Structural Evolution of the Pulaski Thrust System, Southwestern Virginia

Structural evolution of the Pulaski thrust system, southwestern Virginia

MERVIN J. BARTHOLOMEW Montana Bureau of Mines and Geology, Montana College of Mineral Science and Technology, Butte, Montana 59701

ABSTRACT clinorium. The classic, ubiquitous Max Mead- minimum shortening for the Pulaski thrust ows breccias generally are confined to the sheet is -80%, whereas that for the western The Pulaski fault system is a complex se- broken formation, which forms the basal part part of the Valley and Ridge province is only ries of Alleghanian thrusts which has juxta- of the complexly deformed plate and which is ~10%-20%. The Pulaski fault system is inter- posed rocks of the Pulaski thrust sheet over interpreted as an exhumed lower level décol- preted to have originated below the Ap- rocks as young as Early to middle Mississip- lement zone formed during an earlier Alle- palachian-wide basal décollement zone near pian (Maccrady Formation) of the Saltville ghanian stage and transported during a later the contact between the Cambrian Rome and thrust sheet The Salem branch, a later mqjor Alleghanian stage of deformation to a much Elbrook Formations. branch of the Pulaski fault system within the higher structural level. Pulaski thrust sheet, displaced the complexly Minimum displacement of the complexly INTRODUCTION deformed plate containing a broken forma- deformed plate is on the order of 100- tion (a lithotectonic assemblage of complexly 110 km, on the basis of a new palinspastic This paper is aimed at (1) elucidating rela- folded and faulted Elbrook and Rome car- reconstruction of the Pulaski thrust sheet. On tionships among major tectonic features of the bonates and shales with associated breccias) the basis of this reconstruction and of two Pulaski thrust sheet from Fincastle to Pulaski, over rocks as young as Early Mississippian balanced and restored cross sections for the Virginia (Fig. 1); (2) determining structural evo- (lower Price Formation) of the Salem syn- western portion of the Valley and Ridge, lution of the Pulaski sheet during the course of

Figure 1. Index map showing location of area relative to some tectonic features of the southern Appala- chians. MCW = Mountain City window, GMW = Grandfather Mountain window, SMA = Sauratown Mountain anticlinorium, SRA = Smith River allochthon, KMB = Kings Mountain belt, RB = Raleigh belt, ESB = eastern slate belt.

Geological Society of America Bulletin, v. 99, p. 491-510, 22 figs., 1 table, October 1987.

491

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 Figure 2. Generalized geologic map of the Pulaski thrust sheet at the northern end of the southern Appalachians near the Roanoke recess; modified after Bartholomew and others (1980). DDZ = ductile deformation zone, S = slice, IMW = Ingles Mountain window, BMW = Barringer Mountain window, ERW = East Radford window, CCA = Crab Creek allochthon, YSF = Yellow Sulphur fault, GRF = Green Ridge fault, GMF = Glebe Mills fault, PC = Peak Creek fault, RV = Roanoke Valley fault, RMA = Read Mountain allochthon, CMA = Coyner Mountain allochthon, BW = Bonsack window, GCW = Glade Creek window, CG = Chilhowee Group, €S = Cambrian-age Shady Formation, €R = Cambrian-age Rome Formation, €E/€R = Cambrian-age Elbrook Formation with Rome Formation, CO = Upper Cambrian/Lower Ordovi- cian carbonates, O = Middle and Upper Ordovician carbonates and calcareous shales, SD = Silurian sandstones and Devonian shales, D = Devonian sandstones and shales, M = Mississippian sandstones and shales, open teeth denote faults of the Pulaski fault system, solid teeth denote faults of the Salem branch, "T" on hanging wall of all other thrusts. Complexly deformed plate of Pulaski thrust sheet is shaded. Mapping credit: McGuire (1970)—STR (Strom), ER (Eagle Rock), ORI (Oriskany), SAL (Salisbury); Spencer (1968)—SM (Sugarloaf Mountain), BUC (Buchanan); McGuire (1976)—DAL (Daleville); all of the above quadrangles partially remapped by W. S. Henika (1977-1980), Henika (1981)—VIL (Villamont), MON (Montvale); Amato (1974)—SAL (Salem); Bartholomew and Hazlett (1981)—ROA (Roanoke); Bartholomew (1981)—STE (Stewartsville); reconnaissance mapping by Bartholomew and Lewis (1984)—IRV (Irving), HAR (Hardy), GC (Garden City), BM (Bent Mountain), CAL (Callaway), ELL (Elliston); reconnaissance mapping by Bartholomew, thesis maps by Bauerlein (1967), Broughton (1971), Edwards (1960), Eubank (1967), Murphy (1969), and detailed strip by Bartholomew and Schultz (1980), Bartholomew and others (1987b)—LOO (Looney), CAT (Catawba), GLE (Glenvar), MM (McDonalds Mill), IRO (Ironto), ELL (Elliston), PUL (Pulaski); Bartholomew and Lowry (1979)—BLA (Blacksburg); Schultz and Bartholomew (1987)—STA (Staffordsville), RN (Radford North); Schultz and others (1986)—PEA (Pearisburg), NAR (Narrows), EGG (Eggleston); Bartholomew and others (1987b) and Schultz and others (1986)—NEW (Newport), WG (White Gate); quadrangles mapped, but manuscripts not yet completed: A. P. Schultz—DUB (Dublin), partially adapted from Schultz (1983); S. E. Lewis, M. J. Bartholomew, and P. B. Kaygi—RIN (Riner), partially adapted from Lewis (1975) and Kaygi (1979); M. J. Bartholomew, P. M. Dove, and C. A. Walsh-Stovall—PIL (Pilot), CHE (Check); RS (Radford South)—northern part mapped by A. P. Schultz for city of Radford, southern half reconnaissance mapping by M. J. Bartholomew and S. E. Lewis.

Figure 3. Generalized geologic map of the Pulaski thrust sheet in the Pulaski to Blacksburg area, showing age of rocks in allochthonous window duplexes derived from the footwall of the Pulaski thrust ramp; also shows distribution of Max Meadows breccias (black areas) relative to the faults of the Pulaski fault system and to the upper detachment surface of the broken formation. Other abbreviations, mapping credits, and quadrangle names are the same as those shown in Figure 2. Cs = Shady Dolomite containing siliceous breccia, B = Middle Cambrian to Middle Ordovician carbonate rocks, CI = Middle and Upper Ordovician calcareous shales, C2 = Upper Ordovician to Devonian shales and sandstones, D = Upper Devonian to Lower Mississippian sandstones. UDS = upper detachment surface of a broken formation.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 494 M. J. BARTHOLOMEW

Figure 4. Generalized geologic map of the Pulaski thrust sheet in the Roanoke area, showing age of rocks in allochthonous window duplexes and distribution of Max Mead- ows breccias (black areas) relative to faults of the Pulaski fault system. Abbreviations, mapping credits, and quadrangle names are the same as those shown in Figure 2. Cs = Shady Dolomite; B, CI, and C2 are the same as shown in Figure 3.

its emplacement across Cambrian to Mississip- pian footwall strata to an upper level glide horizon in Mississippian strata during the Alleghanian event; (3) determining minimum displacements of various plates within the Pulaski thrust sheet by utilizing a new palinspastic reconstruction of these component plates; and (4) calculating percent shortening across this part of the Valley and Ridge province utilizing the palirispastic reconstruction for the Pulaski thrust sheet plus both balanced and restored cross sections for strata structurally beneath the Pulaski sheet. The Pulaski fault system includes all of the faults, bounding or cutting the Pulaski thrust sheet (Table 1; Fig. 1). Originally, the Pulaski fault was mapped from Marion to Blacksburg, Virginia, by Campbell and others (1925), who named it for exposures near the town of Pulaski (Fig. 2). Butts (1933) delineated much of the Pulaski fault system in southwestern Virginia; he also extended the Pulaski fault system northeastward. to near Purgatory Mountain (Fig. 2) and then into the central Appalachian part of the Valley and Ridge province. Cooper (1970) and Rodgers (1970) extended its trace 120 km southwestward into northeastern Tennessee,

