 1976; Thomas, 1976, 1977; Wehr and suggest a spreading-center shift at the begin- tives, because syn-rift intracratonic fault systems Glover, 1985; Rast and Kohles, 1986; Schwab, ning of the Cambrian Period from the Blue must be (1) intracratonic projections of either 1986; Simpson and Eriksson, 1989). In the Ridge rift to the Ouachita rift southwest of transform faults or rift segments or (2) the failed northwestern part of the Blue Ridge, rift-related the Alabama-Oklahoma transform fault. arms of three-armed radial rifts. Components of rocks are in laterally discontinuous and variable From Early to Early Late Cambrian, a small the Appalachian-Ouachita rift are diachronous. accumulations that overlie Precambrian (-1.0 For example, rift-related rocks in the Appala- Ga and older) crystalline basement rocks and chian Blue Ridge are overstepped by post-rift are overlain by post-rift strata in the Lower •Present address: Department of Geological Sci- strata of Early Cambrian age (Simpson and ences, University of Kentucky, Lexington, Kentucky Cambrian Chilhowee Group (Fig. 3). Along the 40506. Eriksson, 1989), whereas rift-related igneous southeastern side of the Blue Ridge, rift-related

Geological Society of America Bulletin, v. 103, p. 415-431, 6 figs., 1 table, March 1991.

415

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/3/415/3381149/i0016-7606-103-3-415.pdf by guest on 26 September 2021 J- y & t ^r northeastern limit \PENNy of Catoctin outcrop ^ along Blue Ridge southwestern limit of ^ Swift Run-Catoctin along SJ y ^ ,N/ northwest limb of Blue Ridge J\ //7s of EXPLANATION —v— cratonward limit of Appalachian-Ouachita detachment —»— thrust fault anticline - M> cratonward limit of Appalachian accreted intracratonic basement fault margin of Gulf and Atlantic Coastal Plains

outline of Altamaha magnetic anomaly

Marathon outcrop

Figure 1. Outline map of Appalachian-Ouachita orogenic belt and intracratonic fault systems. Locations of rift-related rocks are shown in present structural position. Map and locations of structures and rocks compiled from references cited in text. End points of cross sections of Figures 3,4, and 5 indicated by letters.

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rifted margin of continental crust O— • transform fault Intracratonlc basement fault

crustal-scale southeast-dipping seismic reflectors

palinspastically restored width of passive-margin shelf fades

OKLA

foc, %Q abrupt margin of \ i continental crust ARK. °°c, (PASSCAL data) cK ...

% V ST^S. \ %/>

^ MARATHON %>CPROMONTORY - SCALE EMBAYMENT^ ll 0 100 km

Figure 2. Outline map of interpreted late Precambrian-eariy Paleozoic continental margin as bounded by rift segments and transform faults. Map includes locations of observations that provide control for the reconstruction of the continental margin and intracratonic fault systems (compiled from references cited in text). Intersections between rift segments and transform faults are drawn orthogonally as a simplifying generalization. End points of cross sections of Figures 3,4, and 5 indicated by letters.

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# Knox # Knox • Mount Simon # Elbrook # Rome Base of transgressive Sauk sequence: # Shady Late Cambrian # Chilhowee

EXPLANATION Base of transgressive Sauk sequence: DOMINANT ROCK TYPES Early Cambrian OF LITHQSTRATIGRAPHIC UNITS • siliciclastic rocks # Grove # carbonate rocks # Frederick v volcanic rocks Araby

LOCATIONS OF KEY STRUCTURES present leading edge • palinspastic leading edge Rome trough sedimentary 10 km -, Early Late Cambrian and older SCALE (3.2 km) •v Mechum River (0.7 km) • Fauquier Goochland • Blue Ridge 50 km

B'

ROME TROUGH BLUE RIDGE RIFT • Knox • Conasauga • Rome • Shady • Chilhowee Shady (Lower Cambrian) shelf edge

Rome trough sedimentary Early Late Cambrian and older (1.9 km) •v Grandfather Mountain Ocoee (9 km) (12 km)

Kings Mountain belt Blue Ridge I

C Great Smoky Mountains Blue Ridge Corbin-Salem Church basement massif northern end of Catoctin outcrop DATUM: TOP OF CHILHOWEE Chilhowee Precambrian basement

• Ocoee v Catoctin v Mount Rogers c • Swift Run srr,b. ^ (12 km) (3 km) ar> b, (1 km) basalt in Unicoi (lower part of Chilhowee)

Figure 3. Palinspastic cross sections of the Blue Ridge rift and Rome trough. Cross sections A-A' and B-B' are perpendicular to strike; cross section C-C' is parallel with strike of the present Blue Ridge structures. Names of lithostratigraphic units in rift-related successions (maximum thickness in parentheses) are plotted below each cross section, and names of units in post-rift succession are plotted above each cross section. Data for cross sections compiled from references cited in text. End points of cross sections shown by letters in Figures 1 and 2.

