Similarities Between the Thick-Skinned Blue Ridge Anticlinorium and the Thin-Skinned Powell Valley Anticline

Similarities between the thick-skinned Blue Ridge anticlinorium and the thin-skinned Powell Valley anticline

LEONARD D. HARRIS U.S. Geological Survey, Reston, Virginia 22092

ABSTRACT a nearly continuous sequence with sedimentary rocks of the Valley and Ridge. South of Roanoke, Virginia, in the southern Appalachi- The Blue Ridge anticlinorium in northern Virginia is a part of an ans, the continuity of the sequence is broken by a series of great integrated deformational system spanning the area from the Pied- thrust faults that have transported Precambrian rocks of the Blue mont to the Appalachian Plateaus. Deformation intensity within Ridge westward in Tennessee at least 56 km (35 mi), burying rocks the system decreases from east to west. Differences of opinion have of the Valley and Ridge province. Although surface relations in the emerged concerning the central Appalachians as to whether the southern Appalachians clearly demonstrate that basement rocks basement rocks exposed in the core of the Blue Ridge anticlinorium are involved in thrusting, surface relations in the central Appala- are rooted or are allochthonous. Available surface and subsurface chians are less definitive. Consequently, differences of opinion have stratigraphie and structural data suggest that the anticlinorium may emerged in the central Appalachians concerning whether, in the be a rootless thick-skinned analogue to the rootless thin-skinned subsurface, basement rocks beneath the Blue Ridge are rooted or Powell Valley anticline in the Valley and Ridge. Both structures involved in thrusting. As an example, Cloos (1947, 1972) consid- were produced during the Alleghenian orogeny by similar defor- ered that the Blue Ridge anticlinorium in northern Virginia and mational processes. The form of the Powell Valley anticline is at- Maryland is rooted and was produced by shear folding. He tributed to duplication of about 4,575 m (15,000 ft) of sedimentary suggested that distribution of slip along many closely spaced cleav- rock during approximately 16 km (10 mi) of northwest movement age planes and large shear zones is the mechanism responsible for above a subhorizontal décollement. Similarly, the form of the Blue uplift and transport of the Blue Ridge anticlinorium about 16 km Ridge anticlinorium is attributed to duplication of about 9,150 m (10 mi) westward. In contrast, Root (1970, 1973) suggested that (30,000 ft) of igneous, metamorphic, and sedimentary rock during basement rocks beneath the Blue Ridge in Pennsylvania are a minimum of 59 km (37 mi) of northwest movement above an allochthonous and are displaced westward about 80 km (50 mi) eastward continuation of a subhorizontal décollement within above an eastward continuation of the master décollement that is Cambrian sedimentary rocks beneath the Valley and Ridge. Thus, beneath the Valley and Ridge. in northern Virginia there is a mixing of structural styles: the Whether the Blue Ridge anticlinorium in the central Appalachi- thick-skinned rootless Blue Ridge anticlinorium sits above a thin- ans is nearly autochthonous, as viewed by Cloos, or allochthonous, skinned detachment. This relationship implies that thin- and thick- as viewed by Root, has a direct bearing on the fundamental role of skinned styles are simply end members of a complex deformational the Blue Ridge in the development of the entire Appalachian Moun- process that includes a transition zone, where characteristics of tain system. If autochthonous, the uplift of the rooted Blue Ridge as both styles commingle. a shear fold was the principal cause for the deformation of the Val- ley and Ridge in the central Appalachians. If allochthonous, the INTRODUCTION Blue Ridge, rather than being the cause of the deformation of the Valley and Ridge, was actually a part of the shortening process. It is In the eastern part of the United States there is a noticeable the purpose of this paper to evaluate current data concerning the change in the style of deformation at the surface from the Valley origin of the Blue Ridge in northern Virginia, where it forms a con- and Ridge province eastward into the Blue Ridge province. Style in tiguous sequence with rocks of the Valley and Ridge, in an attempt the Valley and Ridge, documented by drilling and seismic data to more clearly understand the place of the Blue Ridge anti- along the west edge of the Valley and Ridge from Pennsylvania to clinorium in the development of the Appalachian Mountain sys- Tennessee (Gwinn, 1964; Jacobeen and Kanes, 1974, 1975; Perry, tem. 1964, 1975; Harris, 1976) is characterized by flexure folding and thrust faults rooted in a master décollement that is entirely within REGIONAL STRUCTURAL RELATIONS the sedimentary cover (thin-skinned). Style in the Blue Ridge, on the basis of surface relations, is thought to be shear folding and re- Northern Virginia and the adjacent parts of West Virginia (along lated faulting involving both basement and cover as a unit (thick- the line of section A—A', Fig. 1) are uniquely suited for studying the skinned). This style change is best seen in the central Appalachians change in style of deformation from the Appalachian Plateaus in northern Virginia, Maryland, and southern Pennsylvania, where through the Blue Ridge anticlinorium. Rocks in that area form a the Blue Ridge anticlinorium, cored with Precambrian rocks, forms nearly complete lateral sequence that has been adequately de-

Geological Society of America Bulletin, Part I, v. 90, p. 525-539, 7 figs., June 1979, Doc. no. 90605.

525

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scribed in several reports (Cloos, 1947, 1972; King, 1950; Brent, placement above a décollement limited to the area west of the Blue 1960; Nelson, 1962; Gwinn, 1964; Perry, 1964, 1975; Smith and Ridge. Seismic and drilling data (Perry, 1964, 1975; Gwinn, 1964,' others, 1964; Rader and Perry, 1976). Collectively, these papers 1970; Jacobeen and Kanes., 1974, 1975) confirms that in this area demonstrate the decrease in intensity of deformation from east to the Valley and Ridge is underlain by a décollement at the Cambrian west. level. Cloos (1972), who studied in detail the northwest limb of the The construction of my regional cross section (Fig. 2) is based on Blue Ridge anticlinorium, concluded that uplift and westward the concept that lateral movement above a master décollement, movement of 13 to 16 km (8 to 10 mi) of the basement core of the which changes its stratigraphie position westward from Cambrian Blue Ridge would be sufficient to produce the structural pattern shown in the present Valley and Ridge in northern Virginia, Mary- land, and southern Pennsylvania. Although he did not construct a cross section to illustrate his concept of the subsurface relations from the Blue Ridge to the Appalachian Plateaus, he did suggest that folding in the Valley and Ridge was probably related to dis-

Figure 1. Generalized structure map of central and southern Appalachians (modi- fied from King and Beikman, 1974).

