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Home , Allochthon, Aulacogen, Basement (geology), Foreland basin, Graben, Laurentia

The Iapetan rifted margin of southern

William A. Thomas* Department of and Environmental Sciences, University of , Lexington, Kentucky, 40506-0053, USA

ABSTRACT Geophysical modeling supports a steep conti- and of Atlantic-Gulf -stage extension and nental margin along the Alabama-Oklahoma passive-margin subsidence. The objective of The Iapetan rifted margin of southern transform, and a similar structure can be this article is to summarize the relevant data for Laurentia includes the northeast-striking inferred for the transform. The Blue interpretation of the trace, structure, and age of Blue Ridge, Ouachita, and Marathon , Ridge rift north of the Alabama promontory the Iapetan rifted margin of southern Laurentia, which are offset by the northwest-striking is dated by synrift volcanic rocks as young using the large-scale elements of the rifted mar- Alabama-Oklahoma and Texas transform as 564 Ma, and passive-margin transgres- gin as an outline. faults, framing the Alabama and Texas sion beginning in earliest is docu- The large-scale framework of the Iapetan promontories and the Ouachita and Mara- mented along the Alabama promontory and rifted margin of Laurentia is interpreted in the thon embayments of the continental margin. farther north. The age of the Ouachita rift context of northeast-trending rift segments Interpretations of the original trace, struc- is documented by the 530–539 Ma synrift offset by northwest-trending transform faults tural style, and age of the rifted margin rest volcanics of the transform-parallel intra- (Fig. 1) (e.g., Thomas, 1976, 1977, 1991, on identifi cation of synrift rocks and struc- cratonic Southern Oklahoma system, 2006). In southern Laurentia, intersections of tures, as well as continental-shelf and off- by Early Cambrian synrift along two large-scale transform faults (Alabama- shelf sedimentary deposits on the passive the conjugate rift margin in the Argentine Oklahoma and Texas transforms) with rift seg- margin. Both late Ouachita- Precordillera, and by late synrift -fi ll ments (Blue Ridge, Ouachita, Marathon) outline Appalachian and post-orogenic of Early to early Late Cambrian age in the two promontories (Alabama and Texas) and two Atlantic-Gulf passive-margin deposits cover rift-parallel intracratonic Mississippi embayments (Ouachita and Marathon) of the the Iapetan rift margin, necessitating the and Birmingham graben systems, as well as rifted margin (Fig. 1). Inboard from the rifted use of data from deep wells and geophysi- by subsidence history of the margin, late synrift intracratonic fault systems cal surveys along with geologic maps of the on the Texas promontory. The diachroniety include rift-parallel extensional faults (Missis- exposed Ouachita-Appalachian thrust belts of rifting refl ects an inboard shift from the sippi Valley and Birmingham graben systems) to characterize the synrift and post-rift rocks Blue Ridge rift to the Ouachita rift along and transform-parallel faults (Southern Okla- and structures. The continental margin and the Alabama-Oklahoma transform and rift- homa fault system). In addition, the rift and passive-margin shelf strata are primarily ing of the Argentine Precordillera from the transform margins of the Ouachita embayment in the footwall of the Ouachita ; Ouachita embayment. are conjugate to the Iapetan rift margin of the however, some Ouachita thrust faults dis- Argentine Precordillera microcontinent (Fig. 2) placed shelf-margin and cover. INTRODUCTION (Thomas and Astini, 1996), and the rift history Appalachian thrust faults imbricate synrift of the Precordillera is complementary to that of fi ll of the intracratonic Birmingham graben The late –Cambrian Iapetan rifted southern Laurentia (Thomas and Astini, 1999). and the passive-margin shelf. Palinspastic margin, as well as the subsequent Cambrian- restoration of thrust-belt structures uses bal- passive margin, of southern Lau- CORNER OF ALABAMA anced cross sections to locate the original rentia is covered by late Paleozoic Ouachita- PROMONTORY trace of the Iapetan margin. Thickness and Appalachian allochthons (emplaced during the subsidence history of the passive-margin assembly of ) and by In the northeast-striking, northwest-verging successions, as well as a general lack of pre- -Cenozoic synrift and passive-margin Appalachian thrust belt in Alabama and Geor- served synrift deposits, indicate an upper- strata of the Gulf Coastal Plain (deposited dur- gia, the décollement is near the base of the plate structure along the Blue Ridge rift ing opening of the Atlantic Ocean and Gulf of Paleozoic sedimentary succession above Pre- on the Alabama promontory and along the Mexico) (summary in Thomas, 2006). Because cambrian crystalline basement rocks, and a Ouachita rift on the Texas promontory. The of the younger tectonic and sedimentary cover, Cambrian-Ordovician passive-margin (Iapetan upper plate on the Texas promontory is con- interpretations of the geometry and tectonic ele- post-rift) succession with upward transition jugate to a lower-plate rift structure on the ments of the Iapetan margin are based on data from clastic to carbonate rocks is imbricated Argentine Precordillera. Although data are from deep wells and geophysical surveys. Reso- in the allochthon (Figs. 1, 3, and 4C) (Thomas limited, the evolution of the passive margin lution of the Iapetan margin in the subsurface and Bayona, 2005). Along the trailing (south- along the Marathon rift in the Marathon requires palinspastic reconstruction of the early eastern) edge of the sedimentary thrust belt, the embayment suggests a lower-plate structure. post-rift passive margin to remove the effects lower-greenschist Talladega slate belt includes of Ouachita-Appalachian orogenesis, includ- similar passive-margin stratigraphy (Tull et al., *[email protected]. ing subsidence of synorogenic foreland basins, 1988). In the Appalachian Piedmont, southeast

Geosphere; February 2011; v. 7; no. 1; p. 97–120; doi:10.1130/GES00574.1; 12 fi gures.

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95°W Rough to the Pine internal basement massif, Creek Rome and a basement-rooted beneath the Ozark 37°N massif apparently merges into the Appalachian décollement above the extensive shallow base- Nashville ment (Fig. 3) (McBride et al., 2005). Mississippi Arkoma Valley dome Southern Oklahoma B D zones (Goat and Bartletts Black Ferry fault zones) along the southeast side of the Bhm Bt Warrior BB A Pine Mountain internal basement massif mark ALABAMA-OKLAHOMA the leading edge of a relatively wide Suwannee- Ouachita PM 32°N Wiggins zone between Laurentian embayment C BLUE RIDGE Fort E GEORGI on the northwest and African crust and sedi- Worth A mentary cover of the Suwannee on the Val Verde Llano Wa G SuwanneeAlabama terrane southeast (Figs. 1 and 3) (summary in Thomas, F uplift palinspastic site of OUACHITA promontory 2010). To the south beneath the Gulf Coastal Lu PRECORDILLERA DR Plain, the suture zone is imaged seismically as Texas MarathonTEXAS a wide band of southeast-dipping refl ectors that promontory embaymen extend down to the Moho (Nelson et al., 1985; 85°W McBride et al., 2005), suggesting that the suture MARATHON 0 200 400 600 km t zone consists of highly tectonized lithons inter- laced with mylonite zones. Crystallization ages APPALACHIAN-OUACHITA OROGEN IAPETAN RIFTED MARGIN of the most southerly exposed metamorphic leading edge of Appalachian rocks (Uchee belt, southeast of the exposed TRANSFORM and Ouachita thrust belts mylonite zones) indicate a peri-Gondwanan arc RIFT leading edge of Appalachian terrane, suggesting comparisons with the peri- accreted metamorphic Gondwanan Carolinia terrane to the northeast along Appalachian Piedmont strike and with zones in Appalachian the Suwannee terrane across strike to the south intracratonic fault Piedmont metamorphic terranes (Fig. 1) (Steltenpohl et al., 2008). The footwall leading edge of Appalachian of the leading edge of the Suwannee-Wiggins GULF AND ATLANTIC COASTAL PLAINS external basement massifs suture zone forms the present limit of Lauren- tian crust at the corner of the Alabama promon- edge of Gulf and Atlantic possible limits of tory (Figs. 1 and 3). Coastal Plains Suwannee-Wiggins suture zone Palinspastic restoration (minimum resto- Figure 1. Outline map of palinspastically restored Iapetan rifted margin of southern Lau- ration, using line-length and area balancing) rentia, synrift intracratonic basement faults, and palinspastic site of Argentine Precordillera of thrust sheets in the sedimentary thrust belt terrane (modifi ed from Thomas, 1991, 2006); of leading edge of Ouachita and Appalachian places the trailing thrust sheets approximately thrust belts, which overlie and/or deform the Iapetan rifted margin; and of the edge of Gulf at the present location of the Pine Mountain and Atlantic Coastal Plains, which cover pre-Mesozoic rocks and structures. Map shows internal basement massif, showing that the the restored trace of the Blue Ridge rift along the Alabama promontory, where Laurentian early Paleozoic passive-margin carbonate-shelf crust has been truncated at the leading edge of the Suwannee-Wiggins suture, and the succession (now imbricated in the thrust belt) interpreted successive locations of the Marathon rift margin. Intracratonic basement fault covered all of the area of shallow crystalline systems are labeled in green letters, abbreviation: Bhm—Birmingham graben. Locations basement rocks (now beneath the thrust belt of Ouachita-Appalachian basement uplifts (thrust-ramp ) are shown by abbre- and Piedmont metamorphic terranes) (Fig. 3) viations in blue letters: DR—Devils River uplift; Lu—Luling uplift; Wa—Waco uplift; (Thomas and Bayona, 2005). The extent of the BB—Broken Bow uplift; Bt—Benton uplift; and PM—Pine Mountain internal basement palinspastically restored passive-margin succes- massif. Locations of Ouachita-Appalachian late Paleozoic synorogenic foreland basins are sion leaves little or no space on the present shal- shown by names in red letters. Locations of intracratonic basement domes are shown by low basement for the palinspastic location of names in black letters. Black lines labeled A through G show locations of cross sections in the passive-margin in the Talladega slate Figures 3, 5–8, 11, and 12. belt, indicating that the Laurentian continental shelf originally extended father southeast than the present location of Pine Mountain (Fig. 3). The Pine Mountain internal basement massif of the Talladega slate belt, northwest-directed, of metasedimentary passive-margin facies and may be a thrust sheet from near the rifted mar- accreted metamorphic terranes rest on the same laterally discontinuous synrift meta-clastic gin of Laurentian crust (e.g., Thomas, Neathery, footwall décollement as that beneath the sedi- rocks (Fig. 3) (Steltenpohl, 1988; Steltenpohl and Ferrill, in Hatcher et al., 1989), a Lauren- mentary thrust belt to the northwest (Fig. 3). et al., 2008). Although interrupted by some rift- tian microcontinent thrust over the rifted margin Along the southeastern (trailing) edge of the related normal faults, the top of Precambrian (Thomas, 1977; Steltenpohl et al., 2004), or a metamorphic terranes, the Pine Mountain inter- basement beneath the allochthon dips gradu- far-traveled allochthonous basement terrane nal basement massif includes Grenville-age ally southeastward at a relatively shallow level (McBride et al., 2005). If the Pine Mountain basement rocks and isoclinally infolded cover from the Appalachian foreland on the northwest basement massif is Laurentian, the palinspastic

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transgression over the rifted margin of Lau- ROUGH CREEK ROME rentia. The base of the sedimentary succession is deeply buried in the Appalachian footwall SOUTHERN on the Alabama promontory, and the oldest OUACHITA OKLAHOMA EMBAYMENT exposed rocks are Lower Cambrian VIR P GI RO NIA MO (Chilhowee Group) (Fig. 4C). Farther northeast VIR NT GIN ORY IA- along the Blue Ridge, the Chilhowee Group TEN TENNESSEETR NE MISSISSIPPI AN SSE rests on synrift rocks and oversteps rift-stage VALLEY SF E EMBAYMENT ORM BIRMINGHAM faults onto Precambrian basement (summary TEXAS PROMONTORY ALABAMA in Thomas, 1991); the transition from rift to PROMONTORY GEORGI passive margin is within the lowermost Chil- A TRANSFORM howee Group (Unicoi Formation) (Simpson and

