Middle Ordovician Knox Unconformity, Virginia Appalachians

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Sedimentology and development of a passive- to convergent-margin unconformity: Middle Ordovician Knox unconformity, Virginia Appalachians

Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia 24061

ABSTRACT ters in the Appalachians (Colton, 1970; Read, 1980) influenced unconformity development. The Knox unconformity, central and southern Appalachians, Other ancient and modern (evolving) foreland thrust-fold belts have marks the transition from passive-margin to convergent-margin sedi- similar regional unconformities that separate stable platform sequences mentation, possibly during global sea-level lowering. The unconform- from overlying foreland basin sequences, for example, in the Timo:: Sea ity developed on Early to Middle Ordovician Knox-Beekmantown (Veevers, 1971; Crostella and Powell, 1975), the Persian Gulf (Murris, carbonates, and it has > 140-m erosional relief in southwest Virginia, 1980), and the Antler Orogenic Belt, western United States (Poole and decreasing to <20 m in northern Virginia. Decrease in erosional relief Sandberg, 1977; Poole and others, 1977). is accompanied by rapid depositional thickening of Lower Ordovician Hydrocarbons may be associated with these unconformities, bemuse and earliest Middle Ordovician units into a depocenter in Penn- source beds undergo rapid thermal maturation due to high sedimentation sylvania. rates and thrust-sheet loading in the foreland basin (Bally and Snelson, Paleokarst features include topographic highs (tens of metres 1980). Such unconformities also may control distribution of leacl-zinc relief), breccia- an d mud-filled sinkholes and caves that extend down deposits associated with permeable, subunconformity breccia horizons that to 65 m below the unconformity, and subconformity intraformational act as conduits for warm basinal brines expelled from shales following dolomite breccias that formed after dissolution of limestone interbeds. deep burial beneath synorogenic deposits. Detritus on the unconformity surface formed veneers of regolith, subaerial debris flows:, and mud-flat deposits, and locally it was reworked SETTING during transgression. The unconformity influenced the distribution of postunconform- The Knox-Beekmantown carbonates of the Appalachian Valley and ity carbonates, including Middle Ordovician build-ups. It also influ- Ridge Province (Fig. 1) formed on a passive continental margin bordering enced later Zn mineralization and possible localization of petroleum either a marginal basin or a major ocean basin (Glover and others, 1978; reservoirs in the basin. Development of regional unconformities at Hatcher, 1978). The unconformity developed during arc-continent or passive-to-convergent-margin transitions is common in other oro- microplate-continent collision during the Middle Ordovician (Jacobi, gens, reflecting gentle warping and uplift of the shelf prior to founder- 1981; Shanmugam and Lash, 1982). ing and burial beneath synorogenic clastics. The Knox unconformity in the Valley and Ridge Province is exposed within imbricate thrust sheets that moved from southeast to northwest, INTRODUCTION with displacements as much as tens of kilometres. The province lies between overthrust Precambrian and Lower Cambrian igneous and metased- This paper describes the Knox-Beekmantown unconformity ("Knox imentary rocks of the Blue Ridge to the southeast and nearly flat-lying late unconformity" hereinafter), which is the major stratigraphic break in the Paleozoic sediments of the Appalachian Plateau to the northwest (Fig. 1). Paleozoic sequence in the Appalachian Valley and Ridge Province, occur- Subunconformity Ordovician carbonates are referred to as "Upper ring between Lower to Middle Ordovician Knox-Beekmantown carbon- Knox Group" (as much as 1,000 m thick) in southwest Virginia and ates and overlying Middle Ordovician limestones. The unconformity is of "Beekmantown Group" (as much as 1,200 m thick) in northern Virginia. economic interest because it influenced localization of economic base- The unconformity is overlain by transgressive Middle Ordovician carbon- metal deposits (Harris, 1971; Collins and Smith, 1975) and possible hy- ates (Fig. 2). Depocenters centered in Tennessee and Pennsylvania drocarbon reservoirs in the Eastern Overthrust Belt of the Appalachians. (Colton, 1970; Thomas, 1977; Read, 1980, his Fig. 2) appear to have The Knox unconformity marks transition from shelf-carbonate depo- strongly influenced unconformity development and thickness of post- sition on a passive margin (Rodgers, 1968; Bird and Dewey, 1970) to unconformity units. deposition in a foreland basin associated with a convergent margin (Shanmugam and Walker, 1980; Read, 1980). It probably resulted from Age Relations Adjacent to Unconformity deformation of the passive margin during initial collision (early phase of the Taconic Orogeny of Rodgers, 1971; Quinlan and Beaumont, 1984), Conodont biostratigraphy indicates that uppermost Knox carbonates possibly during global lowering of sea level. Actively subsiding depocen- in southwest Virginia are Lower Ordovician, Canadian age, and arc overlain by Chazyan limestones (Fig. 2). The unconformity here probably •Present address: 8584 Rabbit Brush Way, Parker, Colorado 80134. spanned —10 m.y. (duration of the Whiterockian; Ross and others, 1982).