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to younger levels, is responsible for the development of an integ- tended to form the same fan patterns observed for cleavage, circles rated deformation system spanning the area from the Piedmont to became ellipses (deformed oolites), intensity of deformation the Appalachian Plateaus. Near Wills Mountain at the west margin gradually decreased away from the hinged area, and incipient de- of the Valley and Ridge, the décollement changes stratigraphie po- velopment of a bedding-plane thrust took place. In addition, sition from Cambrian along a major tectonic ramp to the Mar- although not intended, his experiment shows that he was dealing tinsburg Formation (Ordovician). The detachment continues at the with a flexure fold adjusting to compression, as indicated by the level of the Martinsburg beneath the Appalachian Plateaus to the offsetting of tops of circles in opposite directions, which took place Elkins Valley anticline, where it tectonically ramps upward to the between the layers of clay separated by Saran Wrap (Cloos, 1972, Silurian. Displacement of the northwest limb of the Wills Mountain Fig. 5). Thus, surprisingly, Cloos experimentally demonstrated that anticline from its ramp zone appears to be about 21 km (13 mi). all structures he assumed to be the result of shear folding can be The splay anticlines west of Wills Mountain are the principal sub- produced by a flexure fold. If the Blue Ridge anticlinorium is a kind surface take-up zones that compensate for the Wills Mountain dis- of thick-skinned flexure fold, it may be instructive to compare the placement. East of Wills Mountain the surface of the Valley and similarities and the differences between it and the rootless thin- Ridge is cut by two major thrust faults — the Little North Moun- skinned Powell Valley anticline in the Pine Mountain thrust sheet in tain and the Pulaski-Staunton. Seismic profiles, deep drilling, and the Valley and Ridge. geologic relations (Jacobeen and Kanes, 1975; Rader and Perry, 1976) suggest that rocks above the Little North Mountain fault COMPARISON OF THE POWELL VALLEY ANTICLINE have moved about 27 km (17 mi). A klippe 6 km (4 mi) west of the AND THE BLUE RIDGE ANTICLINORIUM Pulaski-Staunton fault indicates that displacement on the Pulaski- Staunton is at least 6 km (4 mi). In addition, the apparent subsur- The Pine Mountain thrust fault, which is the standard model face duplication and folding may account for an additional 5 km (3 (Rich, 1934) for thin-skinned tectonics in the southern Appalachi- mi) of displacement. ans, is thought to have been initiated as a subhorizontal décolle- Summing of the displacements on all thrust faults suggests that a ment in an incompetent zone just above basement, and to have minimum of shortening in the Valley and Ridge immediately in shifted upward across competent units along a steeply dipping front of the Blue Ridge is about 59 km (37 mi). Although Perry tectonic ramp to relocate as a subhorizontal décollement at a higher (1975) and Rader and Perry (1976) showed many more com- stratigraphie level and finally to cut to the surface at Pine Mountain plexities in the subsurface above the master décollement than are (Fig. 3). Because the initial detachment did not form everywhere at shown in my section (Fig. 2), estimated shortening in their sections, the same stratigraphie level, displacement above this irregular sur- exclusive of the Pulaski-Staunton fault, is about 56 km (35 mi). face moves rocks from the lower level detachment up the tectonic Neither of these estimates takes into account an additional incre- ramp and onto the higher level detachment surface to produce the ment of shortening that may have occurred through penetrative de- rootless Powell Valley anticline. Although surficially the Powell formation. However, the magnitude of shortening, about 59 km Valley anticline is similar to a flexure fold under compression, it is (37 mi), in the Valley and Ridge implies that the Blue Ridge, be- actually a rootless flexure confined to the allochthonous sheet. cause it forms a contiguous sequence with the Valley and Ridge, The Powell Valley anticline can be divided into three distinct must be displaced northwestward an equal distance. Thus, it seems parts — the northwest limb, the crestal region, and the southeast possible that the Blue Ridge may be a thick-skinned rootless anti- limb (Fig. 3). Each of these parts formed in response to different clinorium sitting above a thin-skinned detachment. mechanical elements within the system. The most active part of the Although it is reasonable to assume that the décollement, as fold is the northwest limb. This limb is formed by a combination of shown in Figure 2, beneath the Valley and Ridge must continue uplift and transport as truncated beds ride up the tectonic ramp southeastward beneath the Blue Ridge, its attitude is uncertain. The zone and rotate at a hinge line formed where truncated beds in the fault either can be a subhorizontal thrust or it may turn steeply allochthonous sheet abut against the subhorizontal surface of the downward into basement. Cloos's (1972) experimental attempt to next higher level of the detachment fault in the autochthonous simulate the upturning of the Blue Ridge has a direct bearing on the plate. During the development stage of the northwest limb, its at- question of the attitude of this fault. In that experiment, he con- titude progressively changes from low dipping to relatively steeply structed a block of clay separated into layers by three layers of dipping in response to gradual uplift as older beds in the truncated Saran Wrap above a hinged trap door. He then marked the sides of zone are moved onto the upper level detachment. Vertical growth the clay block with a series of parallel horizontal lines and circles, of the northwest limb as well as the Powell Valley anticline ceases so that he could measure the effect of deformation on these fea- when all beds in the truncated zone are moved to the upper level tures. Finally, he sprayed the model with Krylon, so that he could detachment surface. From this point on, the Powell Valley anticline observe the crenulation pattern produced by compression. Appar- expands only laterally as subhorizontal beds originally above the ently, he assumed that the hinged board was the contact between lower level detachment are duplicated above the subhorizontal the Precambrian crystalline rocks and its cover rocks (the clay upper level detachment surface. Minimal movement of lower level block), so that his experiment dealt exclusively with the effect of units at first form a narrow crested anticline, but in the final stage uplift on a layered sequence. By lifting the trap door, he bent the of development, the amount of displacement (several kilometres) clay block and carefully studied the deformational effect at various results in a broad, slightly undulatory crestal region, within which stages. By a series of experiments, he was able to duplicate many of no single axis defines the fold (Harris and Milici, 1977). Drag in the the detailed structural features he previously had carefully de- hinge zone where truncated beds at the base of the northwest limb scribed in the Blue Ridge anticlinorium (Cloos, 1971). Crenulation intersect the upper level detachment surface tends to steepen the