TABLE 1. FAULTS OF THE PULASKI FAULT SYSTEM IN THE TYPE REGION

Pulaski fault uystem 1. Pulaski fault 2. Catawba fault 3. Max Meadows branch a. Max Meadows fault b. Roanoke Valley fault c. Back Creek fault d. M01 Creek fault e. unnuned fault northwest of Mill Creek fault 4. Salem branch a. Salem fault b. Yellow Sulphur fault Figure 5. Transverse Yellow Sulphur fault exposed at reference locality 24 of Bartholomew c. Green Ridge fault and Lowry (1979 ). The fault surface dips westward (left) at -30° and juxtaposes typical Max d Glebe Mills fault 5. Peak Creek and associated faults Meadows carbonate breccia (top left) and lower Elbrook dolomite of the broken formation 6. Slate Branch and associated faults 7. all roof, floor, and internal faults (hanging wall) over well-bedded, east-dipping dolomite of the middle Elbrook Formation of of allochthonous duplexes the Salem synclimorium. The small fold (lower center of photograph) in the footwall indicates 8. upper detachment surface and internal faults of broken formation an eastward thrust component. Photographed by T. M. Gathright II.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 STRUCTURAL EVOLUTION OF PULASKI THRUST SYSTEM, VIRGINIA 495

where both Rodgers (1970) and Milici (1975) showed the Pulaski fault system overlapped by the Blue Ridge thrust sheet. On the basis of some tectonic breccias, a number of workers (Cooper, 1946, 1970; Ed- mundson, 1958; Bick, 1960) suggested that the Staunton fault of the central Appalachians might be the northern extension of the Pulaski thrust system, and this designation has been followed by subsequent researchers (Harris, 1979; Bick, 1986). Although Kulander and Dean (1986) followed this designation, they suggested that the North Mountain fault is the structural equivalent of the Pulaski fault system. If correlation of the Pulaski fault system with either the Staunton fault or the North Mountain fault proves tena- ble, then as pointed out by Rodgers (1970), the Pulaski fault system is the only major fault system of the southern Appalachian Valley and Ridge province to be traced around the Roanoke recess into the central Appalachians. The faults bordering the other major thrust sheets (Cumberland, St. Clair, Narrows-Copper Creek, and Saltville) of the southern Appala- chians all terminate as surface features near the Figure 6. The southeast-dipping to vertical roof fault of the Claytor Lake aUochthons as 30° bend at Roanoke (Figs. 1 and 2). exposed along the southeast side of Interstate 81 -1.6 km east of New River in the Radford Bartholomew and others (1980) pointed out a South quadrangle. Overturned Ordovician carbonates of the allochthon form most of the 5- to fundamental difference between the Pulaski and 7-m-high roadcut. The overturned and brecciated Elbrook Formation of the hanging wall is more westerly faults in that the latter all propa- within the tight syncline between the Claytor Lake allochthons and the Ingles Mountain gate from décollements in the Rome Formation window. whereas the Pulaski fault system, like faults of the Blue Ridge, stems from a structurally lower level and thus did not necessarily originate as a an uninterrupted stratigraphic section beneath The presence of rocks older than the Rome décollement. Within the area, Henika (1981), the Rome Formation in the Pulaski thrust sheet precludes the structural derivation of the Pulaski Spencer (1968), and Bartholomew and others (Butts, 1933; Currier, 1935; Epenshade and oth- thrust sheet solely from above the Rome décol- (1987a) have mapped Shady Dolomite (Figs. 3 ers, 1975; Rankin and others, 1972; Stose and lement. Harris and others' (1981) interpretation and 4) structurally below the Blue Ridge thrust Stose, 1957). Farther to the southwest, within notwithstanding, the Pulaski thrust sheet may, in sheet in apparent stratigraphie continuity with the Mountain City window (Fig. 1), the Pulaski fact, involve basement at a significant distance Rome Formation of the Pulaski thrust sheet. thrust sheet contains Shady Dolomite and Chil- southeast of the leading edge of the Blue Ridge Furthermore, between Pulaski and Marion, Vir- howee Group rocks beneath the Rome in nu- thrust sheet (Boyer and Elliott, 1982; Kulander ginia (Fig. 1), both Shady Dolomite and the merous imbricates (King and Ferguson, 1960; and Dean, 1986). An understanding of both the older Chilhowee Group are mapped as part of Rankin and others, 1972). lithotectonic character and the emplacement his-

„PULASKI FAUL

Figure 7. Diagrammatic cross section showing relationships of Blue Ridge fault and associated stage A footwall imbricate fan to allochthonous folded hinterland-dipping duplexes (GCW, BW, and RMA/CMA) associated with later stages of structural development of the Pulaski, Catawba (CF), and Salem (SF) faults. The GCW contains only Cambrian/Ordovician carbonates welded to the base of the Pulaski thrust sheet during stage B; BW = Ordovician rocks-stage B; RMA/CMA = Upper Ordovician to Devonian rocks are stage C, but structurally highest slice of Cambrian/Ordovician rocks is stage B; BP = Branch Point.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 496 M. J. BARTHOLOMEW

Figure 8. The Roanoke Valley fault exposed in the east wall of a small landslide chute on the east side of U.S. Highway 460 at the junction with Virginia State Road 696 in the Stewartsville quadrangle -1.6 km west of the Webster brick pit shown in Figure 6. The pocket watch, just below the fault, is -4.5 cm in diameter. The fault dips -45° southeast at this locality and places mottled red and green mudstone of the Rome Formation over sheared dolomite of the Elbrook Formation.

and is the structurally higher branch of the Pulaski fault system (Figs. 2 and 3). To avoid confusion, faults related to this structurally higher branch of the Pulaski fault system are simply designated as part of the Salem branch, and "Pulaski fault" is restricted to the floor fault surface of the Pulaski fault system west (hinter- landward) of the branch line of the Salem branch with the Catawba fault. "Pulaski fault" is not used as it traditionally has been for roof faults around the other windows exposing allochthonous rocks. These faults, around the Christiansburg-Crab Creek, Ingles Mountain-Barringer Mountain, East Rad- ford, and Glade Creek windows, are all part of the Pulaski fault system but are themselves folded (Fig. 6) and cut off at depth by the Pulaski fault, which is the floor fault surface (Boyer and Elliott, 1982) of the Pulaski fault system (Bartholomew and Lowry, 1979; Schultz, 1983 ). Hence, these older, folded faults should be simply referred to as "border faults" of the respective windows to avoid perpetuating nomenclatural obfuscation (see Rader and tory of the Pulaski thrust sheet may thus provide the Yellow Sulphur fault) near Blacksburg others, 1987). The fault flanking the southeast- a firmer basis by which to relate both the geome- (Figs. 2 and 3). The Pulaski fault also frames ern side of the Read Mountain-Coyner Mountain try and the sequence of deformation in the most of the Price Mountain window, but the allochthon and the roof fault of the Bonsack southern Appalachians to that of the central north-south-trending branch line crosses the window also bear a similar relationship to the Appalachians. east end of the window (Bartholomew and Salem fault (Bartholomew and Hazlett, 1981). Lowry, 1979). East of the branch line, the As herein mapped and defined, the Pulaski FAULT NOMENCLATURE Catawba fault (Ritter, 1970; Cooper, 1971; thrust sheet is a complex, composite sheet within Williams, 1978) is the structurally lower branch which different plates were juxtaposed during Pulaski Fault System of the Pulaski fault system. The transverse, distinguishable stages of Alleghanian thrusting. westward-dipping (Fig. 5) Yellow Sulphur fault As such, recognition the Pulaski fault system As used in this pa per, the term "Pulaski fault" (Ritter, 1970; Bartholomew and Lowry, 1979) is not based exclusively on the presence or ab- sensu stricto is used only for the fault from developed above the lateral, eastward-dipping sence of a unique rock type (such as the Max Draper Mountair. northeastward through Catawba ramp (Fig. 2 of Bartholomew and Meadows Breccia) in the overriding thrust sheet Pulaski to the majoi branch line (which parallels Lowry, 1979; Bartholomew and Brown, 1987) but rather on juxtaposition of a variety of more

Figure 9. Diagrammatic cross section illustrating relationship of footwall imbricate fan developed in Rome Formation beneath Blue Ridge thrust sheet during stage A (Figs. 4,10, and 12). Shaded zone is broken formation developed near Rome/Elbrook contact; RVF, BCF, and MCF are the Roanoke Valley, Back Creek, and Mill Creek faults, respectively.

Figure 10. Diagrammatic sketch showing structural stages in Alleghanian development and emplacement of Pulaski thrust sheet. Stage A is deformation associated with ramp up to, and formation of, the lower level décollement near the Rome/Elbrook contact; stages B, C, and D deformation associated with the Pulaski thrust sheet ramping across €-0,0-D, and D-M strata; stage E deformation associated with, or after, emplacement of Pulaski thrust sheet at the upper level glide horizon in the Mississippian Maccrady Formation; stages correspond to those letters used in Figures 3,4, and 18. UDS = upper detachment surface of broken formation; small letters M, D, S and O are Mississippian, Devonian, Silurian, and Ordovician; € is Cambrian units above the Rome Formation (€r).

easterly derived lithotectonic assemblages (such as the complexly deformed plate containing the broken formation with Max Meadows Breccia) over rocks of the Saltville thrust sheet or its equivalent. The basal fault of the Pulaski fault system, as thus defined, is the southeastern border fault of the Saltville thrust sheet; its pos- sible correlation with thrusts in the central Ap- palachian Valley and Ridge province (north of Purgatory Mountain) could be based on determination of the southeastern limit of rocks structurally equivalent (see Kulander and Dean, 1986) to those in the Saltville thrust sheet.