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rocks and continental basement rocks are trun- on average finer (Rankin, 1970; Rankin and the Blue Ridge are ~1.0 Ga (Grenville) age and cated by thrust faults at the northwestern bound- others, 1989). older (Bartholomew and Lewis, 1984), and are ary of accreted-terrane rocks (Fig. 1) (Williams Along the northeastern Blue Ridge (on the intruded by the Crossnore (plutonic) Complex, and Hatcher, 1982; Horton and others, 1989; northwest limb and around the nose of the for which U-Pb and Rb-Sr studies indicate a 690 Rankin and others, 1989). northeast-plunging anticlinorium), the Catoctin ± 10 Ma age of crystallization (Odom and Full- Rift-Related Rocks. The Ocoee Supergroup Formation (Figs. 1, 3) consists of a basal thin agar, 1984). Other rocks correlated with the (Figs. 1, 3) of clastic sedimentary rocks extends veneer of alluvial clastic sedimentary rocks (also Crossnore Complex are cut by presumed feeder -270 km along the Blue Ridge and pinches out called Swift Run Formation) and a subaerial dikes of the Catoctin volcanic rocks and may be northeastward along present structural strike bimodal volcanic suite containing sedimentary as young as 630 to 650 Ma (Mose and Nagel, (Hadley, 1970; King, 1970; Rankin and others, interbeds (Reed, 1955; Schwab, 1986; Rankin 1984; Mose and Kline, 1986). The Fauquier, 1989). Much of the Ocoee clastic was and others, 1989). Rhyolite is abundant at the Mechum River (Lukert and Banks, 1984), and eroded from basement rocks on the northeast, northeasternmost exposures along the Blue Grandfather Mountain Formations (Bryant and but the lower part had a provenance of base- Ridge anticlinorium, but basalt dominates far- Reed, 1970) contain clasts of Crossnore-type ment rocks on the east and southeast (Hadley ther southwest (Rankin, 1975, 1976). The Ca- granite; therefore, the age of the Crossnore pro- and Goldsmith, 1963; King, 1964, 1970; Had- toctin gradually pinches out southwestward be- vides a maximum age of rifting (Wehr and ley, 1970). Deep-water turbidites compose most tween Precambrian basement rocks and the Glover, 1985). Volcanic rocks of the Catoctin of the Ocoee, but the lower part includes fluvial overlying Lower Cambrian Chilhowee Group Formation have an Rb-Sr whole-rock age of to shallow-marine deposits (King, 1964; Hadley, along the northwest limb of the Blue Ridge (Fig. 570 ± 36 Ma (Badger and Sinha, 1988), indicat- 1970; DeWindt, 1975; Rast and Kohles, 1986; 3) (Brown, 1970). Alluvial to shallow-water ing the time of youngest syn-rift volcanism. The Schwab, 1986). A general lack of volcanic and clastic deposits of the Fauquier Formation range of ages of the Crossnore and Catoctin sug- volcaniclastic rocks distinguishes the Ocoee (Swift Run equivalent) extend along the north- gests two stages of rifting (Badger and Sinha, from the other rift-related rocks of the Blue eastern part of the southeast limb of the Blue 1988). Although stratigraphic position beneath Ridge and indicates deposition in a basin that Ridge (Brown, 1970; Wehr and Glover, 1985). the Lower Cambrian Chilhowee Group clearly indicates a late Precambrian age for part of the was separated from other sites of rift-related ac- Along the northeastern part of the crest of the rift-fill accumulations, biostratigraphic data from cumulation (Fig. 2) (Rankin, 1975; Rankin and Blue Ridge anticlinorium, the Mechum River the syn-rift rocks are not definitive. Acritarchs others, 1989). The basement-rock provenance Formation (Figs. 1, 3) comprises a narrow belt collected from the Ocoee Supergroup were con- on the southeast further indicates a -and- of sedimentary rocks > 100 km long surrounded sidered to be of late Precambrian age (Knoll and graben structure within continental crust. by basement outcrops (Schwab, 1974). Sedi- Keller, 1979); however, those forms may range The Mount Rogers Formation (Figs. 1, 3) mentary structures and paleocunent indicators into the Paleozoic (A. H. Knoll, personal com- contains a bimodal suite of peralkaline rhyolite suggest alluvial deposition in a narrow rift valley mun., cited by Unrug and Unrug, 1990). Re- and basalt, geochemically indicative of continen- with basement-rock sediment sources on both cently discovered Paleozoic fossils in rocks tal rifting (Rankin, 1970, 1975, 1976). The sides (Schwab, 1974). previously mapped as upper Ocoee (Walden Mount Rogers also contains clastic sedimentary Tectonic Framework of Deposition. Distri- Creek Group) near the thrust front in the rocks mostly of alluvial origin, and includes butions of thickness and rock types of the late southwestern Blue Ridge require «interpreta- some glaciogenic rhythmites and dropstones Precambrian Ocoee, Mount Rogers, and Grand- tion of a possible fault or unconformable contact (Rankin, 1970; Schwab, 1976). Subaerial rhyo- father Mountain successions in the southwestern between the fossil-bearing strata and the bulk of lite ash flows and alluvial sedimentary deposits Blue Ridge suggest local steep-sided depositional the Ocoee (Unrug and Unrug, 1990). indicate a dominantly terrestrial origin for the basins framed by steep faults (Hadley, 1970; Mount Rogers (Rankin, 1970; Schwab, 1976; Rankin, 1975, 1976; Schwab, 1974, 1976, Age of Rift-to-Passive-Margin Transition. Wehr and Glover, 1985), and petrography of 1977,1986; Wehr and Glover, 1985; Rast and Regionally outside the extent of rift-related sed- clasts indicates a basement-rock provenance. Kohles, 1986). Clastic sediment derived from imentary and volcanic accumulations, sand- The Lower Cambrian Chilhowee Group over- up-faulted basement blocks locally accumulated stones at the base of the Chilhowee Group rest steps the Mount Rogers onto Precambrian to thickness in excess of 10 km (Fig. 3), indicat- nonconformably on basement rocks. The Chil- basement rocks both southwestward and north- ing the probable minimum vertical separation of howee unconformably overlies parts of the rift- eastward along strike, as well as across strike to some of the basement faults. Lower Cambrian related accumulations, but in some places, the northwest (Fig. 3) (Rankin, 1970). (Chilhowee Group) overlie late Pre- the contact between the Mount Rogers and The Grandfather Mountain Formation (Figs. cambrian rift-fill deposits and extend beyond the Chilhowee and that between the Ocoee and 1, 3), composed of sedimentary and volcanic boundary faults onto upthrown Precambrian Chilhowee are evidently gradational and con- rocks, is exposed only within the Grandfather basement rocks, indicating that fault movement formable (King, 1970; Rankin, 1970). Biostrati- Mountain window, where it overlies Precam- and basin filling pre-dated deposition of all but graphic data document an Early Cambrian age brian basement gneisses (Bryant and Reed, the oldest Chilhowee beds (lower part of Unicoi for all but the lower part of the Chilhowee 1970; King, 1970; Rankin, 1970, 1975). The Formation). The gradual pinch out of the Swift Group (Resser, 1938; Laurence and Palmer, sedimentary rocks are alluvial, and paleocurrent Run-Catoctin along the northeastern Blue 1963; Simpson and Sundberg, 1987). The base data suggest centripetal drainage in a steep-sided Ridge may be a result of truncation of relatively of the Chilhowee commonly has been consid- (fault-bounded?) basin (Schwab, 1977). The extensive rift-related plateau basalts (Reed, ered to mark the post-rift unconformity and the Grandfather Mountain Formation is composi- 1955; Schwab, 1986) outside the deep graben transition from rift to passive margin (Thomas, tionally somewhat similar to the Mount Rogers, system. 1977; Wehr and Glover, 1985; Fichter and but it differs in that basalt is more abundant than Age of Rift. The ages of rift-related rocks are Diecchio, 1986). In part of the southwestern rhyolite, and the sedimentary components are documented in a few places. Basement rocks of Blue Ridge, the lower part of the Chilhowee

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D PRESENT STRUCTURE D' MISSISSIPPI EMBAYMENT APPALACHIAN THRUST BELT (-) (LATE PALEOZOIC) 1 2 34 5 6 DATUM: SEA LEVEL

MISSISSIPPI VALLEY GRABEN

BIRMINGHAM BASEMENT FAULT SYSTEM

D PALINSPASTIC RESTORATION D' BIRMINGHAM MISSISSIPPI VALLEY BASEMENT FAULT GRABEN SYSTEM # several named units # Knox # Knox • Lamotte # Conasauga = Conasauga = Rome Base of transgressive Base of transgressive # Shady Sauk sequence: Sauk sequence: # Chilhowee , latest Middle to Middle Cambrian Early Late Cambrian Base of transgressive Sauk sequence: Early Cambrian DATUM: TOP OF LOWER ORDOVICIAN

Precambrian basement

Mississippi Valley graben sedimentary fill: Early Late Cambrian and older

Figure 4. Structural cross sections and palinspastic cross sections of the Mississippi Valley graben and Birmingham basement fault system. Cross sections are based on well data (wells identified by number in Table 1), proprietary seismic reflection profiles, and references cited in text. Structure of the basement beneath the Appalachian allochthon is interpreted from balanced structural cross sections based on outcrop geology, preserved stratigraphie thicknesses in Appalachian synclines, seismic reflection profiles, and sparse wells. Wells are indicated by vertical lines that show depth of drilling in the cross sections of present structure, and stratigraphie interval penetrated in the cross sections of palinspastic restoration. End points of cross sections shown by letters in Figures 1 and 2.

Group (lower part of Unicoi Formation) con- faults, and overstep of the faults by upper Unicoi howee is overlain by the transgressive Shady sists of alluvial-fan deposits and related sedi- deposits. Dolostone, and the Shady is overlain by fine- mentary facies and locally contains basalt flows Passive Margin. The Chilhowee succession grained clastic rocks of the Rome (Lower Cam- (Fig. 3) (Simpson and Eriksson, 1989). The from basal alluvial-fan deposits to marine depos- brian) and Conasauga (Middle Cambrian) basalt flows are stratigraphically below the its at the top (Brown, 1970; Schwab, 1972; Formations, the distribution of which indicates a horizon of the oldest Cambrian fossils, and may Mack, 1980; Simpson and Eriksson, 1989) re- source on the craton to the northwest (Rodgers, be equivalent to the upper part of the Catoctin flects transgression related to post-rift thermal 1953, 1968; Palmer, 1971). The lower part of (Simpson and Eriksson, 1989) and possibly subsidence (Bond and others, 1984), as well as the Rome Formation grades southeastward into uppermost Ocoee. Therefore, the transition from eustatic sea-level rise (Vail and others, 1977). a carbonate facies above the Shady Dolostone in rift to passive margin is within the Unicoi For- The Chilhowee and overlying Cambrian and southwestern Virginia, and in the most south- mation (lower Chilhowee) rather than below it; Lower Ordovician rocks constitute the eastern- easterly preserved strata in the footwall of the however, the age of the transition is very near most part of a craton-wide transgressive se- Blue Ridge frontal thrust fault, Shady facies de- the Precambrian-Cambrian boundary (Simpson quence, the Sauk sequence of Sloss (1963). The fine a shelf edge and a slope to the southeast and Eriksson, 1989). The thickest lower Unicoi base of the Sauk sequence is of Early Cambrian (Fig. 3) (Rodgers, 1968; Pheil and Read, 1980). reported by Simpson and Eriksson (1989) over- age along the Blue Ridge, but it is latest Cam- The clastic facies of the Conasauga grades east- lies the Mount Rogers Formation, suggesting brian in the interior of the North American ward to a carbonate facies (for example, El- continued movement of the basin-boundary craton (Fig. 3) (Sloss, 1963, 1988). The Chil- brook Formation) in the Appalachians of

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E PRESENT STRUCTURE E' MISSISSIPPI EMBAYMENT EXPLANATION (MESOZOIC-CENOZOIC) UNITS ON STRUCTURAL CROSS SECTIONS 11 12 13 14 15 _DATUM: M Mesozoic-Cenozoic SEA LEVEL mp Upper -Pennsylvanian o Middle Ordovician-Lower Mississippian k Knox Group and equivalent units c Cambrian below the Knox Group b Precambrian basement