Geological Society of America Bulletin, v. 97, p. 282-295, 19 figs., 1 table, March 1986.

282

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Figure 1. Valley and Ridge Province, Virginia Appalachians, showing location of measured sections and major thrust faults; SC = St. Oair; CC = Copper Creek; S = Saltville; P = Pulaski; BR = Blue Ridge; N = Narrows; NM = North Mountain; ST = Staunton. Localities are: 1, Tumbez; 2, Avens Bridge; 3, Lebanon; 4 and S, Rich Valley; 6, Chatham Hill; 7,8, and 9, Marion; 10, East River Mountain; 11, Draper; 12, Narrows; 13, Eggleston; 14, Newport; 15, Lusters Gate; 16, Ellett; 17 and 18, Fincastle; 19, Park View; 20, Cedar Grove Church; 21 and 22, Harrisonburg; 23, east Broadway; 24, Madden Quarry at New Market; 25, Leaksville; 26, Woodstock; 27, Tumbling Run. Details of locations and measured sections are given in Mussman (1982).

Given that rates of carbonate dissolution in karst terranes range from 10 to NW SENW SE 100 mm/1,000 yr (Sweeting, 1972), from 1 to 10 m.y. would be needed NORTHERN QMRMU WESTERN VIRGINIA to form the observed unconformity relief. In northern Virginia, uppermost VIRGINIA SOUTHV Beekmantown carbonates are early Middle Ordovician (Whiterockian) age and are overlain by latest Whiterockian-earliest Chazyan limestone LINCOLNSHIRE LINCOLN SHIRE (Suter and Tillman, 1973; Tillman, 1976; A. Harris, personal commun.). LST . PART ) LENOIR No break is evident here on the basis of paleontologic data, yet sedimento- FIVE OAKS logical evidence suggests that subaerial emergence, even of short duration ORO . TUMBEZ CHAZYA N NEW MARKET — JIOSHEIM (LOWE R (few tens to hundreds of thousands of years), formed distinctive karstic MI D H J= ) 1 features in northern Virginia. ORDOVICIA N

SUBUNCONFORMITY KNOX-BEEKMANTOWN LITHOFACIES MIDDL E WHITEROCKIAN -J o Upper Knox-Beekmantown beds consist of cyclic carbonates. The |

Knox Group (southwest Virginia) is mainly dolomite, with abundant ' m JEEKMANTOWN bedded and nodular chert and quartz sand stringers, whereas the Beekman- GROUP UPPER KNOX GROUP