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northwest limb, so that it may become slightly overturned. The dip (15,000 ft) of rock, extends for about 150 km (95 mi) in parts of of the beds in the southeast limb is a reflection of the dip of the southwest Virginia and Tennessee. In addition to a difference in tectonic ramp in the autochthonous block. size, the type and abundance of strain marker s within each of these Although the northwest limb of the Powell Valley anticline was folds are different. Cloos (1971) has carefully shown that such formed in a laterally moving deformational system, its stages of structural features as cleavage, deformed oolites, and lineations are growth are comparable with the stages of growth in the stationary characteristic of the northwest limb of the Blue Ridge anti- experimental system used by Cloos (1972) to simulate the de- clinorium. None of these features has been noted in the Powell Val- velopment of the northwest limb of the Blue Ridge anticlinorium ley anticline. This obvious difference in kind and abundance of (Fig. 4). In the Cloos experiment, upturning to form the simulated structural features is probably related to the relative positions of northwest limb of the Blue Ridge anticlinorium was accomplished the Blue Ridge anticlinorium and the Powell Valley anticline within by gradually bending a partly fixed-hinged board. In contrast, up- the original deformational system. Cloos (1971) clearly showed turning of the northwest limb of the Powell Valley anticline was ac- that deformational intensities gradually decrease westward from complished along a hinge line by lateral transport that gradually the Blue Ridge. Thus, the Blue Ridge anticlinorium, which displays moved more than 1,525 m (5,000 ft) of truncated beds in the an abundance of strain markers, formed near the center of the allochthonous sheet up a tectonic ramp and onto a higher subhori- orogenic disturbance, whereas the Powell Valley anticline, which zontal detachment surface in the autochthonous plate. The pro- has few strain markers, is a distal feature that formed near the west gressive steepening of the northwest limb of the Powell Valley an- edge of the system. ticline produced by gradual duplication is similar to the progressive Detailed studies of the Powell Valley anticline indicate that it is a steepening of the simulated northwest limb of the Blue Ridge anti- rootless structure, whose form is directly related to miles of dis- clinorium produced by slightly bending the hinged board to steeply placement of beds by thrusting over an irregular surface. If the Blue bending the board (Fig. 4). Thus, the Cloos experiment may be Ridge is rootless, subdivisions of that fold, because they are directly equally applicable to some flexure folds formed in situ, as well as to related to the mechanics of thrusting, should be comparable with rootless flexure folds formed through duplication of beds by lateral the subdivisions of the rootless Powell Valley anticline. As a matter movement in thin-skinned tectonics. This, of course, raises the of fact, both the anticline and the anticlinorium have a steeply dip- question of whether the Blue Ridge anticlinorium is a rootless an- ping northwest limb, a broad crestal region within which no single ticline similar to the rootless Powell Valley anticline. axis defines the fold, and a more gently dipping southeast limb (Fig. A comparison of the characteristics of the Blue Ridge anti- 5). Because of the similarities in form, it seems likely that the clinorium and the Powell Valley anticline indicates that there are configuration of the décollement surface beneath the Blue Ridge differences as well as similarities between the two structures. One anticlinorium is comparable with that beneath the Powell Valley of the main differences is size. Cloos (1972) suggested that about anticline. Limited data (Smith and others, 1964; Ern, 1968; Brown, 9,150 m (30,000 ft) of rock in the anticlinorium are upturned along 1969) suggest that cleavage is oriented relative to the inferred posi- a hinge line extending for 385 km (240 mi) from southern Pennsyl- tion of the tectonic ramp zone in the autochthonous plate beneath vania to the James River in Virginia (Fig. 1). The Powell Valley an- the southeast limb of the Blue Ridge anticlinorium. In general, ticline, a much smaller structural feature involving about 4,575 m cleavage in the crestal region of the anticlinorium is nearly upright;

APPALACHIAN PLATEAUS VALLEY AND

A Elkins Valley anticline Wills Mountain anticline Little North Mountain thrust r-»——

15 MILES PRECAMBRIAN —I

15 KILOMETERS

Crystalline basement Mechum River Formation Lynchburg as used by Milici Formation and others (1963)

Figure 2. Structure section showing integrated deformational model spanning area from Piedmont to Appalachian Plateaus.