Salem Branch

Within the area of Figure 2, the Pulaski thrust sheet contains a younger internal fault system, the Salem branch, which separates the complexly deformed plate (Figs. 2,3,4, and 5) from the Salem synclinorium. In addition to the main Salem fault, the Salem branch includes (1) the Yellow Sulphur (Fig. 5) and Glebe Mills fault zones (two cross faults similar to the Jack Mountain and Russel Fork faults of the Cum- berland thrust sheet) and (2) the Green Ridge fault that separates the subsidiary Green Ridge plate from the main Salem synclinorium. The largest fault of this branch, the Salem Figure 11. Southeast-dipping (10°-15°) thrust exposed on the west side of Virginia State fault, dips -25° southeast (Schultz and Bar- Road 659 at the western end of the Price Mountain window in the Radford North quadrangle. tholomew, 1980) and borders the southern flank This thrust (stage E) dies out within the Price Mountain anticline (Bartholomew and Lowry, of the Salem synclinorium and subsidiary Green 1979) and, at this locality, places the Maccrady Formation (Mississippian) over brecciated Ridge plate from the Roanoke area (Amato, carbonates of the Elbrook Formation (Cambrian). Schultz (1979a) described these exposures 1974; Bartholomew and Hazlett, 1981; Bar- in detail and showed that the snow-covered rocks in the foreground are part of one of the tholomew, 1981; Henika, 1981; McGuire, 1976) highly deformed slices of Ordovician, Silurian, and Devonian rocks which lie along the Pulaski southwestward to near Yellow Sulphur, just fault along the southern and western flanks of the Price Mountain window in the Radford south of Blacksburg. From Yellow Sulphur North and Blacksburg quadrangles.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 498 M. J. BARTHOLOMEW

Figure 12. Structural-trend map showing mapped folds and faults of the Pulaski thrust sheet. Quadrangle abbreviations are the same as those in Figure 2. Hacliured lines in PUL, STA, RN, RS, and BLA quadrangles mark the UDS of the broken formation; hachured line in DAL quadrangle marks another, structurally higher detachment surface in the Martinsburg Formation along eastern flank of Salem synclinorium. Letter designations assigned to some folds and faults correspond to structural stages shown in Figures 3,4, and 10; Blue Ridge thrust sheet is shaded; curving sirrows indicate plunge and vergence directions of small folds, <-*— right, —left; straight arrows indicate folds that have vertical axial surfaces; dotted lines mark trend changes in fold patterns.

southwestward, previous workers had shown the Salem fault, between the Christiansburg and along the leading edges of both the Salem and the Salem fault continuing along a variety of Price Mountain windows southwest of the inter- Yellow Sulphur faults. All of these slices are traces generally as a high-angle reverse fault of section of the Salem and Yellow Sulphur faults. derived from rocks of the Salem synclinorium, small displacement (Cooper, 1961,1963,1964, Both Bartholomew and Lowry (1979) and which forms the footwall of the Salem branch. 1968; Lowry, 1971, 1979). Detailed mapping Schultz and Bartholomew (1980), as well as The Read Mountain-Coyner Mountain al- by Bartholomew and Lowry (1979) showed no Broughton (1971), Glass (1970), Hazlett (1968), lochthon, southeast of the Green Ridge subsid- major low-angle thirust fault, of the magnitude of and Ritter (1970), reported numerous slices iary plate, is a horse along the Salem fault. The

Figure 13. Dia|>rammatic cross section showing relationships of the broken formation (stippled) of the Pulaski thrust sheet to the Roanoke Valley fault (RVF) formed during stage A and duplexes formed during successive stages B, C, and D. Stage B = Christiansburg window (CW), stage C = Ingles/Barringer Mountain window (I/BW), and stages B to D = East Radford window (ERW); the Slate Branch fault is an E2 thrust fault (mapped in IPulaski and Blacksburg quadrangles, Fig. 12) that cuts the Pulaski fault; the UDS of the broken formation is folded about the stage B, C, and D antiforms and is cut by the ERW roof fault; Mississippian Maccrady Formation lies below the Pulaski fault

sedimentary fades within the allochthon indicate that it was derived from Saltville plate rocks originally overridden by the Salem synclinorium and subsequently exhumed along the Salem fault (Bartholomew, 1981; Bartholomew and Hazlett, 1981), as illustrated in Figures 2,4, and 7. Rocks within the Bonsack window are interpreted as part of this horse. The faults along the Figure 14. A small syn- southeastern flank of the Read Mountain- cline that has a near- Coyner Mountain allochthon as well as the roof vertical axial surface fault around the Bonsack window are thus part (dashed) within highly of a folded, hinterland-dipping duplex (Boyer disrupted and brecciated and Elliott, 1982) and are cut off at depth by the rock. This small fold, Salem fault, which is the floor fault of this du- within the tightly infolded plex (Bartholomew and Hazlett, 1981). Elbrook between the Claytor Lake allochthons Max Meadows Branch (stage B) and the Ingles Mountain window (stage The Max Meadows fault, as shown by C), is exposed on the Cooper (1963), Rodgers (1970), and Lowry eastbound on ramp of In- (1971, 1979), was also called the "Christians- terstate 81 -0.6 km east burg fault" by Cooper (1961, 1968) and Die- of New River in the Rad- trich (1954) and was firstmappe d and described ford South quadrangle. by Cooper (1939). This contact, as previously mapped, is not a single continuous fault but throughout its length, reflects a change in the fold style, including local interference patterns, at both outcrop and map scale. Bartholomew and others (1980) referred to this fold style change as the "Max Meadows line." The actual

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 500 M. J. BARTHOLOMEW

On the basis of crosscutting relationships among folds and faults, the Alleghanian deformation of rocks of the Pulaski thrust can be divided into five more-or-less discrete stages. Within each stage, discernable sequences of folding and faulting can in many cases be recognized as well. The stages are keyed to the structural position of the Pulaski thrust sheet relative to the footwall rocks (Fig. 10). These stages are (A) deformation which preceded development of the ramp from the Rome Formation to Maccrady Formation; (B) deformation as the Pulaski sheet ramped across Cambrian-Ordovician strata to décollements in Middle to Upper Ordovician shales; (C) deformation as the Pulaski sheet ramped across Upper Ordovician to Devonian rocks to décollements in Devonian shales; (D) deformation as the Pulaski sheet ramped across Figure 15. Recumbent fold in lower Elbrook Formation at reference locality 17 of Bartho- uppermost Devonian to Mississippian rocks, fi- lomew and Lowry (1979), showing Max Meadows Breccia within the disrupted hinge zone of nally reaching décollements in the coal measures a fold in the complexly deformed plate. Sedimentary structures (near shadow) in the light gray of the Price Formation (along the Catawba dolomite (L) indicate that lower dolomite is upside down, whereas those in the lithologically fault) and in the stratigraphically higher Mac- similar upper light gray dolomite (L) (near center) indicate that it is right side up; both light crady Formation (along the Pulaski fault); and gray dolomites stiratigraphically overlie dark gray dolomite (D) which rests on argillaceous (E) post-emplacement deformation of the dolomite (S) enclosing the breccia (B). Photographed by T. M. Gathright II. coupled Pulaski/Saltville sheet. The crosscutting relationships used to establish these stages are basic ones such as those Max Meadows fault originally mapped by follows the Mill Creek fault, a lower thrust of noted by Perry (1978). For example, folding of Cooper (1939) has not been traced beyond the this fan. The Max Meadows branch includes the the roof fault of a duplex (Fig. 6) containing Draper Mountain area (Figs. 2 and 3), where he Max Meadows fault per se, the Roanoke Valley only Lower Ordovician and Cambrian rocks determined that it predated the Pulaski fault. fault, and the associated faults of the imbricate must have occurred during stage B as that part of Eastward from Draper Mountain, the Peak fan, notably, the Mill Creek and Back Creek the Pulaski thrust sheet crossed the lower Pa- Creek fault (Schulfci, 1983) marks this structural faults. leozoic part of the footwall. Moreover, if a fault, discordance from tinat place to a point where it such as the Slate Branch fault (Figs. 3 and 11), overrides the fault along the Roanoke Valley as STRUCTURAL EVOLUTION cuts a window roof fault, then that fault must well as other northeast-trending structures (Figs. OF PULASKI THRUST SHEET postdate the roof fault. These types of relation- 2 and 3). The Peak Creek fault, itself the upper ships were used to construct a structural-trend thrust of a diastomcsing imbricate fan, then con- Structural Stages in the Evolution map (Fig. 12) in which many of the larger folds tinues eastward and. merges with the trace of the of the Pulaski Thrust Sheet and faults are labeled to indicate during which Blue Ridge fault southeast of Christiansburg. stage they developed. This map includes revi- This fault trace is thus significantly southeast of The structural development of the Pulaski sions of earlier attempts by Bartholomew and the Roanoke Valley fault (Figs. 2 and 3) and thrust sheet can be separated into distinguish- others (1982), Schultz (1983, 1986), and Gib- generally marks a sharp change in structural able, structural stages as the sheet ramped over son and Gray (1985) to establish a deforma- trends (but not fold style) within the Rome parts of the footwall (Fig. 10), and slices from tional chronology. Formation, whereas the Max Meadows line fol- the footwall (Figs. 2,3, and 4) were incorporated Of particular significance to determining the. lows the Roanoke Valley fault through the Roa- into the base of the sheet. That is, the sheet must sequence of deformation is the upper detach- noke area (Figs. 2 and 4). have first crossed the thick, Cambrian/Ordo- ment surface of the "broken formation" of The Roanoke Valley fault (Figs. 8) is very vician carbonate portion of the footwall (stage Schultz (1983). On a regional basis, the upper similar to the Max Meadows fault as described B, Fig. 10), in order for duplexes at higher detachment surface (Fig. 12) can be considered by Cooper (1939) and may, in fact, be its contin- structural levels to contain these rocks (Figs. 3 as the top of a décollement zone of the Pulaski uation north of the younger Peak Creek fault. and 4). Moreover, the roof fault (Boyer and fault thai, was an active zone of deformation In the Roanoke area (Figs. 2, 4, and 9), the Elliott, 1982) if folded above a duplex, is then prior to, and perhaps during, incipient ramping Roanoke Valley fault is the uppermost thrust of unlikely to experience more thrust motion. during earlier Alleghanian stages of deforma- an imbricate fan (includes the Back Creek and Crosscutting fold and fault relationships with tion. Although not unequivocally recognized as Mill Creek faults and an unnamed fault to the roof faults of the numerous duplexes, as well as a fault by Schultz (1983, 1986), his maps and northwest of the Mill Creek fault) and merges other thrusts, thus establish a chronology for cross sections clearly support a regional upper with the Blue Ridge fault east of the Glade many of the structural features of the Pulaski detachment surface within the Cambrian car- Creek window, whereas the fold style change thrust. bonates (probably at a nearly continuous strati-

graphic horizon in the Elbrook Formation) of the complexly deformed plate (Fig. 2). Schultz (1983, 1986) and Gibson and Gray (1985) implied that intense deformation within the broken formation took place over the continuum of Alleghanian emplacement of the Pulaski thrust sheet and that breccias and associated complex folds and faults developed continuously during both ramping from the Rome to Maccrady and later as the Pulaski sheet moved along the upper level glide horizon in the Maccrady Formation. In support of this interpretation, Schultz (1983) showed that both the Maccrady footwall and the Elbrook/Conoco-