Figure 2. Simplified stratigraphic chart, Early and early Middle ORDOVICIA N Ordovician, Virginia. Note that "Whiterockian" is used in the restricted sense of Sweet and others (1971) and includes beds between IBEXIAN/CANADIA N LOWE R

the Canadian and Chazyan. |

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town Group (northern Virginia) has many limestone interbeds (locally as erosional break was found. Disconformable contacts appear conformable much as 50%). Knax-Beekmantown cycles are 2 to 9 m thick, and consist where Beekmantown dolomites are overlain by fine detrital dolomites (as of, from top to bottom: (4) cryptalgal, laminated dolomite and fenestral much as 10 m thick); these pass into unconformity breccias along strike limestone (0 to 5.5 m thick); (3) thick, laminated dolomite (1 to 4 m (Iocs. 21, 22 in Fig. 1). Pseudoconformable contacts also occur where thick); (2) massive and burrowed, thin-bedded dolomite (0.5 to 3 m thick); Beekmantown limestones are overlain by New Market Limestone (loc. 19 (1) coarsely crystalline dolomite and thrombolites (0.5 to 4 m thick). in Fig. 1), the unconformity being evidenced only by local limestone Upper Knox-Beekmantown beds are cyclic, upward-shallowing peri- breccia. Where well developed, the unconformity has paleokarstic highs tidal sequences. Se miarid setting and hypersaline conditions are indicated and lithoclastic breccias, conglomerates, and red beds that form sheets and by the association of silicified evaporite nodules, abundant cryptalgal 1am- fill unconformity depressions. There also are subunconformity, cave and inites, and abundant dolomite that largely predates unconformity sinkhole fills, and intraformational breccias. development.

KNOX UNCONFORMITY

In most areas in Virginia, the Knox-Middle Ordovician limestone contact is a disconformity. Locally, however, the contact is an angular unconformity (Figs. 3 A, 3B), with discordance of as much as 12° (Iocs. 2, TENN. A NC. 23, 25, 27 in Fig. 1). The unconfoimity increases in magnitude across strike from southeast A DISTANCE (kms) A' to northwest (Fig. 4B). Along strike, erosional relief decreases from 140 m in southwest Virginia (Webb, 1959) to a few metres in northern Virginia; o 100 600 this decrease in relief is accompanied by rapid thickening of Lower Ordo- vician and Middle Ordovician beds (Fig. 4A). Neuman (1951) and o Lowry (1957) suggested that the contact was locally conformable in -y \rrt -WR.-CAN. IJDY. northern Virginia. In all sections examined, however, some evidence of an 150 UPPER KNOX GRP. UPPER 300 « BEEKMANTOWN ® CHEPULTEPEC GRP. A NE E 450 « « 600 CONOCOCHEAGUE Z 750 * O — 900

H 1050 \ L = 1200 240 m DISTANCE (kms)

200 300 400 500 TT TT B sw NE

Figure 3. Angular unconformities. Note ex- B aggerated vertical scale in all diagrams. A. Broad- way, Virginia. Note di- »L vergence of beds above 120 m and below contact. B. Avens Bridge, Virginia.

MEASURED SECTION 1050

KNOX/BEEKMANTOWN Figure 4. Erosional and stratigraphic relief on unconformity. A. CARBONATES H Northeast-southwest strike section (line A-A', inset map); plot of MEW MARKET thicknesses of subunconformity formations against horizontal dis- LIMESTONE tance. Unconformity used as datum. Relief on unconformity ¡shown LENOIR LIMESTONE schematically. B. Across-strike (northwest-southeast) section (line B-B' in inset map); plot of thicknesses of subunconformity formations MARKER BEDS against palinspastic distance. Unconformity used as datum.