it fans across the inferred tectonic ramp zone and again becomes tic material as much as 3,050 m (10,000 ft) thick (Brown, 1970). nearly upright within the James River synclinorium (Fig. 5). Cleav- The Swift Run Formation, a thin 30±-m (100-ft) clastic unit found age fanning possibly outlines the area in the subsurface where rocks on parts of the crest and northwest limb of the anticlinorium, has moving up the inclined ramp zone are flexed into an anticlinal fold been considered to be a much thinned equivalent of the Lynchburg as they move onto the subhorizontal surface of the décollement de- (Brown, 1970). However, detailed work by Reed (1969) suggests veloped in Cambrian rocks. By analogy, the Blue Ridge anti- that the Swift Run is a basal alluvial sedimentary unit of the Catoc- clinorium, which has been displaced about 59 km (37 mi) north- tin Formation that accumulated in low places on an erosion surface west and above the Valley and Ridge décollement, appears to be a and locally is found as thin beds interstratified with volcanic rocks rootless thick-skinned anticline formed by processes during the well up in the Catoctin. Assuming that Reed is correct, there is no Alleghenian orogeny similar to those that produced the thin- equivalent to the Lynchburg in the crest or northwest limb of the skinned rootless Powell Valley anticline (Fig. 2). Blue Ridge anticlinorium. Because that part of the Lynchburg For- mation preserved beneath the Catoctin Formation on the southeast PARTIAL GEOLOGIC HISTORY OF THE limb of the Blue Ridge anticlinorium is interpreted to be a deep- BLUE RIDGE ANTICLINORIUM water deposit (Brown, 1970), the possibility exists that the original IN PART OF NORTHERN VIRGINIA Lynchburg was much more extensive and included other com- plementary facies. Erosion prior to the accumulation of the Catoc- Structural data presented earlier strongly imply that the Blue tin produced an extensive upland on Precambrian crystalline base- Ridge anticlinorium is a rootless thick-skinned structure formed by ment which had a relief of about 305 m (1,000 ft) (Reed, 1969) and thrusting during the Alleghenian orogeny. However, the rocks in- removed that part of the Lynchburg deposited on crystalline base- volved in this structure, as well as the adjacent Piedmont rocks, had ment west of its present outcrop. The Mechum River Formation, as a prior complex geologic history. Appraisal of these early geologic used by Milici and others (1963), a Precambrian alluvial sequence events offers an alternative method to both substantiate the time of as much as 1,000 m (3,000 ft) thick exposed in a narrow graben development inferred from structural data and assess the influence along the Blue Ridge (Schwab, 1974), may have accumulated along earlier events had on the final form of the anticlinorium. A com- with the Swift Run Formation during this erosion interval. plete discussion of the geologic history of the Blue Ridge from Apparently, during Catoctin time, the Precambrian crystalline Pennsylvania to Georgia is beyond the scope of this report, which is upland of Reed (1969) was bordered on the east by an ocean basin limited to the general area of northern Virginia. Figure 6 is an at- (Brown, 1970). As a consequence, basalts and sediments of the tempt to summarize the general geologic history in the vicinity of Catoctin accumulated under both subaerial conditions, on the section A—A' (Fig. 2), beginning with the basal Precambrian western upland, and subaqueous conditions, in the eastern marine sedimentary deposits and proceeding upward to the construction basin (Fig. 6, A). Near Lynchburg, Virginia, greenstones of the by thrusting of the Blue Ridge anticlinorium. Catoctin are interbedded with sedimentary sequences, suggesting to The basal metasedimentary rock on the southeast flank of the Brown (1955) that the Lynchburg and Catoctin interfinger. Appar- Blue Ridge anticlinorium is the Lynchburg Formation, which has ently, sediments derived from erosion of the western upland of been interpreted to be a deep-water deposit of graywacke and peli- Reed (1969) could have been the source for the sediments interbed-

RIDGE BLUE RIDGE PIEDMONT

Blue Ridge anticlinorium James River synclinorium Pulaski-Staunton thrust

MIDDLE CAMBRIAN TO UPPERMOST MIDDLE 0RD0VICIAN PENNSYLVANIAN 0RD0VICIAN PRECAMBRIAN (?) TO UPPER CAMBRIAN THROUGH SILURIAN

Catoctin Formation Rome Formation Carbonate sequence Shady Dolomite Evington Group Chilhowee Group Figure 2. (Continued).

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ded with the Catoctin. Thus, while erosion was taking place in the Lynchburg, acted as a hinge line. This hinge line apparently per- upland of Reed, continuous sedimentation from the Lynchburg sisted through Cambrian and into Early Ordovician time and rep- into the Catoctin may have occurred within the oceanic basin. The resents the transition zone between the early shallow Paleozoic shoreline zone on the southeast limb of the Blue Ridge anti- miogeosynclinal shelf on the west and the eugeosynclinal deep clinorium, where the Catoctin interfingers as well as overlaps the basin on the east (Brown, 1970).

Initial form of the Pine Mountain décollement

Maximum amplitude attained when all cross-cut beds in tectonic ramp zone are duplicated on the upper level décollement surface

ROOTLESS POWELL VALLEY ANTICLINE

Northwest limb Crestal region Southeast limb Tends to tighten by drag Increases in width by massive duplication Reflects the dip of and may be slightly of rocks that were originally above the lower tectonic ramp in the overturned locally level de'collement autochthonous plate ^ / m SE

Miles of displacement Figure 3. Stages in development of rootless Powell Valley anticline. Shape and amplitude of fold were controlled by initial form of Pine Mountain décollement and amount of northwest displacement.

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OF THE BLUE RIDGE ANTICLINORIUM Uplift by movement up a tectonic ramp m SE (Cloos, 1972)

Uplift by bending a simulated layered sequence ^Upper level décollement-'

Tectonic ramp

Layered sequence of clay block Initial form of the Pine Mountain décollement

Gradual uplift occurs as cross-cut beds are duplicated and rotated to form a moderately dipping northwest limb Slight bending results in uplift and a moderately dipping limb

Continued transport produces maximum uplift and gradual steeping of beds in the northwest limb, until all beds in cross-cut zone are duplicated Increased bending results in gradual uplift and a steeply dipping limb Figure 4. Similarities between progressive uplift produced by bending a partly fixed board in the Cloos (1972) experiment and progressive uplift of northwest limb of rootless Powell Valley anticline produced by transport and duplication of beds.

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1 2 3 4 5 MILES

0 5 KILOMETERS

BLUE RIDGE ANTICLINORIUM

Figure 5. Subdivisions of both Powell Valley anticline and Biue Ridge anticlinorium are directly comparable, suggesting that both structures are rootless folds produced by similar processes. pC = Precambrian crystalline rocks; p€m = Mechum River Formation as used by Milici and others (1963); pCly = Lynchburg Formation; pCc = Catoctin Formation, €p€rch = Rome Formation, Shady Dolomite, and Chilhowee Group; €e = Evington Group; Cr = Rome Formaton; OC = Middle Ordovician limestone, Beekmantown Group, Con- ococheague Limestone, and Elbrook Dolomite; OCk = Middle Ordovician limestone and Knox Group; O = Ordovician rocks; SO = Silurian and Ordovician rocks; MD = Mississippian and Devonian rocks; IP = Pennsylvanian rocks.