Figure 16. Geologic map (modified after Figs. 34 and 35 of Bartholomew and others, 1982) of area around Webster and Weblite brick pits (hachured lines) in Stewartsville quadrangle, showing structural trends associated with the Roanoke Valley fault (RVF) which places Rome over Elbrook. Max Meadows Breccia = black, dolomite = dark shading, grayish-green Rome mudstone = light shading, maroon Rome mudstone = not shaded; lettered locations referred to in text.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 502 M. J. BARTHOLOMEW

younger faults (stages D-l and E-2) and folds (stages D-l and E-l). Formation of the upper detachment surface thus definitely preceded stage C-2. (6) A necessary consequence of a ubiquitous upper detachment surface bounding the broken formation is that the surface must be folded above (or cut by) the roof faults (stage B) of the Christiansburg window (Figs. 12 and 13). The intimately infolded breccias (Fig. 14) between folded roof faults of the Christiansburg/Crab Creek (stage B) and Barringer Mountain (stage C) windows also support their development during or prior to stage B. Indeed, only the recognition of the overturned fault, instead of a high-angle fault, is needed for Cooper's cross section (1970) to show the folded upper detachment surface adjacent to the Barringer Mountain window. (7) The projection of the upper detachment surface must be structurally truncated by the Roanoke Valley fault (Figs. 12 and 13). This fault is part of the stage A imbricate fan and Figure 17. Complexly folded Rome mudstones at reference locality 17 of Bartholomew and parallels the structural trend of the slightly Lowry (1979), showing older axial surface (parallel to axe handle in lower center) coaxially younger (stage B) antiformal Glade Creek and folded about younger axial surface (parallel to geologic hammer in left center of photograph). Christiansburg windows. The much younger Fold style in Rome shales of the broken formation (Schultz, 1983) is the same as fold styles in Peak Creek fault (stage D-2) truncates this struc- the Rome Formation southeast of the Max Meadows branch (Gibson and Gray, 1985). Photo- tural trend. graphed by T. M. Gathright O. (8) Although breccia cuts across many folded thrusts within the broken formation (Schultz, 1983), no examples of breccia cutting later stage cheague carbonates above the detachment leghanian movement (stage D-6) of the Salem faults are yet documented. Also, layers of Max surface have similar fold orientations. The branch is measured in only metres (Fig. 5) or Meadows Breccia are found folded (Fig. 14) intervening broken formation, in part, reflected tens of metres (Bartholomew and Schultz, 1980; about younger Alleghanian folds (Bartholomew the same orientations (Schultz, 1983). Schultz and Bartholomew, 1980), in stark con- and Lowry, 1979; Bartholomew and Schultz, By contrast, my own view (Bartholomew, trast to the 300- to 500-m-thick broken 1980; Bartholomew and others, 1982; Schultz, 1979, 1985; Bartholomew and others, 1980, formation. 1983). The exceptionally thick broken formation 1982) is that the 300- to 500-m-thick broken (3) Extensive breccia is intimately associated with its tectonic breccia thus probably devel- formation represents a lower-level décollement with the early Alleghanian (stage A) imbricate oped during earlier stages of deformation before zone largely formed prior to ramping (Fig. 10) fan of the Max Meadows branch developed in the Pulaski thrust sheet ramped across middle to in earlier Alleghman time and then passively the Rome Formation as the Blue Ridge thrust upper Paleozoic rocks. transported, simply as another stratigraphie unit, sheet ramped across the Rome Formation (Figs. up to the flat in the Maccrady Formation. The 9 and 12) to the lower level décollement. Geo- Development of the Max Meadows Breccia broken formation was then, of course, deformed metric constraints similar to those cited by Boyer Tectonic breccia was previously mapped and along with other contiguous stratigraphie units and Elliott (1982) require a break-forward jux- described by Cooper (1939 and 1970) and during later Alleghanian deformation as noted taposition of the Blue Ridge sheet over the foot- Cooper and Haff (1940) as areally extensive. by Schultz (1983). wall imbricates during earlier Alleghanian time Detailed mapping, however, has shown that tec- My interpretation is supported by the follow- (stage A), thus necessitating formation of breccia tonic breccia actually constitutes only a small ing points. during stage A. part of the complexly deformed plate between (1) The deformation of the Maccrady foot- (4) The folded roof fault (stage B) of the the Blue Ridge fault and the Pulaski and Salem wall, to which Schultz (1983) referred, took Glade Creek window reflects a regional anti- faults. Breccia occurs as large masses only lo- place after the Pulaski thrust sheet reached the formal arch in the Blue Ridge fault (Fig. 7) and cally along some segments of the Pulaski fault Maccrady décollement (stage D). Similar fold associated footwall imbricate fan (Henika, 1981; system and is virtually absent in the Salem and orientations thus ¡.imply reflect deformation dur- Bartholomew, 1981). This folded fan implies Fincastle synclinoria. As noted by Bartholomew ing stages D and E. that development of the associated broken for- and others (1980) and Henika (1981) and con- (2) At only a few localities (Schultz, 1983) mation predated stage B. firmed by Schultz (1983, 1986), the breccia is can breccias be found involving footwall rocks (5) The upper detachment surface in the confined to a stratigraphic interval of several of the Pulaski or Salem faults, and such zones Blacksburg and Radford North quadrangles hundred metres at the base of the Elbrook For- are typically < 1 rn thick. Indeed, the entire zone (Fig. 12) is cut by the roof fault of the East mation and subjacent Rome shale. The specific of intense deformation associated with the Al- Radford window (stage C-2) as well as by types of occurrence are (1) along faults, particu-

Figure 18. Palinspastic reconstruction of the Pulaski thrust sheet near the Roanoke re-entrant. PMW = Price Mountain window, ERW = East Radford window, IM/BMW = Ingles Mountain/Barringer Mountain window, CLA = Claytor Lake allochthons, DMA = Draper Mountain allochthons, CW = Christiansburg window, RM/CMA = Read Mountain/Coyner Mountain allochthon, BW = Bonsack window, SS = Salem synclinorium, GR = Green Ridge plate, FS = Fincastle synclinorium, GCW = Glade Creek window, ERA = Eagle Rock ailochthon, PM = Purgatory Mountain; coarse stipple is complexly deformed plate of the Pulaski thrust sheet, fine stipple near PMW is known extent of Maccrady Formation, dashed lines = fades changes, fine-dotted lines = pinch outs, coarse-dotted line = thickness change.

larly stage A faults of the Max Meadows branch indurated pods of breccia occur within large Bartholomew (1980), Cooper (1939,1970), and such as the Roanoke Valley fault and Max breccia masses, and the Rome-Elbrook contact Cooper and Haff (1940), as well as to the refer- Meadows fault (Cooper, 1939); (2) within fold is a frequent locus of breccia. For more detailed ences for Figure 2. cores (Fig. 15); and (3) as dikes and sills with or descriptions of the structural loci of the breccia, To summarize, tectonic breccia is abundant without deformation of the wall rock (Barthol- the reader should refer to Schultz (1983,1986), only locally along the leading edge of the Pu- omew and Schultz, 1980; Schultz, 1983). Well- Bartholomew and Schultz (1980), Schultz and laski thrust sheet. A dearth of breccia is most

