geological society of america bulletin 1991 thomas 415 31

The Appalachian-Ouachita rifted margin of southeastern North America

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

ABSTRACT

Promontories and embayments along the late Precambrian-early Paleozoic Appala-chian-Ouachita continental margin of south-eastern North America are framed by a northeast-striking rift system offset by northwest-striking transform faults. Inboard from the continental margin, basement fault systems have two sets of orientation; one is northeast parallel with rift segments, and the other is northwest parallel with transform faults.

Late Precambrian clastic and volcanic syn-rift rocks overlie Precambrian basement rocks along the Appalachian Blue Ridge. Lower Cambrian sandstone at the base of a transgressive passive-margin succession over-steps the rift-fill successions and basement rocks, defining the time of transition from an active rift to a passive margin along the Blue Ridge. Locally thick Early Late Cambrian and older sedimentary rocks fill downthrown blocks of the intracratonic Mississippi Val-ley-Rough Creek-Rome graben system and Birmingham basement fault system. These basement fault systems, which indicate north-west-southeast extension like the Blue Ridge rift, are overstepped by Upper Cambrian strata. The northwest-striking Southern Ok-lahoma fault system is interpreted to be a transform fault that propagated into the con-tinent from the Ouachita rift. Early and Mid-dle Cambrian rift-related igneous rocks along the fault system and adjacent Precambrian basement are overstepped by Upper Cam-brian sandstone.

The differences in age of rift-related rocks suggest a spreading-center shift at the begin-ning of the Cambrian Period from the Blue Ridge rift to the Ouachita rift southwest of the Alabama-Oklahoma transform fault. From Early to Early Late Cambrian, a small

Present address: Department of Geological Sci-ences, University of Kentucky, Lexington, Kentucky 40506.

component of extension propagated north-eastward to form the intracratonic fault systems northeast of the transform fault, but most of the extension of the Ouachita rift was transformed along the Alabama-Oklahoma transform fault to the Mid-Iapetus Ridge outboard from the Blue Ridge passive margin.

INTRODUCTION

Late Precambrian-early Paleozoic rifting and opening of the Iapetus (proto-Atlantic) Ocean produced a North American continental margin along which the late Paleozoic Appalachian-Ouachita orogenic belt subsequently formed (Figs. 1, 2). Several interpretations have con-verged on the conclusion that a zigzag trace of the Appalachian-Ouachita rifted margin out-lines large-scale promontories and embayments in the edge of North American continental crust (for example, Hoffman and others, 1974; Cebull and others, 1976; Rankin, 1976; Thomas, 1976, 1977, 1985a; Lowe, 1985); however, these in-terpretations differ in detail and in mechanisms of rifting. Among the more significant differ-ences, each of the large-scale embayments in the continental margin is interpreted (1) as framed by an intersection of the rift with a transform fault (Thomas, 1976, 1977) or (2) as formed at the intersection of two "successful" arms of a three-armed radial rift (rift-rift-rift triple junction) (Burke and Dewey, 1973; Hoffman and others, 1974; Rankin, 1976). The trace and nature of intracratonic fault systems that extend from the Appalachian-Ouachita orogen into the craton ("aulacogens" as defined by N. S. Shatski; see discussion in Hoffman and others, 1974) are critical to discrimination between these alterna-tives, because syn-rift intracratonic fault systems must be (1) intracratonic projections of either transform faults or rift segments or (2) the failed arms of three-armed radial rifts. Components of the Appalachian-Ouachita rift are diachronous. For example, rift-related rocks in the Appala-chian Blue Ridge are overstepped by post-rift strata of Early Cambrian age (Simpson and Eriksson, 1989), whereas rift-related igneous

rocks of Early and Middle Cambrian age along the Southern Oklahoma fault system are over-stepped by post-rift strata of Late Cambrian age (Ham and others, 1964). The purposes of this article are to synthesize available data into an interpretation of the mechanisms controlling the shape of the rifted margin and to consider the implications of differences in age of rifting.

RIFT-RELATED ROCKS AND STRUCTURES

Blue Ridge

General Setting. The Blue Ridge is an elon-gate external basement massif (Fig. 1) along which late Precambrian syn-rift sedimentary and volcanic rocks, as well as older basement rocks, have been translated and deformed by younger Appalachian compressional structures, espe-cially large-scale Alleghanian (late Paleozoic) thrust faults. Westward-directed thrust faults of large displacement characterize the southern part of the Blue Ridge. Toward the northeast along strike, the surface structure is a northeast-plunging anticlinorium above a blind detach-ment. Although rift-related rocks are in several separate thrust sheets, along-strike distribution is defined by mapping, and across-strike distribu-tion can be inferred from restored cross sections (for example, see Rast and Kohles, 1986).

The tectonic framework of accumulation of late Precambrian sedimentary and volcanic rocks along the Blue Ridge is generally inter-preted in the context of fault-bounded basins along an Atlantic-type rifted continental margin (for example, Hatcher, 1972, 1978; Rankin, 1975, 1976; Thomas, 1976, 1977; Wehr and Glover, 1985; Rast and Kohles, 1986; Schwab, 1986; Simpson and Eriksson, 1989). In the northwestern part of the Blue Ridge, rift-related rocks are in laterally discontinuous and variable accumulations that overlie Precambrian (-1.0 Ga and older) crystalline basement rocks and are overlain by post-rift strata in the Lower Cambrian Chilhowee Group (Fig. 3). Along the southeastern side of the Blue Ridge, rift-related

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

4 1 5

on September 17, 2015gsabulletin.gsapubs.orgDownloaded from

EXPLANATION

v cratonward limit of Appalachian-Ouachita detachment thrust fault

anticline - M> cratonward limit of Appalachian accreted terranes

intracratonic basement fault

margin of Gulf and Atlantic Coastal Plains

southwestern limit of ^ Swift Run-Catoctin along SJ y ^ ,N/ northwest limb of Blue Ridge J \ / /7s of

\PENNy

J - y & t ^ r northeastern limit

of Catoctin outcrop ^ along Blue Ridge

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.


EXPLANATION

Orifted margin of continental crust transform fault

Intracratonlc basement fault

crustal-scale southeast-dipping seismic reflectors

palinspastically restored width of passive-margin shelf fades

O K L A foc, %Q abrupt margin of \ i continental crust ARK.

c, (PASSCAL data) c K

...

% V TEXAS S T ^ S .

\ %/>

^ MARATHON % > C P R O M O N T O R Y -EMBAYMENT^ ll

SCALE 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.


A R O M E T R O U G H

A' B L U E R I D G E RIFT

E X P L A N A T I O N

DOMINANT ROCK TYPES OF LITHQSTRATIGRAPHIC UNITS siliciclastic rocks # carbonate rocks v volcanic rocks

LOCATIONS OF KEY STRUCTURES present leading edge

palinspastic leading edge

10 km -, SCALE

# Knox Mount Simon

Base of transgressive Sauk sequence: Late Cambrian

# Knox # Elbrook # Rome # Shady # Chilhowee

Base of transgressive Sauk sequence: Early Cambrian

# Grove # Frederick

Araby

Rome trough sedimentary Early Late Cambrian and older (3.2 km)

50 km

v Mechum River (0.7 km) Fauquier

Goochland terrane Blue Ridge

B'

R O M E T R O U G H B L U E R I D G E R IFT

Knox Conasauga Rome Shady Chilhowee Shady (Lower Cambrian)

shelf edge

Rome trough sedimentary Early Late Cambrian and older (1.9 km)

Ocoee (12 km)

v Grandfather Mountain (9 km)

Kings Mountain belt Blue Ridge I

Corbin-Salem Church basement massif

Great Smoky Mountains Blue Ridge C

DATUM: TOP OF CHILHOWEE northern end of Catoctin outcrop

Ocoee csrr,b. ^ (12 km)

ar> b,

Chilhowee Precambrian basement

v Mount Rogers (3 km)

basalt in Unicoi (lower part of Chilhowee)

v Catoctin Swift Run

(1 km)

Figure 3. Palinspastic cross sections of the Blue Ridge rift and R o m e trough. Cross sect ions A - A ' and B - B ' are perpendicular to strike; cross section C - C ' is parallel with strike o f the present Blue Ridge structures. Names of lithostratigraphic units in rift-related successions (maximum thickness in parentheses) are plotted b e l o w 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 s h o w n by letters in Figures 1 and 2.


APPALACHIAN-OUACHITA RIFTED MARGIN 4 1 9

rocks and continental basement rocks are trun-cated by thrust faults at the northwestern bound-ary of accreted-terrane rocks (Fig. 1) (Williams and Hatcher, 1982; Horton and others, 1989; Rankin and others, 1989).