A Ericksson, 1989) and is of earliest Cambrian LA BA MA age (Laurence and Palmer, 1963; Simpson and TEXAS TRANSF -O T KL RA AH Sundberg, 1987). An upward transition from N OM SFO A RM Chilhowee to the middle Lower O PRECORDILLERA RM Cambrian (e.g., Sloss, 1963; Palmer, 1971; Mack, 1980) is consistent with early post-rift thermal subsidence and - ward transgression over a passive margin. Above the Shady Dolomite, however, the Rome Formation (upper Lower Cambrian) of red Figure 2. Block diagram, showing interpretation of rifting of the Argentine Precordillera and green mudstones, sandstones, and carbon- from the Ouachita embayment of southern Laurentia (modifi ed from Thomas and Astini, ates indicates a cratonic supply of clastic sedi- 1999). Diagram illustrates a low-angle detachment with the Precordillera on the lower plate ment (e.g., Thomas et al., 2004). The overlying and the Texas promontory on the upper plate; the Alabama-Oklahoma Middle to lower Upper Cambrian Conasauga forms a steep continental margin orthogonal to the rift. Formation (Fig. 4C) encompasses an upward transition from clastic to carbonate deposition; however, the stratigraphic level of the transi- location must be southeast of the present loca- the Talladega slate belt or sedimentary thrust tion varies laterally from near the base to near tion and could be either southeast or northwest belt. The stratigraphic comparisons and con- the top of the Conasauga, consistent with active of the palinspastic location of the Talladega slate trasts favor restoring the Talladega slate belt extension along basement faults (Thomas et al., belt (Fig. 3). Similarity of the Talladega passive- adjacent to the trailing part of the sedimentary 2000a). The Upper Cambrian–Lower Ordovi- margin stratigraphy to that in the trailing part thrust belt, thereby placing Pine Mountain base- cian Knox Group (Fig. 4C) of massive car- of the sedimentary thrust belt (Figs. 3 and 4C) ment and cover outboard from the palinspastic bonates constitutes the fully developed passive (Tull et al., 1988) suggests that originally these Talladega and at or near the original rifted mar- margin, sometimes called the Great American successions may have been in close proximity. gin of Laurentian crust (Fig. 3). Alternatively, a Carbonate Bank. The Talladega fault truncates several of the trail- geometrically acceptable reconstruction places ing sedimentary thrust sheets along strike, how- the Pine Mountain massif between the sedi- BIRMINGHAM GRABEN ever, indicating that some unquantifi ed space for mentary thrust belt and Talladega belt. In either AND ASSOCIATED SYNRIFT the excised thrust sheets must separate the palin- alternative, the relatively thin and mature Pine BASEMENT FAULTS spastic location of the Talladega slate belt from Mountain passive-margin cover indicates a sep- that of the present immediate footwall (Fig. 3). arate basement block (Fig. 3). More than 150 km inboard from the Lauren- In contrast, the highly metamorphosed sedimen- Palinspastic restoration of the passive-margin tian rifted margin and the Suwannee-Wiggins tary cover of the Pine Mountain basement mas- facies in the sedimentary thrust belt, Talladega suture, seismic refl ection profi les image the sif differs from the successions in the trailing slate belt, and Pine Mountain massif shows that Birming ham basement graben beneath the Appa- sedimentary thrust sheets and in the Talladega the original trace of the Iapetan rifted margin at lachian sedimentary thrust belt in Alabama slate belt (Figs. 3 and 4C). In Pine Mountain, the southeast corner of the Alabama promontory (Figs. 1 and 3) (Thomas and Bayona, 2005; a compositionally mature stratigraphi- must have been at least 80 km southeast of the Thomas, 2007). Large-scale thin-skinned cally overlies that may represent less present truncated margin of Laurentian crust frontal ramps of the sedimentary thrust sheets mature late synrift or very early post-rift sedi- along the Suwannee-Wiggins suture (Figs. 1 (Big Canoe Valley thrust sheet, and Jones Val- ment (Steltenpohl et al., 2004). A marble strati- and 3). No passive-margin shelf-edge facies or ley thrust sheet along strike to the southwest) graphically above the quartzite may represent off-shelf facies have been recognized; therefore, rise northwestward over the down-to-southeast the Cambrian passive-margin transgression; the restored extent of the continental shelf is a basement faults (Birmingham basement fault), however, the state of deformation precludes minimum. The Suwannee-Wiggins suture pen- and over thick ductile duplexes (Gadsden mush- accurate stratigraphic correlation of the marble. etrates through the entire crust, indicating that wad, and others along strike to the southwest) Although penetrative deformation precludes an Laurentian crust was excised and replaced by of the -dominated Conasauga Formation accurate estimate of original thickness, the evi- Gondwanan (African) crust. (Fig. 3) (Thomas, 2001). Palinspastic restora- dent volume of the Pine Mountain cover rocks The early Paleozoic stratigraphy in the sedi- tion of the thrust sheets and ductile duplexes indicates lesser stratigraphic thickness than in mentary thrust belt records passive-margin places shale-dominated facies (dark-colored

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shale and thin-bedded dark-colored fi ne-grained ) of the Conasauga Formation in the SEA LEVEL SEA Birmingham graben (Thomas and Bayona, 2005), where the formation is >2000 m thick (Fig. 4C) (Thomas et al., 2000a). In contrast, both northwest and southeast of the graben (in palinspastic location), the Conasauga Forma- gins tion is a dolomitized succession characterized

no verti- by ooid grainstones and intraclastic grainstones restored and Cook

INTERNAL (Thomas et al., 2000a), and the formation is mylonite zones and tectonized metamorphic lithons PINE MOUNTAIN

Pine Mountain basement generally <800 m thick. The facies distribution ′ BASEMENT MASSIF SUWANNEE-WIGGINS SUTURE ZONE AND SUTURE ZONE SUWANNEE-WIGGINS TERRANES ACCRETED GONDWANAN A indicates high-energy shallow-marine environ- ZONES T ments on upthrown fault blocks and deeper WANNEE- FAUL SU WIGGINS Y water environments in the graben. Locally, SUTURE ZONE BELT SLATE SLATE carbonate-clast debris fl ows within the graben TALLADEGA SOUTHEAST Gulf Coastal Plain succession indicate of the shallow- LETTS FERR at Suwannee-Wiggins suture Uchee terrane Hatcher et al. (1989); Thomas et al. (1989); Hatcher water facies and into the SUTURE ZONE in AND ACCRETED deeper water in the graben (Astini et al., 2000). SUWANNEE-WIGGINS restored width of Laurentian crust truncated

GONDWANAN TERRANES The indicated paleobathymetric relief, as well

ROCK AND BART T as the anomalously great thickness in the gra-

GOA ben, documents synsedimentary fault move- T L

U LLADEGAT

FA TA UL WALIGA TO FA ment on the basement faults during deposition MASSIF INTERNAL BASEMENT of the Conasauga Formation through Middle PINE MOUNTAIN Cambrian? passive-margin quartzite and marble Late Precambrian? synrift meta-clastic rocks Precambrian crystalline basement Laurentian to early Late Cambrian. The Conasauga For-

PINE MOUNTAIN INTERNAL BASEMENT MASSIF BASEMENT INTERNAL PINE MOUNTAIN mation is overlain by middle Upper Cambrian shallow-marine carbonates of the basal Knox Group across the graben and boundary faults, constraining the age of the end of rift-stage fault movement (Figs. 3 and 4C) (Thomas, 1991). The décollement in these thrust sheets is in the lower part of the Conasauga Formation, and the stratigraphy of any older units (i.e., Rome ARD ZONE

APPALACHIAN PIEDMONT BREV Formation) in the footwall is unknown. Seismic

Appalachian Piedmont metamorphic accreted terranes refl ection profi les of the basement graben show some additional sedimentary fi ll in the footwall of the décollement, but no deep wells are avail- able to document the age or character of that ACHOPCO LT ENIT sediment, leaving the age of initiation of the FAU S graben boundary faults not specifi cally defi ned. thrust fault trajectory of future thrust fault (palinspastic reconstruction) basement fault HOLLINULT FA In the Helena thrust sheet (Fig. 3), the BELT SLATE SLATE décolle ment is in the Lower Cambrian Rome TALLADEGA T TALLADEGA Formation, which is characterized generally by FAUL red and green mudstones, sandstones, and lime-

Cambrian-(?) marble and meta-clastic rocks stones. In the subsurface in the trailing part of

T the Helena thrust sheet, deep wells document as TALLADEGA SLATE BELT SLATE TALLADEGA PIEDMONT APPALACHIAN L U HELENAFA much as 160 m of anhydrite interbedded with BIRMINGHAM BIRMINGHAM dolostone and mudstone in the Rome Formation BASEMENT GRABEN BASEMENT GRABEN (Thomas et al., 2001). The relative thickness MUSHWAD

eroded roof of of the anhydrite suggests that this part of the the GADSDEN (ductile duplex) D WA Y Rome Formation may represent deposition in a FAULT HELENA FAULT FAULT

APPALACHIAN THRUST BELT separate synsedimentary graben southeast of the BASEMENT BASEMENT BIRMINGHAM BIRMINGHAM Birmingham basement graben. Seismic refl ec- T GADSDEN MUSH BIG CANOE VALLE FAUL tion profi les image the northwestern bound- BIG CANOE VALLEY FAULT Figure 3. Structural cross section of Appalachian structures, including Pine Mountain internal basement massif and Suwannee-Wig Appalachian structures, section of 3. Structural cross Figure Alabama ( in Appalachian orogen of Iapetan margin along the Blue Ridge rift beneath and palinspastic reconstruction suture, Thomas and Bayona (2005); Steltenpohl et al. (2008). Thomas et al. (2000a); (1991, 2001); cal exaggeration; line of cross section shown in Fig. 1). Cross section compiled from data in Neathery and Thomas (1983); Sears data in Neathery and section compiled from section shown in Fig. 1). Cross cal exaggeration; line of cross and Ferrill, Thomas, Neathery, et al. (1988); Tull (1984); Nelson et al. (1985, 1987); Steltenpohl (1988); ary fault of a graben beneath the trailing thrust sheets in the southwestern part of the thrust belt; however, most of the extent of the sedimento- Appalachian thrust front Appalachian thrust front logically indicated graben is beneath the present Upper Cambrian–Lower Ordovician passive-margin carbonate Lower–lower Upper Cambrian passive-margin facies and synrift graben fill Precambrian crystalline basement Laurentian continental crust Middle Ordovician–Middle distal Taconic and Acadian synorogenic clastic facies and interlayered carbonate-shelf facies Mississippian-Pennsylvanian synorogenic shallow-marine to deltaic clastic facies and Mississippian carbonate facies cover of the metamorphic terranes in the Appa- NO VERTICAL EXAGGERATION NO VERTICAL APPALACHIAN THRUST THRUST BELT APPALACHIAN A NORTHWEST Black Warrior lachian Piedmont, beyond the extent of detailed 0 –5

ELEVATION (km) –10 ELEVATION seismic refl ection profi les.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/1/97/3342031/97.pdf by guest on 01 October 2021 The Iapetan rifted margin of southern Laurentia Woodford Sh. Woodford Sylvan Sh. Grp. Viola Simpson Grp. Arbuckle Grp. Honey Creek Ls. Reagan Ss. Precambrian crystalline basement Hunton Grp. n ara- fault system volcanic and plutonic rocks 530–539 Ma Southern Oklahoma marble, tectonically thickened Precambrian crystalline basement quartzite Waco uplift, Waco Ouachita orogen Ouachita rift, Mississippian above unconformity Johnson et al. (1988); Arbenz (1989a); and Johnson et al. (1988); in Texas promontory Texas vertical scale 500 m not to scale horizontally Riley Fm. foreland shelf Hickory Ss. autochthonous and Precambrian crystalline basement Wilberns Fm. Ellenburger Grp. Fort Worth foreland basin, Fort Worth Precambrian crystalline basement Bliss Ss. late Precambrian metasedimentary and metavolcanic rocks Ellenburger Grp. Simpson Grp. above unconformity uplift, Devils River Marathon orogen ). Correlation chart of representative stratigraphic sections along the Iapetan rifted margin of chart of representative ). Correlation Texas transform Texas Woods Hollow Sh. Woods Alsate Sh. Marathon Ls. Dagger Flat Ss. Fort Peña Sh. Gaptank Fm. Dimple Fm. Fm. Tesnus Maravillas Chert Haymond Fm. Caballos Novaculite limestone sandstone dark-colored shale chert base of Marathon allochthon Marathon thrust belt, on this and following two pages two on this and following allochthonous off-shelf Figure 4 ( Figure the Marathon embayment (including a list of Mississippian-Pennsylvanian formations in M A—Sections from southern Laurentia. thon synorogenic clastic wedge), Texas promontory, and Southern Oklahoma intracratonic fault system; chart compiled from data i and Southern Oklahoma intracratonic fault system; chart compiled from promontory, Texas clastic wedge), thon synorogenic Rozendal and Erskine (1971); Nicholas (1975); (1983); Denison, (1986); and Mankin (1986). Adler Hills and Kottlowski (1983); from McBride (1989); and correlations chert shale, mudstone limestone, dolostone sandstone Marathon embayment Bliss Ss. cratonic shelf facies facies off-shelf foreland shelf autochthonous A Canutillo Ls. EXPLANATION: dominant EXPLANATION: Precambrian crystalline basement Woodford Sh. Woodford Montoya Grp. Simpson Grp. Fusselman Ls. Ellenburger Grp. (El Paso Grp.)