PALEOKARST FEATURES SW LINCOLNSHIRE LIMESTONE NE

Paleotopographic Highs

Paleohighs (Iocs. 3, 5, and 22 in Fig. 1) are a few metres to 30 m in relief, being highest in southwestern Virginia. They are marked by thinning or pinch-out of postunconformity beds (tens of metres of thinning over a a0ln \ \ V's. \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ few hundred metres along strike; Fig. 5A). Peritidal facies adjacent to ' •> s ///;//// r(Grow,t976ï£ — highs may pass outward into subtidal beds (Fig. 5B). Some unconformity highs form isolated "islands" of Knox dolomite surrounded by peritidal LINCOLNSHIRE LIMESTONE beds (Webb, 1959) (Fig. 5A). Paleohighs are veneered by thin (as much as B 30 cm) beds of mud-supported, chert-dolomite breccia that grades outward into fine, detrital dolomite or peritidal limestone. Origin. Paleotopographic features were formed by differential erosion of Knox-Beekmantown carbonates. They were positive elements during Middle Ordovician transgression, and some were undercut by marine erosion. Highs controlled thickness and distribution of tidal-flat deposits, (Grovtr. 1976) V / V V V V V V V V V V V and some may have localized later Middle Ordovician downslope build- S V < > > < < > > < > > > ups (Read, 1982). Similar paleohighs (as much as 1 km wide by 50 m high) in the Indiana subsurface contain hydrocarbons (Patton and Daw- FENESTRAL LIMESTONE F-F-^ KNOX DOLOMITE son, 1969; Keller and Abdulkareem, 1980). Dawson (1967) suggested that these formed mainly by physical weathering (deflation), because of their SKELETAL LIMESTONE MEASUREO SECTION association with postunconformity St. Peter Sandstone and because their buttelike shapes are characteristic of arid landforms. Common corroded Figure 5. Paleotopographic highs. A. Thinning of fenestral lime- clast margins in unconformity breccias in Virginia and lack of postuncon- stones toward unconformity high of Knox Dolomite, Ellett Valley (loc. formity sands, however, indicate chemical dissolution was the main 16 in Fig. 1). Modified from Grover (1976). B. Fenestral limestone weathering process here. thinning onto Knox unconformity high and grading out into skeletal limestone (location ~8 km northeast of loc. 16 in Fig. 1). Modified Detrital Carbonate-Filled Depressions (Sinkholes) from Grover (1976). Nonfossiliferous Fillings. Rare exposures of narrow (3 to 35 m wide, as much as 65 m deep), breccia-filled depressions on the unconformity (loc. 17 in Fig. 1) are mainly vertical to bedding, but some are Origin. Depression fills of detrital carbonate resemble modern col- horizontal at depth (Table 1). Sides are subparallel, and contacts between lapse dolines (sinkholes). Some dolines develop by dissolution along joints, host beds and the fills are sharp and irregular (Fig. 6). The fillings consist fractures, and(or) bedding planes. Others that form by collapse of cavern of lithoclast breccia with rare pods and lenses of granule conglomerate in roofs have near-vertical sides and high depth-to-width ratios and contain detrital dolomite matrix. They lack skeletal remains and are poorly sorted. megabreccias (Quinlan, 1972). The breccias are mainly mud-supported, but some are clast-supported; Most Knox sinkhole fills are nonmarine. Detritus was shed from they contain angular, rotated carbonate blocks (as much as 2 m diam) and gravel- to cobble-sized, poorly rounded carbonate and angular chert clasts (Fig. 7A). Some blocks show in-place brecciation, with fractures filled by dolomite mud. Clasts commonly are corroded (Fig. 7B). Matrix within breccias is quartzose, fine to medium crystalline dolomite. Rare, granule-carbonate conglomerate fills scours cut into dolomite mud. The conglomerates are clast-supported, with clasts of <5 mm diam, and interstitial dolomite mud and rare calcite cement. In extreme eastern exposures in the Valley and Ridge, some breccia-filled depressions covered by Middle Ordovician basinal shale contain abundant iron sulfide (Cooper and Diggs, 1953). Fossiliferous Fillings. A few depressions are filled by fossiliferous lime sands and muds, with minor breccia. One such depression (loc. 25 in Fig. 1) is 35 m wide and 15 m deep and overlies extensive intraformational breccias in Beekmantown carbonates. Contacts with host beds are sharp, and the fills consist of interlayered, black, lime mudstone beds (as much as 50 cm thick) and lime sand beds (as much as 15 cm thick) that have soft-sediment faults and recumbent folds directed toward the center of the MIOOLE ORDOVICIAN FENESTRAL LIMESTONE depression (Fig. 8). The mudstones are pyritic, argillaceous, and slightly fissile to unlaminated, and they contain abundant, fine quartz sand; rare, KNOX / BEEKMANTOWN CARBONATES rounded, sand-sized dolomitic mudstone clasts; and minor ostracode and trilobite debris. Lime sands are intraclast grainstone-packstone composed Figure 6. Map view of breccia-filled sinkholes on unconformity of rounded, sand- and granule-sized mudstone clasts and skeletal debris. near Fincastle (loc. 19 in Fig. 1; remapped after Campbell, 1975). Packstones are stylolitized and have abundant quartz silt. Rectangles on this and other maps are farm buildings.