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 504 M. J. BARTHOLOMEW

GL PI Mbs Mp Mh

LITHOLOGIC Mbf -C/R = 1.35 PMW SYMBOLS

notable in the Salem and Fincastle synclinoria. Moreover, breccias are virtually absent from Roanoke Valley fault is itself deformed into footwall rocks of both the Pulaski fault and open to tight folds (areas A-A' and B-B' on Fig. Salem branch but not of the Max Meadows 16), in many cases exposing Max Meadows branch. The braxias are an integral part of the Breccia in the domal cores. Classic dome and broken formation in the complexly deformed basin interference patterns are developed in the plate in which many folds and breccias, pre- 1984, 1985) closely linked with the Max Mea- Webster pit (X-Z' on Fig. 16), where earlier viously formed, were cut through and trans- dows branch, which has breccia along portions isoclinal folds were refolded about northeast- ported along faults of the Pulaski fault system of its trace. The relationship of the breccias to striking axial surfaces (near X in Fig. 16). The and Salem branch. Neither was uniquely re- folds and faults of the Max Meadows branch is northeast-trending open dome (Z-Z') and basin sponsible for the development of the breccias, well illustrated in the Webster brick pit (Fig. 16) (X-X") are parallel to the stage B antiform of the which are more likely to be lithology dependent along the Roanoke Valley fault (Fig. 8). Here, nearby Glade Creek window, thus suggesting than specifically related to emplacement of the early, tight to isoclinal folds (stage A) are coax- that the breccia formed prior to stage B. Hence, Pulaski thrust sheet at the structurally higher ially refolded (area A'-B' on Fig. 16) and trun- folding of crudely layered carbonate breccias glide horizons. cated by the Roanoke Valley fault. Similar about later stage folds is associated with Al- The tectonic origin of the classic Max Mead- coaxially refolded folds are found throughout leghanian ramping of the Pulaski thrust sheet ows Breccia most likely is coincident with an the Rome Formation (Fig. 17) of the Pulaski across Cambrian to Mississippian strata to its earlier stage(s) of Alleghanian deformation of sheet and, south of the Max Meadows branch, uppermost glide horizon. the Pulaski thrust sheet (Bartholomew, 1979, were described by Gibson and Gray (1985). The The presence of earlier (stage A) tight to iso-

Figure 19. Fence diagram showing facies relationships and stratigraphic sections in relative pre-thrusting positions illustrated in Figure 18. Outline of Figure 18 shown for reference. Approximate geographic locations of sections are the positions of the top of the Martinsburg Formation (Omb) except for SC, N, and S, which are moved slightly to the left, and PM, to the right, to avoid overlap of sections. GL = Glen Lyn syncline below St. Clair fault, SC = St. Clair thrust sheet, N = Narrows thrust sheet, S = Saltville thrust sheet, PMW = Price Mountain window, IBW = Ingles-Barringer Mountain window, CW = Christiansburg window, DM = Draper Mountain, WSS = western Salem synclinorium, RM = Read Mountain anticline, BW = Bonsack window, CMA = Coyner Mountain allochthon, CM = Catawba Mountain (northeastern Salem synclinorium), PM = Purgatory Mountain, ER = Eagle Rock allochthon, GR = Green Ridge plate, F - Fincastle synclinorium, C/R = coal with reflectance value, CZ = Crepicephalus Zone, PI - Lee Formation, Mbs = Bluestone Formation, Mp = Princeton Sandstone, Mh = Hinton Formation, Mbf = Bluefield Formation, Mg = Greenbrier Group, Mmc - Maccrady Formation, Mpr = Price Formation, Dch = Chemung Formation, Db = Brallier Formation, Dm = Millboro Formation, Dss = undifferentiated Devonian strata—mostly sandstone, chert, or carbonate, Ske = Keefer Formation (locally has some Silurian limestone above Keefer), Sr = Rose Hill Formation, Stu = Tuscarora Formation, Oj = Juniata Formation, Oo = Oswego Formation, Omb = Martinsburg Formation, Oeg = Eggleston Formation, Om = Moccasin Formation, Oba = Bays Formation, Ofc = Fincastle Conglomerate, Olh = Liberty Hall Formation, Op = Paperville Shale, Ols = undifferentiated Middle Ordovician limestones, Oku = undivided upper Knox Group, Ob = Beekmantown Formation, Ccr = Copper Ridge Formation, Ceo = Conococheague Formation, Cn = Nolichucky Shale, Ch = Honaker Dolomite, Ce = Elbrook Formation, Cr = Rome Formation. Lithologic symbols: 1 = interbedded red sandstone/mudstone with minor dolomite, 2 = interbedded gray sandstone/mudstone with minor red mudstone and dolomite, 3 = coal, 4 = red mudstone with limestone or dolomite, 5 = debris and mudflows/ conglomerate with sandstone and gray shale, 6 = thin units of sandstone, chert, limestone, and mudstone, 7 = orthoquartzite/sandstone, 8 = coarser grained marine and/or deltaic sandstone, 9 = quartz-pebble/cobble conglomerate, 10 = finer grained sandstone with mudstone, 11 = interbedded black mudstone/siltstone/fine-grained sandstone, 12 = black mudstone, 13 = gray mudstone with limestone, 14 = limestone, IS = interbedded dolomite/limestone, 16 = dolomite. Sections compiled from Amato (1974), Bartholomew (1981), Bartholomew and Lowry (1979), Bartholomew and others (1982,1987b), Cooper (1939,1961,1963,1971), Derby (1965), Gathright and Rader (1981), Henika (1981), Hergenroder (1966), Hower and others (1987), Karpa (1974), Kreisa (1980,1981), Kreisa and Bambach (1973), McGuire (1970, 1976), Schultz (1979a, 1979b), Schultz and Bartholomew (1987), Schultz and others (1986), Spencer (1968), Tillman (1963), and Tillman and Lowry (1968).

clinal folds in both the Elbrook carbonates and suggested by breccia dikes lacking wall-rock de- window in Figs. 2, 3, and 13), the older phyllitic Rome mudstones of the complexly formation (Bartholomew and Schultz, 1980). rocks are in structurally higher segments of deformed plate (Bartholomew and Lowry, Likewise, because breccias are most abundant in duplex structures which in many cases contain 1979; Bartholomew, 1981; Bartholomew and the stratigraphic interval near the Rome/El- overturned strata. The ramp zone is most dra- Hazlett, 1981; Henika, 1981; Bartholomew and brook contact, contrasts in fluid pressure be- matically illustrated in the Draper Mountain others, 1982) further suggests that principal tween phyllitic Rome mudstones and Elbrook structure where the slices contain Cambrian development of the breccias began during carbonates may have enhanced localized shear- through Devonian strata. stage A. Breccia in sheared-out hinge zones of ing and breccia development. Tectonic breccias As thus defined, the ramp zone is -40 to some horizontal, tight to isoclinal folds (Fig. 15) thus probably formed under high fluid-pressure 45 km wide and 55 to 60 km wide in a north- and in large tight, reclined folds both near conditions both along thrusts and in tightly west-southeast direction in the Pulaski-Chris- Blacksburg (Bartholomew and Lowry, 1979) compressed fold cores as the overturned to re- tiansburg and in the Roanoke-Fincastle areas, and near Christiansburg (Bartholomew and cumbent limbs became greatly attenuated or lo- respectively. This is the zone from which slices Schultz, 1980) suggests that breccia develop- cally sheared out. were detached and incorporated into the base of ment is genetically linked to the early stage of the Pulaski thrust sheet as duplex structures and folding and faulting. Locally, some breccia may Ramp Zone of the Pulaski Fault then transported to structurally higher levels as have developed or been modified during the part of that sheet. In the Roanoke area, the later stages of Alleghanian thrusting. Most of the relatively parautochthonous rocks complex Read Mountain-Coyner Mountain Although breccias are found in early-stage exposed within windows of the Pulaski thrust duplex structure also was derived from this fold cores, no breccia layers are known to be sheet are derived from and located on the Salt- ramp but was transported along the Salem involved in these earliest folds (stage A); thus, an ville plate (Fig. 18) over which the Pulaski branch. exclusively sedimentary origin for the breccias is thrust sheet ramped from a lower décollement untenable, as previously indicated by Cooper near the Rome/Elbrook contact to its upper- Palinspastic Reconstruction of (1970). However, if sedimentary breccia was most décollement in the Maccrady Formation the Pulaski Thrust Sheet originally formed in evaporite horizons (Rod- (Fig. 18). The ramp zone (a series of ramps and gers, 1970) but subsequently mobilized and em- flats) is inferred from the fact that the Pulaski Because determination of displacement on the placed under high fluid pressure as tectonic thrust must have cut across the entire strati- Pulaski fault system is dependent upon a breccia, then such breccia could not be distin- graphic section in order for successively older reasonable interpretation of the relative pre- guished from breccia formed solely by cataclas- rocks to be exposed in windows in a south- thrust locations of the component plates, a palin- tic processes, given the types of studies made to easterly direction from the Price Mountain spastic map of these plates was constructed using date in the Pulaski thrust sheet. High fluid- window to the Christiansburg window (Figs. 2, both structural and lithofacies data (Figs. 18,19, pressure indicators are noted by Gibson and 3, and 4). Similarly, within the more complex and 20). The palinspastic map was constructed Gray (1985) and Schultz (1983) and are further windows (see, for example, the East Radford along the following structural premises.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 506 M. J. BARTHOLOMEW

GL/S Figure 20. Selected stratigraphie sections from Figure area!». This produces slightly divergent restora- 19, showing the principal stratigraphie changes (above the tion vectors; hence, the complexly deformed Rome Formation) used in reconstruction of Pulaski thrust plate was split into separate pieces to accommo- sheet (Figs. 18 and 19). Symbols and abbreviations are the date the divergence. If the Roanoke bend existed same as those in Figure 19; approximate vertical scale prior to thrusting, then the plate occupied a sig- shown. nificantly larger area before thrusting and the area reduction during thrusting must have been accommodated by an increase in thickness due to internal plate deformation. Alternatively, the Roanoke bend may have developed after emplacement of the Pulaski thrust sheet; if so, a greater northeast-southwest area is not necessarily required during restoration. (4) Most fades changes, pinchouts, and other depositional changes are also considered to be regional changes that may be approximated by straight or slightly curved lines across the study area. Finally, in order to get a closer approximation to actual total displacement during emplacement of the Pulaski thrust sheet, the plates must be placed in positions on the basis of not only depositional changes but also their probable minimum locations relative to the ramp zone as indicated on the balanced and restored cross sections. For instance, the allochthonous window rocks are derived from the ramp zone whereas the complexly deformed thrust plate perse (Fig. 2) is clearly the southeasternmost component inasmuch as it contains rocks of the Rome and lowermost Elbrook Formations which are lacking in the intermediate Salem synclinorium, Green Ridge, and Fincastle synclinorium plates where the Pulaski fault system had climbed to the middle of the Elbrook Formation. My (Bartholomew, 1979) original estimate of 53 km of minimum displacement of the complexly deformed plate is less than that for this revised paleogeographic reconstruction (Fig. 18) which yields a displacement figure of -100- 110 km for the leading edge of the Pulaski thrust sheet in the Pulaski-Roanoke area.