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/1/97/3342031/97.pdf by guest on 01 October 2021 Thomas Knox Grp. Penters Chert Precambrian crystalline basement Ft. Payne Chert Conasauga Fm. Black Warrior Black Warrior Ouachita and foreland basin, Appalachian foreland transform margin Alabama-Oklahoma Valley Valley graben Mississippi Knox Grp. Precambrian crystalline basement Penters Chert Conasauga Fm. Chattanooga Sh. Boone Fm. Lafferty/ Lafferty/ St. Clair Cason Sh. Grp. Viola Simpson Grp. Arbuckle Grp. Bonneterre Fm. Lamotte Ss. Penters Chert Elvins Grp. Precambrian crystalline basement Chattanooga Sh. foreland shelf autochthonous Arkoma foreland basin, vertical scale 500 m not to scale horizontally Mississippian Middle and Upper Ordovician Lower Ordovician Middle Cambrian Devonian Upper Cambrian Lower Cambrian CHRONOSTRATIGRAPHY base of Ouachita allochthon Ouachita embayment B Blakely Ss. Collier Sh. Mazarn Sh. Arkansas Novaculite Womble Sh. Womble Blaylock Ss. Bigfork Chert Polk Creek Sh. Crystal Mtn. Ss. Missouri Mtn. Sh. Ouachita thrust belt, allochthonous off-shelf allochthonous off-shelf ). B—Sections from Ouachita ). B—Sections from Woodford Sh. Woodford Sylvan Sh. Grp. Viola Simpson Grp. Arbuckle Grp. Honey Creek Ls. Reagan Ss. Precambrian crystalline basement Hunton Grp. Johnson et al. (1988); Thomas (1988, Johnson et al. (1988); continued in fault system Southern Oklahoma volcanic and plutonic rocks 530–539 Ma 1989a, 1991); and Arbenz (1989a); and correla- 1989a, 1991); and Mankin (1986). tions from embayment, Alabama-Oklahoma transform mar- embayment, gin, and Southern Oklahoma Mississippi intracratonic fault systems; chart com- Valley and Lantz (1953); data in Maher piled from Denison, Figure 4 ( Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/1/97/3342031/97.pdf by guest on 01 October 2021 The Iapetan rifted margin of southern Laurentia Chewacla Marble Cheaha Quartzite Halawaka/ Sparks Precambrian crystalline basement Pine massif internal Mountain basement Kahatchee Mtn. Grp. Sylacauga Marble Grp. Fayetteville Phyllite Jumbo Dol. base of Talladega Talladega base of thrust sheet slate belt Talladega Talladega Knox Grp. Rome Fm. Shady Dol. Chilhowee Grp. Conasauga Fm. trailing Alabama promontory sedimentary Appalachian base of Appalachian base of allochthon thrust sheets Taconic, Acadian, and/or Alleghanian Taconic, synorogenic clastic wedges and equivalent foreland carbonates Conasauga Fm. Knox Grp.

base of Appalachian base of allochthon Birmingham basement fault basement Birmingham Birmingham Appalachian thrust belt and basement graben Rome Fm. Precambrian crystalline basement Conasauga Fm. ). C—Sections from Alabama-Oklahoma transform margin and ). C—Sections from vertical scale 500 m not to scale horizontally continued Knox Grp. Penters Chert Precambrian crystalline basement Ft. Payne Chert Conasauga Fm. Black Warrior Black Warrior Ouachita and foreland basin, C Appalachian foreland transform margin Figure 4 ( Figure in Butts (1926); data and correlations chart compiled from Alabama promontory; Thomas et al. (2000a); et al. (1988); Tull Thomas (1988, 1989a, 1991); Mack (1980); Thomas and Bayona (2005); Steltenpohl et al. (2008). Alabama-Oklahoma

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The Birmingham basement graben is bounded B by northeast-striking faults parallel with the

Blue Ridge rift farther southeast, and northwest- SOUTH southeast extension is co-axial with Iapetan rift- ing along the Blue Ridge rift. The ages of initial passive-margin cover over the rift-stage faults LEVEL SEA (Early Cambrian along the Blue Ridge rift, and middle Late Cambrian along the Birmingham graben) show that extension on the Birming- ham graben was later than rifting along the Blue

Ridge rift. FAULT ALABAMA- FAULT OKLAHOMA TRANSFORM ALABAMA- OKLAHOMA ALABAMA-OKLAHOMA TRANSFORM TRANSFORM FAULT

The trace and geometry of the Alabama- of Iapetan margin along the Alabama-Oklahoma of Iapetan margin along the Oklahoma transform fault were interpreted data in Nelson et al. section compiled from Cross initially from palinspastic reconstructions of s (1989), Thomas (1991), and Mickus and Keller (1992). Thomas (1991), and Mickus Keller s (1989),

early Paleozoic passive-margin shelf deposits southern thrust belt Gulf Coastal Plain and coeval off-shelf, continental slope and rise deposits (Cebull et al., 1976; Thomas, 1976, 1977; Viele and Thomas, 1989), and have been thrust fault trajectory of future thrust fault (palinspastic reconstruction) documented more recently by seismic velocity basement fault Alabama-Oklahoma transform fault and gravity models (Keller et al., 1989a; Mickus and Keller, 1992; Harry et al., 2003; Harry and Londono, 2004). The Alabama-Oklahoma

transform fault is in the footwall of the late Benton uplift Paleozoic Ouachita allochthon; along most of the trace of the transform, the Ouachita alloch- thon is covered by post-orogenic Mesozoic- Cenozoic sediment of the Gulf Coastal Plain (Fig. 1). Interpretation of the structure of the

Ouachita thrust belt from outcrop geology, deep OUACHITA THRUST BELT

wells, and seismic refl ection profi les shows that Mesozoic-Cenozoic Gulf Coastal Plain strata Mississippian-Pennsylvanian synorogenic Cambrian–Middle Mississippian passive-margin deposits off-shelf the off-shelf sedimentary rocks were thrust over the shelf edge onto passive-margin-shelf facies, leaving the passive-margin shelf and the trans- form margin of Laurentian crust in the Ouachita footwall (e.g., Viele and Thomas, 1989).

Northwestern Part of Alabama-Oklahoma Transform Margin, Arkansas-Oklahoma

In Arkansas, the exposed Ouachita thrust belt frontal thrust belt verges northward into the northward shallowing Arkoma foreland basin, which contains an upper Paleozoic synorogenic clastic wedge (Fig. 5). Up dip to the north of the Arkoma basin, Pre-

cambrian basement rocks exposed in the crest Mississippian-Pennsylvanian synorogenic shallow-marine to deltaic clastic facies Cambrian–Middle Mississippian passive-margin carbonate shelf Precambrian crystalline basement Laurentian continental crust of the intracratonic Ozark dome belong to the

Granite-Rhyolite province with ages of 1.38– EXPLANATION 1.48 Ga (Lidiak, Bickford, and Kisvarsanyi, in Van Schmus et al., 1993). Overlying the base- ment rocks around the Ozark dome, a classic passive-margin succession (Fig. 4B) includes a basal quartzarenite (Lamotte) of late Middle Cambrian age (Denison, in Johnson et al., 1988) and shallow-marine carbonate rocks (Bonne- NO VERTICAL EXAGGERATION NO VERTICAL B NORTH terre, Elvins, Arbuckle), the Sauk sequence of Arkoma foreland basin 0

–5 ELEVATION (km) ELEVATION –10

Sloss (1963). The Upper Cambrian to Lower (including Benton uplift) and palinspastic reconstruction section of Ouachita thrust-belt structures 5. Structural cross Figure section shown in Fig. 1). Arkansas (no vertical exaggeration; line of cross transform fault beneath the Ouachita thrust belt in Thoma and Viele (1989), Viele et al. (1989a), Nielsen (1989), Arbenz (1989b, 2008), Keller (1982), Lillie et al. (1983),

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Ordovician succession is a massive carbonate; The ages of the boulders suggest that the been identifi ed, and the trace and restored posi- various parts contain quartz sand both as scat- transform locally cut across the Grenville front, tion of the shelf edge are constrained only by the tered grains in carbonate rocks and as sandstone leaving rocks of the Granite-Rhyolite province palinspastic reconstruction of the continental- interbeds. The craton-wide post-Sauk uncon- along the transform margin (e.g., Bickford and shelf and off-shelf facies. formity (Sloss, 1963) marks the top of the mas- Anderson, in Van Schmus et al., 1993). The sive carbonate; however, shallow-marine-shelf sedimentary facies, shelf-derived detritus, and Southeastern Part of Alabama-Oklahoma deposition persisted into the Mississippian associated ultramafi c rocks are consistent with a Transform Margin, Mississippi-Alabama (Viele and Thomas, 1989). Drill data show that steep continental margin along a transform fault the Cambrian-Ordovician and younger passive- and with deposition of -dominated sedi- Southeastward from Arkansas, the traces of margin shelf-carbonate facies extends southward ment on the continental slope and rise over thin the Alabama-Oklahoma transform fault and the in the subsurface beneath the Arkoma foreland transitional or oceanic crust. Submarine can- Ouachita thrust belt pass eastward beneath a basin and Ouachita thrust belt; however, the yons evidently penetrated the passive-margin southward thickening cover of the Gulf Coastal southward limit of the passive-margin facies shelf edge and cut into crystalline basement Plain (Fig. 1), and are relatively deep in the sub- is not closely constrained because of the great rocks at the transform margin of Laurentia. surface across Mississippi. Gravity models along depth (>8 km) and lack of deep wells. South- A seismic velocity model from the wide- two profi les across the subsurface Ouachita ward extent of the carbonate facies beneath the angle refl ection/refraction PASSCAL survey thrust belt in Mississippi show abrupt southward Ouachita thrust belt is inferred from the charac- and a gravity model, extending across the thinning of the crust from ~35 km thickness to ter of seismic refl ectors (Lillie et al., 1983). Ouachita thrust-belt structures in Arkansas <10 km thickness within a distance of <50 km The exposed Ouachita thrust belt consists and southward beneath the Gulf Coastal Plain, (Fig. 6) (Harry et al., 2003; Harry and Londono, of disharmonically deformed thrust sheets of show an abrupt southern margin of Laurentian 2004). The abrupt thinning of the crust and the early and middle Paleozoic deep-water off- continental crust at the location of the Alabama- transition from thick continental crust to thin shelf passive-margin facies and late Paleozoic Oklahoma transform fault (Keller et al., 1989a; transitional or oceanic crust defi ne the location synorogenic turbidites (Figs. 4B and 5) (sum- Mickus and Keller, 1992). The crust thins south- and geometry of the Alabama-Oklahoma trans- mary in Arbenz, 2008). The oldest strata in ward, within a distance of ~25 km, from thick form fault. the allochthon comprise the Upper Cambrian (~39 km) continental crust to thin transitional The Ouachita thrust front curves from east- Collier Shale (Fig. 4B). The Cambrian–Middle or oceanic crust, indicating a steep boundary ward strike in outcrop in Arkansas to south- Mississippian off-shelf passive-margin succes- consistent with the geometry of a near-vertical eastward strike in the subsurface in eastern sion is exposed almost exclusively in the cen- transform fault. Mississippi along the southwest side of the tral Benton and Broken Bow uplifts, and the In the Ouachita thrust belt, the off-shelf rocks Black Warrior foreland basin, where the lead- late Paleozoic synorogenic turbidites dominate are thrust over the carbonate-shelf rocks, which ing edge of the Ouachita allochthon is as much outcrops of both the frontal and trailing (south- remained in the footwall of the Ouachita alloch- as 170 km inboard from the transform margin ern) parts of the thrust belt. Within the dishar- thon (Fig. 5) (Viele, 1979; Arbenz, 1989a, 2008; of Laurentian crust (Fig. 1). The Black War- monically folded thrust sheets of the off-shelf Viele and Thomas, 1989). The regional detach- rior basin is a southwest-dipping passive-margin strata, rare tectonically bounded ment is in the lower part of the off-shelf passive- broken by northwest-striking normal faults; the pods of ultramafi c rocks (serpentinite) probably margin succession and ramps upward into the homocline dips beneath the northwest-striking are fragments of the oceanic crust on which the late Paleozoic synorogenic turbidites toward Ouachita thrust front (Fig. 6). Drill data indi- deep-water were deposited (Morris the foreland. Beneath the central uplifts, in cate that, as in the outcrops in Arkansas, the and Stone, 1986; Nielsen et al., 1989). The which the stratigraphically and structurally Ouachita allochthon consists of deep-water Cambrian–Lower Ordovician passive-margin lower components of the allochthon are exposed, facies, and the allochthon was emplaced over off-shelf succession is characterized by dark- broad ramp anticlines are associated with thrust the passive-margin carbonate succession, which colored shale, and includes sandstone, calcareous faults in basement rocks (e.g., Nelson et al., remained in the footwall (Thomas, 1973, 1985). mudstone, chert, and carbonate-clast conglom- 1982; Lillie et al., 1983; Viele, 1989; Arbenz, The Ouachita thrust faults ramp over the normal erates (summary in Arbenz, 1989a; Viele and 2008). The basement ramp anticlines warped the faults in the Black Warrior basin (Fig. 6). Thomas, 1989). The carbonate detritus (both Ouachita detachment and overlying allochthon Deep wells in the Black Warrior basin show clasts and mud) and quartzose sand link the (Nelson et al., 1982; Lillie et al., 1983; Viele, that a relatively thick carbonate succession (Fig. off-shelf facies to a supply of sediment from 1989; Arbenz, 2008). The shortening of the base- 4C) overlies a generally thin and laterally dis- the nearby shelf. One sandstone unit (Middle ment rocks may be as much as 23 km, an order continuous basal sandstone, which rests on Pre- Ordovician Blakely Sandstone, Fig. 4B) con- of magnitude less than that of the thin-skinned cambrian crystalline basement rocks (Thomas, tains boulders of granite and meta-arkose, Ouachita allochthon (Fig. 5) (Arbenz, 2008). 1988, 1989a). The age of the base of the sedi- indicating a supply of clasts from steep scarps The basement-rooted thrust faults and associ- mentary cover is unconstrained biostratigraphi- that exposed basement rocks along the Lau- ated ramp anticlines represent the only deforma- cally, but farther east in the Appalachian thrust rentian continental margin (Stone and Haley, tion of the continental crust along the transform belt in Alabama, the oldest documented Paleo- 1977). The granite boulders have U-Pb zircon margin; otherwise the rift-stage geometry of the zoic strata are Early Cambrian age (Butts, 1926; ages of 1284 ± 12, 1350 ± 30, 1407 ± 13, and transform fault has been preserved. Copeland and Raymond, 1984). If the basal 1300–1350 Ma (Bowring, 1984). Although The off-shelf rocks in the Ouachita allochthon sandstone in the Black Warrior basin is Early Grenville-age (1.0–1.35 Ga) rocks extend along must be palinspastically restored on the out- Cambrian, it represents post-rift transgression most of the Laurentian rifted margin, data are board side of the carbonate-shelf facies (Fig. 5), that is much earlier than that across the Arkansas not adequate to closely constrain the trace of the and the restored shelf-edge facies boundary is segment of the transform margin (Fig. 4B); how- Grenville front in the subsurface in the inferred to mark the edge of continental crust ever, the shelf is partitioned by the late synrift northeast of the Alabama-Oklahoma transform. along the transform. No shelf-edge facies have intracratonic Mississippi Valley graben between

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C the Arkansas outcrops and the Black Warrior basin (Fig. 1). The southwestward extent of the shelf carbonate succession toward the transform margin beneath the Ouachita allochthon in Mis- sissippi is unconstrained by available data. Dis-