DOMINANT ROCK TYPES OF LITHOSTRATIGRAPHIC UNITS • sandstone MISSISSIPPI VALLEY = fine-grained clastic rocks GRABEN # carbonate rocks

E PALINSPASTIC RESTORATION E' 2 km SCALE MISSISSIPPI VALLEY GRABEN # several named units # Knox 0 50 km • Lamotte # Conasauga # basal sandstone Base of transgressive Sauk sequence: Base of transgressive latest Middle to Sauk sequence: Early Late Cambrian Middle(?) Cambrian DATUM: Figure 4. (Continued). TOP OF LOWER ORDOVICIAN graphic data must be considered in palinspastic Precambrian basement location as determined from balanced structural cross sections. The Cambrian succession in the Appalachian # Knox (dark-colored fine-grained limestone) thrust belt in Alabama is similar to that adjacent = mudstone, fine-grained limestone to the Blue Ridge. A Lower and Middle Cam- # dolostone, limestone brian succession extends across the Birmingham # arkosic to quartzose sandstone basement fault system, but on the downthrown Mississippi Valley graben sedimentary fill: side (in palinspastic location), it is more than Early Late Cambrian and older twice as thick as on the upthrown side (Fig. 4) (Thomas, 1986). In thrust sheets in the south- eastern part of the thrust belt (palinspastically Tennessee and Virginia (Palmer, 1971). Above belt, down-to-northwest basement faults define southeast of the larger basement faults), the the Rome-Conasauga and equivalent carbonate the southeastern side of a graben along part of Lower Cambrian Chilhowee Group (sandstone, facies, carbonate-shelf deposits of the Upper the fault system. The regional décollement of the conglomerate, and mudstone) is >750 m thick Cambrian-Lower Ordovician Knox Group ex- Appalachian thrust belt is near the base of the (Fig. 4); however, the complete thickness is un- tend throughout the region west of the Blue Paleozoic stratigraphie succession, and strati- known because the lower part is detached at Ridge. A slope-to-shelf transition is included in the Frederick Limestone on the southeastern TABLE 1. WELLS IDENTIFIED BY NUMBER IN FIGURE 4 side of the northeastern Blue Ridge (Reinhardt,

1974). The shelf edge indicated by Shady Well Location Source (southwestern Blue Ridge) and Frederick (north- of data eastern Blue Ridge) facies suggests the position I. U.S. Bureau Mines No. 1 Olivet Sec. 29, T. 22 N, R. 11 E„ New Madrid Co., Mo. a, b of the boundary between thick continental crust 2. Strake No. 1 Russell Sec. 24, T. 19 N„ R. 11 E„ Pemiscot Co., Mo. b.c 3. Benz No. 1 Merritt Sec. 3, T. 4 S„ R. 1 E„ Lake Co., Tenn. d and attenuated continental crust or oceanic crust 4. Henderson No. 1 Rice Sec. 22, T. 4 S, T. 1 E„ Dyer Co., Tenn. a, b to the east (Rodgers, 1968; Thomas, 1977). 5. Big Chief No. 1 Taylor Sec. 19, T. 5 S., R. 6 E., Gibson Co., Tenn. d, e 6. du Pont No. 2 Fee Sec. 14, T. 6 S.. R. 19 E., Humphreys Co., Tenn. d 7. No. 1 Beeler Sec. 4, T. 15 S, R. 29 E„ Giles Co., Tenn. tU 8. Saga No. 1 Skidmore Sec. 36, T. 1 S„ R. I W„ Morgan Co., Ala. f,g Birmingham Basement Fault System 9. Saga No. 1 Hudson Sec. 16, T. 10 S„ R. 2 E„ Blount Co, Ala. f. 6 IO. ARCO No. 1 Edgmon Sec. 6, T. 7 N, R. 12 W, Faulkner Co., Ark. 11. Cockrell No. 1 Carter Sec. 4, T. 4 N, R. 1 E., St. Francis Co, Ark. e, h The Birmingham basement fault system in- 12. Pan American No. 1 Bosnick Sec. 1, T. 2 N, R. 1 E, Lee Co, Ark. c 13. Amerada No. 1 Abbay Sec. 21, T. 4 S„ R. 11 W, Tunica Co, Miss. c cludes several northeast-striking faults in the 14. Smith & Hess No. I Waldrop Sec. 15, T. 5 S„ R. 7 W, Tate Co, Miss. e subsurface beneath the Appalachian thrust belt 15. Pruet & Hughes No. \ Dunlap Sec. 18, T1 S„ R. 1 W, Lafayette Co, Miss.

(Figs. 2, 4) (Thomas, 1985b, 1986; Ferrill, Source of data:

1989). Large-scale frontal thrust ramps (for ex- a. Missouri Geological Survey open file. f. Alabama Geological Survey open file. ample, Birmingham anticlinorium) are posi- b. Grohskopf, 1955. g. Neathery and Copeland, 1983. c. Sample description by author. h. Denison, 1984. tioned over down-to-southeast basement faults d. Tennessee Division of Geology open file. i. B. R. Haley, unpublished data. (Figs. 1,4). Farther southeast beneath the thrust e. Unpublished industry report. j. Mellen, 1977.