VALLEY AND RIDGE DISPLACEMENT AND SHORTENING

If one uses both the palinspastic reconstruction of the Pulaski sheet, showing a minimum of 100-11.0 km displacement of the leading edge, and balanced cross sections for strata below the (1) Folds and faults of structurally lower imated by gently curved lines which are sub- Pulaski thrust sheet (Figs. 21 and 22), then the thrust sheets are not restored; hence, structural parallel to the Roanoke bend. Locally, in the approximate percentage of shortening due to all features such as the Roanoke bend, the St. Clair, Blacksburg area, the cutoffs of Upper Devonian of the principal faults of the Valley and Ridge Narrows, and Siltville faults, and the Price and Mississippian strata are sharply curved, province can be determined by constructing Mountain windo w remain fixed relative to the reflecting the lateral ramp above which devel- restored sections in a manner similar to that used restored Pulaski tlirust sheet. oped the Salem branch. by Elliott and Johnson (1980) and Boyer and (2) Most of the cutoffs of footwall strata are (3) The component plates were moved hin- Elliott (1982) for other thrust systems. The considered as the intersection of a regionally terlandward approximately perpendicular to palinspastic reconstruction is necessary for calcu- smooth Pulaski fault surface and approximately principal fold trends at successive stages in both lation of shortening of the Pulaski sheet and horizontal bedding and, hence, may be approx- the Pulaski-Blacksburg and Roanoke-Fincastle ramp zone because both the erosion level in the

A 4'

5 MILES 8 KM V R

8 KM

Figure 21. Balanced (top) and restored (middle and bottom) cross sections along A-A' (Fig. 2). Modified after cross section J-J' of Butts (1933) and Milici (1973); stratigraphie sections taken from Figure 19. V = base of Mississippian strata at hinge of Glen Lyn synciine (McDowell, 1982) at the Allegheny structural front, R = intersection of base of Mississippian strata of footwall with Catawba fault, B = branch point of Pulaski fault to form Catawba and Salem branches, P = leading edge of Pulaski thrust sheet, S = leading edge of Blue Ridge thrust sheet, Z = intersection of the Elbrook/Rome contact with the Pulaski fault surface as determined from palinspastic reconstruction, C = Cambrian units above the Rome, RFVDZ = Rockfish Valley DDZ, CG = Chilhowee Group, CF = Catawba fault, GRF = Green Ridge fault, SF = Salem fault, SCF = St. Clair fault, NF = Narrows fault, SF = Saltville fault.

far-traveled Pulaski thrust sheet and abundant reduction spots, vary from about N29°W to in the same part of Tennesee, (5) the 1.5° to 3° noncylindrical folds render a simple restoration N37°W around the fold and are about N31°W used by Chappie (1978) in his calculations for on the basis of balanced sections rather difficult. near the section line which trends N23°W per- the Valley and Ridge, and (6) the 1° used by On the other hand, for strata structurally below pendicular to cleavage trends determined by Kulander and Dean (1986) for their sections the Pulaski thrust sheet, restored sections made them along the section line. By analogy, section across this region. from balanced sections are quite feasible. The A-A' trends N50°W, also is nearly perpendicu- For the Pulaski thrust sheet (P-S in Figs. 21 two sections constructed (Figs. 21 and 22) fol- lar to regional fold axes, and is inferred to lie in and 22), the ratios of restored width minus ac- low the classic section lines I-I' and J-J' of the plane of strain as well, although detailed tual width (entire quantity) to restored width are Butts (1933), from what is still the most detailed structural data are lacking here. For a first ap- used to determine the percent shortening. In a regional map available for part of the area. proximation to compare regional shortening similar manner, ratios are used to calculate the Milici (1973) also constructed a balanced sec- across the western Valley and Ridge, these two percent shortening across the western portion of tion along I-I'. classic section lines are thus sufficient. In the the Valley and Ridge (V-R in Figs. 21 and 22). Few strain data are currently available in the balanced sections, the regional dip on the basal If one uses the above method of calculation in western part of the Valley and Ridge here. Sec- décollement is assumed to be ~2° southeast. the Blacksburg area, the shortening of the Pu- tion B-B', however, lies nearly perpendicular to This dip is consistent with (1) interpretation of laski sheet is -85%, whereas in the Roanoke regional trends of fold axes and cleavage gravity data in this region (Stovall, 1984), (2) area, it is -90%. surfaces, which vary from about N60°E to the 2° slope determined in Tennessee (Roeder A necessary and important consequence of N80°E. Additionally, Simon and Gray (1982) and Witherspoon, 1978) from aeromagnetic the restoration is that if point P is restored to provided considerable detail on strain across a data, (3) a l°-2° slope in Tennessee from a point Z, then the part of the Pulaski sheet origi- minor fold, the Clover Hollow anticline, located seismic-reflection survey (Harris and others, nally existing between Z and R may have ex- just northwest of the Saltville fault along section 1981), (4) the 1° slope used by Boyer and Elliott tended westward of point P, perhaps to the B-B'. Their Z axis orientations, determined from (1982) in construction of their balanced section vicinity of the St. Clair fault, after thrusting and

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 508 M. J. BARTHOLOMEW

5 MILES 8 KM

Figure 22. Balanced (top) and restored (middle and bottom) cross section along B-B' (Fig. 2) -40 kia southwest of A-A'. Modified after cross section I-r of Buitts (1933) and Milici (1973); stratigraphie sections taken from Figure 19. FDZ = Fries DI>Z, X = intersection of glide horizon in Mississippian Maccrady Formation with the Pulaski fault surface south of the Price Mountain window, P = leading edge of Pulaski thrust sheet, PF = Pulaski fault, R = intersection of base of Mississippian strata in footwall with Pulaski fault. Other symbols are the same as those in Figures 2 and 21.

has since been removed by erosion. This would spectively. Moreover, along cross section H-H' Again, a comparison with Roeder and With- reduce shortening for each section to 77% and of Butts (1933), which is -20 km southwest of erspoon's (1978) work is useful. If their cross 69%, respectively. These figures provide the my section B-B', Lewis and Bartholomew section T8 and maps are used, the over-all minimum shortening because reflectance values (1984) calculated -30% shortening for the same shortening for the western portion of the Valley for Mississippian coal (Hower and others, 1987) western thrust package. All of these figures, and Ridge province in Tennessee is -46%, again indicate that the Pulaski thrust did not extend which are derived from balanced and restored using my method of calculation. If one considers beyond the St. Clair fault. cross sections, account for the regionally impor- that Roeder and Witherspoon's work is -150- If one uses cross section T2 and the palinspas- tant folds as well as faults and thus (unlike the 200 km along strike to the southwest but in the tic map of Roeder and Witherspoon (1978), the figures for the Pulaski sheet) reflect some of the direction of increasing tectonic transport on shortening for the Pulaski and related thrusts in internal plate shortening of the competent strata, faults of the western thrust package, then a pro- northeastern Tennessee is -73%, calculated in such as the thick sequences of Lower Ordovi- gressive: increase in the amount of shortening is the manner I have: used. If one considers that all cian to Cambrian carbonates, the Silurian quite consistent with the results reported herein. of these figures do not reflect internal deforma- quartzites, and the Mississippian sandstones. It tion within the component plates of the Pulaski is, of course, recognized that incompetent strata, CONCLUSIONS sheet but rather only that due to the relative such as the Ordovician and Devonian shales, movement of these major plates, it is thus likely locally may undergo considerably more shorten- (1) A palinspastic reconstruction of the Pu- that over-all shortening for the Pulaski sheet is ing (Gray, 1981; Simon and Gray, 1982) in laski thrust sheet indicates that the minimum on the order of 80%. compressed hinge zones and adjacent to faults, displacement of the complexly deformed plate is By comparison, shortening percentages for and this is reflected in cross sections A-A' and -100-110 km. This would restore this plate to the Devonian/Mississippian boundary of the B-B' but is considered, like the internal defor- the vicinity of Danville and South Boston, Vir- western thrust package (Saltville, Narrows, and mation of the plates of the Pulaski sheet, of lesser ginia, which are near the interpreted margin of St. Clair) of the Valley and Ridge province (V-R concern for regional comparison of over-all the Noirth American craton (Hatcher and Zietz, on Figs. 21 and 22) are only 8% and 19%, re- shortening of the more competent strata. 1980). This restoration thus places these rocks in