A nonconformity is inferred at the base of the Chilhowee Group kind of relationship holds in Maryland and Pennsylvania, where an (Rankin and others, 1969) because regionally it overlaps Precamb- offshore section Z is compared with a nearer shore section Y. Thus, rian rocks of great antiquity (crystalline basement, 1,050 m.y. old; strike variation in thickness of the Chilhowee is readily related to Catoctin and Mount Rogers Formations 820 m.y. old). Only the distribution of facies and apparently not structurally controlled in upper part of the Chilhowee contains Early Cambrian fossils; thus, aulacogens as suggested by Rankin (1976). the possibility exists that the lower part may be Precambrian in age. Both Brown (1970) and Schwab (1972) have considered the Because outcrops of the Chilhowee Group are essentially confined Chilhowee Group to be a lateral equivalent of the deeper water to a strike section stretching from Pennsylvania to Georgia, a dep- pelitic deposits of the Candler Formation in the lower part of the ositional model derived from study of that belt is necessarily an in- Evington Group in the Piedmont. The Evington Group is overlain ferred generalization (Schwab, 1972). In summarizing several years unconformably by the possibly Middle to Late Ordovician Arvo- of study of the eastern outcrop belt of the Chilhowee, Schwab nia Formation. Pavlides and others (1974) have tentatively corre- (1972) concluded that the Chilhowee consists of a series of lated the upper part of the Evington Group from the Candler to fluviatile and nearshore sandstones interfingering eastward with the Middle Ordovician unconformity with the Chopawamsic For- deeper water shale and siltstone (Fig. 7). In general, as marine mation (lower Paleozoic) of northern Virginia. This correlation transgression progressed westward the character of the sandstone raises the possibility that the Chilhowee Group, the Shady Dolo- changed upward from arkosic and conglomeratic sandstone to or- mite, and perhaps the Rome Formation in the Valley and Ridge, thoquartzite. rather than being the lateral equivalents of only the lower part of Schwab (1972) indicated that the marked variation in thickness the Evington Group, are equivalent to the entire Evington Group. of the Chilhowee Group shown in his eastern strike belt section The fact that the Chilhowee Group thickens rapidly eastward at (Fig. 7) could be accounted for by slight shifts of the present-day an apparent hinge line (Fig. 7) suggests that the Lynchburg- outcrop across the depositional strike. Such shifts would emphasize Catoctin hinge line may have been effective in actively segregating differences, because units from different parts of the basin of depo- the miogeosynclinal shelf deposits of the Chilhowee Group, Shady sition would be exposed. In contrast, Rankin (1976), evidently dis- Dolomite, and Rome Formation from their deeper water pelitic de- counting Schwab's depositional controls, suggested that the varia- posits on the east (Fig. 6, B). In addition, this hinge line may have tion in thickness along strike, especially near the Maryland- been the mechanism that limited the development of the later Pennsylvania border and in northeast Tennessee, were evidence of Middle Cambrian to Lower Ordovician carbonate bank, the edge failed-arm troughs (sediment-filled rift complexes termed aulaco- of which, as suggested by Rodgers (1968), may have been quite gens). However, as pointed out by both Schwab and Rankin, their abrupt. respective interpretations were based on a two-dimensional model. Sometime after the deposition of the Chopawamsic Formation Surprisingly, King and Ferguson (1960) had previously supplied the equivalent, a granodiorite mass (Hatcher Complex of Brown, third dimension but had been completely overlooked. These men, 1969) invaded the eugeosynclinal deposits. Although the time of working in the complexly faulted part of northeast Tennessee, rec- crystallization has been estimated to be 595 ± 80 m.y. ago (Ful- ognized that kilometres of northwest movement by separate thrust lagar, 1971), the intrusion apparently had little immediate effect on sheets had juxtaposed rocks deposited in different parts of the orig- the sedimentary process or structure of the area. Higgins and others inal basin of deposition of the Chilhowee Group. By determining (1977) have pointed out that metaplutonic rocks in the central Ap- the sequence of faulting and then arranging sections of the palachian Piedmont may have inherited old radiogenic lead derived Chilhowee Group as they were before thrusting, they were able to from older basement. Thus, ages as determined may have little re- construct a restored section for the Chilhowee. Although they did lation to the actual age. Perhaps the Hatcher Complex of Brown not intend to construct a depositional model for the Chilhowee (1969) fits into this category, and its age of intrusion may be more Group, in effect that is exactly what they did. The Chilhowee dep- nearly Late Cambrian or Early Ordovician. At any rate, uplift in- ositional model shown in Figure 7 is based on the original King and volving the intruded area did take place until late Early Ordovician Ferguson (1960, p. 35) compilation. However, their section has or early Middle Ordovician (Fig. 6, C). It is unlikely that this uplift been enlarged by including several of their measured sections not was due exclusively to the Hatcher Complex; rather, it was part of previously used. In addition, Rankin and others (1972) have a regional process of uplift that initiated the widespread discon- slightly revised the sequence of faulting in the area by recognizing formity recorded at the top of the Cambrian and Lower Ordovician that a large slice occurs within the southwest part of the Mountain carbonate sequence throughout much of the eastern half of the City window. United States. That the amount of uplift and erosion was greater in The restored depositional model of the Chilhowee Group in the Hatcher Complex area is suggested by the fact that Cambrian northeast Tennessee is strikingly similar to the inferred model of and Lower Ordovician carbonate rock occurs beneath the discon- Schwab (1972) for central Virginia. By relating the various parts of formity in the present-day Valley and Ridge and the northwest limb both models to the sections used by Schwab in his strike profile, it is of the Blue Ridge anticlinorium, whereas Lower Cambrian rock quite clear that variation in thickness is related to chance inclusion and the Hatcher Complex (Cambrian) are beneath the disconfor- of sections from different parts of the model. As an example, in mity in the Piedmont (Fig. 6). northeast Tennessee, Schwab used section 15, which is mainly rep- The Hatcher upland in the Piedmont appears to have persisted resentative of the offshore, deeper water part of the model, to corre- until late Middle Ordovician time, because early Middle Ordovi- late with section Y, a thin nearshore part of the model in central cian rocks are deposited above Lower Ordovician rocks im- Virginia. In northeast Tennessee, had Schwab used either of the mediately west of the Blue Ridge, but in the Hatcher area possible thin nearshore sections of King and Ferguson (1960), sections 20 or Middle Ordovician and definite Upper Ordovician rocks (Tillman, 21, to correlate with his central Virginia nearshore Y section, the 1970) were deposited above Cambrian rocks. During early Middle strike variation in thickness would have been minimal. The same Ordovician time, a deep basin formed west of the Hatcher upland