NORTHEAST tinctive seismic refl ectors allow tracing of the carbonate beneath the frontal Ouachita thrust ippi Mesozoic (possibly reactivated) basement fault Appalachian thrust fault Ouachita thrust fault late Paleozoic (and possibly Cambrian) basement fault Alabama-Oklahoma transform fault ama- sheets; however, where the Ouachita allochthon thickens southwestward to >7 km, seismic reso- a, 1989b), lution of the sub-detachment rocks is lost. Some indication of the extent of the car- bonate facies may be gained from the Appala- chian thrust sheets, which can be traced (using seismic refl ection data and deep wells) from the outcrops in Alabama westward in the sub- surface (Thomas, 1973; Thomas et al., 1989; Surles, 2007). In eastern Mississippi beneath les. the Gulf Coastal Plain, the westward striking Appalachian thrust front truncates southeast- striking Ouachita thrust faults (Fig. 1), indicating that northeast-directed Ouachita thrusting along ection profi ection Mesozoic-Cenozoic Gulf Coastal Plain strata Mississippian-Pennsylvanian synorogenic turbidites Cambrian–Middle Mississippian passive-margin deposits off-shelf oceanic crust the southwest side of the southwest-deepening Black Warrior basin preceded northwest-directed Appalachian thrusting (Thomas, 1989a; Whit- ing and Thomas, 1994; Thomas and Whiting, 1995). Although Ouachita thrust faults em placed

subsurface foreland basin Black Warrior off-shelf sedimentary facies over the shelf edge, leaving the passive-margin shelf facies in the footwall (“Ouachita-style structure”), Appalachian thrust sheets are detached in mud-dominated strata near the base of the passive-margin carbonate succession; and the Cambrian-Ordovician massive carbonate unit is translated within Appalachian thrust sheets (“Appalachian-style structure”). These char- acteristics distinguish the Appalachian and Ouachita thrust sheets in the subsurface, and the subsurface Ouachita thrust sheets Appalachian thrust sheets are internally much more coherent than the Ouachita thrust sheets. The massive carbonate unit serves as a regional stiff layer, controlling the geometry of Appala- Mississippian-Pennsylvanian synorogenic shallow-marine to deltaic clastic facies Cambrian–Middle Mississippian passive-margin carbonate shelf Cambrian-Devonian(?) slate belt (marble and meta-clastic rocks) Talladega Precambrian crystalline basement Laurentian continental crust chian thrust sheets and thrust ramps; and palin- spastic restoration of Appalachian thrust sheets

EXPLANATION can rely on line-length balancing. Restoration of the Appalachian thrust sheets shows that the passive-margin carbonate facies extended to near the projected trace of the Alabama- Oklahoma transform fault on the corner of the Alabama promontory (Thomas, 1991).

subsurface Appalachian thrust sheets thrust northwest from out-of-plane MISSISSIPPI VALLEY GRABEN

The Mississippi Valley graben (also called Figure 6. Structural cross section of Ouachita and Appalachian thrust-belt structures emplaced on Iapetan margin along the Alab emplaced on Iapetan margin along the Appalachian thrust-belt structures section of Ouachita and 6. Structural cross Figure basin beneath the Gulf Coastal Plain in Mississ foreland Warrior Oklahoma transform fault along the southwest side of Black Thomas (1973, 1988, 1989 data in section compiled from section shown in Fig. 1). Cross (no vertical exaggeration; line of cross Harry et al. (2003), Harry and Londono (2004), Surles (2007), and interpretation of seismic refl Harry et al. (2003), and Londono (2004), Surles (2007), interpretation Reelfoot rift in some publications) is an intra- cratonic basement fault system now beneath the of the Gulf Coastal Plain (Figs. 1 and 7). The graben is parallel with the Blue Ridge rift, which is ~500 km to the Gulf Coastal Plain C SOUTHWEST

0

NO VERTICAL EXAGGERATION NO VERTICAL southeast. The early Paleozoic stratigraphy on –5 ELEVATION (km) –10 ELEVATION –20 –15 the opposite shoulders of the Mississippi Valley

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D D′ WEST EAST structural cross section MISSISSIPPI EMBAYMENT (MESOZOIC-CENOZOIC) Cretaceous and Cenozoic Pennsylvanian– Upper Mississippian Middle Ordovician to Middle Mississippian

Upper Cambrian–Lower Ordovician Knox Group

Lower and Middle Cambrian

Precambrian basement

2 km MISSISSIPPI VALLEY GRABEN (CAMBRIAN)

50 km palinspastic restoration: Cambrian Mississippi Valley graben

Upper Cambrian–Lower Ordovician Knox Group

Lower and Middle Cambrian

Precambrian basement

Figure 7. Structural cross section of rift-parallel Mississippi Valley intracratonic graben beneath the Mississippi Embayment of the Gulf Coastal Plain, and palinspastic reconstruction of rift-stage graben (vertical exaggeration shown by vertical and horizontal scales, line of cross section shown in Fig. 1). Cross section compiled from data in Thomas (1991), which includes a list of wells shown by vertical black lines in the cross section.

graben is similar to that in the Black Warrior and lent to the upper Conasauga Formation in the sissippi Valley and Birmingham refl ect Arkoma foreland basins, respectively: a basal Birmingham graben and Appalachian foreland extension co-axial with that of the Blue Ridge sandstone, and overlying shallow-marine car- (Grohskopf, 1955; Palmer, 1962; Weaverling, rift; however, the Mississippi Valley and Bir- bonate rocks (Fig. 4B). Although the basal sand- 1987). The age of the older part of the graben- mingham grabens record a later stage of exten- stone (late Middle Cambrian) west of the graben fi ll is not documented biostratigraphically, but sion (Early to early Late Cambrian) than that of evidently is younger than that to the east, where it may be as old as the Lower Cambrian Rome the Blue Ridge rift (late Precambrian–earliest the basal sandstone may be of Early Cambrian Formation. The Conasauga-equivalent strata are Cambrian). The Mississippi Valley graben is a age, the post-Sauk unconformity caps the Lower overlain by shallow-marine carbonate rocks of rift-parallel intracratonic fault system that is Ordovician carbonates regionally. the Upper Cambrian–Lower Ordovician Knox approximately perpendicular to the Alabama- The Cambrian stratigraphy in the graben con- Group, which oversteps the boundary faults of Oklahoma transform fault (Fig. 1). trasts markedly with that outside the graben the graben, indicating the end of fault movement (Fig. 4B). The graben-fi ll succession includes (Fig. 7) (Thomas, 1991). CORNER OF OUACHITA EMBAYMENT laterally variable components of mudstone, Within the limits of resolution, the end of sandstone, and limestone (summary in Thomas, movement on the basement faults of the Mis- In southeastern Oklahoma, the Ouachita 1991). The upper part of the succession is bio- sissippi Valley graben is the same age as the frontal thrust belt curves nearly 90° from a west- stratigraphically documented as lower Upper end of movement on the boundary faults of the erly trend in Arkansas to a south-southwesterly Cambrian (Dresbachian = Steptoean), equiva- Birmingham basement graben. Both the Mis- trend in Oklahoma, defining the Ouachita

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salient of the thrust belt; that bend of the alies extend along the Ouachita interior zone, ate generally less than 1000 m thick (Fig. 4A) thrust belt has been the basis for interpretation around an abrupt curve from south-southwest (summary in Thomas and Astini, 1999). The of the shape of the Ouachita embayment of strike east of the Llano uplift to west-north- basal beds overlap a paleotopographic surface the Iapetan rifted margin, which is outlined by the west strike south of the Llano uplift (Fig. 1) with more than 200 m of relief on the Precam- intersection of the northwest-striking Alabama- (Keller et al., 1989b). Although several possible brian basement (Barnes et al., 1972). The age Oklahoma transform fault and the northeast- sources may be proposed for the potential fi eld of the base of the passive-margin succession striking Ouachita rift (Fig. 1) (Thomas, 1976, anomalies, modeling requires a major transi- shows that post-rift thermal subsidence began 1977, 2006). The lower Paleozoic passive- tion in crustal structure, indicating the location later along the Ouachita rift than along the margin succession of sandstone and limestone of the continental margin. Specifi c palinspastic Blue Ridge rift, where the basal transgressive extends southeastward in the footwall beneath reconstruction of the crustal structure of the succession is of Early Cambrian age (Fig. 4). the Ouachita allochthon of off-shelf facies. The Ouachita rift is somewhat uncertain because of The age of passive-margin transgression onto northeast-trending Broken Bow uplift in the the overprint of Mesozoic rifting and opening the Laurentian margin along the Ouachita rift apex of the Ouachita salient (Fig. 1) is similar to of the along the same alignment indicates a time of rifting consistent with the the Benton uplift in that the older off-shelf facies (Mickus et al., 2009). age of igneous rocks along the Southern Okla- are exposed above a ramp of basement A lower Paleozoic passive-margin succes- homa fault system and with the stratigraphically rocks on a basement-rooted thrust fault (Arbenz, sion of basal sandstone and overlying carbonate constrained age of the end of 2008). Two deep wells on the Broken Bow uplift (Fig. 4A) dips eastward in the Fort Worth basin movement along the Mississippi Valley and Bir- penetrated meta-carbonate rocks (interpreted to and from the Llano uplift beneath the Ouachita mingham grabens. be Ordovician shelf facies) beneath thrust sheets thrust front (summary in Thomas and Astini, of Ouachita off-shelf facies, showing that the 1999). As in the Ouachita thrust belt in Arkan- SOUTHERN OKLAHOMA shelf carbonate extends at least as far southeast sas and Oklahoma, off-shelf rocks have been FAULT SYSTEM as the palinspastic location of the basement thrust over the shelf edge onto the shelf carbon- rocks in the Broken Bow uplift (Nicholas and ate facies (Fig. 8). The Southern Oklahoma fault system extends Waddell, 1989; Arbenz, 2008). A linear positive In east Texas, east of the Fort Worth foreland >500 km northwesterly into the Laurentian gravity anomaly extends along the Broken Bow basin, the Waco uplift (Fig. 1) is a subsurface craton from the Ouachita thrust front ~100 km uplift (Kruger and Keller, 1986). basement structure, in which basement and south of the abrupt bend in strike of the Ouachita In the foreland of the Ouachita embayment, cover rocks are signifi cantly uplifted relative to salient (Fig. 1) (Ham et al., 1964; Johnson et al., the transgressive passive-margin succession the elevation of comparable rocks beneath the 1988; Keller and Stephenson, 2007). The fault rests directly on basement, and no synrift sedi- leading part of the Ouachita thrust belt (Fig. 8) system is most evident in outcrop because of mentary rocks are known. A large gravity mini- (Rozendal and Erskine, 1971; Nicholas and large-magnitude basement faults of late Paleo- mum just north of the apex of the curved trace of Waddell, 1989). A deep well on the Waco uplift zoic age. A bimodal suite of igneous rocks with the Ouachita thrust-belt salient, and northwest penetrated thrust sheets of “Ouachita facies” off- crystallization ages of 530–539 Ma (Hogan and of the gravity high along the Broken Bow uplift, shelf strata, below which marble and quartzite Gilbert, 1998; Thomas et al., 2000b) records a indicates a subsurface mass of low-density overlie basement (Figs. 4A and 8). The well data Cambrian-age synrift component of the fault rocks (Kruger and Keller, 1986). The gravity indicate that the shallow-marine shelf-carbonate system. Linear, northwest-trending, high- minimum may be the expression of a locally facies extends to the palinspastic location of the amplitude short-wavelength gravity and mag- thick synrift sedimentary accumulation (Keller Waco uplift and that the palinspastic location of netic anomalies outline a steeply bounded zone et al., 1989b), which is now deep in the sub- the Ouachita thrust sheets of deep-water facies of dense igneous rocks ~65 km wide (Gilbert, surface and along the northwesterly projected must lie farther east beyond the shelf edge at the 1983; Coffman et al., 1986; Denison, 1989; trace of the Alabama-Oklahoma transform fault. rifted margin of Laurentian crust (Fig. 8). Keller and Stephenson, 2007). The anoma- Similarly thick synrift sediment accumulations Farther south, southeast of the Llano uplift, lies end abruptly southeastward, indicating the are associated with other transform faults along COCORP seismic refl ection profi ling images intersection of the zone of mafi c rocks with the the Laurentian margin (Thomas, 2006). the Luling uplift (Fig. 1) beneath the Ouachita rifted continental margin (Keller et al., 1989b). thrust belt (Culotta et al., 1992). The uplift is Evidence of Cambrian synrift faults is found in OUACHITA RIFT ZONE interpreted to be similar in geometry and com- displacements of the Cambrian volcanic rocks position to the Waco uplift. Along with the Bro- with respect to Precambrian basement, faults Southward from the abrupt bend in the ken Bow uplift, the Waco and Luling uplifts within the volcanic rocks, and angular discor- Ouachita thrust belt in southeastern Oklahoma, suggest an alignment of basement thrust ramp dances within the layered igneous rocks (Ham the thrust belt trends south-southwestward and anticlines along most of the Ouachita rift margin et al., 1964; McConnell and Gilbert, 1986). passes southward beneath the cover of the Gulf of Laurentia. These basement structures may be The igneous rocks include gabbro, , Coastal Plain (Fig. 1). The relatively straight similar to the northern Blue Ridge basement granite, and rhyolite (Hogan and Gilbert, trace of the thrust front in the subsurface has ramp anticline in Virginia (e.g., Harris, 1979; 1998); the composition refl ects deep sources in been used to infer the shape of the pre-orogenic Pratt et al., 1988; Costain et al., 1989; Thomas the upper mantle. The steep gradients on both rifted margin. Abundant drill data document a and Becker, 2007); however, in contrast, the sides of the potential fi eld anomalies document frontal thrust belt of off-shelf sedimentary facies basement uplifts along the Ouachita rift margin steep boundaries of the mafi c rocks in the shal- like those in the Ouachita outcrops in Arkansas are covered by thrust sheets of off-shelf facies. low crust. The geometry and composition indi- and Oklahoma, and a trailing metasedimen- The passive-margin succession exposed cate crust-penetrating near-vertical fractures tary belt (low-grade metamorphic rocks of the around the Llano uplift and drilled in the Fort as conduits, consistent with a leaky “Ouachita interior zone”) (Thomas et al., 1989). Worth basin has a basal sandstone of latest transform fault. The evident large volume and Prominent positive gravity and magnetic anom- Middle Cambrian age, and an overlying carbon- short time span of magma production, however,

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suggest a possible convergence of multiple causes of melting. A transgressive passive-margin succession of SEA LEVEL SEA basal sandstone and overlying shallow-marine carbonates overlaps the Cambrian igneous rocks (Fig. 4B), and the age of the base of the transgressive succession is middle Late Cam- brian (Denison, in Johnson et al., 1988). Within the limits of resolution, the onlap here is coeval f Iapetan Rozendal

(no vertical with the overstep of the graben boundary faults n of the Shell of the Mississippi Valley and Birmingham gra- bens by the carbonate rocks of the basal Knox Group (Fig. 4) (Thomas, 1991). Above the basal sandstone along the Southern Oklahoma fault system, the overlying carbonate succession is ′ exceptionally thick (Fig. 4A), consistent with E synrift thermal uplift followed by post-rift cool- EAST ing of the shallow igneous rocks (Thomas and Astini, 1999).