Rift-Related Rocks. The Ocoee Supergroup (Figs. 1, 3) of clastic sedimentary rocks extends - 2 7 0 km along the Blue Ridge and pinches out northeastward along present structural strike (Hadley, 1970; King, 1970; Rankin and others, 1989). Much of the Ocoee clastic sediment was eroded from basement rocks on the northeast, but the lower part had a provenance of base-ment rocks on the east and southeast (Hadley and Goldsmith, 1963; King, 1964, 1970; Had-ley, 1970). Deep-water turbidites compose most of the Ocoee, but the lower part includes fluvial to shallow-marine deposits (King, 1964; Hadley, 1970; DeWindt, 1975; Rast and Kohles, 1986; Schwab, 1986). A general lack of volcanic and volcaniclastic rocks distinguishes the Ocoee from the other rift-related rocks of the Blue Ridge and indicates deposition in a basin that was separated from other sites of rift-related ac-cumulation (Fig. 2) (Rankin, 1975; Rankin and others, 1989). The basement-rock provenance on the southeast further indicates a horst-and-graben structure within continental crust.

The Mount Rogers Formation (Figs. 1, 3) contains a bimodal suite of peralkaline rhyolite and basalt, geochemically indicative of continen-tal rifting (Rankin, 1970, 1975, 1976). The Mount Rogers also contains clastic sedimentary rocks mostly of alluvial origin, and includes some glaciogenic rhythmites and dropstones (Rankin, 1970; Schwab, 1976). Subaerial rhyo-lite ash flows and alluvial sedimentary deposits indicate a dominantly terrestrial origin for the Mount Rogers (Rankin, 1970; Schwab, 1976; Wehr and Glover, 1985), and petrography of clasts indicates a basement-rock provenance. The Lower Cambrian Chilhowee Group over-steps the Mount Rogers onto Precambrian basement rocks both southwestward and north-eastward along strike, as well as across strike to the northwest (Fig. 3) (Rankin, 1970).

The Grandfather Mountain Formation (Figs. 1, 3), composed of sedimentary and volcanic rocks, is exposed only within the Grandfather Mountain window, where it overlies Precam-brian basement gneisses (Bryant and Reed, 1970; King, 1970; Rankin, 1970, 1975). The sedimentary rocks are alluvial, and paleocurrent data suggest centripetal drainage in a steep-sided (fault-bounded?) basin (Schwab, 1977). The Grandfather Mountain Formation is composi-tionally somewhat similar to the Mount Rogers, but it differs in that basalt is more abundant than rhyolite, and the sedimentary components are

on average finer (Rankin, 1970; Rankin and others, 1989).

Along the northeastern Blue Ridge (on the northwest limb and around the nose of the northeast-plunging anticlinorium), the Catoctin Formation (Figs. 1, 3) consists of a basal thin veneer of alluvial clastic sedimentary rocks (also called Swift Run Formation) and a subaerial bimodal volcanic suite containing sedimentary interbeds (Reed, 1955; Schwab, 1986; Rankin and others, 1989). Rhyolite is abundant at the northeasternmost exposures along the Blue Ridge anticlinorium, but basalt dominates far-ther southwest (Rankin, 1975, 1976). The Ca-toctin gradually pinches out southwestward be-tween Precambrian basement rocks and the overlying Lower Cambrian Chilhowee Group along the northwest limb of the Blue Ridge (Fig. 3) (Brown, 1970). Alluvial to shallow-water clastic deposits of the Fauquier Formation (Swift Run equivalent) extend along the north-eastern part of the southeast limb of the Blue Ridge (Brown, 1970; Wehr and Glover, 1985).

Along the northeastern part of the crest of the Blue Ridge anticlinorium, the Mechum River Formation (Figs. 1, 3) comprises a narrow belt of sedimentary rocks > 100 km long surrounded by basement outcrops (Schwab, 1974). Sedi-mentary structures and paleocunent indicators suggest alluvial deposition in a narrow rift valley with basement-rock sediment sources on both sides (Schwab, 1974).

Tectonic Framework of Deposition. Distri-butions of thickness and rock types of the late Precambrian Ocoee, Mount Rogers, and Grand-father Mountain successions in the southwestern Blue Ridge suggest local steep-sided depositional basins framed by steep faults (Hadley, 1970; Rankin, 1975, 1976; Schwab, 1974, 1976, 1977,1986; Wehr and Glover, 1985; Rast and Kohles, 1986). Clastic sediment derived from up-faulted basement blocks locally accumulated to thickness in excess of 10 km (Fig. 3), indicat-ing the probable minimum vertical separation of some of the basement faults. Lower Cambrian sandstones (Chilhowee Group) overlie late Pre-cambrian rift-fill deposits and extend beyond the boundary faults onto upthrown Precambrian basement rocks, indicating that fault movement and basin filling pre-dated deposition of all but the oldest Chilhowee beds (lower part of Unicoi Formation). The gradual pinch out of the Swift Run-Catoctin along the northeastern Blue Ridge may be a result of truncation of relatively extensive rift-related plateau basalts (Reed, 1955; Schwab, 1986) outside the deep graben system.

Age of Rift. The ages of rift-related rocks are documented in a few places. Basement rocks of

the Blue Ridge are ~1.0 Ga (Grenville) age and older (Bartholomew and Lewis, 1984), and are intruded by the Crossnore (plutonic) Complex, for which U-Pb and Rb-Sr studies indicate a 690 10 Ma age of crystallization (Odom and Full-agar, 1984). Other rocks correlated with the Crossnore Complex are cut by presumed feeder dikes of the Catoctin volcanic rocks and may be as young as 630 to 650 Ma (Mose and Nagel, 1984; Mose and Kline, 1986). The Fauquier, Mechum River (Lukert and Banks, 1984), and Grandfather Mountain Formations (Bryant and Reed, 1970) contain clasts of Crossnore-type granite; therefore, the age of the Crossnore pro-vides a maximum age of rifting (Wehr and Glover, 1985). Volcanic rocks of the Catoctin Formation have an Rb-Sr whole-rock age of 570 36 Ma (Badger and Sinha, 1988), indicat-ing the time of youngest syn-rift volcanism. The range of ages of the Crossnore and Catoctin sug-gests two stages of rifting (Badger and Sinha, 1988). Although stratigraphic position beneath the Lower Cambrian Chilhowee Group clearly indicates a late Precambrian age for part of the rift-fill accumulations, biostratigraphic data from the syn-rift rocks are not definitive. Acritarchs collected from the Ocoee Supergroup were con-sidered to be of late Precambrian age (Knoll and Keller, 1979); however, those forms may range into the Paleozoic (A. H. Knoll, personal com-mun., cited by Unrug and Unrug, 1990). Re-cently discovered Paleozoic fossils in rocks previously mapped as upper Ocoee (Walden Creek Group) near the thrust front in the southwestern Blue Ridge require interpreta-tion of a possible fault or unconformable contact between the fossil-bearing strata and the bulk of the Ocoee (Unrug and Unrug, 1990).

Age of Rift-to-Passive-Margin Transition. Regionally outside the extent of rift-related sed-imentary and volcanic accumulations, sand-stones at the base of the Chilhowee Group rest nonconformably on basement rocks. The Chil-howee unconformably overlies parts of the rift-related accumulations, but in some places, the contact between the Mount Rogers and Chilhowee and that between the Ocoee and Chilhowee are evidently gradational and con-formable (King, 1970; Rankin, 1970). Biostrati-graphic data document an Early Cambrian age for all but the lower part of the Chilhowee Group (Resser, 1938; Laurence and Palmer, 1963; Simpson and Sundberg, 1987). The base of the Chilhowee commonly has been consid-ered to mark the post-rift unconformity and the transition from rift to passive margin (Thomas, 1977; Wehr and Glover, 1985; Fichter and Diecchio, 1986). In part of the southwestern Blue Ridge, the lower part of the Chilhowee


4:20 W. A. THOMAS

D PRESENT STRUCTURE MISSISSIPPI EMBAYMENT

(MESOZOIC-CENOZOIC) 1 2 3 4 5 6

MISSISSIPPI VALLEY GRABEN

D' APPALACHIAN THRUST BELT

(LATE PALEOZOIC) DATUM: SEA LEVEL

BIRMINGHAM BASEMENT FAULT

SYSTEM

D PALINSPASTIC RESTORATION


# several named units Lamotte

D'

Base of transgressive Sauk sequence: latest Middle to Early Late Cambrian

DATUM: TOP OF LOWER ORDOVICIAN

# Knox # Conasauga

Base of transgressive Sauk sequence: Middle Cambrian

BIRMINGHAM BASEMENT FAULT

S Y S T E M # Knox = Conasauga = Rome # Shady # Chilhowee ,

Base of transgressive Sauk sequence: Early Cambrian

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-sists of alluvial-fan deposits and related sedi-mentary facies and locally contains basalt flows (Fig. 3) (Simpson and Eriksson, 1989). The basalt flows are stratigraphically below the horizon of the oldest Cambrian fossils, and may be equivalent to the upper part of the Catoctin (Simpson and Eriksson, 1989) and possibly uppermost Ocoee. Therefore, the transition from rift to passive margin is within the Unicoi For-mation (lower Chilhowee) rather than below it; however, the age of the transition is very near the Precambrian-Cambrian boundary (Simpson and Eriksson, 1989). The thickest lower Unicoi reported by Simpson and Eriksson (1989) over-lies the Mount Rogers Formation, suggesting continued movement of the basin-boundary

faults, and overstep of the faults by upper Unicoi deposits.