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within the former Cambrian to Lower Ordovician shelf. Polymictic Complex area rather than the western part of the Blue Ridge as conglomerates containing clasts ranging from the Chilhowee previously suggested by Tillman and Lowry (1971). Later, the Group to the Cambrian and Lower Ordovician carbonate rock are Middle Ordovician basin widened eastward across the Hatcher up- found in deep-water graptolitic black shale of Middle Ordovician land, so that possible Middle and Upper Ordovician sedimentary age (Liberty Hall Formation) in the Valley and Ridge near Fincas- rocks (Arvonia Slate) were deposited above parts of the Evington tle, Virginia (Kellberg and Grant, 1956). Because the Hatcher up- Group and the Hatcher Complex (Fig. 6, D; Brown, 1970; Tillman, land persisted until late Middle Ordovician time, the source for 1970). these conglomerates was the west flank of the uplifted Hatcher Although there are no documented Paleozoic sedimentary rocks

Valley and Ridge Blue Ridge Piedmont o r —— Ce pCm Hatcher pCc t—? Complex ocv p€ €r of Brown (1969)

." CpCch

pCc H. Present-day rootless Blue Ridge anticlinorium produced by thrusting and duplication of beds

F. Accumulation of delta complex during Devonian through part of the Pennsylvanian. PMD, Pennsylvanian, Mississippian, and Devonian rocks

E. Uplift and accumulation of fluviatile and nearshore clastic deposits of Silurian and Early Devonian age derived from erosion of Middle and Upper Ordovician sedimentary deposits. DS, Lower Devonian and Silurian rocks Figure 6. Geologic history of rocks of Blue Ridge anticlinorium.

younger than Ordovician in the Piedmont, sedimentation processes that prior to the deposition of the Silurian strata, the present-day obviously continued to be active in the area, because on the west eastern Valley and Ridge may have been the leading edge of a much edge of the Valley and Ridge today there are more than 3,660 m larger eastern positive area (Fig. 6, E). Studies of the Tuscarora (12,000 ft) of post-Ordovician rocks. The Silurian in this general Sandstone (Silurian), Pocono Sandstone (Devonian and Mississip- area is represented by a relatively thin 366-m (1,200 ft) sequence. pian), and Pottsville Formation (Pennsylvanian) all suggest that West of the Little North Mountain fault (Fig. 1), a basal sandstone much of the present-day Blue Ridge and the immediately adjacent unit (called Lower Silurian and Upper Ordovician by Rader and part of the Piedmont during Silurian to Pennsylvanian time was an Perry, 1976) contains zones of polymictic conglomerate composed alluviating coastal plain bordered on the east by an upland (Pel- of clasts of silicified oolitic chert, slate, radiolarian chert, and vol- letier, 1958; Yeakel, 1962; Meckel, 1967; Whisonant, 1977). That canic rock. East of the Little North Mountain fault, the Massanut- this same eastern positive area existed during Early Devonian time ten Sandstone (Silurian and younger?) lies disconformably on the can be inferred from polymictic conglomerate in the Ridgeley Martinsburg Formation (Ordovician). This relationship suggests Sandstone (Lower Devonian) near Blacksburg, Virginia (Eubank,

0. Development of basin during Middle and Late Ordovician within the former Cambrianf?) to Early Ordovician shelf. 0, Ordovician rocks

C. Intrusion, uplift, and erosion—late Early or Middle Ordovician time

B. Development of shelf and basin sequence during the Cambrian (?) to Early Ordovician. €p€ch, Chilhowee Group,- €e, Evington Group; Cs, Shady Dolomite; Cr, Rome Formation; 0C, Lower Ordovician and Cambrian carbonate rocks

Upland Plateau Subaerial , Hinge line » * » * * * Subaequeous p€c p« _ _ _ _

A. Uplift and accumulation of basaltic flows and pyroclastics of the Catoctin Formation (Precambrian; pCc). p€ly, Lynchburg Formation; p€m, Mechum River Formation as used by Milici and others (1963); and pC, Precambrian crystalline basement Figure 6. (Continued).

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1969). Source of the clasts in the Ridgeley has been tentatively and were recycled, removing the carbonate. The insoluble identified as sandstone from the Clifton Forge Sandstone Member sandstone of the Unicoi and vein quartz clasts formed parts of the of the Keyser Limestone (Lower Devonian), sandstone from the polymictic conglomerates of the Ridgeley. Evidently, that part of Bays Formation (Middle Ordovician), black shale pebbles from the the eastern positive area within the present-day Blue Ridge during Liberty Hall Formation (Middle Ordovician), feldspathic Early Devonian time, instead of being a highland, was a low area sandstone from the Unicoi Formation (Lower Cambrian?), and underlain by Silurian and Ordovician rocks. Perhaps farther east, vein quartz from a Precambrian crystalline terrane (Eubank, 1969; beyond the scope of this report, older rocks were exposed. Tillman and Lowry, 1971). The presence of Lower Cambrian(P) Meckel (1970) pointed out that the Devonian to Pennsylvanian Unicoi and vein quartz clasts in the Lower Devonian conglomerates rocks in the Valley and Ridge form a sedimentary sequence that led Eubank (1969), Tillman and Lowry (1971), and Lowry (1974) changes from dominantly alluvial on the east to thinner marine in to conclude that the source area for these clasts must have been the the west. That the nonmarine part of the model continues to Blue Ridge anticlinorium. However, the fact that the polymictic thicken eastward to near the boundary of the Valley and Ridge and conglomerates of the Ridgeley contain clasts in the sequence from Blue Ridge suggests that sedimentation must have been continuous Middle Ordovician to Lower Devonian but include no representa- well beyond the Blue Ridge and into the Piedmont (Fig. 6, F). Ful- tive clasts from older formations except the Cambrian Unicoi and lagar (1971) noted a strontium isotopic equilibrium at 340 m.y. Precambrian vein quartz suggests that the source may have been the ago in the Hatcher Complex of Brown (1969) and suggested that recycling of the polymictic conglomerate of the Liberty Hall For- this was either a time of metamorphism or of deep burial of the mation rather than the Blue Ridge anticlinorium. Thus, the Middle complex. Conodont color-alteration isograds compiled for the Ap- Ordovician rocks in the Liberty Hall Formation, which contains palachian basin (Epstein and others, 1977) tend to support the deep Cambrian and Lower Ordovician carbonate clasts as well as burial suggested by Fullagar. In that study, five distinct, progres- quartzite of the Unicoi and vein quartz clasts in its polymictic con- sive, irreversible color changes in conodonts were used to charac- glomerates (Kellberg and Grant, 1956), were exposed to erosion terize the levels of organic metamorphism within the Appalachian