Gulf Coastal Plain The Southern Oklahoma fault system (com- monly also called the Southern Oklahoma aulaco gen) was interpreted previously to be a failed rift, specifi cally the failed arm of a three- armed radial-rift (e.g., Burke and thrust fault trajectory of future thrust fault (palinspastic reconstruction) basement fault Dewey, 1973; Hoffman et al., 1974), an inter- pretation that is supported by a three-armed Waco uplift Waco pattern of linear gravity highs at the junction of the Southern Oklahoma fault system and the Ouachita orogen (Keller and Stephenson, 2007). Prominent linear gravity highs defi ne three inter- secting arms, each refl ecting a different source: a relatively short, northeast-trending arm along the Broken Bow basement uplift; a northwest- trending arm, extending into the conti nent along the mafi c igneous rocks of the Southern Okla- Mesozoic-Cenozoic Gulf Coastal Plain strata Mississippian-Pennsylvanian synorogenic turbidites Cambrian–Middle Mississippian passive-margin deposits off-shelf Ouachita interior zone Paleozoic metasedimentary rocks homa fault system; and a south-southwest trend- OUACHITA THRUST BELT ing arm, extending along the subsurface interior zone of the Ouachita orogen in east Texas and curving abruptly to the northwest around the cor- ner of the Texas promontory. The concept of a failed rift was supported in part by interpretation of a subsurface, thick clastic metasedimentary succession as rift-fi ll sediment; however, more detailed work has shown that the metasedi- frontal thrust belt mentary succession is >1.0 Ga (age of meta- morphism) and is unrelated to Cambrian rifting (Muehlberger et al., 1967; Denison et al., 1984; Coffman et al., 1986). Previous analogy with the of West as the failed arm of a rift triple junction also has been superceded by

Mississippian-Pennsylvanian synorogenic shallow-marine to deltaic clastic facies Cambrian–Middle Mississippian passive-margin carbonate shelf Precambrian crystalline basement Laurentian continental crust the interpretation of the Benue trough as a strike- slip system, projecting into the African

EXPLANATION from transform faults that offset the margin along the Ouachita rift beneath the Ouachita thrust belt and post-orogenic cover of the Gulf Coastal Plain in east Texas of the Gulf Coastal Plain in east cover margin along the Ouachita rift beneath thrust belt and post-orogenic Figure 8. Structural cross section of Ouachita thrust-belt structures (including Waco uplift) and palinspastic reconstruction o uplift) and palinspastic reconstruction Waco (including section of Ouachita thrust-belt structures 8. Structural cross Figure uplift shows location and depth of penetratio Waco black line on Vertical section shown in Fig. 1). exaggeration; line of cross data in Rozendal and Erskine (1971), Nicholas section compiled from well (Rozendal and Erskine, 1971). Cross No. 1 Barrett (1975), Nicholas (1989), Nicholas and Waddell (1989), Thomas et al. (1989), Culotta (1992), and Mickus (2009). (1989), Waddell (1975), Nicholas (1989), and rifted margin of West Africa (e.g., Francheteau Fort Worth foreland basin Fort Worth and Le Pichon, 1972; Mascle et al., 1988, 1992; Benkhelil et al., 1998). The latter observations provide an appropriate analog for the Southern Oklahoma fault system as a transform-parallel E WEST NO VERTICAL EXAGGERATION NO VERTICAL intracratonic fault projecting into the conti- 0 –5

ELEVATION (km) ELEVATION –10 nent from the Iapetan margin (Thomas, 1991);

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however , synrift magmatism dominated the dillera carbonate succession, however, is much the thrust belt has been interpreted to parallel Southern Oklahoma fault system. thicker than the carbonate-shelf succession on the pre-orogenic rifted margin of continental The Southern Oklahoma fault system is paral- the Texas promontory of Laurentia, and the crust (e.g., Thomas, 1977); however, docu- lel but not aligned with the Alabama-Oklahoma base of the carbonate succession in the Pre- mentation for the actual location of the margin transform fault; instead the Southern Oklahoma cordillera is older than that on the Texas is limited. The gravity and magnetic anoma- transform-parallel fault system intersects the promontory (Fig. 9) (Thomas and Astini, 1999). lies associated with the Ouachita interior zone Ouachita rift ~120 km south of the corner of the Stratigraphic comparisons show that passive- curve ~90° around the Texas promontory and Ouachita embayment (Fig. 1). The near coinci- margin subsidence and transgression began ear- extend northwestward along the northwest- dence in age of the Southern Oklahoma igneous lier on the Pre cor dillera than on the Texas prom- trending Ouachita thrust belt in south Texas. rocks with that of the sedimentary fi ll of the Bir- ontory, and that the magnitude of subsidence These anomalies include a crustal boundary mingham and Mississippi Valley intracratonic was greater on the Precordillera. This seeming (Keller et al., 1989b), interpreted to be the extensional grabens suggests a regional system paradox is consistent with the observations of Ouachita rift and Texas transform, which out- of northwest-southeast extension partitioned complementary asymmetry of subsidence on line the Texas promontory (Fig. 1). by northwest-striking transform faults. In this the conju gate rift margins of a simple-shear low- The Devils River uplift (Fig. 1), a basement context, the Southern Oklahoma fault system is angle-detachment rift system (Fig. 10) (sum- uplift along part of the northwest-trending a transform-parallel, intracratonic, leaky trans- mary in Thomas and Astini, 1999). The lack Ouachita thrust belt, appears to be generally form fault. of synrift sedimentary deposits, as well as the similar, with some exceptions, to other basement paleotopographic relief beneath the basal trans- uplifts along the Ouachita system. Deep wells on ARGENTINE PRECORDILLERA gressive sandstone on the Texas promontory, is the uplift have drilled through a meta-carbonate characteristic of upper-plate margins, in contrast succession into a metasedimentary-metavolcanic The Argentine Precordillera is an exotic ter- to lower-plate margins, which are characterized succession and underlying basement rocks (Figs. rane now in the eastern foothills of the by graben-fi lls of synrift sediment followed by 4A and 11) (Nicholas and Rozendal, 1975; Deni- in northwestern Argentina (Ramos et al., 1986; earlier subsidence and transgression as in the son et al., 1977; Nicholas and Waddell, 1989). Astini et al., 1995, 1996). A wide variety of evi- Precordillera (Fig. 10). Specifi cally, these con- The details of structure are somewhat uncer- dence indicates that the Precordillera was rifted trasts show that the Texas promontory was an tain, and available data may be interpreted as a from the Ouachita embayment of Laurentia in upper-plate margin and the Precordillera was a northeast-directed basement-cored ramp anti- Cambrian time and accreted to western Gond- lower-plate margin (Thomas and Astini, 1999). cline (shown in Fig. 11), or alternatively as a wana in Ordovician time (Thomas and Astini, The age of synrift rocks in the Precordillera fault-bounded basement horst (Nicholas, 1983). 1996, 2003; Astini and Thomas, 1999; Ramos, constrains the time of rifting from Laurentia to In either interpretation, the palinspastic location 2005). Indications of age and mechanism of rift- Early Cambrian, the age of the Cerro Totora red- of the Devils River uplift is not far south of the ing show that the rifted margin of the Precor- beds and (Astini et al., 1995; Thomas present location, unless the basement uplift has dillera is conjugate to the Ouachita rift margin and Astini, 1996, 1999). This age of rifting is been displaced along the orogen by strike-slip of the Texas promontory of southern Laurentia consistent with the age of synrift igneous rocks motion. Unlike the other basement uplifts along (Figs. 1 and 2) (Thomas and Astini, 1999). along the Southern Oklahoma transform-parallel the Ouachita orogen, the Devils River uplift is Paleozoic rocks in the Precordillera are in fault system and with the ages of the graben at the leading edge of the thrust belt and is not the hanging walls of Andean thrust faults, and fi lling synrift sediment in the Mississippi Val- covered by thrust sheets of deep-water facies; the contact with basement rocks is not exposed. ley and Birmingham grabens. Initial rifting in however, off-shelf rocks are recognized in the The oldest Paleozoic rocks exposed in the Pre- earliest Cambrian led to opening of an Ouachita frontal thrust sheets to both northwest and south- cordillera are a synrift succession of redbeds, ocean fl oor and migration of the Precordillera east along Ouachita strike. The Devils River evaporites, and carbonates of the Lower Cam- microcontinent along the Alabama-Oklahoma basement rocks and cover are juxtaposed with brian Cerro Totora Formation (Fig. 9) (Astini transform fault (Thomas, 1991; Thomas and synorogenic foreland-basin deposits (Nicholas and Vaccari, 1996). The Cerro Totora Forma- Astini, 1996). Movement along the transform and Waddell, 1989). tion has an olenellid fauna identical to that in fault is consistent with episodic reactivation of The carbonate cover on the Devils River the Lower Cambrian Rome Formation in the the basement faults of the Mississippi Valley basement marks the minimum extent of the southern Appalachians (Butts, 1926; Palmer, and Birmingham grabens until the Ouachita passive-margin shelf toward the transform mar- 1971; Astini and Vaccari, 1996; Astini et al., mid-ocean ridge migrated past the corner of gin, thereby constraining the location of the 1996). Lithologic similarities include red mud- Laurentian crust on the Alabama promontory. shelf edge. The age of the lower part of the car- cracked sandstones, Salterella-bearing lime- The end of basement fault extension marks the bonate cover is in dispute: either Middle Cam- stones, and evaporites. Strontium isotopes from time of separation of the Precordillera micro- brian (Palmer et al., 1984), or Late Cambrian Cerro Totora and Rome evaporites indicate plate from Laurentia. (Nicholas and Waddell, 1989). The age is criti- similar ages and similar depositional settings cal for constraining the time of initial post-rift (Thomas et al., 2001). Detrital-zircon popula- TEXAS TRANSFORM FAULT passive-margin transgression along this part of tions from sandstones in the Cerro Totora and the Laurentian rifted margin, but no defi nitive in the Rome are similar and suggest a similar South of the Llano uplift beneath the biostratigraphic data are available. Below the car- source, which is consistent with cratonic Lau- cover of the Gulf Coastal Plain, the trace of bonate cover, a metasedimentary-metavolcanic rentia (Thomas et al., 2004). The carbonate suc- the Ouachita thrust belt bends from south- (metarhyolite, metadacite) unit (~850 m thick) cession above the Cerro Totora clastic- southwest to west-northwest and extends in overlies Precambrian (1121–1246 Ma, Rb/Sr facies corresponds in age to southern Appala- that direction in the subsurface across south whole rock isochron dates) basement (Nicholas chian carbonates from middle Lower Cambrian Texas to the exposed thrust belt in the Mara- and Waddell, 1989). Geochronological analyses through Lower Ordovician (Fig. 9). The Precor- thon region of west Texas (Fig. 1). The trace of of the volcanic rocks include Rb/Sr whole rock

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Texas Argentine Alabama promontory Precordillera promontory terrane Ouachita Appalachian foreland thrust belt Ouachita rift transform fault Alabama-Oklahoma

San Juan Ls.

Ellenburger Grp.

La Silla Ls. Knox Grp. Wilberns Fm.

Riley Fm. La Flecha Ls. Hickory Ss. Precambrian crystalline basement

Zonda Dol.

Conasauga Fm. vertical scale 500 m

not to scale horizontally

La Laja Ls. Rome Fm. EXPLANATION:

dominant lithology Shady Dol.

limestone, dolostone Chilhowee Grp. redbeds, evaporites

Cerro Totora Fm. sandstone

base of Andean base of Appalachian allochthon allochthon

Figure 9. Correlation chart comparing a stratigraphic section from the Argentine Precordillera terrane with a section on the conjugate rift margin on the Texas promontory and a section on the Alabama promontory. Chart compiled from data and correlations in Astini et al. (1995) and Thomas and Astini (1999). Color code for chronostratigraphic subdivisions same as in Figure 4.