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TABLE 1. CHARACTERISTICS OF KARSTRELATED LITHOFACIES

Subunconformity Postunconformity Filled sinkholes Filled caves Intraformational breccias Breccias and conglomerates Thick, fine detrital carbonat:, Blackford Formation

Geometry Narrow and subvertical (as much as Subvertical or parallel to host Lenticular to stratiform, up to Breccias: thicken into unconformity Thickens into depressions, thir s and 35 m wide, 65 m deep) or bedding, up to 2 m wide and 200 m wide and long, and lows, thin onto highs; up to over paleohighs; 0 to 70 m hick Thickness V-shaped (35 m wide, 15 m deep). 12 m long. No visible 35 m thick in subsurface; up to 2 m thick connection to unconformity surface 10 m long and 5 m thick in outcrop Conglomerates: fill channel-form depressions on the unconformity surface; up to 6 m thick

Distribution Southwestern Virginia Northern Virginia Throughout upper Southwestern Virginia Confined mainly to western bilts. Knox/Beekmantown beds in Southwestern Virginia; occurs Virginia and Tennessee rarely in southeastern belts beneath peritidal carbonate:

Lithology Nonmarine fills: chert/dolomite Mainly finely lam'd detrital Angular dolomite breccia in Chert, dolomite and limestone Massive fine detrital dolomite. breccias (blocks as much as 2 m dolomite with scour-filling medium to coarse dolomite clasts in matrix of detrital graded lithoclastic dolomite. diam) in fine detrital dolomite pebble conglomerate, detrital matrix and/or coarse blocky dolomite, skeletal grainstone. quartz siltstone, bentonitic matrix; rare scour-filling pebble limestone blocks, and chert- calcite or dolomite cement or fenestral lime mudstone/ shale, algal limestone, rare conglomerates dolomite breccias in cavities pellet packstone algal tufas near unconformity Marine fills: interbedded, black, skeletal intraclast grainstone and lime mudstone

Color Nonmarine fills: tan and Light brown Clasts are gray to light brown; Red, gray, or beige Red, gray, or beige blue-gray clasts, light brown matrix and cements are white or yellow matrix to yellow

Marine fills: dark gray to black

Bedding Nonmarine fills: lack bedding Lack bedding but are Bedding and structures absent Breccias: medium to thick bedded. Thin to thick-bedded; thin to and and grading, and are poorly finely laminated internally; some are graded thick lamination, tepees, sorted contain soft-sediment mudcracks fenestrae, crypu.lgal structures slumps, small scours, and Conglomerates: thick bedded at base, lamination, and burrowmo:tling Marine fills: thin to thick rare ripple cross-lamination thinning upwards; hardgrounds, clast bedded; grainstones contain imbrication, rare ripple cross- soft-sediment folds and faults lamination