A, Strike section Figure 7. Strike section and depositional models of Chilhowee Group, showing regional variation in thickness and lithology because of discordance between present outcrop and original depositional strike. Section numbers and letters in part A are keyed to lettered and numbered sections in depositional models (B and C).

basin up to the Blue Ridge. Conodont color index 5 characterizes which, is within the temperature range necessary to initiate the the Middle Ordovician rocks in the north limb of the Blue Ridge greenschist metamorphic facies. Espenshade (1970) indicated that anticlinorium in northern Virginia. An Ahrrenius plot of experi- in this area, rocks in the Blue Ridge anticlinorium from Precamb- mental and field data indicates that alteration of conodont color to rian to Chilhowee Group contain minerals indicative of the an index of 5 takes place in the temperature range of 300 to 400 °C, greenschist facies. Analysis of overburden thickness in the Appala- chian basin necessary to indicate a conodont color alteration index of 5 ranges from 7,700 to 9,150 m (25,000 to 30,000 ft), suggesting that a similar thickness of rock must have been present above Middle Ordovician rocks in the north limb of the Blue Ridge anticlinorium to initiate the greenschist facies. On the basis of the assumption that the configuration of the thrust fault beneath the Blue Ridge anticlinorium is similar to that beneath the rootless Powell Valley anticline, it is possible to envi- sion the rocks involved in the Blue Ridge back in their general place of origin. In doing this (Fig. 6, G), it becomes readily apparent that the Arvonia Slate of Ordovician age in the Piedmont occupies B, Inferred depositional model for the Chilhowee Group about the same elevation relative to present-day sea level as do the in central Virginia (Schwab, 1972)

Mountain City Window

Doe River Slice within the Shady Valley Buffalo Mountain Inner Window Mountain City Window Thrust sheet Thrust sheet

Section 21 Section 18 Section 17 Section 4 Section 15 METERS FEET

- 1000

- 2000

3000

1000 —

- 4000

- 5000

Restored depositional model for the 7000 Chilhowee Group in northeast Tennessee. Stratigraphie data from King and Ferguson (1960).

Structural data from Rankin, Espenshade, and L- 8000 Neuman (1972)

10 KILOMETERS Figure 7. (Continued).

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rocks of Devonian age in the Valley and Ridge (Fig. 2). Evidently, Epstein, A. G., Epstein, J. B., and Harris, L. D., 1977, Conodont color there was a fold of unknown magnitude in the Piedmont just prior alteration — An index to organic metamorphism: U.S. Geological to thrusting. Survey Professional Paper 995, 27 p. Em, E. H., 1968, Geology of the Buckingham quadrangle, Virginia: Rankin and others (1969) noted an episodic lead loss from zir- Virginia Division of Mineral Resources Report of Investigations 15, cons at about 240 m.y. ago in samples of Precambrian rocks overly- 45 p. ing basement taken from the Blue Ridge anticlinorium in South Espenshade, G. H., 1970, Geology of the northern part of the Blue Ridge Mountain, Pennsylvania, and the Blue Ridge thrust sheet in the anticlinorium, in Fisher, G. W., and others, eds., Studies of Appala- chian geology — Central and southern: New York, Interscience Pub- Grandfather Mountain—Mount Rogers areas of Virginia, Tennes- lishers, p. 199-211. see, and North Carolina. These authors suggested that shearing as- Eubank, R. T., 1969, Basal conglomerate in the Ridgeley Sandstone (Lower sociated with movement of the Blue Ridge thrust sheet in the south- Devonian) near Blacksburg, Virginia [abs.]: Geological Society of ern Appalachians disturbed the zircon ages at about 240 m.y. ago. America Special Paper 121, p. 435. They interpreted this lead loss to mean that the Blue Ridge thrust Fullagar, P. D., 1971, Age and origin of plutonic intrusions in the Piedmont of the southeastern Appalachians: Geological Society of America Bul- was an integral part of the episode of thrusting affecting the Valley letin, v. 82, p. 2845-2862. and Ridge during the late Paleozoic Appalachian orogeny. How- Gwinn, V. E., 1964, Thin-skinned tectonics in the Plateau and northwestern ever, because thrusting could not be verified in the South Moun- Valley and Ridge provinces of the central Appalachians: Geological tain, Pennsylvania, area, they were noncommittal as to what rela- Society of America Bulletin, v. 75, p. 863-900. tionship the lead loss had to the Blue Ridge anticlinorium in the 1970, Kinematic patterns and estimates of lateral shortening, Valley and Ridge and Great Valley provinces, central Appalachians, south- central Appalachians. Data presented in this paper suggest that the central Pennsylvania, in Fisher, G. W., and others, eds., Studies of Ap- Blue Ridge anticlinorium in northern Virginia did form during the palachian geology — Central and southern: New York, Interscience late Paleozoic thrusting, so that the 240-m.y.-ago lead loss may Publishers, p. 127-146. well indicate the time of development of the anticlinorium. Harris, L. D., 1976, Thin-skinned tectonics and potential hydrocarbon traps — Illustrated by a seismic profile in the Valley and Ridge province of Tennessee: U.S. Geological Survey Journal of Research, v. 4, SUMMARY p. 379-386. Harris, L. D., and Milici, R. C., 1977, Characteristics of thin-skinned style Shortening in the Valley and Ridge immediately in front of the of deformation in the southern Appalachians and potential hydrocarbon traps: U.S. Geological Survey Professional Paper 1018, Blue Ridge anticlinorium is estimated to be a minimum of 59 km (37 40 p. mi). Because rocks of the Blue Ridge form a nearly uninterrupted se- Higgins, M. W., Sinha, A. K., Zartman, R. E., and others, 1977, U-Pb zir- quence with rocks of the Valley and Ridge, it seems likely that the con dates from the central Appalachian Piedmont: A possible case of Blue Ridge is allochthonous and was part of the shortening process, inherited radiogenic lead: Geological Society of America Bulletin, rather than the cause. The Blue Ridge anticlinorium appears to be a v. 88, p. 125-132. Jacobeen, F., Jr., and Kanes, M. H., 1974, Structure of Broadtop rootless thick-skinned structure sitting above a subhorizontal de- synclinorium and its implications for Appalachian structural style: tachment fault similar to the thin-skinned rootless Powell Valley American Association of Petroleum Geologists Bulletin 58, p. 362- anticline. Conodont color-alteration isograds along the west edge 375. of the Blue Ridge anticlinorium suggest that prior to the develop- 1975, Structure of Broadtop synclinorium, Wills Mountain anticlinorium and Allegheny frontal zone: American Association of Petro- ment of thrusting, rocks of the Blue Ridge were buried beneath at leum Geologists Bulletin 59, p. 1136-1150. least 7,700 to 9,150 m (25,000 to 30,000 ft) of post-Middle Or- Kellberg, J. M., and Grant, L. F., 1956, Coarse conglomerates of the Mid- dovician sedimentary rocks. Thus, the Blue Ridge anticlinorium is a dle Ordovician in the southern Appalachian Valley: Geological Society relatively late Paleozoic structure formed during thrusting. of America Bulletin, v. 67, p. 697-716. King, P. B., 1950, Geology of the Elkton area, Virginia: U.S. Geological Survey Professional Paper 230, 82 p. REFERENCES CITED King, P. B., and Beikman, H. M., compilers, 1974, Geologic map of the United States: U.S. Geological Survey, scale 1:2,500,000, 2 sheets. Brent, W. B., 1960, Geology and mineral resources of Rockingham County: King, P. B., and Ferguson, H. W., 1960, Geology of northeast Tennessee: Virginia Division of Mineral Resources Bulletin 76, 174 p. U.S. Geological Survey Professional Paper 311, 136 p. Brown, W. R., 1955, Geology and mineral resources of the Lynchburg Lowry, W. 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L., and others, 1974, Correlation others, eds., Appalachian structures; origin, evolution, and possible between geophysical data and rock types in the Piedmont and Coastal potential for new exploration frontiers: Morgantown, W. Va., West Plain of northeast Virginia and related areas: U.S. Geological Survey Virginia University and West Virginia Geological and Economic Sur- Journal of Research, v. 2, p. 569-580. vey, p. 17-37. Pelletier, B. R., 1958, Pocono paleoenvironments in Pennsylvania and