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LAURENTIA-- PRECORDILLERA marine deposits (Ross, 1986; Muehlberger and TEXAS PROMONTORY Tauvers, 1989a; McBride, 1989). The extent of sea level the passive-margin shelf southeastward beneath UPPER PLATE LOWER early Early Cambrian PLATE the allochthon is not documented by deep drill- ing (Fig. 12), and available data are not adequate 1. rifting along low-angle detachment; listric blocks of upper plate remain on lower plate. to support a quantitative palinspastic reconstruc- tion of the locations of the shelf margin and of sea level the rifted margin of continental crust. Because Early Cambrian the allochthon contains exclusively off-shelf passive-margin facies and is thrust over the 2. isostatic subsidence of thinned crust; thermal uplift caused by high heat flow from rift. passive-margin shelf facies, the regional detach- ment must cut through the shelf edge some- sea level where beneath the allochthon. end of Early Cambrian Internally the Marathon thrust belt includes a disharmonic array of thrust faults and tight 3. residual thermal uplift on upper plate; passive-margin subsidence on lower plate. folds of various wavelengths; internal shorten- ing within the allochthon is estimated to be ~3:1 sea level (King, 1937; Muehlberger and Tauvers, 1989a). Middle mid-ocean ridge Cambrian In addition to the internal shortening, the lead- ing edge of the allochthon has been translated 4. residual thermal uplift on upper plate; passive margin on lower plate. an unknown distance over the passive-margin shelf. Complete palinspastic reconstruction of sea level the allochthon requires restoring the cratonward mid-ocean Late Cambrian ridge translation over the footwall, as well as the inter- nal shortening within the allochthon. The palin- 5. thermal decay and passive-margin subsidence on upper plate. spastic site of deposition of the strata now in the Marathon allochthon is interpreted to be in an Figure 10. Sequential, schematic cross sections illustrating countering effects of thermal off-shelf setting on transitional or oceanic crust uplift and isostatic crustal subsidence on the conjugate plates at a low-angle detachment relative to a passive-margin shelf on Laurentian during continental rifting and breakup in simple shear (modifi ed from Thomas and Astini, continental crust (e.g., McBride, 1989), and the 1999, and references cited therein). shelf edge at the rifted margin of continental crust marks the boundary between the contrast- ing facies. isochron dates of 524 ± 31, 529 ± 31, and 699 ± reconstruction of the rifted margin. A prominent, The oldest strata in the allochthon, Upper 26 Ma (Nicholas and Rozendal, 1975; Denison relatively narrow gravity high extends along the Cambrian Dagger Flat Sandstone (Figs. 4A and et al., 1977; Nicholas and Waddell, 1989). These subsurface Ouachita interior zone northwest- 12), include coarse arkosic sandstone, shale, dates suggest synrift contemporane- ward to the outcrops of the Marathon salient quartzose sandstone, calcarenite, and conglom- ous either with rifting along the Ouachita rift or (Handschy et al., 1987; Keller et al., 1989b), erate, which contains clasts of limestone, shale, with early rifting along the Blue Ridge rift; how- marking the trace of the Texas transform (Fig. 1). chert, sandstone, granite, and mafi c igneous ever, U-Pb zircon data are needed to constrain The gravity high bends abruptly southward rocks (McBride, 1989). The overlying Lower the correlation. within the arc of the Marathon salient, extending Ordovician Marathon Formation (Fig. 4A) along the Ouachita interior zone and marking a includes limestone, shale, sandstone, limestone- CORNER OF MARATHON crustal boundary (Handschy et al., 1987), which clast conglomerate, and boulder beds (McBride, EMBAYMENT is interpreted to be the Marathon rifted margin 1989). One distinctive olistostrome, dominated of Laurentian crust (Figs. 1 and 12). The abrupt by dolostone olistoliths, was originally mapped In the Marathon topographic basin in west curve of the gravity high outlines the Marathon as the Monument Spring Dolomite Member of Texas, the Ouachita thrust belt is exposed in a rel- embayment at the intersection of the Texas trans- the Marathon Limestone (King, 1937). Other- atively small area surrounded by extensive Meso- form and Marathon rift (Fig. 1). A gravity low, wise, the boulders include a variety of lime- zoic strata of the Gulf Coastal Plain (Fig. 1). inside the arc of the Ouachita-interior-zone high, stones, dolostones, and other Strike of the thrust belt curves locally from north- marks the location of the accreted ter- types. Signifi cantly, the boulders contain fossils westerly to southwesterly, outlining the Mara- rane, the emplacement of which has obscured of shallow-water forms in contrast to the deeper thon structural salient. The abrupt curve of the the deep crustal structure in much of the Mara- water graptolite faunas of the interbedded ; thrust front has been the basis for interpretation thon embayment (Dickinson and Lawton, 2001). however, the biostratigraphic ages are indistin- of the location of the Marathon embayment, out- The Marathon allochthon, containing a strati- guishable. The Dagger Flat and Marathon strata lined by intersection of the Texas transform with graphic succession of Cambrian-Mississippian are time-equivalent to a basal sandstone and the Marathon rift, in the Iapetan rifted margin off-shelf passive-margin facies (Fig. 4A) and massive carbonate (Ellenburger Group) of the of Laurentia (Thomas, 1977). Only limited data Mississippian- synorogenic turbidites, passive-margin shelf facies (Fig. 4A). Sedimen- from deep wells are available (e.g., Ross, 1986; is thrust over Cambrian-Mississippian carbon- tary structures indicate deposition by turbidity Muehlberger and Tauvers, 1989a, 1989b), and no ate-dominated passive-margin shelf facies and fl ows from sources along the shelf edge to the seismic refl ection surveys are available to support Mississippian-Permian synorogenic shallow- northwest of the depositional site (McBride,

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1989). The large size and abundance of the dolostone olistoliths suggests that the deposi- tional site was near the shelf edge; however, the palinspastically restored distance from the lead- ing edge of the allochthon through the extent of SEA LEVEL SEA the dolostone olistostrome is ~70 km (Fig. 12). The Middle Ordovician Fort Peña Formation

south and Woods Hollow Shale are dominated by shale but include interbeds of sandstone, lime- ruction of

in Nicholas stone, chert, and conglomerate. Both units con-

ion and depth tain boulder beds, which are most numerous near the top of the Woods Hollow. The boulders include shallow-water carbonate, sandstone, felsite porphyry, and schist (McBride, 1989); and fossils in the carbonate clasts document a range of Late Cambrian and Early Ordovician ages (King, 1937; Wilson, 1954; summary in

Ouachita thrust fault trajectory of future fault (palinspastic reconstruction) transform fault Texas McBride, 1989). Considering the palinspasti- cally restored length of off-shelf facies within ′

F the internally deformed allochthon, the depo- sitional site of the boulders of shallow-water carbonate reached at least 70 km from the shelf SOUTH edge, which must have been the source of sub- marine slumps and debris fl ows that transported Gulf Coastal Plain the boulders into deeper water (Fig. 12). The

Ouachita interior zone clasts of granite and metamorphic rocks suggest that submarine canyons in the shelf edge eroded Mesozoic-Cenozoic Gulf Coastal Plain strata down into basement rocks. Boulders in the Marathon synorogenic clastic wedge, specifi cally in the Pennsylvanian Hay- mond Formation in the middle part of the alloch- thon, provide further insight into the nature of the passive margin. The Haymond boulders are of three distinct types: intrabasinal fragments of Devils River uplift older strata of the Marathon allochthon, exotic

OUACHITA THRUST BELT igneous (369–457 Ma Rb/Sr isochron, Denison et al., 1969) and metamorphic rocks of unknown source, and limestone boulders with Middle Cambrian (Marjuman) distinctive of the seaward margin of a carbonate shelf (King, Ouachita interior zone Paleozoic metasedimentary rocks Cambrian–Middle Pennsylvanian passive-margin deposits off-shelf oceanic crust 1937; Palmer et al., 1984; Ross, 1986; McBride,

frontal thrust belt 1989). The boulders are within a suc- cession, all of which is generally interpreted as derived from the southeast of a foreland basin (e.g., Denison et al., 1969; Ross, 1986). All of the boulders are inferred to have come from distinct thrust sheets within the oro- gen, and both the intrabasinal clasts and exotic rocks fi t readily into that interpretation. The boulders of Middle Cambrian limestone, which are not in the same Marathon thrust sheet as the intrabasinal or exotic boulders (Fig. 12) (Palmer et al., 1984), however, suggest important con- Texas (no vertical exaggeration; line of cross section shown in Fig. 1). Vertical black line on Devils River uplift shows locat black line on Devils River Vertical section shown in Fig. 1). (no vertical exaggeration; line of cross Texas Figure 11. Structural cross section of Ouachita thrust-belt structures (including Devils River uplift) and palinspastic reconst (including Devils River section of Ouachita thrust-belt structures Structural cross 11. Figure of the Gulf Coastal Plain in cover transform beneath the Ouachita thrust belt and post-orogenic Texas Iapetan margin along the data section compiled from of penetration the Shell No. 1 Stewart well (Nicholas and Rozendal, 1975; Nicholas, 1983). Cross Thomas et al. (1989). (1989), and Waddell and Rozendal (1975), Nicholas (1983, 1989), straints on both the age of rifting and the loca- tion of the rifted margin/shelf edge. Middle Pennsylvanian–Lower Permian synorogenic turbidites Cambrian–Middle Pennsylvanian passive-margin carbonate shelf Neoproterozoic–Lower Cambrian metasedimentary and metavolcanic rocks Precambrian crystalline basement Laurentian continental crust The Middle Cambrian (Haymond Val Verde foreland basin Verde Val boulders) are older than the base of the trans-

EXPLANATION gressive basal sandstones farther inboard around F NORTH NO VERTICAL EXAGGERATION NO VERTICAL

0 the Ouachita embayment and Texas promontory –5 –10

ELEVATION (km) ELEVATION (on the Ozark dome and Llano uplift) (Fig. 1)

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s thrust sheet, source of Haymond boulders thrust sheet, source of Haymond boulders Ouachita interior zone Paleozoic metasedimentary rocks Mississippian-Pennsylvanian synorogenic turbidites Cambrian-Mississippian passive-margin deposits off-shelf depositional site of Haymond boulders palinspastic depositional site of Haymond boulders late (syn-Haymond) thrust fault breaks through shelf-edge strata in Marathon footwall late (syn-Haymond) thrust fault breaks through shelf-edge strata in Marathon footwall

extent of boulders of shelf-facies and basement rock in Cambrian-Ordovician off-shelf strata in Marathon allochthon Mississippian-Pennsylvanian synorogenic shallow-marine facies Cambrian-Mississippian passive-margin carbonate shelf Middle Cambrian and older(?) passive-margin carbonate shelf Precambrian crystalline basement Laurentian continental crust s . Cross section compiled from data in King (1937), Flawn et al. (1961), Ross section compiled from . Cross ′ EXPLANATION Late Cambrian–Ordovician shelf edge early (pre-Haymond) Marathon allochthon strata of off-shelf thrust sheet, source of Haymond boulder G' Ouachita interior zone Gulf Coastal Plain SOUTHEAST depositional site of Haymond boulders profile of shelf-edge submarine canyon, sedimentary deposits supplying clasts to off-shelf late (syn-Haymond) thrust fault breaks through shelf-edge strata in Marathon footwall early (pre-Haymond) Marathon allochthon strata of off-shelf early (pre-Haymond) Marathon allochthon strata of off-shelf Middle Cambrian limestone leading edge of Marathon allochthon with no internal shortening boulders, Haymond Formation intrabasinal and exotic rocks Figure 12. Structural cross section of Marathon thrust-belt structures and alternative interpretations of palinspastic reconstr and alternative interpretations section of Marathon thrust-belt structures 12. Structural cross Figure Iapetan margin along the Marathon rift beneath the Marathon thrust belt and post-orogenic cover of Gulf Coastal Plain in west T of Gulf Coastal Plain in west cover Iapetan margin along the Marathon rift beneath thrust belt and post-orogenic aligned at the northwest sections are All palinspastic cross section shown in Fig. 1). (no vertical exaggeration; line of cross point G at the northwest end of the structural cross section G–G point G at the northwest end of structural cross (1986), Palmer et al. (1984), McBride (1989), Muehlberger and Tauvers (1989a, 1989b), and Thomas et al. (1989). (1989a, 1989b), and Tauvers and et al. (1984), McBride (1989), Muehlberger (1986), Palmer MARATHON THRUST BELT leading edge of Marathon allochthon with 2:1 internal shortening leading edge of Marathon allochthon with 2:1 internal shortening . PALINSPASTIC RECONSTRUCTION (see text alternative 5) Middle Cambrian . Late Cambrian–Ordovician boulders, Hollow Formation Woods and Marathon Formation . PALINSPASTIC RECONSTRUCTION (see text alternative 2) . PALINSPASTIC RECONSTRUCTION (see text alternative 1) . PALINSPASTIC RECONSTRUCTION OF DEPOSITIONAL SETTING BOULDER BEDS minimum extent of passive-margin shelf facies documented by drilling G NORTHWEST B C A D-1 D-2 0