Biota Nonmarine fills: barren Barren Barren Breccias: detrital dolomite matrix— Barren barren; skeletal matrix—open Marine fills: abundant trilobites. marine (echinoderm, bryozoan, ostracodes, and echinoderms, trilobite, ostracode, and and rare algae, brachiopod and brachiopod debris); fenestral bryozoan debris matrix—restricted marine (ostracodes. algae, delicate corals)

Conglomerates: detrital dolomite or chert/quartz sand matrix—barren; limestone matrix—open to restricted assemblages (as above)

Diagenesis Compaction and pressure solution Compaction features Silification, saddle dolomite, Absent Dolomitization (?), pressure features sulfide ores, liquid hydrocarbon solution

Primary Minor in pebble conglomerates, Minor in pebble conglomerates Trace to 30% Breccias: trace porosity abundant in grainstone beds Trace (cement-filledj Conglomerates: trace to 5%

walls and roofs of cavities or was washed in from the unconformity during Most fillings are nonfossiliferous, fine dolomite, although local breccias heavy rains. Som; blocks were incipiently brecciated after collapse, possi- occur near the unconformity. bly by impact or weathering, or by later deep burial compaction. Fills are brown and generally massive, but some are finely laminated Fossiliferousi sinkhole fills (northern Virginia) contain marine biota (Table 1). Layering may be inclined toward the center of cavities with and accumulated during Middle Ordovician transgression. Such sinkholes slopes as much as 40°, especially adjacent to walls, and may contain small are similar to those on the Pleistocene Florida Keys (Dodd and Siemers, slumps and truncation surfaces. Common small scours (a centimetre to a 1971) and may have been connected to the sea by caves or tidal channels, metre wide) are filled with laminated dolomite mud that is concordant to or they may have formed deep ponds surrounded by tidal flats. Abraded the basal scour (Fig. 1 IB). Other scours are smooth-walled, anastomosing, clasts in the Ordovician fills probably were rounded on the unconformity and subvertical, and they are filled with carbonate conglomerate (F ig. 10), prior to transport into sinkholes during storms or high tides. Soft-sediment which also forms rare sheets (as much as 15 cm thick) within or at bases of deformation was caused by subsidence of sinkhole floors and slumping of laminated beds. Conglomerates are clast- to mud-supported, composed of sediments from inclined walls in sinkholes. rounded carbonate clasts (as much as 8 mm diam), interstitial fine dolomite, and equant calcite cement. Breccias are poorly sorted and Discordant Detiital Carbonate Bodies (Caves, Collapse Dolines) clast-supported and lack bedding and grading. Clasts (as much as 25 cm diam) are subangular to rounded dolomite and limestone (locally with Thin, sheetKke to irregular bodies (as much as 12 m long, and 2 m concentric banding) and rare, angular chert, in a sandy, argillaceous, fine wide) of detrital dolomite within Knox-Beekmantown carbonates extend dolomite matrix. down to 35 m below the unconformity (Iocs. 13 and 25 in Fig. 1). They Origin. Discordant detrital carbonate bodies beneath the unconform- are mainly subvertical to bedding, but some are randomly oriented and ity surface in Virginia are probably cross sections of cave fills and col- lack connection to the unconformity surface in outcrop (Fig. 9). Contacts lapse dolines. Caves that form by dissolution along joint planes tend to be with host carbonates are sharp and highly irregular (Figs. 9, 10, 11 A). high, narrow, winding to vertical slits, whereas in flat-lying beds, where

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UPPER 3EEKKANT0HN GROUP

FINELY LAMINATED DOLOMITE; RARE CONGLOMERATE AND BRECCIA

Figure 9. Outcrop map of upper Beekmantown Group, unconformably overlain by New Market Limestone near Leaksville (loc. 25 in Fig. 1), showing relationships between host carbonates, intraformational breccias, and cave fills. Location of Figures 10 and 12 shown.