Maryland: Geological Society of America Bulletin, v. 69, p. 1033- 830. 1064. 1973, Structure, basin development, and tectogenesis in the Pennsyl- Perry, W. J., Jr., 1964, Geology of the Ray Sponaugle well, Pendleton vania portion of the folded Appalachians, in Dejong, K. A., and Schol- County, West Virginia: American Association of Petroleum Geologists ten, R., eds., Gravity and tectonics: New York, John Wiley & Sons, Bulletin, v. 48, p. 659-669. p. 343-360. 1975, Tectonics of the western Valley and Ridge foldbelt, Pendleton Schwab, F. L., 1972, The Chilhowee Group and the late Precambrian-early County, West Virginia — A summary report: U.S. Geological Survey Paleozoic sedimentary framework in the central and southern Appala- Journal of Research, v. 3, p. 583-588. chians, in Lessing, Peter, and others, eds., Appalachian structures; Rader, E. K., and Perry, W. J., Jr., 1976, Reinterpretation of the geology of origin, evolution, and possible potential for new exploration frontiers: Brocks Gap, Rockingham County, Virginia: Virginia Minerals, v. 22, Morgantown, West Virginia University and West Virginia Geological no. 4, p. 37-45. and Economic Survey, p. 59—101. Rankin, D. W., 1976, Appalachian salients and recesses: Late Precambrian 1974, Mechum River Formation: Late Precambrian(?) alluvium in the continental breakup and opening of the Iapetus ocean: Journal of Blue Ridge province of Virginia: Journal of Sedimentary Petrology, Geophysical Research, v. 81, p. 5604-5619. v. 44, p. 862-871. Rankin, D. W., Stern, T. W., Reed, J. C., Jr., and others, 1969, Zircon ages Smith, J. W., Milici, R. C., and Greenberg, S. S., 1964, Geology and mineral of felsic volcanic rocks in the upper Precambrian of the Blue Ridge, resources of Fluvana County: Virginia Division of Mineral Resources central and southern Appalachian Mountains: Science, v. 166, Bulletin 79, 62 p. p. 741-744. Tillman, C. G., 1970, Metamorphosed trilobites from Arvonia, Virginia: Rankin, D. W., Espenshade, G. H., and Neuman, R. B., 1972, Geologic Geological Society of America Bulletin, v. 81, p. 1189-1200. map of the west half of the Winston-Salem quadrangle, North Tillman, C. G., and Lowry, W. D., 1971, The Salem synclinorium — A Carolina, Virginia and Tennessee: U.S. Geological Survey Miscellane- treasury of Appalachian tectonic history, in Guidebook to contrasts in ous Investigations Map I-907A, scale 1:250,000, 1 sheet. style of deformation of the southern and central Appalachians of Reed, J. C., Jr., 1969, Ancient lavas in Shenandoah National Park near Virginia: Virginia Polytechnic Institute and State University Depart- Luray, Virginia: U.S. Geological Survey Bulletin 1265, 43 p. ment of Geological Science, Guidebook no. 6, p. 23-68. Rich, J. L., 1934, Mechanics of low-angle overthrust faulting as illustrated Whisonant, R. C., 1977, Lower Silurian Tuscarora (Clinch) dispersal pat- by Cumberland thrust block, Virginia, Kentucky, and Tennessee: terns in western Virginia: Geological Society of America Bulletin, American Association of Petroleum Geologists Bulletin, v. 18, v. 88, p. 215-220. p. 1584-1596. Yeakel, L. S., 1962, Tuscarora, Juniata, and Bald Eagle paleocurrents and Rodgers, J., 1968, The eastern edge of the North American continent dur- paleogeography in the central Appalachians: Geological Society of ing Cambrian and Early Ordovician, in Zen, E-an, and others, eds., America Bulletin, v. 73, p. 1515-1540. Studies of Appalachian geology — Northern and maritime: New York, Interscience Publishers, p. 141—149. MANUSCRIPT RECEIVED BY THE SOCIETY JULY 28, 1977 Root, S. I., 1970, Structure of the northern terminus of the Blue Ridge in REVISED MANUSCRIPT RECEIVED FEBRUARY 2, 1978 Pennsylvania: Geological Society of America Bulletin, v. 81, p. 815- MANUSCRIPT ACCEPTED MARCH 30, 1978

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