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thrust faults (Mack, 1980). The depth to the margin succession along the northwestern side atically distributed; and contacts are gradational base of sedimentary rocks as determined from of the Blue Ridge. The time of initial movement both vertically and laterally. The arkosic sand- seismic reflection profiles! indicates no thick rift- along the Birmingham system is unknown; stone is interpreted as alluvial-fan deposits, and fill succession comparable to the Ocoee (Fig. 4). however, absence of thick late Precambrian rift- the upward transition to quartzose sandstone The Chilhowee includes fluvial to shallow- related rocks suggests Early Cambrian initiation. and carbonate rocks indicates transgression marine clastic derived from the craton Continued fault movement until Early Late through shoreline and shallow-marine environ- (Mack, 1980) and is overlain by the transgres- Cambrian clearly post-dates the time (Early ments (Howe, 1985; Weaverling, 1987). The sive Lower Cambrian Shady Dolostone. The Cambrian) of the rift-to-passive-margin transi- dark-colored, fine-grained rocks toward the top Shady is overlain by shallow-marine, fine- tion along the Blue Ridge, and overstep of the and on the northeast reflect a deeper shelf setting grained clastic rocks of the Rome and Cona- faults by the Knox Group marks the end of a in the graben. In a shallow in the sauga Formations, the youngest part of which is phase of extension along the Birmingham base- southeasternmost part of the graben in Missis- of Early Late Cambrian age (Resser, 1938; ment fault system. sippi, the succession is mostly limestone and Palmer, 1971). Part of the Conasauga clastic dolostone but contains some dark-colored mud- facies locally grades into a carbonate facies. Dis- Mississippi Valley Graben stone, dark-colored argillaceous limestone, an- tribution of thickness and facies within the hydrite, and a relatively thin basal sandstone Conasauga is generally related to separate Well data, gravity and magnetic data, and (Mellen, 1977). basement fault blocks (Ferrill, 1989). As docu- seismic surveys outline the southwest-striking Trilobites from three wells indicate an Early mented by deep wells northwest of the basement Mississippi Valley graben in Precambrian base- Late Cambrian (Dresbachian) age for the upper fault system, the Lower Cambrian Rome For- ment rocks and Paleozoic sedimentary rocks be- part of the clastic sequence in the Mississippi mation rests on Precambrian basement rocks, neath Mesozoic-Cenozoic cover in the Missis- Valley graben (Grohskopf, 1955; Palmer, 1962; and the Chilhowee and Shady are unconform- sippi Embayment of the Gulf Coastal Plain Weaverling, 1987; Missouri Geol. Survey open ably absent (Fig. 4) (Kidd and Neathery, 1975; (Figs. 1, 2) (Ervin and McGinnis, 1975; Harris, file), but the age of the oldest part of the graben Thomas, 1988). Northwestward onto the cra- 1975; Kane and others, 1981; Schwalb, 1982a, fill is unknown. The oldest rocks of the trans- ton, the sedimentary succession thins gradually, 1982b; Keller and others, 1983; Mooney and gressive Sauk sequence northwest (cratonward) and the base is progressively younger (Fig. 4). others, 1983; Hildenbrand, 1985; Howe, 1985; of the graben are a thin basal sandstone of latest Thickness and facies variations in the Chil- Thomas, 1985a, 1988). Fault separation of the Middle or Early Late Cambrian age (A. R. howee-Shady-Rome-Conasauga succession in- top of basement rocks is as much as 1.8 km (Fig. Palmer, 1989, personal commun.). These allu- dicate synsedimentary movement along the 4), but separation of Ordovician and younger vial to shallow-marine strata (Houseknecht and Birmingham basement fault system. The overly- Paleozoic strata is generally no more than 0.5 Ethridge, 1978) are equivalent in age to the ing Middle Upper Cambrian to Lower Ordovi- km, indicating late Paleozoic reactivation of upper part of the dark-colored mudstone within cian carbonate unit (Knox Group) extends Cambrian faults. A southwest-plunging late Pa- the graben, indicating an abrupt increase in across the fault system with no variations that leozoic arch parallels the fault system and is ap- water depth across the graben boundary. suggest synsedimentary fault movement. Local proximately coaxial with the broad southwest- Middle Late Cambrian and younger carbon- truncation of the upper Knox and stratigraphic plunging Mesozoic-Cenozoic syncline of the ate rocks overlie the graben-fill succession and variations in post-Knox rocks indicate episodic Mississippi Embayment (Fig. 4). Internal struc- extend widely on the craton as part of the reactivation of the basement fault system from tures within the graben include a steep, late Pa- craton-wide transgressive Sauk sequence (Sloss, Middle Ordovician to Pennsylvanian (Thomas, leozoic anticline (Howe, 1985) that is positioned 1988). Thickness of the Upper Cambrian- 1986; Ferrill, 1989), and the Birmingham base- over a basement fault. Kinematics and timing Lower Ordovician carbonate unit (Knox ment fault system is overridden by post-Middle suggest that the late Paleozoic structures were Group) generally has only regional-scale varia- Pennsylvanian (Alleghanian) Appalachian thrust caused by compression from the Ouachita oro- tions across the Mississippi Valley graben (Fig. faults. genic belt. The late Paleozoic structures are 4), indicating no substantial post-rift subsidence truncated by the sub- unconformity No shelf-edge facies have been recognized in in the region around the graben. Continued sub- which, along with the covering strata, is warped the Chilhowee-Knox succession, indicating that sidence after Early Late Cambrian time within into the Mississippi Embayment syncline. shelf deposition extended at least as far southeast the graben, however, is indicated by (1) locally as the palinspastic location of the trailing edge of The Mississippi Valley graben contains a thicker Upper Cambrian-Lower Ordovician the thrust belt. The Talladega slate belt (a low- Cambrian sedimentary fill that is > 1 km thick carbonate succession in the graben, suggesting grade metamorphic thrust sheet along the trail- on downthrown fault blocks and is lacking continued fault movement rather than regional ing edge of the thrust belt; Fig. 1) contains a outside the graben system (Fig. 4) (Kersting, downwarp; and (2) fine-grained, dark-colored marble (Sylacauga Marble Group) that is equiv- 1982; Schwalb, 1982a, 1982b; Howe, 1985; limestones in the southwestern part of the alent to the Shady through Knox succession Weaverling, 1987; Thomas, 1988; Houseknecht, graben, suggesting deeper water (Fig. 4) (Tull and others, 1988), indicating an even more 1989). Along the northwestern side of the (Thomas, 1988). By Late Cambrian time, the southeastwardly extensive shelf facies. Beneath graben, the graben-fill succession consists of a region around the Mississippi Valley graben evi- the marble, the Kahatchee Mountain Group of basal sandstone that is generally arkosic, but dently was in a stable cratonic setting at some clastic rocks is equivalent to the Chilhowee and quartzose toward the top; a middle unit of light- distance from any active rift. perhaps part of the Ocoee, but it is probably colored, partly oolitic limestone and dolostone; about 1.5 km thick (Tull, 1982; Tull and others, and an upper, dark-colored, partly calcareous Rough Creek Graben 1988). mudstone (Fig. 4) (Denison, 1984; Weaverling, Synsedimentary movement along the Bir- 1987; Thomas, 1988; Houseknecht, 1989). In The Rough Creek graben is bounded by east- mingham basement fault system during deposi- the northeastern part of the graben, in contrast, striking faults that are traced eastward from the tion of the Chilhowee-Shady-Rome-Conasauga siltstone and mudstone, fine-grained sandstone, northern end of the Mississippi Valley graben on (Early Cambrian to Early Late Cambrian) coin- and dark-colored fine-grained partly silty to ar- the basis of geophysical data and some subsur- cides temporally with deposition of the passive- gillaceous limestone apparently are nonsystem- face data (Figs. 1, 2). Maximum structural relief

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of the graben is uncertain, but possibly is several Northwest of the Rome trough, Upper Cam- the Ouachita embayment (Viele, 1973; Thomas, kilometers (Harris, 1975; Soderberg and Keller, brian-Lower Ordovician carbonate rocks of the 1976; Viele and Thomas, 1989). Distribution of 1981; Schwalb, 1982a, 1982b; Collinson and Knox Group generally overlie a thin sandstone the contrasting early Paleozoic carbonate-shelf others, 1988). The eastern part of the Rough unit (Upper Cambrian Mount Simon Sand- and deep-water facies indicates a shelf edge Creek graben is not well known, but the trend of stone) that rests unconformably on basement around the Ouachita region, and the deposi- the fault system suggests a connection with the rocks (Fig. 3) (Woodward, 1961; Webb, 1980). tional framework implies that a rifted margin of Rome trough across the Cincinnati arch in cen- In contrast, within the trough, the Knox Group continental crust controlled the location of the tral Kentucky (Harris, 1975). is underlain by a clastic sequence > 1 km thick shelf edge (Thomas, 1976,1977). No wells have The Rough Creek graben contains a Cam- dominated by sandstones and mudstones. The penetrated to depths necessary to sample any brian sedimentary fill >2.5 km thick (Schwalb, lower part of the sedimentary fill of the trough possible rift-related rocks beneath the Paleozoic 1982a; Collinson and others, 1988). Along the consists of a basal arkosic sandstone and an shelf edge. Seismic data indicate an abrupt northern part of the Rough Creek graben, espe- overlying carbonate unit interpreted by Webb southward decrease in crustal thickness from cially on the west at the connection to the Mis- (1980) to represent pre-fault deposition of the normal continental crust to oceanic crust or sissippi Valley graben, the sedimentary fill is Lower Cambrian Chilhowee and Shady, there- highly attenuated transitional crust beneath the mainly arkosic sandstone which is interpreted as by limiting the maximum age of fault move- Ouachita thrust belt (Figs. 1, 2) (Keller and oth- alluvial-fan deposits (Schwalb, 1982a; Weaver- ment; however, the ages of these rocks are not ers, 1989a). ling, 1987). In the central and southern part of documented biostratigraphically. Above the the Rough Creek graben, the succession is dom- basal units, a succession of siltstone, sandstone, Southern Oklahoma Fault System inated by mudstone, indicating persistence of mudstone, and carbonate rocks is laterally vari- deeper marine environments (Schwalb, 1982a; able (Webb, 1980). Middle to Late Cambrian The Southern Oklahoma fault system strikes Weaverling, 1987). A late Middle Cambrian age trilobites are reported from the fill of the eastern northwest from the Arbuckle Mountains of for part of the fill is documented by trilobites part of the trough (Donaldson and others, southern Oklahoma through the Wichita Moun- identified from one well (Schwalb, 1982b; 1975). Carbonate rocks of the Knox Group tains to the subsurface Amarillo uplift in Weaverling, 1987). The thick graben-fill succes- cross the Rome trough, and relatively uniform northwestern Texas (Fig. 1). Precambrian base- sion is overstepped by Upper Cambrian car- thickness distribution suggests neither substantial ment rocks and a Paleozoic sedimentary succes- bonate rocks, and north (cratonward) of the synsedimentary fault movement nor broad post- sion are displaced by a fault system which has graben, the lower part of the Sauk sequence fault downwarp in the region of the trough vertical separation of >12 km and probably consists of a thin basal sandstone and overlying (Fig. 3). The Cambrian succession southeast of larger strike-slip separation (see review by Perry, transgressive carbonate rocks. Facies and age re- the Rome trough is thinner than the graben-fill 1989; McConnell, 1989). Thick accumulations lationships indicate a history of Cambrian fault succession, but it is thicker and includes older of coarse clastic sediments indicate late Paleo- movement like that of the Mississippi Valley strata than the succession northwest of the zoic fault movement (extensive literature re- graben. Late Paleozoic reactivation along some trough (Fig. 3) (Harris, 1975; Webb, 1980), a viewed by Johnson and others, 1988; Perry, of the faults included inversion of vertical sepa- distribution that is consistent with northwest- 1989), which was associated with compression ration and strike-slip separation (Krausse and ward transgression onto the craton during the during the Ouachita orogeny (Hoffman and Treworgy, 1979); reactivation of the basement Cambrian and Early Ordovician. others, 1974; Kluth and Coney, 1983; Kluth, faults presumably was a result of northwest- 1986). An earlier phase of fault movement is directed compression from the Appalachian Ouachita Thrust Belt and Foreland indicated by an alignment of Cambrian igneous Tennessee salient and/or north-directed com- rocks that characterize the Southern Oklahoma pression from the Ouachita salient. No rift-related sedimentary or volcanic rocks fault system as rift related. comparable to those along the Blue Ridge have Syn-rift igneous rocks associated with the Rome Trough been recognized along the Ouachita thrust Southern Oklahoma fault system include an The Rome trough is traced from eastern Ken- belt, where the autochthonous passive-margin older part consisting of a layered gabbro com- tucky northeastward into on the carbonate-shelf facies extends southward be- plex, gabbro plutons, and basalt-spilite; and a basis of subsurface and geophysical data (Figs. 1, neath allochthonous deep-water rocks (Viele, younger part consisting of granite and rhyolite 2) (Woodward, 1961; Harris, 1975; Kulander 1979; Nelson and others, 1982; Lillie and others, (Gilbert, 1983). The lithologic association indi- and Dean, 1978; Ammerman and Keller, 1979; 1983; Lillie, 1985). In contrast to the Early cates a continental rift environment (Gilbert, Webb, 1980; Donaldson and Shumaker, 1981). Cambrian age of the base of the transgressive 1983). Crystallization age of the older, layered Structural relief of the top of Precambrian shelf facies of the Sauk sequence adjacent to the gabbro is interpreted to be 577 Ma (Rb-Sr and basement rocks is > 1 km between the deepest Blue Ridge, no shelf-facies strata older than Late Sm-Nd) (Lambert and Unruh, 1986), and the graben blocks and the boundaries of the trough Cambrian (or latest Middle Cambrian; A. R. younger gabbros yield an age of 552 ± 7 Ma (Fig. 3). A thick clastic graben-fill sequence, Palmer, 1989, personal commun.) have been (U-Pb zircon) (Bowring and Hoppe, 1982). The lacking outside the trough, clearly indicates sig- identified in the Ouachita foreland. The Upper granite and rhyolite are interpreted to be 525 ± nificant synsedimentary fault movement during Cambrian-Lower Ordovician carbonate facies 25 Ma (Rb-Sr) (Ham and others, 1964), and are the Middle Cambrian and possibly earlier, and at the top of the Sauk sequence extends west- intruded by diabase dikes. filling of the graben blocks by Late Cambrian ward throughout the Appalachian thrust belt The Cambrian igneous rocks evidently are time. Post-Late Cambrian to late Paleozoic and foreland basins of the eastern craton, and it restricted to a zone —65 km wide as indicated by reactivation of some of the faults is reflected by extends southward beneath the front of the distinct linear gravity and magnetic anomalies displacement of the younger Paleozoic rocks Ouachita allochthon. In the Ouachita thrust (Fig. 2) (Coffman and others, 1986). Displace- (Dever and others, 1977; Dever, 1986); how- belt, the oldest known rocks are of Late Cam- ment of Cambrian volcanic rocks with respect to ever, the reactivation produced displacements brian age, and the Cambrian-Ordovician succes- older basement rocks in the Arbuckle Moun- smaller than those of the earlier basement sion is an off-shelf deep-water facies deposited tains (Ham and others, 1964), as well as faults faulting. beyond the margin of continental crust within within some of the Cambrian volcanic rocks,