Passive Margin. The Chilhowee succession from basal alluvial-fan deposits to marine depos-its at the top (Brown, 1970; Schwab, 1972; Mack, 1980; Simpson and Eriksson, 1989) re-flects transgression related to post-rift thermal subsidence (Bond and others, 1984), as well as eustatic sea-level rise (Vail and others, 1977). The Chilhowee and overlying Cambrian and Lower Ordovician rocks constitute the eastern-most part of a craton-wide transgressive se-quence, the Sauk sequence of Sloss (1963). The base of the Sauk sequence is of Early Cambrian age along the Blue Ridge, but it is latest Cam-brian in the interior of the North American craton (Fig. 3) (Sloss, 1963, 1988). The Chil-

howee is overlain by the transgressive Shady Dolostone, and the Shady is overlain by fine-grained clastic rocks of the Rome (Lower Cam-brian) and Conasauga (Middle Cambrian) Formations, the distribution of which indicates a source on the craton to the northwest (Rodgers, 1953, 1968; Palmer, 1971). The lower part of the Rome Formation grades southeastward into a carbonate facies above the Shady Dolostone in southwestern Virginia, and in the most south-easterly preserved strata in the footwall of the Blue Ridge frontal thrust fault, Shady facies de-fine a shelf edge and a slope to the southeast (Fig. 3) (Rodgers, 1968; Pheil and Read, 1980). The clastic facies of the Conasauga grades east-ward to a carbonate facies (for example, El-brook Formation) in the Appalachians of


APPALACHIAN-OUACHITA RIFTED MARGIN 421

E PRESENT STRUCTURE MISSISSIPPI EMBAYMENT

(MESOZOIC-CENOZOIC) 11 12 13 14 15


E'

_DATUM: SEA LEVEL

EXPLANATION

UNITS ON STRUCTURAL CROSS SECTIONS M mp o

k

c

b

Mesozoic-Cenozoic Upper Mississippian-Pennsylvanian Middle Ordovician-Lower Mississippian Knox Group and equivalent units Cambrian below the Knox Group Precambrian basement

DOMINANT ROCK TYPES OF LITHOSTRATIGRAPHIC UNITS sandstone = fine-grained clastic rocks # carbonate rocks

E PALINSPASTIC RESTORATION

# several named units Lamotte

Base of transgressive Sauk sequence: latest Middle to Early Late Cambrian



Precambrian basement

E'

# Knox # Conasauga # basal sandstone

Base of transgressive Sauk sequence: Middle(?) Cambrian

2 km

# Knox (dark-colored fine-grained limestone) = mudstone, fine-grained limestone # dolostone, limestone # arkosic to quartzose sandstone

Mississippi Valley graben sedimentary fill: Early Late Cambrian and older

Tennessee and Virginia (Palmer, 1971). Above the Rome-Conasauga and equivalent carbonate facies, carbonate-shelf deposits of the Upper Cambrian-Lower Ordovician Knox Group ex-tend throughout the region west of the Blue Ridge. A slope-to-shelf transition is included in the Frederick Limestone on the southeastern side of the northeastern Blue Ridge (Reinhardt, 1974). The shelf edge indicated by Shady (southwestern Blue Ridge) and Frederick (north-eastern Blue Ridge) facies suggests the position of the boundary between thick continental crust and attenuated continental crust or oceanic crust to the east (Rodgers, 1968; Thomas, 1977).

Birmingham Basement Fault System

The Birmingham basement fault system in-cludes several northeast-striking faults in the subsurface beneath the Appalachian thrust belt (Figs. 2, 4) (Thomas, 1985b, 1986; Ferrill, 1989). Large-scale frontal thrust ramps (for ex-ample, Birmingham anticlinorium) are posi-tioned over down-to-southeast basement faults (Figs. 1,4). Farther southeast beneath the thrust

SCALE

belt, down-to-northwest basement faults define the southeastern side of a graben along part of the fault system. The regional dcollement of the Appalachian thrust belt is near the base of the Paleozoic stratigraphie succession, and strati-

0 50 km

Figure 4. (Continued).

graphic data must be considered in palinspastic location as determined from balanced structural cross sections.

The Cambrian succession in the Appalachian thrust belt in Alabama is similar to that adjacent to the Blue Ridge. A Lower and Middle Cam-brian succession extends across the Birmingham basement fault system, but on the downthrown side (in palinspastic location), it is more than 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 southeast of the larger basement faults), the Lower Cambrian Chilhowee Group (sandstone, conglomerate, and mudstone) is >750 m thick (Fig. 4); however, the complete thickness is un-known because the lower part is detached at

TABLE 1. WELLS IDENTIFIED BY NUMBER IN FIGURE 4

Well Location Source of data

I. U.S. Bureau Mines No. 1 Olivet Sec. 29, T. 22 N , R. 11 E New Madrid Co., Mo. a, b 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 4. Henderson No. 1 Rice Sec. 22, T. 4 S , T. 1 E Dyer Co., Tenn. a, b 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. California No. 1 Beeler Sec. 4, T. 15 S , R. 29 E Giles Co., Tenn. t U 8. Saga No. 1 Skidmore Sec. 36, T. 1 S R. I W Morgan Co., Ala. f , g 9. Saga No. 1 Hudson Sec. 16, T. 10 S R. 2 E Blount C o , 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 C o , Ark. e, h 12. Pan American No. 1 Bosnick Sec. 1, T. 2 N , R. 1 E , Lee C o , Ark. c 13. Amerada No. 1 Abbay Sec. 21, T. 4 S R. 11 W , Tunica C o , Miss. c 14. Smith & Hess No. I Waldrop Sec. 15, T. 5 S R. 7 W , Tate C o , Miss. e 15. Pruet & Hughes No. \ Dunlap Sec. 18, T 1 S R. 1 W , Lafayette C o , Miss.

Source of data:

a. Missouri Geological Survey open file. f. Alabama Geological Survey open file. b. Grohskopf, 1955. g. Neathery and Copeland, 1983. c. Sample description by author. h. Denison, 1984. d. Tennessee Division of Geology open file. i. B. R. Haley, unpublished data. e. Unpublished industry report. j. Mellen, 1977.


422 W. A. THOMAS

thrust faults (Mack, 1980). The depth to the base of sedimentary rocks as determined from seismic reflection profiles! indicates no thick rift-fill succession comparable to the Ocoee (Fig. 4). The Chilhowee includes fluvial to shallow-marine clastic sediments derived from the craton (Mack, 1980) and is overlain by the transgres-sive Lower Cambrian Shady Dolostone. The Shady is overlain by shallow-marine, fine-grained clastic rocks of the Rome and Cona-sauga Formations, the youngest part of which is of Early Late Cambrian age (Resser, 1938; Palmer, 1971). Part of the Conasauga clastic facies locally grades into a carbonate facies. Dis-tribution of thickness and facies within the Conasauga is generally related to separate basement fault blocks (Ferrill, 1989). As docu-mented by deep wells northwest of the basement fault system, the Lower Cambrian Rome For-mation rests on Precambrian basement rocks, and the Chilhowee and Shady are unconform-ably absent (Fig. 4) (Kidd and Neathery, 1975; Thomas, 1988). Northwestward onto the cra-ton, the sedimentary succession thins gradually, and the base is progressively younger (Fig. 4).