REFERENCES CITED logical Sciences Memoir 1, p. 81-114. a geologically likely location for the accumula- 1968, Profile of the folded Appalachians of western Virginia: University tion of the Cambrian/Lower Ordovician shelf of Missouri at Rolla Journal 1, p. 27-64. Amato, R. V., 1974, Geology of the Salem quadrangle, Virginia: Virginia 1970, The Max Meadows Breccias: A reply, in Fischer, G. W., and sequence. Division of Mineral Resources Report of Investigation 37,40 p. others, eds., Studies of Appalachian geology: Central and southern: New Bartholomew, M. J., 1979, Thrusting component of shortening and a model for York, Interscience Publishers, p. 179-191. (2) The shortening calculated for the Pulaski thrust fault development at the central/southern Appalachian junction: 1971, Appalachian structural and topographic front between Narrows thrust sheet is on the order of 80%, which is Geological Society of America Abstracts with Programs, v. 11, no. 7, and Beckley, Virginia and West Virginia: Virginia Polytechnic Institute p. 384-385. and State University Department of Geological Sciences Guidebook 5, significantly greater than the l0%-20% shorten- 1981, Geology of the Roanoke and Stewartsvillc quadrangles, Virginia, p. 89-142. with geologic map of the Roanoke quadrangle and a summary of Cooper, B. N., and Haff, J. C., 1940, Max Meadows fault breccia: Journal of ing calculated for the western thrust package. tectonic history—Valley and Ridge by Bartholomew, M. J., and Geology, v. 48, p. 945-974. Hazlett, W. H., Jr.: Virginia Division of Mineral Resources Publication Currier, L. W., 1935, Zinc and lead region of southwestern Virginia: Virginia (3) Structural stages (pre-, syn-, and post- 34, 23 p. Division of Mineral Resources Bulletin 43,122 p. ramp) can be recognized to be related to ramp- 1984, Structural evolution of a portion of the eastern (Appalachian) Derby, J. R., 1965, Paleontology and stratigraphy of the Nolichucky Forma- overthrust belt—A new model: Geological Society of America Ab- tion in southwestern Virginia and northeastern Tennessee (Ph.D. dis- ing of the Pulaski fault system from the lower stracts with Programs, v. 16, no. 6, p. 438. sert.]: Blacksburg, Virginia, Virginia Polytechnic Institute and State level décollement (in the Rome Formation), 1985, Mechanics of Alleghanian emplacement of thrust sheets along the University, 468 p. Blue Ridge-Valley and Ridge boundary: Geological Society of America Dietrich, R.*V., 1954, Geology of the Pilot Mountain area, Virginia: Virginia across Cambrian to Mississippian strata, to the Abstracts with Programs, v. 17, no. 7, p. 520. Polytechnic Institute Bulletin, Engineering Experiment Station Series Bartholomew, M. J., and Brown, K. E., 1987, Structure and stratigraphy of 91,32 p. upper level décollement (in the Maccrady Devonian and Mississippian strata in Pulaski and Montgomery Coun- Edmundson, R. S., 1958, Industrial limestones and dolomites in Virginia; ties, Virginia: Virginia Division of Mineral Resources Publication, in James River district west of the Blue Ridge: Virginia Division of Min- Formation). press. eral Resources Bulletin 73,137 p. (4) The 300- to 500-m-thick broken forma- Bartholomew, M. J., and Hazlett, W. H., Jr., 1981, Geologic map of the Edwards, J., Jr., 1960, The geology of the upper Roanoke valley area, Mont- Roanoke quadrangle and a summary of tectonic history—Valley and gomery and Roanoke Counties, Virginia [M.S. thesis]: Blacksburg, Vir- tion, which contains the classic Max Meadows Ridge, in Bartholomew, M. J., Geology of the Roanoke and Stewarts- ginia, Virginia Polytechnic Institute and State University, 90 p. ville quadrangles, Virginia: Virginia Division of Mineral Resources Pub- Elliott, D., and Johnson, M.R.W., 1980, Structural evolution in the northern tectonic breccias, formed during the earlier lication 34,23 p. part of the Moine thrust belt, NW Scotland: Royal Society of Edin- stage(s) of Alleghanian deformation when the Bartholomew, M. J., and Lewis, S. E., 1984, Evolution of Grenville massifs in burgh Transactions, Earth Sciences, v. 71, p. 69-96. the Blue Ridge geologic province, southern and central Appalachians, in Espenshade, G. H., Rankin, D. W., Shaw, K. W., and Neuman, R. B., 1975, Pulaski fault system was developing well to the Bartholomew, M. J., and others, eds., The Grenville event in the Appa- Geologic map of the east half of the Winston-Salem quadrangle, North lachians and related topics: Geological Society of America Special Carolina-Virginia: U.S. Geological Survey Miscellaneous Investigations southeast, geographically, of the present-day ex- Paper 194, p. 220-254. Map I-709-B, scale 1:250,000. posures and at a lower level glide horizon in Bartholomew, M. J., and Lowry, W. D., 1979, Geology of the Blacksburg Eubank, R. T., 1967, Geology of the southwestern end of the Catawba syndine, quadrangle, Virginia: Virginia Division of Mineral Resources Publica- Montgomery County, Virginia [M.S. thesis]: Blacksburg, Virginia, Vir- Cambrian strata. tion 14, GM-81B (1:24,000 map with text). ginia Polytechnic Institute and State University, 91 p. Bartholomew, M. J., and Schultz, A. P., 1980, Deformation in the hanging wall Gathright, T. M., II, and Rader, E. K., 1981, Field guide to selected Paleozoic (5) The Pulaski thrust system, like the Blue of the Pulaski thrust sheet near Ironto, Montgomery County, Virginia rocks, Valley-Ridge province, Virginia: Part 1, Roanoke, Gifton Forge, (Part B, Sheet 3), in Geologic structure and hydrocarbon potential along Front Royal areas: Virginia Division of Mineral Resources Virginia Ridge fault system, initially developed structur- the Saltville and Pulaski thrusts in southwestern Virginia and northeast- Minerals, v. 27, no. 3, p. 17-23. ally below the lower level décollement in the ern Tennessee: Virginia Division of Mineral Resources Publication 23 Gibson, R. G„ and Gray, D. R., 1985, Ductile-to-brittle transition in shear (6 sheets with text). during thrust sheet emplacement, southern Appalachian thrust belt: Rome Formation. Bartholomew, M. J., Milici, R. C, and Schultz, A. P., 1980, Regional structure Journal of Structural Geology, v. 7, no. 5, p. 513-525. and hydrocarbon potential (Part A, Sheet 1), in Geologic structure and Glass, F. R., Jr., 1970, Structural geology of the Christiansburg area, Mont- hydrocarbon potential along the Saltville and Pulaski thrusts in south- gomery County, Virginia [M.S. thesis]: Blacksburg, Virginia, Virginia western Virginia and northeastern Tennessee: Virginia Division of Min- Polytechnic Institute and State University, 81 p. eral Resources Publication 23 (6 sheets with text). Gray, D. R., 1981, Compound tectonic fabrics in singly folded rocks from Bartholomew, M. J., Schultz, A. P., Henika, W. S„ and Gathright, T. M., II, southwest Virginia, U.S.A.: Tectonophysics, v. 78, p. 229-248. ACKNOWLEDGMENTS 1982, Geology of the Blue Ridge and Valley and Ridge at the junction Harris, L. D., 1979, Similarities between the thick-skinned Blue Ridge anticli- of the central and southern Appalachians, in Lyttle, P. T., ed., Central norium and the thin-skinned Powell Valley anticline: Geological Society Appalachian geology, NE-SE GSA '82 field trip guidebooks: American of America Bulletin, Part I, v. 90, no. 6, p. 525-539. This paper summarizes the results of a map- Geological Institute, p. 121-170. Harris, L. D., Harris, A. G., deWitt, W., Jr., and Bayer, K. C., 1981, Evaluation Bartholomew, M. J., Dove, P. M., and Walsb-Stovall, C. A., 1987a, Relation- of southern eastern overthrust belt beneath Blue Ridge-Piedmont thrust: ping program in the Blacksburg to Pulaski area, ships at the southern terminus of the Pedlar and Lovingston massifs in American Association of Petroleum Geologists Bulletin, v. 65, no. 12, Virginia, which I directed while in charge of the the Virginia Blue Ridge: Geological Society of America Abstracts with p. 2497-2505. Programs, v. 19, no. 2, p. 75. Hatcher, R. D., Jr., and Zietz, I., 1980, Tectonic implications of regional Blacksburg regional office of the Virginia Divi- Bartholomew, M. J., Brown, K. E., Ingram, G. R., and Schultz, A. P., 1987b, aeromagnetic and gravity data from the southern Appalachians, in Geologic maps of Devonian and Mississippian rocks in Pulaski and Wones, D. R., ed., The Caledonides in the USA: Virginia Polytechnic sion of Mineral Resources (VDMR) at Virginia Montgomery Counties, Virginia: Virginia Division of Mineral Re- Institute and State University Department of Geological Sciences Polytechnic Institute and State University. For- sources, 1:100,000-scale map, in press. Memoir 2, p. 235-244. Bauerlein, H. J., 1967, Geology of the Millers Cave area, Roanoke, Craig, and Hazlett, W. H., Jr., 1968, Structural evolution of the Roanoke, Virginia, area mer students of Virginia Tech who assisted with Montgomery Counties, Virginia [M.S. thesis]: Blacksburg, Virginia, Vir- [Ph.D. dissert]: Blacksburg, Virginia, Virginia Polytechnic Institute and ginia Polytechnic Institute and State University, 77 p. State University, 266 p. this mapping are K. E. Brown, P. A. Dove, G. R. Bick, K. F., 1960, Geology of the Lexington quadrangle, Virginia: Virginia Henika, W. S., 1981, Geology of VQlamont and Montvale quadrangles, Vir- Ingram, A. P. Schultz, C B. Stanley, and C. A. Division of Mineral Resources Report of Investigations 1,40 p. ginia: Virginia Division of Mineral Resources Publication 35,18 p. 1986, Structure of the Sugarloaf Mountain area: Intersecting trends of Hergenroder, J. D., 1966, The Bays Formation (Middle Ordovician) and re- Walsh-Stovall. Other Virginia Tech students the northeast flank of the Roanoke reentrant, Virginia, in McDowell, lated rocks of the southern Appalachians [Ph.D. dissert]: Blacksburg, R. C., and Glover, L., Ill, eds., The Lowry volume: Studies in Ap- Virginia, Virginia Polytechnic Institute and State University, 325 p. whose work is incorporated herein include P. L. palchian Geology: Virginia Polytechnic Institute and State University Hower, J. C., Lewis, S. E., and Trinkle, E. J., 1987, Vitrinite reflectance and Broughton, F. R. Glass, Jr., W. H. Hazlett, Jr., Department of Geological Sciences Memoir 3, p. 27-36. anisotrophy of Mississippian-age coals in Montgomery, Pulaski, and Boyer, S. £., and Elliott, D., 1982, Thrust systems: American Association of Giles Counties, Virginia: Virginia Division of Mineral Resources Publi- P. B. Kaygi, and G. S. Ritter. W. S. Henika, Petroleum Geologists Bulletin, v. 66, p. 1196-1230. cation, in press. Broughton, P. L, 1971, Structural geology of the Pulaski-Salem thrust sheet Karpa, J. B., Ill, 1974, The Middle Ordovician Fincastle Conglomerate north W. D. Lowry, and S. E. Lewis each contributed and the eastern end of the Cbristiansburg window, southwestern Vir- of Roanoke, Virginia, and its implications for Blue Ridge tectonism ginia [M.S. thesis}: Blacksburg, Virginia, Virginia Polytechnic Institute [M.S. thesis]: Blacksburg, Virginia, Virginia Polytechnic Institute and significantly to the mapping. Discussions with, and State University, 126 p. State University, 164 p. and/or reviews by, the following VDMR staff Butts, C., 1933, Geologic map of the Appalachian Valley of Virginia with Kaygi, P. B., 1979, The Fries fault near Riner, Virginia: An example of a explanatory text: Virginia Division of Mineral Resources Bulle- polydeformed, ductile deformation zone [M.S. thesis]: Blacksburg, Vir- were helpful in formulating and revising this tin 42, 56 p. ginia, Virginia Polytechnic Institute and State University, 165 p. manuscript: R. C. Milici, E. K. Rader, T. M. Campbell, M. R., and others, 1925, The Valley Coal Fields of Virginia: Virginia King, P. B., and Ferguson, H. W., 1960, Geology of north easternmost Tennes- Division of Mineral Resources Bulletin 25,322 p. see: U.S. Geological Survey Professional Paper 311,136 p. Gathright II, and W. S. Henika. Reviews by Chappie, W. M., 1978, Mechanics of thin-skined fold-and-thrust belts: Geolog- Kreisa, R. D., 1980, The Martinsburg Formation (Middle and Upper Ordovi- ical Society of America Bulletin, v. 89, no. 8, p. 1189-1198. cian) and related fades in southwestern Virginia [Ph.D. dissert.]: S. E. Lewis, W. J. Perry, Jr., and A. P. Schultz Cooper, B. N., 1939, Geology of the Draper Mountain area, Virginia: Virginia Blacksburg, Virginia, Virginia Polytechnic Institute and State Univer- Division of Mineral Resources Bulletin 55,98 p. sity, 335 p. sharpened the focus of this paper, and the formal w M 1946, Metamorphism along (he Pulaski fault in the Appalachian 1981, Storm-generated sedimentary structures in subtidal marine fades GSA reviewers, A. A. Drake, Jr., J. M. Hull, Valley of Virginia: American Journal of Science, v. 244, p. 95-104. with examples from the Middle and Upper Ordovician of southwestern 1961, Grand Appalachian excursion: Virginia Polytechnic Institute and Virginia: Journal of Sedimentary Petrology, v. 41, p. 823-848. and R. T. Faill, shaped the final manu- State University Department of Geological Sciences Guidebook 1, Kreisa, R. D., and Bamback, R. K., 1973, Environments of deposition of the 187 p. Price Formation (Lower Mississippian) in its type area, southwestern script. Some of the work was supported under 1963, Blacksburg syndinorium and Pulaski overthrust: Virginia Poly- Virginia: American Journal of Sdence, Cooper Volume 273-A, U.S. Geological Survey Agreement 14-08-0001- technic Institute and State University Department of Geological Sci- p. 326-342. ences Guidebook 2, p. 19-47. Kulander, B. R., and Dean, S. L, 1986, Structure and tectonics of central and A0076 awarded to M. J. Bartholomew and 1964, Relation of stratigraphy to structure in the southern Appalach- southern Appalachian Valley and Ridge and Plateau provinces, West ians, in Lowry, W. D., ed., Tectonics of the southern Appalachians: Virginia and Virginia: American Association of Petroleum Geologists G. A. Bollinger. Virginia Polytechnic Institute and State University Department of Geo- Bulletin, v. 70, no. 11, p. 1674-1684.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021 510 M. J. BARTHOLOMEW