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suggests rift-bounding faults of >1 km vertical tary deposit within the Cambrian rift-related ig- Plain (Nicholas and Rozendal, 1975; Nicholas separation (McConnell and Gilbert, 1986). Epi- neous complex (Gilbert, 1983). and Waddell, 1989). Northwest of the Devils sodic fault movement is indicated by angular Regionally, Precambrian crystalline basement River uplift, the thrust belt curves 90° to the discordances between the layered complex and rocks and the Cambrian rift-related igneous southwest around the Marathon salient (Fig. 1). the younger gabbros (McConnell and Gilbert, rocks are overlain nonconformably by a trans- Cratonward from the thrust front in the Mara- 1986). gressive sequence consisting of the Upper Cam- thon salient, the present Permian basin is a late The Cambrian intrusive and extrusive igneous brian Reagan Sandstone and overlying carbon- Paleozoic structure superimposed on the early rocks post-date Precambrian crystalline base- ate rocks (Ham and others, 1964; Gilbert, 1983; Paleozoic Tobosa basin (Adams, 1965; Frenzel ment rocks (Tishomingo Granite) of 1.2 to 1.4 Coffman and others, 1986). Within the resolu- and others, 1988). Ga age, as well as strata of the Tillman Metased- tion of available data, the post-rift unconformity Regionally, Precambrian crystalline basement imentary Group (Ham and others, 1964; Bick- at the base of the Reagan Sandstone is the same rocks are nonconformably overlain by Upper ford and Lewis, 1979; Denison, in Johnson and age as the base of the carbonate rocks that over- Cambrian sandstone at the base of the transgres- others, 1988). The Tillman Metasedimentary step the faulted boundaries of the Mississippi sive Sauk sequence that is dominated by car- Group (known only from wells; Ham and oth- Valley-Rough Creek-Rome graben system and bonate rocks (Frenzel and others, 1988); sand- ers, 1964) previously was considered to be part the Birmingham basement fault system. Above stone in a similar stratigraphic position overlies of the fill of a graben that was associated with the Reagan Sandstone, Upper Cambrian and metasedimentary and metavolcanic rocks on the the emplacement of the Cambrian igneous Lower Ordovician carbonate rocks thicken re- Devils River uplift (Nicholas and Rozendal, rocks; however, subsequent work suggests that gionally into an elongate basin, the axis of which 1975). The metasedimentary-metavolcanic suc- the Tillman is as old as 1.0 to 1.2 Ga (Muehl- coincides with the trend of Cambrian igneous cession is -850 m thick and includes metarhyo- berger and others, 1967; Denison and others, rocks (Denison, in Johnson and others, 1988; lites which have ages of 699 ± 26 Ma (Rb-Sr) 1984; Coffman and others, 1986), indicating Perry, 1989). The section in the basin is more (Denison and others, 1977). These possibly rift- that the metasedimentary rocks are part of the than twice as thick as the regional average out- related rocks are underlain by more massive basement (rather than part of the fill) of the side, and the thicker sections are mainly lime- metaigneous-metasedimentary basement rocks, Southern Oklahoma fault system. Seismic re- stone in contrast to dolostone (Fig. 5) (Gate- which have ages of 1246 + 270 to 1121 ± flection profiles show a thick succession (7 to 10 wood, 1970; Denison, in Johnson and others, 244 Ma (Rb-Sr) (Nicholas and Rozendal, 1975; km) of layered reflectors that represent the Till- 1988). Middle Paleozoic rocks also reflect sub- Nicholas and Waddell, 1989). sidence of the basin, but the rate of differential man south of the Wichita uplift, but the layered A strongly positive gravity anomaly indicates subsidence decreased after Middle Ordovician reflectors are absent to the north (Brewer and a mass of dense mafic rocks beneath the present time (Johnson and others, 1988). The post-rift others, 1981,1983). The abrupt northward ter- Central Basin platform within the Permian subsidence evidently is a result of loading by the mination of the Tillman reflectors suggests a basin, prompting comparison with the Southern dense mafic igneous rocks at a shallow crustal Precambrian fault, the presence of which may Oklahoma fault system (Figs. 1, 2) (Keller and level. have influenced the location of the Early to others, 1985). Drill samples of layered gabbro Middle Cambrian Southern Oklahoma fault sys- on the Central Basin platform have ages of 1077 tem (Brewer and others, 1983). The relatively Devils River Uplift and Tobosa Basin ± 2 to 1163 ± 4 (U-Pb), indicating that these thin and discontinuous Meers Quartzite (pre- The Devils River uplift is a basement-cored rocks are not associated with late Precambrian- viously considered equivalent to the Tillman uplift along a northwest-trending segment of the early Paleozoic rifting (Keller and others, Group) is now thought to be the only sedimen- Ouachita orogenic belt beneath the Gulf Coastal 1989b). The early Paleozoic Tobosa basin is a

F' SOUTHERN OKLAHOMA FAULT SYSTEM

* Ellenburger = Arbuckle # Arbuckle • Riley • Reagan #• Timbered Hills

Base of transgressive Base of transgressive Base of transgressive Sauk sequence: Sauk sequence: Sauk sequence: Late Cambrian Late Cambrian Late Cambrian

DATUM: TOP OF LOWER ORDOVICIAN

EXPLANATION Precambrian basement Precambrian basement (includes Tillman Metasedimentary Group) 2 km - DOMINANT ROCK TYPES SCALE OF LITHOSTRATIGRAPHIC UNITS = limestone * dolostone ~~I #• carbonate rocks and sandstone Cambrian igneous rocks 50 km • sandstone

Figure 5. Diagrammatic palinspastic cross section of the Southern Oklahoma fault system. Cross section modified from Gatewood (1970) and Johnson and others (1988). End points of cross section shown by letters in Figures 1 and 2.