Thickness and facies variations in the Chil-howee-Shady-Rome-Conasauga succession in-dicate synsedimentary movement along the Birmingham basement fault system. The overly-ing Middle Upper Cambrian to Lower Ordovi-cian carbonate unit (Knox Group) extends across the fault system with no variations that suggest synsedimentary fault movement. Local truncation of the upper Knox and stratigraphic variations in post-Knox rocks indicate episodic reactivation of the basement fault system from Middle Ordovician to Pennsylvanian (Thomas, 1986; Ferrill, 1989), and the Birmingham base-ment fault system is overridden by post-Middle Pennsylvanian (Alleghanian) Appalachian thrust faults.

No shelf-edge facies have been recognized in the Chilhowee-Knox succession, indicating that shelf deposition extended at least as far southeast as the palinspastic location of the trailing edge of the thrust belt. The Talladega slate belt (a low-grade metamorphic thrust sheet along the trail-ing edge of the thrust belt; Fig. 1) contains a marble (Sylacauga Marble Group) that is equiv-alent to the Shady through Knox succession (Tull and others, 1988), indicating an even more southeastwardly extensive shelf facies. Beneath the marble, the Kahatchee Mountain Group of clastic rocks is equivalent to the Chilhowee and perhaps part of the Ocoee, but it is probably about 1.5 km thick (Tull, 1982; Tull and others, 1988).

Synsedimentary movement along the Bir-mingham basement fault system during deposi-tion of the Chilhowee-Shady-Rome-Conasauga (Early Cambrian to Early Late Cambrian) coin-cides temporally with deposition of the passive-

margin succession along the northwestern side of the Blue Ridge. The time of initial movement along the Birmingham system is unknown; however, absence of thick late Precambrian rift-related rocks suggests Early Cambrian initiation. Continued fault movement until Early Late Cambrian clearly post-dates the time (Early Cambrian) of the rift-to-passive-margin transi-tion along the Blue Ridge, and overstep of the faults by the Knox Group marks the end of a phase of extension along the Birmingham base-ment fault system.

Mississippi Valley Graben

Well data, gravity and magnetic data, and seismic surveys outline the southwest-striking Mississippi Valley graben in Precambrian base-ment rocks and Paleozoic sedimentary rocks be-neath Mesozoic-Cenozoic cover in the Missis-sippi Embayment of the Gulf Coastal Plain (Figs. 1, 2) (Ervin and McGinnis, 1975; Harris, 1975; Kane and others, 1981; Schwalb, 1982a, 1982b; Keller and others, 1983; Mooney and others, 1983; Hildenbrand, 1985; Howe, 1985; Thomas, 1985a, 1988). Fault separation of the top of basement rocks is as much as 1.8 km (Fig. 4), but separation of Ordovician and younger Paleozoic strata is generally no more than 0.5 km, indicating late Paleozoic reactivation of Cambrian faults. A southwest-plunging late Pa-leozoic arch parallels the fault system and is ap-proximately coaxial with the broad southwest-plunging Mesozoic-Cenozoic syncline of the Mississippi Embayment (Fig. 4). Internal struc-tures within the graben include a steep, late Pa-leozoic anticline (Howe, 1985) that is positioned over a basement fault. Kinematics and timing suggest that the late Paleozoic structures were caused by compression from the Ouachita oro-genic belt. The late Paleozoic structures are truncated by the sub-Cretaceous unconformity which, along with the covering strata, is warped into the Mississippi Embayment syncline.

The Mississippi Valley graben contains a Cambrian sedimentary fill that is > 1 km thick on downthrown fault blocks and is lacking outside the graben system (Fig. 4) (Kersting, 1982; Schwalb, 1982a, 1982b; Howe, 1985; Weaverling, 1987; Thomas, 1988; Houseknecht, 1989). Along the northwestern side of the graben, the graben-fill succession consists of a basal sandstone that is generally arkosic, but quartzose toward the top; a middle unit of light-colored, partly oolitic limestone and dolostone; and an upper, dark-colored, partly calcareous mudstone (Fig. 4) (Denison, 1984; Weaverling, 1987; Thomas, 1988; Houseknecht, 1989). In the northeastern part of the graben, in contrast, siltstone and mudstone, fine-grained sandstone, and dark-colored fine-grained partly silty to ar-gillaceous limestone apparently are nonsystem-

atically distributed; and contacts are gradational both vertically and laterally. The arkosic sand-stone is interpreted as alluvial-fan deposits, and the upward transition to quartzose sandstone and carbonate rocks indicates transgression through shoreline and shallow-marine environ-ments (Howe, 1985; Weaverling, 1987). The dark-colored, fine-grained rocks toward the top and on the northeast reflect a deeper shelf setting in the graben. In a shallow fault block in the southeasternmost part of the graben in Missis-sippi, the succession is mostly limestone and dolostone but contains some dark-colored mud-stone, dark-colored argillaceous limestone, an-hydrite, and a relatively thin basal sandstone (Mellen, 1977).

Trilobites from three wells indicate an Early Late Cambrian (Dresbachian) age for the upper part of the clastic sequence in the Mississippi Valley graben (Grohskopf, 1955; Palmer, 1962; Weaverling, 1987; Missouri Geol. Survey open file), but the age of the oldest part of the graben fill is unknown. The oldest rocks of the trans-gressive Sauk sequence northwest (cratonward) of the graben are a thin basal sandstone of latest Middle or Early Late Cambrian age (A. R. Palmer, 1989, personal commun.). These allu-vial to shallow-marine strata (Houseknecht and Ethridge, 1978) are equivalent in age to the upper part of the dark-colored mudstone within the graben, indicating an abrupt increase in water depth across the graben boundary.

Middle Late Cambrian and younger carbon-ate rocks overlie the graben-fill succession and extend widely on the craton as part of the craton-wide transgressive Sauk sequence (Sloss, 1988). Thickness of the Upper Cambrian-Lower Ordovician carbonate unit (Knox Group) generally has only regional-scale varia-tions across the Mississippi Valley graben (Fig. 4), indicating no substantial post-rift subsidence in the region around the graben. Continued sub-sidence after Early Late Cambrian time within the graben, however, is indicated by (1) locally thicker Upper Cambrian-Lower Ordovician carbonate succession in the graben, suggesting continued fault movement rather than regional downwarp; and (2) fine-grained, dark-colored limestones in the southwestern part of the graben, suggesting deeper water (Fig. 4) (Thomas, 1988). By Late Cambrian time, the region around the Mississippi Valley graben evi-dently was in a stable cratonic setting at some distance from any active rift.

Rough Creek Graben

The Rough Creek graben is bounded by east-striking faults that are traced eastward from the northern end of the Mississippi Valley graben on the basis of geophysical data and some subsur-face data (Figs. 1, 2). Maximum structural relief


APPALACHIAN-OUACHITA RIFTED MARGIN 423

of the graben is uncertain, but possibly is several kilometers (Harris, 1975; Soderberg and Keller, 1981; Schwalb, 1982a, 1982b; Collinson and others, 1988). The eastern part of the Rough Creek graben is not well known, but the trend of the fault system suggests a connection with the Rome trough across the Cincinnati arch in cen-tral Kentucky (Harris, 1975).

The Rough Creek graben contains a Cam-brian sedimentary fill >2.5 km thick (Schwalb, 1982a; Collinson and others, 1988). Along the northern part of the Rough Creek graben, espe-cially on the west at the connection to the Mis-sissippi Valley graben, the sedimentary fill is mainly arkosic sandstone which is interpreted as alluvial-fan deposits (Schwalb, 1982a; Weaver-ling, 1987). In the central and southern part of the Rough Creek graben, the succession is dom-inated by mudstone, indicating persistence of deeper marine environments (Schwalb, 1982a; Weaverling, 1987). A late Middle Cambrian age for part of the fill is documented by trilobites identified from one well (Schwalb, 1982b; Weaverling, 1987). The thick graben-fill succes-sion is overstepped by Upper Cambrian car-bonate rocks, and north (cratonward) of the graben, the lower part of the Sauk sequence consists of a thin basal sandstone and overlying transgressive carbonate rocks. Facies and age re-lationships indicate a history of Cambrian fault movement like that of the Mississippi Valley graben. Late Paleozoic reactivation along some of the faults included inversion of vertical sepa-ration and strike-slip separation (Krausse and Treworgy, 1979); reactivation of the basement faults presumably was a result of northwest-directed compression from the Appalachian Tennessee salient and/or north-directed com-pression from the Ouachita salient.