–5

–10 ELEVATION (km) ELEVATION

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(Palmer et al., 1984), where latest Middle ration, the northeast-striking Marathon rift inter- in the off-shelf facies, although shelf-facies Cambrian sandstones are irregularly distrib- sects the Texas transform near the Devils River boulders of Late Cambrian–Early Ordovician uted on an irregular paleotopographic surface uplift (Fig. 1). The direction of translation of the ages and basement boulders are common. In on Precambrian basement rocks. The Middle Marathon allochthon over the passive-margin contrast, the Haymond carbonate boulders are Cambrian limestone boulders indicate an ear- shelf is nearly parallel with the long northwest- only from the Middle Cambrian. This recon- lier post-rift transition to passive-margin depo- trending segment of the subsurface thrust belt struction is similar in the magnitude of transla- sition in the Marathon embayment than that in that includes the Devils River uplift, implying tion to the ~100 km translation in the Ouachita the Ouachita embayment. Progressive post-rift a large strike-slip component in the northwest- thrust belt in Arkansas and Oklahoma (Fig. 5). thermal subsidence and transgression resulted trending thrust belt and >160 km of cratonward Alternative 3: Passive-margin shelf and in more extensive Upper Cambrian carbonate- translation of the frontal Marathon thrust sheets off-shelf deposition may have begun entirely shelf facies. The oldest off-shelf strata in the over the passive-margin shelf (Figs. 1 and 12). around the Marathon embayment as early as Marathon allochthon are Upper Cambrian sand- For the depositional site of the Haymond boul- Middle Cambrian, and boulders from the shelf stones (Dagger Flat Sandstone, Fig. 4A), and ders to be in the footwall of a thrust sheet that edge could have been incorporated in an older the allochthon contains no exposed strata as old carried the Middle Cambrian shelf-edge strata (pre-Dagger Flat) part of the off-shelf succes- as the depositional age of the Middle Cambrian into the orogen, the leading ~125 km (or less sion far out from the shelf edge. The Middle boulders. The Upper Cambrian through Lower if internal shortening is incorporated) of the Cambrian boulders could have been recycled Ordovician and younger off-shelf facies in the Marathon allochthon had been thrust onto the from off-shelf passive-margin boulder beds allochthon contain boulders of age-equivalent shelf before deposition of the boulder beds (Fig. within the orogen into the Haymond in the fore- shelf carbonates, but no clasts as old as the 12-B). Alternatively, if the shelf collapsed and land basin; however, this succession of events Middle Cambrian boulders in the Haymond the shelf edge shifted cratonward by ~125 km requires a remarkable sorting of boulder types. Formation are recognized in the lower Paleo- (or less) at the end of Middle Cambrian, the This reconstruction is unconstrained in terms of zoic boulder beds in the allochthon. The tempo- Upper Cambrian–Lower Ordovician off-shelf the palinspastic location of the shelf edge. Fur- ral relationships of the shelf and off-shelf facies, facies could have been deposited over the col- ther, it requires all off-shelf deposits older than as well as shelf-facies boulders in the off-shelf lapsed Middle Cambrian shelf, requiring no Dagger Flat to have remained in the Marathon facies, show that the Middle Cambrian lime- translation of the allochthon before Haymond footwall, except for those included in the thrust stones record a component of shelf evolution deposition. Contrary to the latter possibility, the sheet that supplied detritus to the Haymond. that is unknown in the other Marathon rocks. Cambrian-Ordovician off-shelf deposits contain Alternative 4: With only minimal translation Further, the Middle Cambrian shelf-margin Upper Cambrian carbonate and basement clasts of the Marathon allochthon onto the shelf, the limestones indicate a passive-margin shelf edge, from basement-penetrating canyons along the depositional site of the Haymond was located the location of which can be inferred only from shelf edge, but no Middle Cambrian limestone generally southwest of the present location of the dispersal of the boulders. clasts. The lack of Middle Cambrian clasts sug- the Devils River uplift, and erosion from the Within the limitations of available data, sev- gests that Middle Cambrian passive-margin continental margin along the Texas transform eral alternatives for palinspastically fi tting a transgression did not reach as much as 125 km at or near the Devils River uplift (Fig. 1) could Middle Cambrian shelf edge along the Mara- inboard from the original rifted margin. have supplied the Middle Cambrian limestone thon rifted margin can be considered: Alternative 2: Marathon thrusting, tectonic boulders to the Haymond depositional site by Alternative 1: If the Middle Cambrian lime- loading, and foreland subsidence evidently southwest-directed debris fl ows and turbidity stone boulders were derived directly from a began in Mississippian time with deposition of currents. Paleocurrent data and thickness dis- thrust sheet within the growing Marathon oro- the thick siliciclastic Tesnus Formation (Ross, tribution within the upper Paleozoic Marathon gen, the shortening within and translation of 1986; McBride, 1989); therefore, thrust transla- synorogenic clastic wedge (McBride, 1989; the allochthon together provide a measure of the tion onto the shelf and internal shortening of the Muehlberger and Tauvers, 1989a) are consis- original extent of the shelf and location of the Marathon allochthon may have progressed sig- tent with southwesterly sediment dispersal margin (e.g., Palmer et al., 1984; Ross, 1986; nifi cantly before Haymond deposition. A thrust from the Texas transform margin of the Mara- Muehlberger and Tauvers, 1989a). The Middle fault propagating through the sub-allochthon thon embayment, as well as from the Marathon Cambrian shelf edge must have been at the shelf edge could have provided a source for the orogenic belt on the southeast. This alternative palinspastic site of the trailing thrust sheets, Haymond boulders, which were deposited on places only general constraints on the location which supplied the boulders to Haymond depo- the trailing part of the allochthon (Fig. 12-C). of the Marathon rift margin, and it relies on sition. Considering 3:1 shortening within the Considering the depositional site of the shelf- evolution of a passive-margin carbonate shelf allochthon, the palinspastic length of the entire derived boulders in off-shelf sediment near the as early as Middle Cambrian along the Texas allochthon is ~220 km (Fig. 12). The length of shelf edge in deep water suggests a minimum transform. Considering the palinspastic length the allochthon between the thrust front and the of ~85 km (present width from Marathon thrust of the Marathon allochthon and minimal trans- Haymond boulder beds is ~42 km and palin- front to Haymond boulders with 2:1 shorten- lation over the shelf, the rifted margin could be spastically ~125 km (Fig. 12-B). A restoration ing) of translation onto the passive-margin between ~160 and ~90 km southeast of the pres- of the allochthon places the depositional site shelf before deposition of the Haymond. This ent thrust front of the Marathon salient (Fig. 12). of the boulder beds and the shelf edge (source reconstruction places the shelf edge at a mini- Alternative 5: The unique age of the Middle of the Middle Cambrian boulders) at a location mum of ~90 km southeast of the present thrust Cambrian limestone boulders, as well as the >120 km southeast of the present location of the front and ~50 km southeast of the present loca- contrast in age with the Upper Cambrian–Lower boulders in the Marathon thrust belt (Palmer tion of the Haymond boulders (Fig. 12-C). Ordovician boulders in the passive-margin off- et al., 1984). That restoration places the rifted The primary problem with this reconstruction shelf boulder beds, suggests tectonic partition- margin/shelf edge southwest of the present is the lack of Middle Cambrian boulders in any ing and diachronous subsidence of the conti- lo cation of the Devils River uplift; in that resto- of the Upper Cambrian–Ordovician formations nental shelf edge. These conditions can be

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met in progressive subsidence of a lower-plate struction of the trace of the rift margin and Structure of the Rifted Margin margin, which is partitioned by multiple lis- shelf edge, which is consistent with palin- tric extensional faults (e.g., Lister et al., 1986). spastic restoration of the distribution of The rifted margin of the Alabama promon- Post-rift thermal subsidence of listric fault passive-margin shelf carbonates and coeval tory is interpreted to be an upper-plate margin blocks accounts for passive-margin transgres- off-shelf facies (Figs. 5 and 8). Geophysi- in a low-angle-detachment simple-shear rift sion during Middle Cambrian and deposition cal modeling confi rms the location of the system (Thomas, 1993). This interpretation of shelf-edge carbonates along the upthrown Alabama-Oklahoma transform fault (Keller is based on the relatively thin passive-margin (seaward-facing) block (Fig. 12-D-1). Contin- et al., 1989a; Mickus and Keller, 1992; Harry shelf cover and a general lack of synrift sedi- ued subsidence may have resulted in drown- et al., 2003; Harry and Londono, 2004). Seis- ment between basement and the passive-margin ing of the shelf in latest Middle Cambrian and mic refl ection profi les image basement ramp facies. Palinspastic reconstruction shows exten- establishment of a new passive-margin shelf anticlines, supporting palinspastic reconstruc- sional normal faults (Fig. 3) antithetic to the on a structurally higher and more inboard lis- tion along the Ouachita rift margin (Rozendal detachment that dips under the upper plate. tric (Fig. 12-D-2). With continued and Erskine, 1971; Culotta et al., 1992). The An abrupt along-strike change to thick synrift thermal subsidence (or sea-level rise), a car- traces and intersection of the rift and trans- stratigraphy northeastward across the Georgia bonate buildup at the shelf edge maintained a form outline the Ouachita embayment (Fig. 1). transform (Fig. 1) indicates a change in polar- steep seaward-facing slope, where submarine The southwestern part of the Blue Ridge rift ity of the low-angle detachment, and an along- canyons cut through the Late Cambrian– defi nes the corner of the Alabama promontory strike change to lower-plate structure north of Ordovician shelf carbonates and underlying at the intersection with the Alabama-Oklahoma the Alabama promontory (Thomas, 1993; Tull basement to supply clasts to the Upper Cam- transform (Fig. 1). Palinspastic reconstruction and Holm, 2005). Although the original rift brian and Ordovician deep-water deposits. of the passive-margin shelf in the Appalachian margin of the Alabama promontory has been Deposition of off-shelf (deep-water) passive- orogen shows that the original continental shelf truncated by the Suwannee-Wiggins suture, the margin facies likely extended seaward, possibly extended as much as 80 km southeast of the pres- upper-plate structure is indicated by the thrust- forming a mud-dominated cover over the older ent limit of Laurentian crust at the Suwannee- translated passive-margin strata (Fig. 3). Middle Cambrian shelf carbonates. The loca- Wiggins suture (Fig. 3). The crust-penetrating The Alabama-Oklahoma transform fault forms tions of the two essential rift-stage faults are Suwannee-Wiggins suture marks continent- a steep boundary of Laurentian continental not rigorously constrained, nor is the distance continent collision between Laurentian crust and crust, as modeled from geophysical data (Keller between them. For emplacement of the Mara- Gondwanan (African) crust of the Suwannee et al., 1989a; Mickus and Keller, 1992; Harry thon allochthon over the shelf, the most inboard terrane, and truncates Laurentian crust. The et al., 2003; Harry and Londono, 2004). An possible location of the Late Cambrian shelf suture forms the present margin of Laurentian abrupt shelf edge shed detritus into the off-shelf edge is ~42 km from the present thrust front; crust, but it is not at the original rifted margin as facies, and boulders of basement rocks indicate to yield the present spacing of thrust sheets, previously mapped (e.g., Thomas, 1991). that submarine canyons penetrated through the the distance between the Middle Cambrian and Intersection of the Ouachita rift with the passive-margin cover at the shelf edge. The age Late Cambrian shelf edges can be no more than Texas transform forms the southern corner of of the boulders (Bowring, 1984) shows that the ~125 km. Propagation of the Marathon décolle- the Texas promontory, and intersection of the transform fault cut across the Grenville front ment in the off-shelf Upper Cambrian facies led Texas transform with the Marathon rift forms into rocks of the Granite-Rhyolite province. to emplacement of the Marathon allochthon. A the Marathon embayment (Fig. 1). The loca- Pods of ultramafi c rocks in the Ouachita alloch- break-forward thrust, cutting into the footwall, tions and traces of the Ouachita rift, Texas thon of off-shelf passive-margin strata represent placed the Middle Cambrian shelf-edge strata in transform, and Marathon rift have been inter- oceanic crust on which the deep-water sediment the orogenic of the Haymond For- preted primarily on the basis of the sinuously was deposited (Morris and Stone, 1986; Nielsen mation (Fig. 12-D-2). curved trace of the Ouachita-Marathon frontal et al., 1989), indicating that by Ordovician time The fi rst four listed alternatives all require thrust belt around the Texas recess and Mara- the transform margin of Laurentian crust faced fortuitous combinations of processes to generate thon salient (e.g., Thomas, 1977), as well as the an opening ocean and spreading oceanic crust in observed distributions of boulders in the Mara- gravity high along the Ouachita interior zone the Ouachita embayment. thon stratigraphy. The last (fi fth) listed alterna- (Handschy et al., 1987; Keller et al., 1989b). The Ouachita rift margin between the tive is designed specifi cally to explain patterns Available data are not adequate to support quan- Alabama-Oklahoma and Texas transforms of clast dispersal. Although it is speculative at titative palinspastic reconstruction of the extent is an upper-plate margin, as indicated by the this stage, this alternative offers the best com- of the passive-margin shelf or the rift-transform composition and relatively small thickness prehensive interpretation of all currently avail- edge of continental crust. As a result both the of the passive-margin-shelf deposits of Late able data (alternative 5, Figure 12-D). trace and location of the continental margin are Cambrian–Early Ordovician age (Thomas and subject to interpretation, largely from recon- Astini, 1999). Other characteristics of an upper DISCUSSION AND CONCLUSIONS structions of the shelf-edge stratigraphy derived plate include paleotopographic relief on base- from composition of boulder beds in the off- ment rocks beneath the passive-margin cover Trace of the Rifted Margin shelf passive-margin strata and synorogenic and lack of preservation of synrift deposits. The turbidites (Fig. 12). Using these constraints, Argentine Precordillera microcontinent is the Both the Ouachita rift margin and the Alabama- the trace of the Texas transform is parallel with conjugate rift margin of the Ouachita rift and Oklahoma transform margin of the Ouachita the northwest-trending segment of the orogenic exhibits the characteristics typical of a lower embayment are preserved in the footwall of the belt, and an intersection of the transform with plate (Thomas and Astini, 1999). For example, Ouachita allochthon, which consists of off- the Marathon rift beneath the bend in the orogen the Precordillera includes an Early Cambrian shelf facies. Palinspastic reconstruction of at the Marathon salient defi nes the Marathon synrift succession that grades upward into basement ramp anticlines provides a recon- embayment of the continental margin. passive-margin carbonates, and the passive-