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broad regional downwarp of uncertain relation- rate depositional basins, respectively. Horsts and The continental margin southeast of the Pine ship to rift-related rocks or structures (Frenzel grabens within the rift are documented further Mountain internal massif coincides with the and others, 1988; Keller and others, 1989b). by sedimentary onlap patterns in the Lower trace of the linear Altamaha magnetic anomaly, Cambrian strata (Simpson and Eriksson, 1989). which is interpreted to be the signature of a late PALINSPASTIC LOCATION OF THE Although the strikes of individual rift-related Paleozoic suture (Fig. 1) (Higgins and Zietz, RIFTED MARGIN structures cannot be reconstructed from avail- 1983; Horton and others, 1984; Nelson and oth- able data, an area of basement fault blocks is ers, 1985; Hooper and Hatcher, 1988; McBride Syn-rift rocks and structures along the rifted well documented. Palinspastic restorations based and Nelson, 1988). The magnetic anomaly ex- margin and within the adjacent craton have on balanced structural cross sections place rift- tends to southwestern Alabama, where a cluster been modified by subsequent Appalachian- related rocks now in the Blue Ridge at a position of deep wells penetrated volcanic, plutonic, and Ouachita orogenesis, requiring palinspastic re- southeast of the present location of the Kings ultramafic rocks, suggesting an obducted arc and construction as an initial step in determining the Mountain belt in the (Hatcher, 1989; complex, probably near the margin trace of the rift. Post-orogenic erosion has de- Hatcher and others, 1989); southeast-dipping of continental crust (Figs. 1, 2) (Neathery and stroyed part of the record of rifting. Parts of the deep reflectors on COCORP seismic reflection Thomas, 1975; Thomas and others, 1989). Pal- late Paleozoic orogenic belt and older rifted con- profiles indicate a similar location of the rifted inspastic restoration of balanced structural cross tinental margin were modified by Mesozoic ex- margin (Fig. 2) (Cook and others, 1979, 1983). sections (Thomas, 1989) places shelf-facies tension and opening of the present Atlantic Autochthonous basement grabens, inboard from strata now in the trailing part of the subsurface Ocean and Gulf of , and parts are cov- the rifted margin and beneath the basal Appala- Appalachian thrust belt at least as far south as ered by Mesozoic-Cenozoic rifted- and passive- chian detachment, have been imaged seismically the trace of the Altamaha magnetic anomaly in margin deposits of the Atlantic and Gulf Coastal (Harris and others, 1981; Lillie, 1984; Favret southwestern Alabama, further suggesting the Plains (Fig. 1). and Williams, 1988), indicating a wide expanse original extent of the and the Large-scale bends in the rifted continental of brittlely extended continental crust adjacent approximate location of the continental margin margin have been recognized in a variety of in- to the rifted margin in the Tennessee embay- (Fig. 2). Inboard from the rifted margin, the terpretations. In this discussion of the location of ment (Figs. 2, 3). The thick rift-fill facies extensional Birmingham basement fault system the margin, the terminology of promontories (Ocoee) extends southwestward as far as the strikes northeastward approximately parallel and embayments is used for location purposes as Corbin-Salem Church external basement massif with the interpreted trace of the southeastern illustrated in Figure 2, but without implying a in Georgia (Fig. 1) (McConnell and Costello, margin of the Alabama promontory (Fig. 2). 1980,1984), but farther southwest, no thick rift- mechanism of origin. Wide-angle reflection/refraction seismic data fill successions are indicated (Fig. 4). The distribution of rift-related rocks and of (PASSCAL), interpreted via velocity models, the passive-margin shelf edge suggests that the Except for displacement by the Birmingham clearly document the southern margin of Pre- northeastern part of the Blue Ridge allochthon basement fault system, the top of basement is cambrian continental crust at a location beneath contains a relatively straight segment of the generally smooth and dips at a low angle from southward-dipping thrust sheets in the southern rifted margin that approximately parallels pres- beneath the foreland thrust belt southeastward part of the Ouachita Mountains of Arkansas ent strike (Rodgers, 1968; Thomas, 1977; Wehr to a position near the Pine Mountain internal (Figs. 1, 2) (Keller and others, 1989a). That and Glover, 1985). The Mechum River graben basement massif on the Alabama promontory location is consistent with COCORP reflection also parallels Blue Ridge strike. The autochtho- (Figs. 1, 2, 4) (as shown on COCORP seismic data and with gravity models (Nelson and oth- nous position of the rifted continental margin, as reflection profiles, Georgia lines 15,23, and 24). ers, 1982; Lillie and others, 1983). Continental interpreted from southeast-dipping deep seismic From the southeastern side of the Pine Moun- crust thins southward within —25 km to thin reflectors, is at a location east of the present tain massif, seismic reflectors dip southeastward transitional or oceanic crust (Keller and others, Goochland internal basement massif (Figs. 1, 2) to the base of the crust, marking the North 1989a). A boundary between regions of con- (Pratt and others, 1988). American continental margin at a boundary trasting magnetic signatures (Zietz, 1982; Hinze Southwest of the pinch-out of the Catoctin with later Paleozoic accreted terranes (Nelson and Braile, 1988) trends northwest-southeast Formation, a relatively thin Chilhowee Group and others, 1985; Hooper and Hatcher, 1988). from the seismically defined edge of continental rests nonconformably on basement rocks In reconstructions of balanced structural cross crust beneath the Ouachitas, outlining the Ala- (Fig. 3). To the southwest along present struc- sections, the Paleozoic shelf-facies strata now in bama promontory and Ouachita embayment tural strike, the Chilhowee thickens and overlaps the Appalachian thrust belt in Alabama spread (Fig. 2). The Mississippi Valley graben strikes the relatively thick Mount Rogers Formation; at least as far southeast as the present location of northeastward into the continent and is approx- farther southwest, the Chilhowee laps onto the the Pine Mountain internal massif (Figs. 1,2,4) imately perpendicular to the northwest-striking Ocoee Supergroup. The abrupt along-strike (Thomas, 1985b; Ferrill, 1989). Both the extent segment of the continental margin (Fig. 2). changes in rift-related rocks indicate that the dep- of the palinspastically reconstructed continental A segment of the continental margin on the ositional boundaries of both the Mount Rogers shelf and the seismic reflection data indicate that southeastern side of the Texas promontory (Fig. and the Ocoee intersect the Blue Ridge at large the present location of the Pine Mountain massif 2) is identified, primarily on the basis of gravity oblique to -90° angles (Fig. 2); therefore, re- coincides approximately with the margin of models, along a northeast-trending linear gravity gardless of the accuracy of palinspastic recon- North American Precambrian crust. Whether the high that is interpreted to mark the edge of con- struction, the general shape of the rifted margin basement and cover rocks of the Pine Mountain tinental crust (Kruger and Keller, 1986). Subsur- must include a right-lateral offset of the edge of internal massif represent an accreted microcon- face geology, seismic reflection profiles, and the rift from the Virginia promontory to the tinent (Thomas, 1977; Neathery and Thomas, gravity modeling indicate that the Waco uplift Tennessee embayment (Figs. 1, 2). 1983; Hooper and Hatcher, 1988) or a fault (Fig. 1) is a basement massif thrust onto the Variations in thickness and facies of the block from the North American margin margin of continental crust (Nicholas and Ocoee, Mount Rogers, and Grandfather Moun- (Schamel and others, 1980; Nelson and others, Rozendal, 1975) and serves as a guide to the tain indicate a complex of basement horsts and 1987), the massif must be near the autochtho- location of the margin. The Southern Oklahoma grabens that served as sediment sources and sepa- nous edge of North American continental crust. fault system is approximately perpendicular to