Rome Trough

The Rome trough is traced from eastern Ken-tucky northeastward into Pennsylvania on the basis of subsurface and geophysical data (Figs. 1, 2) (Woodward, 1961; Harris, 1975; Kulander and Dean, 1978; Ammerman and Keller, 1979; Webb, 1980; Donaldson and Shumaker, 1981). Structural relief of the top of Precambrian basement rocks is > 1 km between the deepest graben blocks and the boundaries of the trough (Fig. 3). A thick clastic graben-fill sequence, lacking outside the trough, clearly indicates sig-nificant synsedimentary fault movement during the Middle Cambrian and possibly earlier, and filling of the graben blocks by Late Cambrian time. Post-Late Cambrian to late Paleozoic reactivation of some of the faults is reflected by displacement of the younger Paleozoic rocks (Dever and others, 1977; Dever, 1986); how-ever, the reactivation produced displacements smaller than those of the earlier basement faulting.

Northwest of the Rome trough, Upper Cam-brian-Lower Ordovician carbonate rocks of the Knox Group generally overlie a thin sandstone unit (Upper Cambrian Mount Simon Sand-stone) that rests unconformably on basement rocks (Fig. 3) (Woodward, 1961; Webb, 1980). In contrast, within the trough, the Knox Group is underlain by a clastic sequence > 1 km thick dominated by sandstones and mudstones. The lower part of the sedimentary fill of the trough consists of a basal arkosic sandstone and an overlying carbonate unit interpreted by Webb (1980) to represent pre-fault deposition of the Lower Cambrian Chilhowee and Shady, there-by limiting the maximum age of fault move-ment; however, the ages of these rocks are not documented biostratigraphically. Above the basal units, a succession of siltstone, sandstone, mudstone, and carbonate rocks is laterally vari-able (Webb, 1980). Middle to Late Cambrian trilobites are reported from the fill of the eastern part of the trough (Donaldson and others, 1975). Carbonate rocks of the Knox Group cross the Rome trough, and relatively uniform thickness distribution suggests neither substantial synsedimentary fault movement nor broad post-fault downwarp in the region of the trough (Fig. 3). The Cambrian succession southeast of the Rome trough is thinner than the graben-fill succession, but it is thicker and includes older strata than the succession northwest of the trough (Fig. 3) (Harris, 1975; Webb, 1980), a distribution that is consistent with northwest-ward transgression onto the craton during the Cambrian and Early Ordovician.

Ouachita Thrust Belt and Foreland

No rift-related sedimentary or volcanic rocks comparable to those along the Blue Ridge have been recognized along the Ouachita thrust belt, where the autochthonous passive-margin carbonate-shelf facies extends southward be-neath allochthonous deep-water rocks (Viele, 1979; Nelson and others, 1982; Lillie and others, 1983; Lillie, 1985). In contrast to the Early Cambrian age of the base of the transgressive shelf facies of the Sauk sequence adjacent to the Blue Ridge, no shelf-facies strata older than Late Cambrian (or latest Middle Cambrian; A. R. Palmer, 1989, personal commun.) have been identified in the Ouachita foreland. The Upper Cambrian-Lower Ordovician carbonate facies at the top of the Sauk sequence extends west-ward throughout the Appalachian thrust belt and foreland basins of the eastern craton, and it extends southward beneath the front of the Ouachita allochthon. In the Ouachita thrust belt, the oldest known rocks are of Late Cam-brian age, and the Cambrian-Ordovician succes-sion is an off-shelf deep-water facies deposited beyond the margin of continental crust within

the Ouachita embayment (Viele, 1973; Thomas, 1976; Viele and Thomas, 1989). Distribution of the contrasting early Paleozoic carbonate-shelf and deep-water facies indicates a shelf edge around the Ouachita region, and the deposi-tional framework implies that a rifted margin of continental crust controlled the location of the shelf edge (Thomas, 1976,1977). No wells have penetrated to depths necessary to sample any possible rift-related rocks beneath the Paleozoic shelf edge. Seismic data indicate an abrupt southward decrease in crustal thickness from normal continental crust to oceanic crust or highly attenuated transitional crust beneath the Ouachita thrust belt (Figs. 1, 2) (Keller and oth-ers, 1989a).

Southern Oklahoma Fault System

The Southern Oklahoma fault system strikes northwest from the Arbuckle Mountains of southern Oklahoma through the Wichita Moun-tains to the subsurface Amarillo uplift in northwestern Texas (Fig. 1). Precambrian base-ment rocks and a Paleozoic sedimentary succes-sion are displaced by a fault system which has vertical separation of >12 km and probably larger strike-slip separation (see review by Perry, 1989; McConnell, 1989). Thick accumulations of coarse clastic sediments indicate late Paleo-zoic fault movement (extensive literature re-viewed by Johnson and others, 1988; Perry, 1989), which was associated with compression during the Ouachita orogeny (Hoffman and others, 1974; Kluth and Coney, 1983; Kluth, 1986). An earlier phase of fault movement is indicated by an alignment of Cambrian igneous rocks that characterize the Southern Oklahoma fault system as rift related.

Syn-rift igneous rocks associated with the Southern Oklahoma fault system include an older part consisting of a layered gabbro com-plex, gabbro plutons, and basalt-spilite; and a younger part consisting of granite and rhyolite (Gilbert, 1983). The lithologic association indi-cates a continental rift environment (Gilbert, 1983). Crystallization age of the older, layered gabbro is interpreted to be 577 Ma (Rb-Sr and Sm-Nd) (Lambert and Unruh, 1986), and the younger gabbros yield an age of 552 7 Ma (U-Pb zircon) (Bowring and Hoppe, 1982). The granite and rhyolite are interpreted to be 525 25 Ma (Rb-Sr) (Ham and others, 1964), and are intruded by diabase dikes.

The Cambrian igneous rocks evidently are restricted to a zone 65 km wide as indicated by distinct linear gravity and magnetic anomalies (Fig. 2) (Coffman and others, 1986). Displace-ment of Cambrian volcanic rocks with respect to older basement rocks in the Arbuckle Moun-tains (Ham and others, 1964), as well as faults within some of the Cambrian volcanic rocks,


4:20 W. A. THOMAS

suggests rift-bounding faults of >1 km vertical separation (McConnell and Gilbert, 1986). Epi-sodic fault movement is indicated by angular discordances between the layered complex and the younger gabbros (McConnell and Gilbert, 1986).

The Cambrian intrusive and extrusive igneous rocks post-date Precambrian crystalline base-ment rocks (Tishomingo Granite) of 1.2 to 1.4 Ga age, as well as strata of the Tillman Metased-imentary Group (Ham and others, 1964; Bick-ford and Lewis, 1979; Denison, in Johnson and others, 1988). The Tillman Metasedimentary Group (known only from wells; Ham and oth-ers, 1964) previously was considered to be part of the fill of a graben that was associated with the emplacement of the Cambrian igneous rocks; however, subsequent work suggests that the Tillman is as old as 1.0 to 1.2 Ga (Muehl-berger and others, 1967; Denison and others, 1984; Coffman and others, 1986), indicating that the metasedimentary rocks are part of the basement (rather than part of the fill) of the Southern Oklahoma fault system. Seismic re-flection profiles show a thick succession (7 to 10 km) of layered reflectors that represent the Till-man south of the Wichita uplift, but the layered reflectors are absent to the north (Brewer and others, 1981,1983). The abrupt northward ter-mination of the Tillman reflectors suggests a Precambrian fault, the presence of which may have influenced the location of the Early to Middle Cambrian Southern Oklahoma fault sys-tem (Brewer and others, 1983). The relatively thin and discontinuous Meers Quartzite (pre-viously considered equivalent to the Tillman Group) is now thought to be the only sedimen-

tary deposit within the Cambrian rift-related ig-neous complex (Gilbert, 1983).

Regionally, Precambrian crystalline basement rocks and the Cambrian rift-related igneous rocks are overlain nonconformably by a trans-gressive sequence consisting of the Upper Cam-brian Reagan Sandstone and overlying carbon-ate rocks (Ham and others, 1964; Gilbert, 1983; Coffman and others, 1986). Within the resolu-tion of available data, the post-rift unconformity at the base of the Reagan Sandstone is the same age as the base of the carbonate rocks that over-step the faulted boundaries of the Mississippi Valley-Rough Creek-Rome graben system and the Birmingham basement fault system. Above the Reagan Sandstone, Upper Cambrian and Lower Ordovician carbonate rocks thicken re-gionally into an elongate basin, the axis of which coincides with the trend of Cambrian igneous rocks (Denison, in Johnson and others, 1988; Perry, 1989). The section in the basin is more than twice as thick as the regional average out-side, and the thicker sections are mainly lime-stone in contrast to dolostone (Fig. 5) (Gate-wood, 1970; Denison, in Johnson and others, 1988). Middle Paleozoic rocks also reflect sub-sidence of the basin, but the rate of differential subsidence decreased after Middle Ordovician time (Johnson and others, 1988). The post-rift subsidence evidently is a result of loading by the dense mafic igneous rocks at a shallow crustal level.