Lewis, S. E., 1975, Geology of the southern part of the Riner quadrangle, Rankin, D. W., Espenshade, G. H., and Neuman, R. B., 1972, Geologic map of M. J., Lewis, S.E., and Evans, N. H., 1986, Geologic map of Giles Montgomery and Floyc Counties, Virginia [M.S. thesis]: Raleigh, North the west half of the Winston-Salem quadrangle, North Carolina, Vir- County, Virginia: Virginia Division of Mineral Resources Publication, Carolina, North Carolira State University, 106 p. ginia, and Tennessee: U.S. Geological Survey Miscellaneous Geologic l:50,00Q-scale map. Lewis, S. E., and Bartholomew. M. J., 1984, Development of classic dome and Investigations Map 1-709-A, scale 1:250,000. Simon, R. I., and Gray, D. R., 1982, Interrelations of mesoscopic structures and basin structure within the eastern (Appalachian) overthrust belt: Geo- Ritter, G. S., 1970, Geology of the Blacksburg area, Montgomery County, smin »cross a small regional fold, Virginia Appalachians: Journal of logical Society of America Abstracts with Programs, v. 16, no. 6, p. 575. Virginia [M.S. thesis]; Blacksburg, Virginia, Virginia Polytechnic Insti- Structural Geology, v. 4, no. 3, p. 271-289. Lowry, W. D., 1971, Appalachian overthrust belt, Montgomery County, tute and State University, 38 p. Spencer, E V/., 1968, Geology of the Natural Bridge, Sugar!oaf Mountain, southwestern Virginia: Virginia Polytechnic Institute and State Univer- Rodgers, J., 1970, The Pulaski fault, and the extent of Cambrian evapontes in Buchanan, and Arnold Valley quadrangles, Virginia: Virginia Division sity Department of Geological Sciences Guidebook 5, p. 143-178. the central and southern Appalachians, in Fisher, G. W., and others, cf Mineral Resources Report of Investigations 13, 55 p. 1979, Nature of thrusting along the Allegheny front near Pearisburg and eds., Studies of Appalachian geology: Central and southern: New York, Stose, A. J., end Stose, G. W., 1957, Geology and mineral resources of the of overthrusting in the Blacksburg-Radford area of Virginia: Virginia Interscience Publishers, p. 175-178. Gossan lead district and adjacent areas in Virginia: Virginia Division of Polytechnic Institute aid State University Department of Geological Roeder, D., and Witherspoon, W. D., 1978, Palinspastic map of east Tennes- irlinend Resources Bulletin 72,291 p. Sciences Guidebook 8, p. a-54. see: American Journal of Science, v. 278, p. 543-550. Stovall, R. L, 1984, The relationships between gravity anomalies and the McDowell, R. C., 1982, The Hurricane Ridge, Gten Lyn, and Caldwell syn- Schultz, A. P., 1979a, Deformation associated with Pulaski overthrusting in the geology of the Blue Ridge province near Floyd, Virginia [M.S. thesis]: clines of southeastern West Virginia and southwestern Virginia: A rein- Price Mountain and East Radford windows, Montgomery County, Blacksburg, Virginia, Virginia Polytechnic Institute and State Univer- terpretation: Southeastern Geology, v. 23, no. 2, p. 83-88. southwestern Virginia [M.S. thesis): Blacksburg, Virginia, Virginia Poly- sity, 64 p. McGuire, O. S., 1970, Geology of the Eagle Rock, Strom, Oriskany, and technic Institute and State University, 134 p. TiUman, C. G., 1963, Late Silurian(7) and Early Devonian positive area, Salem Salisbury quadrangles, Virginia: Virginia Division of Mineral Resources 1979b, Field trip guide—Second day, in Lowry, W. D., Nature of

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/491/3998356/i0016-7606-99-4-491.pdf by guest on 03 October 2021