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the southeastern margin of the Texas promon- ample, the Blue Ridge rift is divided into three it may bend, at the intersection with the South- tory (Fig. 2), and magnetic signatures (Zietz, rift zones (Fig. 2). Smaller scale horst and ern Oklahoma fault system (Fig. 2). 1982) of the Cambrian igneous rocks along the graben blocks within the rift system are indi- The Southern Oklahoma fault system is paral- fault system terminate abruptly southeastward, cated by thickness and compositional variations lel but not aligned with the Alabama-Oklahoma marking the edge of continental crust (Keller in the Ocoee, Mount Rogers, and Grandfather transform fault. The orientation of the fault sys- and others, 1989c; Viele and Thomas, 1989). At Mountain rocks in the Tennessee embayment tem suggests a transform (or continental trans- the interpreted intersection between the South- (Figs. 2,3). These horsts and grabens are proba- fer) fault that propagated into continental crust. ern Oklahoma fault system and the continental bly about the same scale as the fundamental Possibly, the fault system followed a pre-existing margin, the northeast-trending gravity high horsts and grabens, "rift units," within the East crustal boundary between the Tillman Metased- bends abruptly to the east-northeast, crossing the African rift (Rosendahl, 1987). imentary Group and the older Precambrian corner of the Ouachita embayment on the out- The Virginia-Tennessee transform fault (Fig. crystalline rocks on the north. board side of a deep gravity low, and extending 2) is indicated by the offset of the continental The Mississippi Valley-Rough Creek-Rome to the margin of continental crust as indicated by margin from the Virginia promontory to the graben system, the Birmingham basement fault the PASSCAL seismic data (Figs. 1, 2). Tennessee embayment; by the abrupt along- system, and the Blue Ridge rift all trend gener- A northwest-trending segment of the conti- strike change from the relatively thin rift-related ally northeastward and reflect northwest-south- nental margin along the southwestern side of the rocks along the Northern Blue Ridge rift zone on east extension; however, the east-striking Rough Texas promontory is defined primarily by grav- the Virginia promontory to the thick rift-fill ac- Creek graben intersects and offsets the strike of ity models (Keller and others, 1985). The posi- cumulations along the Southern Blue Ridge rift the Mississippi Valley graben and Rome trough tion of the Devils River uplift suggests an zone in the Tennessee embayment; and by the (Fig. 2). The geometry of these structures in the external basement massif as a thrust sheet of northeastward termination of the wide area of context of regional extension suggests that the basement and cover displaced from the margin horsts and grabens that framed separate deposi- Rough Creek graben may be an oblique transfer of continental crust (Nicholas and Rozendal, tional basins of the Ocoee, Mount Rogers, and zone, possibly consisting of several rhomb 1975; Nicholas and Waddell, 1989). Grandfather Mountain. Alternatively, Rankin grabens. (1976) interpreted the Tennessee embayment as The intracratonic Mississippi Valley graben DISCUSSION OF THE TRACE the site of a three-armed rift triple junction, and parallels rift segments of the continental margin, OF THE RIFT AND MECHANISM Wehr and Glover (1985) suggested that the whereas the Southern Oklahoma fault system is OF RIFTING Ocoee was deposited in a separate failed rift parallel with transform faults (Fig. 2). Important inboard from the rifted continental margin. No contrasts between the two are consistent with The large-scale bends in the Appalachian- thick rift-fill deposits are indicated along the the interpretation that they originated by differ- Ouachita rifted margin are interpreted here to be Pine Mountain rift zone, suggesting that the ent mechanisms. The Mississippi Valley graben the result of transform offsets of a northeast- horsts and deep grabens of the Southern Blue is a belt of extensional faults -150 km wide, and striking rift system (Fig. 2) (Thomas, 1976, Ridge rift zone end southwestward at a trans- it is paralleled by the Birmingham basement 1977). The trace of the rifted margin as mapped form fault. fault system, another belt of extensional faults in Figure 2 is patterned after maps of recent At the interpreted position of the Alabama- that is —100 km wide (Figs. 2, 4). Throughout continental rifts and passive margins (for exam- Oklahoma transform fault along the Ouachita these fault systems, the graben blocks are filled ple, Rosendahl, 1987; Mascle and Blarez, 1987; embayment, the PASSCAL wide-angle reflec- with clastic sedimentary rocks to the evident Colletta and others, 1988). At the inception of tion/refraction data document southward thin- exclusion of volcanic rocks. Although gravity rifting, offsets of the rift, along-strike changes in ning of continental crust within a distance of modeling indicates anomalous upper mantle/ fault-block geometry, and differences in dip di- -25 km (Fig. 2) (Keller and others, 1989a). The lower crust beneath the Mississippi Valley rection of extensional faults are linked through narrow zone of transition from continental crust graben (Ervin and McGinnis, 1975; Mooney accommodation zones, transfer zones, or trans- to oceanic crust is typical of a transform fault and others, 1983), mafic magmas evidently did form faults (for example, Le Pichon and Hayes, (Keen, 1982; Scrutton, 1982b; Keen and Ha- not rise into the upper crust or sedimentary 1971; Scrutton, 1982a; Gibbs, 1984; Bosworth, worth, 1985), and it contrasts with the broad cover during Cambrian time. The Mississippi 1985; Lister and others, 1986a; Rosendahl, zone of attenuated continental crust that charac- Valley-Rough Creek-Rome and Birmingham 1987; Cochran and Martinez, 1988). Angles of terizes rifted margins, such as that in the Tennes- fault systems define shallow (as viewed in the intersection between extensional (rift) and shear see embayment. scale of thickness of the crust) grabens within continental crust, signifying much less extension (transform) structures of recent rifts range from The northeast-striking Ouachita rift zone pro- than along the Blue Ridge rift, which is a crustal- perpendicular to oblique (Freund, 1982; Lister jects toward an intersection with the northwest- scale fault system at the margin of continental and others, 1986b; Cochran and Martinez, striking Alabama-Oklahoma transform fault in crust. In contrast, the Southern Oklahoma fault 1988). The trace of the Appalachian-Ouachita southeastern Oklahoma (Fig. 2), but structure of system is more narrow (—65 km, as indicated by rift is not defined with sufficient precision to the intersection is problematic. The local east- gravity and magnetic anomalies), is character- support an interpretation of exact orientations of northeast trend of the gravity high along the ized by layered gabbros and rhyolites, and lacks the various segments; therefore, Figure 2 shows Ouachita margin may indicate an oblique seg- graben-filling sedimentary rocks (Gilbert, 1983; a simple generalization of orthogonal intersec- ment of the rifted margin. The deep gravity low Perry, 1989). The composition and geometry of tions between rift segments and transform faults. in the corner of the Ouachita embayment sug- the igneous rocks indicate crust-penetrating, The largest transform offsets of the Appala- gests a deep sedimentary basin formed during probably steep faults. Relatively rapid post-rift chian-Ouachita rift are spaced 500 to 800 km rifting along the Alabama-Oklahoma transform subsidence along the Southern Oklahoma fault apart, comparable in scale to the larger offsets of fault and parallel with the Southern Oklahoma system produced an elongate downwarp cen- the "rift zones" of the East African rift and other fault system (Kruger and Keller, 1986). The tered on the zone of dense igneous rocks (Fig. rift zones generally (Rosendahl, 1987). For ex- Ouachita rift zone is not notably offset, although