Devils River Uplift and Tobosa Basin

The Devils River uplift is a basement-cored uplift along a northwest-trending segment of the Ouachita orogenic belt beneath the Gulf Coastal

Plain (Nicholas and Rozendal, 1975; Nicholas and Waddell, 1989). Northwest of the Devils River uplift, the thrust belt curves 90 to the southwest around the Marathon salient (Fig. 1). Cratonward from the thrust front in the Mara-thon salient, the present Permian basin is a late Paleozoic structure superimposed on the early Paleozoic Tobosa basin (Adams, 1965; Frenzel and others, 1988).

Regionally, Precambrian crystalline basement rocks are nonconformably overlain by Upper Cambrian sandstone at the base of the transgres-sive Sauk sequence that is dominated by car-bonate rocks (Frenzel and others, 1988); sand-stone in a similar stratigraphic position overlies metasedimentary and metavolcanic rocks on the Devils River uplift (Nicholas and Rozendal, 1975). The metasedimentary-metavolcanic suc-cession is - 8 5 0 m thick and includes metarhyo-lites which have ages of 699 26 Ma (Rb-Sr) (Denison and others, 1977). These possibly rift-related rocks are underlain by more massive metaigneous-metasedimentary basement rocks, which have ages of 1246 + 270 to 1121 244 Ma (Rb-Sr) (Nicholas and Rozendal, 1975; Nicholas and Waddell, 1989).

A strongly positive gravity anomaly indicates a mass of dense mafic rocks beneath the present Central Basin platform within the Permian basin, prompting comparison with the Southern Oklahoma fault system (Figs. 1, 2) (Keller and others, 1985). Drill samples of layered gabbro on the Central Basin platform have ages of 1077 2 to 1163 4 (U-Pb), indicating that these rocks are not associated with late Precambrian-early Paleozoic rifting (Keller and others, 1989b). The early Paleozoic Tobosa basin is a

SOUTHERN OKLAHOMA FAULT SYSTEM

F '

* Ellenburger Riley



= Arbuckle Reagan


# Arbuckle # Timbered Hills


EXPLANATION

DOMINANT ROCK TYPES OF LITHOSTRATIGRAPHIC UNITS = limestone * dolostone # carbonate rocks and sandstone sandstone

Precambrian basement (includes Tillman Metasedimentary Group)

Precambrian basement 2 km -

SCALE

Cambrian igneous rocks ~~I

50 km

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.



broad regional downwarp of uncertain relation-ship to rift-related rocks or structures (Frenzel and others, 1988; Keller and others, 1989b).

PALINSPASTIC LOCATION OF THE RIFTED MARGIN

Syn-rift rocks and structures along the rifted margin and within the adjacent craton have been modified by subsequent Appalachian-Ouachita orogenesis, requiring palinspastic re-construction as an initial step in determining the trace of the rift. Post-orogenic erosion has de-stroyed part of the record of rifting. Parts of the late Paleozoic orogenic belt and older rifted con-tinental margin were modified by Mesozoic ex-tension and opening of the present Atlantic Ocean and Gulf of Mexico, and parts are cov-ered by Mesozoic-Cenozoic rifted- and passive-margin deposits of the Atlantic and Gulf Coastal Plains (Fig. 1).

Large-scale bends in the rifted continental margin have been recognized in a variety of in-terpretations. In this discussion of the location of the margin, the terminology of promontories and embayments is used for location purposes as illustrated in Figure 2, but without implying a mechanism of origin.

The distribution of rift-related rocks and of the passive-margin shelf edge suggests that the northeastern part of the Blue Ridge allochthon contains a relatively straight segment of the rifted margin that approximately parallels pres-ent strike (Rodgers, 1968; Thomas, 1977; Wehr and Glover, 1985). The Mechum River graben also parallels Blue Ridge strike. The autochtho-nous position of the rifted continental margin, as interpreted from southeast-dipping deep seismic reflectors, is at a location east of the present Goochland internal basement massif (Figs. 1, 2) (Pratt and others, 1988).

Southwest of the pinch-out of the Catoctin Formation, a relatively thin Chilhowee Group rests nonconformably on basement rocks (Fig. 3). To the southwest along present struc-tural strike, the Chilhowee thickens and overlaps the relatively thick Mount Rogers Formation; farther southwest, the Chilhowee laps onto the Ocoee Supergroup. The abrupt along-strike changes in rift-related rocks indicate that the dep-ositional boundaries of both the Mount Rogers and the Ocoee intersect the Blue Ridge at large oblique to - 9 0 angles (Fig. 2); therefore, re-gardless of the accuracy of palinspastic recon-struction, the general shape of the rifted margin must include a right-lateral offset of the edge of the rift from the Virginia promontory to the Tennessee embayment (Figs. 1, 2).

Variations in thickness and facies of the Ocoee, Mount Rogers, and Grandfather Moun-tain indicate a complex of basement horsts and grabens that served as sediment sources and sepa-

rate depositional basins, respectively. Horsts and grabens within the rift are documented further by sedimentary onlap patterns in the Lower Cambrian strata (Simpson and Eriksson, 1989). Although the strikes of individual rift-related structures cannot be reconstructed from avail-able data, an area of basement fault blocks is well documented. Palinspastic restorations based on balanced structural cross sections place rift-related rocks now in the Blue Ridge at a position southeast of the present location of the Kings Mountain belt in the Piedmont (Hatcher, 1989; Hatcher and others, 1989); southeast-dipping deep reflectors on COCORP seismic reflection profiles indicate a similar location of the rifted margin (Fig. 2) (Cook and others, 1979, 1983). Autochthonous basement grabens, inboard from the rifted margin and beneath the basal Appala-chian detachment, have been imaged seismically (Harris and others, 1981; Lillie, 1984; Favret and Williams, 1988), indicating a wide expanse of brittlely extended continental crust adjacent to the rifted margin in the Tennessee embay-ment (Figs. 2, 3). The thick rift-fill facies (Ocoee) extends southwestward as far as the Corbin-Salem Church external basement massif in Georgia (Fig. 1) (McConnell and Costello, 1980,1984), but farther southwest, no thick rift-fill successions are indicated (Fig. 4).

Except for displacement by the Birmingham basement fault system, the top of basement is generally smooth and dips at a low angle from beneath the foreland thrust belt southeastward to a position near the Pine Mountain internal basement massif on the Alabama promontory (Figs. 1, 2, 4) (as shown on COCORP seismic reflection profiles, Georgia lines 15,23, and 24). From the southeastern side of the Pine Moun-tain massif, seismic reflectors dip southeastward to the base of the crust, marking the North American continental margin at a boundary with later Paleozoic accreted terranes (Nelson and others, 1985; Hooper and Hatcher, 1988). In reconstructions of balanced structural cross sections, the Paleozoic shelf-facies strata now in the Appalachian thrust belt in Alabama spread at least as far southeast as the present location of the Pine Mountain internal massif (Figs. 1,2,4) (Thomas, 1985b; Ferrill, 1989). Both the extent of the palinspastically reconstructed continental shelf and the seismic reflection data indicate that the present location of the Pine Mountain massif coincides approximately with the margin of North American Precambrian crust. Whether the basement and cover rocks of the Pine Mountain internal massif represent an accreted microcon-tinent (Thomas, 1977; Neathery and Thomas, 1983; Hooper and Hatcher, 1988) or a fault block from the North American margin (Schamel and others, 1980; Nelson and others, 1987), the massif must be near the autochtho-nous edge of North American continental crust.

The continental margin southeast of the Pine Mountain internal massif coincides with the trace of the linear Altamaha magnetic anomaly, which is interpreted to be the signature of a late Paleozoic suture (Fig. 1) (Higgins and Zietz, 1983; Horton and others, 1984; Nelson and oth-ers, 1985; Hooper and Hatcher, 1988; McBride and Nelson, 1988). The magnetic anomaly ex-tends to southwestern Alabama, where a cluster of deep wells penetrated volcanic, plutonic, and ultramafic rocks, suggesting an obducted arc and subduction complex, probably near the margin of continental crust (Figs. 1, 2) (Neathery and Thomas, 1975; Thomas and others, 1989). Pal-inspastic restoration of balanced structural cross sections (Thomas, 1989) places shelf-facies strata now in the trailing part of the subsurface Appalachian thrust belt at least as far south as the trace of the Altamaha magnetic anomaly in southwestern Alabama, further suggesting the original extent of the continental shelf and the approximate location of the continental margin (Fig. 2). Inboard from the rifted margin, the extensional Birmingham basement fault system strikes northeastward approximately parallel with the interpreted trace of the southeastern margin of the Alabama promontory (Fig. 2).