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margin carbonate succession is thicker than that passive margin is consistently at the beginning of the Ouachita ocean resulted in post-rift ther- on the Texas promontory. Furthermore, the base of Cambrian time along the Blue Ridge rift. mal subsidence at different times on the opposite of the passive-margin carbonate succession on The age of the Alabama-Oklahoma trans- conjugate margins, as well as late-synrift exten- the Precordillera is older than that on the Texas form is based on the age of rifting along the sion on rift-parallel intracratonic faults. promontory. The stratigraphy confi rms that Ouachita rift, the age of both rift-parallel and Early Cambrian rifting around the Ouachita passive-margin subsidence on the rifted margin transform-parallel intracratonic fault systems, embayment and Argentine Precordillera con- of the Precordillera began earlier and continued and the age of rifting of the Argentine Precor- trasts with Early Cambrian evolution of a pas- to greater magnitude than that on the Texas prom- dillera. Synrift redbeds and evaporites (Cerro sive margin along the Blue Ridge rift. The ontory, consistent with complementary asymme- Totora Formation) in the Precordillera are diachroneity is interpreted to be a result of a try of subsidence history on the opposite margins Early Cambrian age (Astini et al., 1995; Astini spreading-ridge shift from the segment of the along an asymmetric low-angle-detachment rift and Vaccari, 1996), and the upward transi- Blue Ridge rift south of the Alabama-Oklahoma (Fig. 10) (Thomas and Astini, 1999). tion to passive-margin carbonates is late Early transform to the Ouachita rift at the beginning Lack of data precludes specifi c interpreta- Cambrian (Fig. 9). In contrast, on the Texas of Cambrian time (Thomas, 1991; Thomas and tions of crustal structure of the Texas transform promontory, the base of the passive-margin Astini, 1996). and the Marathon rift, which intersect to form cover is latest Middle Cambrian, and the basal The initial transgressive passive-margin the corner of the Marathon embayment beneath sandstone rests directly on basement (summary carbonate-shelf deposits, overlying Precam- the present Marathon salient of the late Paleo- in Thomas and Astini, 1999). In the context brian basement, around the Texas promontory zoic thrust belt (Fig. 1). Boulders in the off- of complementary asymmetry of subsidence are of late Middle Cambrian age (summary in shelf facies are consistent with derivation from along a low-angle-detachment rift, the time of Palmer et al., 1984); and the oldest off-shelf a steep continental slope as along a steep trans- initial rifting of the Precordillera from the Texas facies in the Marathon allochthon are of Late form fault; however, some or all of the boulders promontory is Early Cambrian (Fig. 10). Cambrian age. Middle Cambrian limestone may have sources along the Marathon rift mar- The rift-parallel intracratonic Mississippi Val- boulders in the Marathon synorogenic deposits, gin. Distinct passive-margin off-shelf facies in ley graben and Birmingham graben are northeast however, indicate evolution of a passive-margin the Marathon allochthon and passive-margin- of and perpendicular to the Alabama-Oklahoma shelf earlier than deposition of any known strata shelf facies in the Marathon footwall, clearly transform fault (Fig. 1). The age of the upper in the region. Metavolcanic rocks in the Devils document a shelf margin. Distribution and part of the fi ll of both grabens is biostratigraphi- River uplift along the Texas transform have composition of boulders in both passive-margin cally documented as Middle to early Late Cam- yielded Rb/Sr isochron dates of 524–529 Ma and synorogenic deep-water deposits are best brian (summary in Thomas, 1991). The fi ll of (Nicholas and Rozendal, 1975), comparable to explained by a diachronous succession of exten- another graben southeast of and parallel with the U-Pb zircon ages of synrift igneous rocks sional faults, the distribution of which indicates the Birmingham graben includes redbeds and along the Southern Oklahoma fault system, and a relatively wide zone of transitional crust. evaporites that are Early Cambrian, biostrati- also of 699 Ma (Denison et al., 1977), com- These characteristics suggest that the Marathon graphically and geochemically equivalent to parable to ages of early components of the Blue rift is on the lower plate of a low-angle detach- the synrift Cerro Totora Formation in the Pre- Ridge rift. Evolution of a passive margin in ment (Fig. 12-D-2). cordillera (Thomas et al., 2004), and the lower Middle Cambrian is compatible with either date part of the fi ll of the Mississippi Valley and Bir- for the volcanic rocks; however, if the younger Age of Rifting mingham grabens may be of the same age. The date of 524–529 Ma signifi es the time of rifting, graben-fi ll successions and graben-boundary the initiation of subsidence as early as Middle The age of rifting along the Blue Ridge rift on faults are overlapped by passive-margin carbon- Cambrian is consistent only with the typical the corner of the Alabama promontory can best ates of middle Late Cambrian age, indicating the subsidence history of a lower-plate structure. be approached using data from the Blue Ridge end of rift extension. Continuing extension on The difference in possible ages allows that rift farther north in Tennessee and Virginia, these fault systems is compatible with rifting in Marathon rifting either was part of the older where synrift rocks are exposed. Multiple epi- Early Cambrian time and movement of the Pre- Blue Ridge phase, which may have been linked sodes of synrift magmatism include components cordillera microcontinent along the Alabama- to the Marathon rift by a longer Texas transform as old as ~750 Ma, and the youngest synrift Oklahoma transform as the ocean opened along south of the pre-rift site of the Precordillera, or igneous rocks along the Blue Ridge in Virginia the Ouachita rift; extensional faulting stopped by was part of the younger Ouachita phase, during have U-Pb zircon ages of 572 ± 5 to 564 ± middle Late Cambrian when the Ouachita mid- which the Precordillera was rifted from Lauren- 9 Ma (Aleinikoff et al., 1995; Walsh and Aleini- ocean ridge migrated past the corner of Lauren- tia. Considering the latter, the reconstructed size koff, 1999). The synrift rocks are overlain by a tian continental crust on the Alabama promontory of the Precordillera microcontinent matches the passive-margin succession of basal sandstone (Thomas, 1991). size of the Ouachita embayment, suggesting and overlying carbonate of Early Cambrian age. Synrift igneous rocks along the transform- that if the Marathon rift were contemporaneous In Tennessee and farther south to the Georgia parallel Southern Oklahoma fault system have with rifting away of the Precordillera, the Texas transform, the Early Cambrian passive-margin crystallization ages of 530–539 Ma (Hogan transform must have separated the Precor dillera succession rests on thick synrift sedimentary and Gilbert, 1998; Thomas et al., 2000b). A from another Laurentian microcontinent, pos- deposits (summary in Thomas, 1991). On the Late Cambrian passive-margin succession over- sibly (Ramos et al., 1986; Ramos, Alabama promontory, the base of the passive- lapped the synrift igneous rocks as a result of 2005). Like consideration of trace and struc- margin succession is not exposed, but the oldest post-rift thermal subsidence (Fig. 4A) (Thomas ture of the margin, the age of rifting around the strata in Appalachian thrust sheets are Early and Astini, 1999). The age of the synrift igneous Marathon embayment requires more data for Cambrian (Fig. 4C) and are stratigraphically rocks corresponds to the time of initial rifting documentation. similar to the passive-margin succession along along the Ouachita rift, as indicated by the age In summary, Iapetan rifting of the margin the Blue Ridge. The age of transition from rift to of synrift deposits in the Precordillera. Spreading of southern Laurentia includes two primary

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phases. The older, the Blue Ridge rift, the south- Astini, R.A., and Vaccari, N.E., 1996, Sucesión evaporítica metamorphic boulders from the Haymond Formation del Cámbrico Inferior de la Precordillera: Signifi cado (Pennsylvanian), Marathon Basin, Texas, and their ern part of which forms the eastern margin of geológico: Revista de la Asociación Geológica Argen- signifi cance: Geological Society of America Bulletin, the Alabama promontory, is dated by synrift tina, v. 51, p. 97–106. v. 80, p. 245–256, doi: 10.1130/0016-7606(1969)80 volcanic rocks as young as 564 Ma and passive- Astini, R.A., Benedetto, J.L., and Vaccari, N.E., 1995, The [245:IAOIAM]2.0.CO;2. early Paleozoic evolution of the Argentine Precor- Denison, R.E., Burke, W.H., Otto, J.B., and Hetherington, margin transgression beginning in earliest Cam- dillera as a Laurentian rifted, drifted, and collided E.A., 1977, Age of igneous and metamorphic activity brian. The younger, the Ouachita rift refl ects an terrane: A geodynamic model: Geological Society of affecting the Ouachita foldbelt, in Stone, C.G., ed., inboard shift from the Blue Ridge rift and rifting America Bulletin, v. 107, p. 253–273, doi: 10.1130/ Symposium on the geology of the Ouachita , 0016-7606(1995)107<0253:TEPEOT>2.3.CO;2. v. l: Arkansas Geological Commission, p. 25–40. of the Argentine Precordillera from the Ouachita Astini, R.A., Ramos, V.A., Benedetto, J.L., Vaccari, N.E., Denison, R.E., Lidiak, E.G., Bickford, M.E., and Kisvar- embayment, beginning at 530–539 Ma. The and Cañas, F.L., 1996, La Precordillera: Un terreno sanyi, E.B., 1984, Geology and geochronology of exotico a : 13th Congreso Geológico Argen- Precambrian rocks in the Central Interior region of the Ouachita rift is temporally and mechanically tino y 3rd Congreso de Exploración de Hidrocarburos, : U.S. Geological Survey Professional associated with the rift-parallel intracratonic Buenos Aires, Actas 5, p. 293–324. Paper 1241-C, 20 p. Mississippi Valley and Birmingham graben Astini, R.A., Thomas, W.A., and Osborne, W.E., 2000, Sedi- Dickinson, W.R., and Lawton, T.F., 2001, mentology of the Conasauga Formation and equivalent to Cretaceous assembly and fragmentation of Mex- systems, as well as with the transform-parallel units, Appalachian thrust belt in Alabama, in Osborne, ico: Geological Society of America Bulletin, v. 113, intracratonic Southern Oklahoma fault system. W.E., Thomas, W.A., and Astini, R.A., eds., The Cona- p. 1142–1160, doi: 10.1130/0016-7606(2001)113 The age of the Ouachita rift is documented by sauga Formation and equivalent units in the Appala- <1142:CTCAAF>2.0.CO;2. chian thrust belt in Alabama: Alabama Geological Francheteau, J., and Le Pichon, X., 1972, Marginal fracture the 530–539 Ma synrift volcanics of the South- Society, 37th Annual Field Trip Guidebook, p. 41–70. zones as structural framework of continental margins ern Oklahoma fault system, by Early Cambrian Barnes, V.E., Bell, W.C., Clabaugh, S.E., Cloud, P.E., Jr., in South Atlantic Ocean: The American Association McGehee, R.V., Rodda, P.U., and Young, K., 1972, of Bulletin, v. 56, p. 991–1007. synrift sediment along the conjugate rift margin Geology of the Llano region and Austin area, fi eld Flawn, P.T., Goldstein, A., King, P.B., and Weaver, C.E., in the Argentine Precordillera, and by late syn- excursion: University of Texas Bureau of Economic 1961, The Ouachita system: University of Texas Publi- rift graben-fi ll of Early to early Late Cambrian Geology Guidebook 13, 77 p. cation 6120, 401 p. Benkhelil, J., Mascle, J., and Guiraud, M., 1998, Sedimentary Gilbert, M.C., 1983, Timing and chemistry of igneous age in the Mississippi Valley and Birmingham and structural characteristics of the Cretaceous along events associated with the Southern Oklahoma aulaco- graben systems. the Cote d’Ivoire-Ghana transform margin and in the gen: Tectonophysics, v. 94, p. 439–455, doi: 10.1016/ Benue trough: A comparison, in Mascle, J., Lohmann, 0040-1951(83)90028-8. ACKNOWLEDGMENTS G.P., and Moullade, M., eds., Proceedings of the Ocean Grohskopf, J.G., 1955, Subsurface geology of the Missis- Drilling Program, Scientifi c Results, v. 159, p. 93–99. sippi Embayment of southeast Missouri: Missouri Bowring, S., 1984, U-Pb zircon ages of granitic boulders Geological Survey, v. 37, 133 p. I greatly appreciate the invitation from Bill Dickin- in the Ordovician Blakely Sandstone, Arkansas, and Handschy, J.W., Keller, G.R., and Smith, K.J., 1987, The son to prepare this article as a summary of research on implications for their provenance, in Stone, C.G., and Ouachita system in northern Mexico: , v. 6, the Iapetan rifted margin of southern Laurentia. My Haley, B.R., eds., A guidebook to the geology of the p. 323–330, doi: 10.1029/TC006i003p00323. research has been done over the years with the col- central and southern , Arkansas: Ham, W.E., Denison, R.E., and Merritt, C.A., 1964, Base- laboration of many geologists and with the support of Arkansas Geological Commission Guidebook 84-2, ment rocks and structural evolution of southern Okla- various agencies and organizations, all of whom are p. 123. homa: Oklahoma Geological Survey Bulletin 95, 302 p. acknowledged in my more detailed publications that Burke, K., and Dewey, J.F., 1973, Plume-generated triple Harris, L.D., 1979, Similarities between the thick-skinned junctions: Key indicators in applying to Blue Ridge anticlinorium and the thin-skinned Powell are cited herein. I am most grateful for these asso- old rocks: The Journal of Geology, v. 81, p. 406–433, Valley anticline: Geological Society of America Bul- ciations, which have stimulated my research. I thank doi: 10.1086/627882. letin, v. 90, p. 525–539, doi: 10.1130/0016-7606 Geosphere reviewers, Richard Hanson and an anony- Butts, C., 1926, The Paleozoic rocks, in Geology of Ala- (1979)90<525:SBTTBR>2.0.CO;2. mous reviewer, for their reviews of this manuscript. In bama: Alabama Geological Survey Special Report 14, Harry, D.L., and Londono, J., 2004, Structure and evolu- particular, the review by Richard Hanson was one of p. 41–230. tion of the central Gulf of Mexico continental margin the most thorough, helpful, and constructive reviews Cebull, S.E., Shurbet, D.H., Keller, G.R., and Russell, L.R., and coastal plain, southeast United States: Geological that I have ever received, and I am especially grateful 1976, Possible role of transform faults in the devel- Society of America Bulletin, v. 116, p. 188–199, doi: for that. opment of apparent offsets in the Ouachita–southern 10.1130/B25237.1. 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