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5). In contrast, after Early Late Cambrian fault regime rather than parts of two separate, plume- beyond the position of the most southwesterly movement along the Mississippi Valley-Rough generated, three-armed rift triple junctions that recognized rift-related rocks in the Blue Ridge Creek-Rome graben system, more limited sub- expired simultaneously. The orientation of the rift. Possibly the rift intersected the Texas trans- sidence was restricted to parts of the area within structures (approximately orthogonal to each form, which may have been active in the late the graben-boundary faults (Figs. 3, 4). other) places rigid geometric constraints on the Precambrian as suggested by the age of metavol- The origin of the Southern Oklahoma fault possible shape of the rifted margin. Movement canic rocks along the Devils River uplift. system as a transform (or continental transfer) of a rectangular block in a direction parallel By the beginning of Cambrian time, the Blue fault may include vertical separation and/or with the interpreted transform direction is kine- Ridge rift had progressed to a stage in which a transtension. Fault-bounded basins are located matically and geometrically balanced (Fig. 2), passive margin was flanked by an open ocean where transform faults (traced from the Mid- but a rigorous geometric model including two basin (Iapetus), and spreading continued at a Atlantic Ridge) offset the present Atlantic con- three-armed rift triple junctions cannot be mid-ocean ridge (Mid-Iapetus Ridge) (Fig. 6B). tinental margins of Africa and South America; balanced geometrically or kinematically. In ad- At the same time, extension was active along the faults result from differential vertical movement dition to these specific problems of local applica- Mississippi Valley-Rough Creek-Rome and of continental crust at different distances from tion, recent discussions discredit three-armed rift Birmingham intracratonic fault systems, and the mid-ocean ridge (Francheteau and Le Pi- triple junctions driven by thermal doming as a mafic magmas were emplaced along the South- chon, 1972). Another possible cause of verti- general mechanism for the initiation of continen- ern Oklahoma fault system. cal separation is implied by simple-shear exten- tal rifting (Mohr, 1982; Rosendahl, 1987). The The difference in age of rifting indicates a shift sion models in which continental transfer faults transform offset of the rift to form the Ouachita in the spreading center at about the beginning partition low-angle extension faults either at embayment and the inboard propagation of of the Cambrian Period from the southwestern fault offsets or between faults of opposite dip transform faults along the Southern Oklahoma part of the Blue Ridge rift to the Ouachita rift directions (Gibbs, 1984; Wernicke, 1985; Lister fault system are similar to the framework of the (Fig. 6B). The age of the spreading-center shift and others, 1986a). The large transform offset of embayment in the present Atlantic margin of is latest Precambrian to Early Cambrian on the the rift from the Alabama promontory to the Africa and the Benue trough, respectively (Fran- basis of age of the igneous rocks (577 Ma) along Ouachita embayment implies that, even if the cheteau and Le Pichon, 1972; Benkhelil and the Southern Oklahoma fault system. Robineau, 1983; Popoff and others, 1983; dip direction is the same, the tectonic level of The spreading-center shift and initiation of Benkhelil and others, 1988). low-angle extension faults would differ greatly Ouachita rifting were accompanied by initiation on opposite sides of the transform. The conti- of the Southern Oklahoma fault system, as well nental crustal block southwest of the transform CONCLUSIONS: HISTORY OF THE as the Alabama-Oklahoma transform fault (con- is near the Ouachita rifted margin and must be RIFTED MARGIN tinental transform fault), which ultimately differentially thinned; it may have been dis- formed the margin of continental crust. The placed vertically with respect to the crust north- Northwest-striking transform faults and north- northeast-striking active segment of the Ouach- east of the transform, which was much farther east-striking rift segments along the Appa- ita rift ended against the Alabama-Oklahoma from the Appalachian rifted margin. The length lachian-Ouachita continental margin are con- transform fault in the Ouachita embayment of the Southern Oklahoma fault system, as well sistent mechanically with northwest-southeast (Fig. 6B). Most of the extension along the Ouach- as the volume of mafic magma emplaced along extension along the entire rift system (Fig. 2). ita rift was transformed along the Alabama- it, argues for transtensional movement. Mass- Post-rift sedimentary overstep marks the end of Oklahoma transform fault to the Mid-Iapetus balance calculations suggest Cambrian extension active rifting along the Blue Ridge rift in the Ridge outboard from the Blue Ridge passive of 17 to 21 km across the Southern Oklahoma earliest Cambrian (Fig. 3). Transgressive Upper margin. A small component of crustal extension fault system (McConnell and Gilbert, 1986). Cambrian carbonate rocks overstep rift-fill suc- propagated northeastward across the Alabama- In the context of a continental margin framed cessions in the intracratonic Mississippi Valley- Oklahoma transform fault into the Mississippi by transform offsets of a rift, the Southern Okla- Rough Creek-Rome and Birmingham fault Valley-Rough Creek-Rome and Birmingham homa fault system is a continental transform systems (Fig. 4). Igneous rocks along the basement fault systems, but the basement fault fault, and the Mississippi Valley graben is a rift Southern Oklahoma fault system as young as systems northeast of the transform failed to open zone. An alternative interpretation attributes the 525 ± 25 Ma are nonconformably overlain by a new ocean (Fig. 6C). As the Ouachita mid- shape of the margin to separate, plume- sandstones of Late Cambrian age (Fig. 5). Post- ocean ridge spread and the Ouachita ocean generated, three-armed rift triple junctions at rift transgression over the Ouachita rifted mar- opened, the northeast end of the ridge migrated which failed arms are represented by the South- gin in the late Middle Cambrian or Late along the Alabama-Oklahoma transform fault ern Oklahoma fault system (Burke and Dewey, Cambrian is suggested by the age of the base of (Fig. 6C). An active continent-ocean transform 1973; Hoffman and others, 1974) and the Mis- the transgressive sequence around the Ouachita prevailed in front of the migrating ridge- sissippi Valley graben (Ervin and McGinnis, foreland (Figs. 4, 5). The differences in ages of transform intersection, and a passive transform 1975). Contrasts in the nature of the fault sys- rift-related rocks and of post-rift sedimentary margin formed behind it (processes described by tems, in the syn-rift rocks, and in the tectonic overstep indicate that the Ouachita margin, as Scrutton, 1982b; modeled by Todd and Keen, history of crustal-scale structures favor different well as the Mississippi Valley-Rough Creek- 1989). processes (as transform and rift, respectively) Rome and Birmingham intracratonic fault sys- By early in Late Cambrian time, the north- rather than a common process (as failed arms of tems, is younger than the Appalachian margin. east end of the Ouachita ridge had moved two separate three-armed rifts) of origin for Rifting began in late Precambrian time along beyond the corner of continental crust on the these two fault systems. The post-rift strata over- the Blue Ridge rift (Fig. 6A). Lack of late Pre- Alabama promontory, and a passive margin stepping the two fault systems are of the same cambrian rift-related rocks along the Ouachita had evolved along the entire rift and transform age (Middle Late Cambrian), suggesting that the margin suggests that the initial rift continued margin (Fig. 6D). A spreading half-rate of 1.3 to two are components of a single, larger kinematic southwestward for some unknown distance 1.6 cm/yr is calculated for the Ouachita rift/

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Figure 6. Sequential diagrammatic maps il- lustrating interpretation olF history of the late Precambrian-early Paleozoic Appalachian- Ouachita rifted margin of southern North America. Outline of state of Arkansas on each map for consistent location. A. Late Precambrian, -580 Ma. B. Early Cambrian, -565 Ma. C. Middle Cambrian, -540 Ma. D. Late Cambrian, -515 Ma.

/fa*

B - EARLY CAMBRIAN

EXPLANATION

11 active rift

Ji— transform fault

:::::::: rift-fill coarse clastic sedimentary rocks

* < rift volcanic and plutonic rocks

rift-fill sedimentary and volcanic rocks

HKrEH inferred thin and/or fine-grained sedimentary rift-fill rocks

graben-fill sedimentary rocks, intracratonic fault system

i i passive-margin shelf facies

passive-margin off-shelf facies (rifted-margin prism)

" Ò ! « oceanic crust

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Figure 6. (Continued).

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Cook, F. A., Brown, L. D., Kaufman, S., and Oliver, J. E., 1983, The CO- physical Research, v. 90, no. B14, p. 12607-12622. ridge from the time of spreading-center shift to CORP seismic reflection traverse across the southern Appalachians: Hinze, W. J., and Braile, L. W., 1988, Geophysical aspects of the craton: U.S., that of establishment of a passive transform American Association of Petroleum Geologists Studies in Geology in Sloss, L. L., ed., Sedimentary cover-North American craton: U.S.: No. 14,61 p. Geological Society of America, The Geology of North America, margin. Denison, R.E., 1984, Basement rocks in northern Arkansas: Arkansas Geologi- Volume D-2, p. 5-24. cal Commission Miscellaneous Publication 18-B, p. 33-49. Hoffman, P., Dewey, J. F., and Burke, K., 1974, Aulacogens and their genetic Denison, R. E., Burke, W. H., Otto, J. B., and Hetherington, E. A., 1977, Age relation to geosynclines, with a example from Great Slave ACKNOWLEDGMENTS of igneous and metamorphic activity affecting the Ouachita foidbelt, in Lake, : Society of Economic Paleontologists and Mineralogists Stone, C. G., ed., Symposium on the geology of the Ouachita Moun- Special Publication 19, p. 38-55. tains, Volume I: Arkansas Geological Commission, p. 25-40. Hooper, R. J., and Hatcher, R. D., Jr., 1988, Pine Mountain terrane, a complex Denison, R. E., Lidiak, E. G., Bickford, M. E., and Kisvarsanyi. E. B., 1984, window in the Georgia and Alabama Piedmont; evidence from the Reviews of the manuscript by Mervin J. Bar- Geology and geochronology of Precambrian rocks in the Central Inte- eastern termination: Geology, v. 16, p. 307-310. tholomew; Lynn Glover III; Robert D. Hatcher, rior region of the : U.S. Geological Survey Professional Horton, J. W., Jr., Zietz, I., and Neathery, T. L., 1984, Truncation of the Paper 1241-C, 20 p. 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MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 27,1989 ages of the plutonic suite of the Crossnore Complex, southern Appala- Sloss, L .L., 1963, Sequences in the cratonic interior of North America: Geolog- REVISED MANUSCRIPT RECEIVED JUNE 26, 1990 chians, and their implications regarding the time of opening of the ical Society of America Bulletin, v. 74, p. 93-114. MANUSCRIPT ACCEPTED JULY 2,1990

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