Wide-angle reflection/refraction seismic data (PASSCAL), interpreted via velocity models, clearly document the southern margin of Pre-cambrian continental crust at a location beneath southward-dipping thrust sheets in the southern part of the Ouachita Mountains of Arkansas (Figs. 1, 2) (Keller and others, 1989a). That location is consistent with COCORP reflection data and with gravity models (Nelson and oth-ers, 1982; Lillie and others, 1983). Continental crust thins southward within 25 km to thin transitional or oceanic crust (Keller and others, 1989a). A boundary between regions of con-trasting magnetic signatures (Zietz, 1982; Hinze and Braile, 1988) trends northwest-southeast from the seismically defined edge of continental crust beneath the Ouachitas, outlining the Ala-bama promontory and Ouachita embayment (Fig. 2). The Mississippi Valley graben strikes northeastward into the continent and is approx-imately perpendicular to the northwest-striking segment of the continental margin (Fig. 2).

A segment of the continental margin on the southeastern side of the Texas promontory (Fig. 2) is identified, primarily on the basis of gravity models, along a northeast-trending linear gravity high that is interpreted to mark the edge of con-tinental crust (Kruger and Keller, 1986). Subsur-face geology, seismic reflection profiles, and gravity modeling indicate that the Waco uplift (Fig. 1) is a basement massif thrust onto the margin of continental crust (Nicholas and Rozendal, 1975) and serves as a guide to the location of the margin. The Southern Oklahoma fault system is approximately perpendicular to


4:20 W. A. THOMAS

the southeastern margin of the Texas promon-tory (Fig. 2), and magnetic signatures (Zietz, 1982) of the Cambrian igneous rocks along the fault system terminate abruptly southeastward, marking the edge of continental crust (Keller and others, 1989c; Viele and Thomas, 1989). At the interpreted intersection between the South-ern Oklahoma fault system and the continental margin, the northeast-trending gravity high bends abruptly to the east-northeast, crossing the corner of the Ouachita embayment on the out-board side of a deep gravity low, and extending to the margin of continental crust as indicated by the PASSCAL seismic data (Figs. 1, 2).

A northwest-trending segment of the conti-nental margin along the southwestern side of the Texas promontory is defined primarily by grav-ity models (Keller and others, 1985). The posi-tion of the Devils River uplift suggests an external basement massif as a thrust sheet of basement and cover displaced from the margin of continental crust (Nicholas and Rozendal, 1975; Nicholas and Waddell, 1989).

DISCUSSION OF THE TRACE OF THE RIFT AND MECHANISM OF RIFTING

The large-scale bends in the Appalachian-Ouachita rifted margin are interpreted here to be the result of transform offsets of a northeast-striking rift system (Fig. 2) (Thomas, 1976, 1977). The trace of the rifted margin as mapped in Figure 2 is patterned after maps of recent continental rifts and passive margins (for exam-ple, Rosendahl, 1987; Mascle and Blarez, 1987; Colletta and others, 1988). At the inception of rifting, offsets of the rift, along-strike changes in fault-block geometry, and differences in dip di-rection of extensional faults are linked through accommodation zones, transfer zones, or trans-form faults (for example, Le Pichon and Hayes, 1971; Scrutton, 1982a; Gibbs, 1984; Bosworth, 1985; Lister and others, 1986a; Rosendahl, 1987; Cochran and Martinez, 1988). Angles of intersection between extensional (rift) and shear (transform) structures of recent rifts range from perpendicular to oblique (Freund, 1982; Lister and others, 1986b; Cochran and Martinez, 1988). The trace of the Appalachian-Ouachita rift is not defined with sufficient precision to support an interpretation of exact orientations of the various segments; therefore, Figure 2 shows a simple generalization of orthogonal intersec-tions between rift segments and transform faults.

The largest transform offsets of the Appala-chian-Ouachita rift are spaced 500 to 800 km apart, comparable in scale to the larger offsets of the "rift zones" of the East African rift and other rift zones generally (Rosendahl, 1987). For ex-

ample, the Blue Ridge rift is divided into three rift zones (Fig. 2). Smaller scale horst and graben blocks within the rift system are indi-cated by thickness and compositional variations in the Ocoee, Mount Rogers, and Grandfather Mountain rocks in the Tennessee embayment (Figs. 2,3). These horsts and grabens are proba-bly about the same scale as the fundamental horsts and grabens, "rift units," within the East African rift (Rosendahl, 1987).

The Virginia-Tennessee transform fault (Fig. 2) is indicated by the offset of the continental margin from the Virginia promontory to the Tennessee embayment; by the abrupt along-strike change from the relatively thin rift-related rocks along the Northern Blue Ridge rift zone on the Virginia promontory to the thick rift-fill ac-cumulations along the Southern Blue Ridge rift zone in the Tennessee embayment; and by the northeastward termination of the wide area of horsts and grabens that framed separate deposi-tional basins of the Ocoee, Mount Rogers, and Grandfather Mountain. Alternatively, Rankin (1976) interpreted the Tennessee embayment as the site of a three-armed rift triple junction, and Wehr and Glover (1985) suggested that the Ocoee was deposited in a separate failed rift inboard from the rifted continental margin. No thick rift-fill deposits are indicated along the Pine Mountain rift zone, suggesting that the horsts and deep grabens of the Southern Blue Ridge rift zone end southwestward at a trans-form fault.

At the interpreted position of the Alabama-Oklahoma transform fault along the Ouachita embayment, the PASSCAL wide-angle reflec-tion/refraction data document southward thin-ning of continental crust within a distance of - 2 5 km (Fig. 2) (Keller and others, 1989a). The narrow zone of transition from continental crust to oceanic crust is typical of a transform fault (Keen, 1982; Scrutton, 1982b; Keen and Ha-worth, 1985), and it contrasts with the broad zone of attenuated continental crust that charac-terizes rifted margins, such as that in the Tennes-see embayment.

The northeast-striking Ouachita rift zone pro-jects toward an intersection with the northwest-striking Alabama-Oklahoma transform fault in southeastern Oklahoma (Fig. 2), but structure of the intersection is problematic. The local east-northeast trend of the gravity high along the Ouachita margin may indicate an oblique seg-ment of the rifted margin. The deep gravity low in the corner of the Ouachita embayment sug-gests a deep sedimentary basin formed during rifting along the Alabama-Oklahoma transform fault and parallel with the Southern Oklahoma fault system (Kruger and Keller, 1986). The Ouachita rift zone is not notably offset, although

it may bend, at the intersection with the South-ern Oklahoma fault system (Fig. 2).

The Southern Oklahoma fault system is paral-lel but not aligned with the Alabama-Oklahoma transform fault. The orientation of the fault sys-tem suggests a transform (or continental trans-fer) fault that propagated into continental crust. Possibly, the fault system followed a pre-existing crustal boundary between the Tillman Metased-imentary Group and the older Precambrian crystalline rocks on the north.

The Mississippi Valley-Rough Creek-Rome graben system, the Birmingham basement fault system, and the Blue Ridge rift all trend gener-ally northeastward and reflect northwest-south-east extension; however, the east-striking Rough Creek graben intersects and offsets the strike of the Mississippi Valley graben and Rome trough (Fig. 2). The geometry of these structures in the context of regional extension suggests that the Rough Creek graben may be an oblique transfer zone, possibly consisting of several rhomb grabens.

The intracratonic Mississippi Valley graben parallels rift segments of the continental margin, whereas the Southern Oklahoma fault system is parallel with transform faults (Fig. 2). Important contrasts between the two are consistent with the interpretation that they originated by differ-ent mechanisms. The Mississippi Valley graben is a belt of extensional faults - 1 5 0 km wide, and it is paralleled by the Birmingham basement fault system, another belt of extensional faults that is 100 km wide (Figs. 2, 4). Throughout these fault systems, the graben blocks are filled with clastic sedimentary rocks to the evident exclusion of volcanic rocks. Although gravity modeling indicates anomalous upper mantle/ lower crust beneath the Mississippi Valley graben (Ervin and McGinnis, 1975; Mooney and others, 1983), mafic magmas evidently did not rise into the upper crust or sedimentary cover during Cambrian time. The Mississippi Valley-Rough Creek-Rome and Birmingham fault systems define shallow (as viewed in the scale of thickness of the crust) grabens within continental crust, signifying much less extension than along the Blue Ridge rift, which is a crustal-scale fault system at the margin of continental crust. In contrast, the Southern Oklahoma fault system is more narrow (65 km, as indicated by gravity and magnetic anomalies), is character-ized by layered gabbros and rhyolites, and lacks graben-filling sedimentary rocks (Gilbert, 1983; Perry, 1989). The composition and geometry of the igneous rocks indicate crust-penetrating, probably steep faults. Relatively rapid post-rift subsidence along the Southern Oklahoma fault system produced an elongate downwarp cen-tered on the zone of dense igneous rocks (Fig.



5). In contrast, after Early Late Cambrian fault movement along the Mississippi Valley-Rough Creek-Rome graben system, more limited sub-sidence was restricted to parts of the area within the graben-boundary faults (Figs. 3, 4).

The origin of the Southern Oklahoma fault system as a transform (or continental transfer) fault may include vertical separation and/or transtension. Fault-bounded basins are located where transform faults (traced from the Mid-Atlantic Ridge) offset the present Atlantic con-tinental margins of Africa and South America; faults result from differential vertical movement of continental crust at different distances from the mid-ocean ridge (Francheteau and Le Pi-chon, 1972). Another possible cause of verti-cal separation is implied by simple-shear exten-sion models in which continental transfer faults partition low-angle extension faults either at fault offsets or between faults of opposite dip directions (Gibbs, 1984; Wernicke, 1985; Lister and others, 1986a). The large transform offset of the rift from the Alabama promontory to the Ouachita embayment implies that, even if the dip direction is the same, the tectonic level of low-angle extension faults would differ greatly on opposite sides of the transform. The conti-nental crustal block southwest of the transform is near the Ouachita rifted margin and must be differentially thinned; it may have been dis-placed vertically with respect to the crust north-east of the transform, which was much farther from the Appalachian rifted margin. The length of the Southern Oklahoma fault system, as well as the volume of mafic magma emplaced along it, argues for transtensional movement. Mass-balance calculations suggest Cambrian extension of 17 to 21 km across the Southern Oklahoma fault system (McConnell and Gilbert, 1986).

In the context of a continental margin framed by transform offsets of a rift, the Southern Okla-homa fault system is a continental transform fault, and the Mississippi Valley graben is a rift zone. An alternative interpretation attributes the shape of the margin to separate, plume-generated, three-armed rift triple junctions at which failed arms are represented by the South-ern Oklahoma fault system (Burke and Dewey, 1973; Hoffman and others, 1974) and the Mis-sissippi Valley graben (Ervin and McGinnis, 1975). Contrasts in the nature of the fault sys-tems, in the syn-rift rocks, and in the tectonic history of crustal-scale structures favor different processes (as transform and rift, respectively) rather than a common process (as failed arms of two separate three-armed rifts) of origin for these two fault systems. The post-rift strata over-stepping the two fault systems are of the same age (Middle Late Cambrian), suggesting that the two are components of a single, larger kinematic

regime rather than parts of two separate, plume-generated, three-armed rift triple junctions that expired simultaneously. The orientation of the structures (approximately orthogonal to each other) places rigid geometric constraints on the possible shape of the rifted margin. Movement of a rectangular block in a direction parallel with the interpreted transform direction is kine-matically and geometrically balanced (Fig. 2), but a rigorous geometric model including two three-armed rift triple junctions cannot be balanced geometrically or kinematically. In ad-dition to these specific problems of local applica-tion, recent discussions discredit three-armed rift triple junctions driven by thermal doming as a general mechanism for the initiation of continen-tal rifting (Mohr, 1982; Rosendahl, 1987). The transform offset of the rift to form the Ouachita embayment and the inboard propagation of transform faults along the Southern Oklahoma fault system are similar to the framework of the embayment in the present Atlantic margin of Africa and the Benue trough, respectively (Fran-cheteau and Le Pichon, 1972; Benkhelil and Robineau, 1983; Popoff and others, 1983; Benkhelil and others, 1988).

CONCLUSIONS: HISTORY OF THE RIFTED MARGIN

Northwest-striking transform faults and north-east-striking rift segments along the Appa-lachian-Ouachita continental margin are con-sistent mechanically with northwest-southeast extension along the entire rift system (Fig. 2). Post-rift sedimentary overstep marks the end of active rifting along the Blue Ridge rift in the earliest Cambrian (Fig. 3). Transgressive Upper Cambrian carbonate rocks overstep rift-fill suc-cessions in the intracratonic Mississippi Valley-Rough Creek-Rome and Birmingham fault systems (Fig. 4). Igneous rocks along the Southern Oklahoma fault system as young as 525 25 Ma are nonconformably overlain by sandstones of Late Cambrian age (Fig. 5). Post-rift transgression over the Ouachita rifted mar-gin in the late Middle Cambrian or Late Cambrian is suggested by the age of the base of the transgressive sequence around the Ouachita foreland (Figs. 4, 5). The differences in ages of rift-related rocks and of post-rift sedimentary overstep indicate that the Ouachita margin, as well as the Mississippi Valley-Rough Creek-Rome and Birmingham intracratonic fault sys-tems, is younger than the Appalachian margin.

Rifting began in late Precambrian time along the Blue Ridge rift (Fig. 6A). Lack of late Pre-cambrian rift-related rocks along the Ouachita margin suggests that the initial rift continued southwestward for some unknown distance

beyond the position of the most southwesterly recognized rift-related rocks in the Blue Ridge rift. Possibly the rift intersected the Texas trans-form, which may have been active in the late Precambrian as suggested by the age of metavol-canic rocks along the Devils River uplift.

By the beginning of Cambrian time, the Blue Ridge rift had progressed to a stage in which a passive margin was flanked by an open ocean basin (Iapetus), and spreading continued at a mid-ocean ridge (Mid-Iapetus Ridge) (Fig. 6B). At the same time, extension was active along the Mississippi Valley-Rough Creek-Rome and Birmingham intracratonic fault systems, and mafic magmas were emplaced along the South-ern Oklahoma fault system.

The difference in age of rifting indicates a shift in the spreading center at about the beginning of the Cambrian Period from the southwestern part of the Blue Ridge rift to the Ouachita rift (Fig. 6B). The age of the spreading-center shift is latest Precambrian to Early Cambrian on the basis of age of the igneous rocks (577 Ma) along the Southern Oklahoma fault system.

The spreading-center shift and initiation of Ouachita rifting were accompanied by initiation of the Southern Oklahoma fault system, as well as the Alabama-Oklahoma transform fault (con-tinental transform fault), which ultimately formed the margin of continental crust. The northeast-striking active segment of the Ouach-ita rift ended against the Alabama-Oklahoma transform fault in the Ouachita embayment (Fig. 6B). Most of the extension along the Ouach-ita rift was transformed along the Alabama-Oklahoma transform fault to the Mid-Iapetus Ridge outboard from the Blue Ridge passive margin. A small component of crustal extension propagated northeastward across the Alabama-Oklahoma transform fault into the Mississippi Valley-Rough Creek-Rome and Birmingham basement fault systems, but the basement fault systems northeast of the transform failed to open a new ocean (Fig. 6C). As the Ouachita mid-ocean ridge spread and the Ouachita ocean opened, the northeast end of the ridge migrated along the Alabama-Oklahoma transform fault (Fig. 6C). An active continent-ocean transform prevailed in front of the migrating ridge-transform intersection, and a passive transform margin formed behind it (processes described by Scrutton, 1982b; modeled by Todd and Keen, 1989).

By early in Late Cambrian time, the north-east end of the Ouachita ridge had moved beyond the corner of continental crust on the Alabama promontory, and a passive margin had evolved along the entire rift and transform margin (Fig. 6D). A spreading half-rate of 1.3 to 1.6 cm/yr is calculated for the Ouachita rift/


A - LATE PRECAMBRIAN 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

J i 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) " ! oceanic crust


D - LATE CAMBRIAN Z E E 3 E i . i . i i i i i i

Figure 6. (Continued).


4:20 W. A. THOMAS

ridge from the time of spreading-center shift to that of establishment of a passive transform margin.

ACKNOWLEDGMENTS

Reviews of the manuscript by Mervin J. Bar-tholomew; Lynn Glover III; Robert D. Hatcher, Jr.; J. Wright Horton, Jr.; G. Randy Keller; Richard L. Nicholas; Frederic L. Schwab; Douglas W. Rankin; and John Rodgers are gratefully acknowledged. Part of this research was supported by a grant; from the National Science Foundation (EAR-8218604).

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