Figure 7. Nonfossiliferous sinkhole fills. A. Polished slab of lithoclast breccia from sinkhole fill at Eggleston, Vir- Figure 8. Fossiliferous sinkhole fills (Leaksville, ginia (loc. 13 in Fig. 1). Mixture of angular and rounded Virginia, loc. 25 in Fig. 1). Synsedimentary faults (A) and clasts of dolomite (outlined in ink) in a dolomite mud matrix. slumps (B) in layered grainstone sinkhole fill. Scale in centimetres. B. Polished slab of sinkhole fill with scalloped and corroded carbonate clasts in a carbonate pebble conglomerate, Fincastle, Virginia (loc. 18 in Fig. 1). Scale in centimetres.

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V v > / > I meter

FINELY LAMINATED OOLOMITE

FILL EFFIJ PEBBLE CONGLOMERATE / BRECCIA I77T1 DETRITAL LIMESTONE BLOCKS HOST PF^ DOLOMITE

Figure 10. Cross section through portion of cave fill within Beekmantown Group, shown in Figure 9. Note different symbols in Figures 9,10.

bedding planes control cave shape, caves are low and wide (Sweeting, 1972, p. 135-136). The Knox caves are mainly narrow slits subvertical to bedding, but there also are large cavities that appear to have formed along bedding planes. The walls are mainly corrosional and lack flowstone or dripstone. The cave fills are nonmarine, locally derived carbonate. Lami- nated dolomite containing scours and slump structures formed by deposition of argillaceous muds from slowly moving, muddy cave waters (compare Jennings, 1971, p. 176). Muds were scoured during periods of increased flow, and scours were filled with upward-fining gravels or, during lower energy conditions, by laminated muds. Slumping occurred where muds accumulated on sloping floors. Some steeply dipping laminae may have been oversteepened during sediment compaction, or during cave-floor subsidence. Similar cave-collapse dolines were described from the western United States by Sando (1974) and Maslyn (1977).

Intraformational Breccias

These breccias occur in the upper 200 to 300 m of Knox- Figure 11. Cave fills, Leaksville (loc. 25 in Fig. 1). A. Beekmantown carbonates (Iocs. 6,25, and 27 in Fig. 1) (Rader and Biggs, Sharp irregular contact (high-lighted with felt marking; 1975; F. Webb, 1.982, personal commun.). They are lenticular to strati- pen) between light-colored, layered, host dolomite (left! form breccias that have irregular lateral and upper contacts with host beds, and brown, fine-grained dolomite of cave fill (right). B. and sharp basal contacts (Figs. 9, 12, 13; Table 1). In outcrop, they are a Thin section showing small scour in finely laminated cave few centimetres to a few metres thick and as much as 10 m long and may nu. contain local base-metal deposits (for example, loc. 6 in Fig. 1). In the subsurface (Fig. 13C), ore-bearing breccias are 200 m long and wide and 35 m thick (Luttrell, 1966). Breccias grade upward and laterally into unaltered host rock through incipiently brecciated (fitted-fabric) dolomite (Figs. 13 A, 13B). Basal contacts have remnants of limestone beds (Fig. 12) or solution-scalloped limestone clasts. Laterally, many contacts with unaltered limestone bi:ds are corrosional, but some show soft-sediment deformation adjacent to the breccia. The breccias typically are clast-supported and poorly sorted and lack grading. The divei-sity of breccia clasts is related to the vertical extent of the breccia body. Clasts are angular (as much as 60 cm diam), and some large

INCIPIENTLY BRECCIATED DOLOMITE Figure 12. Cross section of intraformational breccia near Leaks- FILL ville, Virginia. Breccia fills cavity after dissolution of limestone, relicts DOLOMITE BRECCIA

of which occur ait base of fill. Breccia grades laterally into rocks with LIMESTONE fitted-fabric into unaltered host dolomite. Note symbols in Figures 9 HOST and 12 are different. DOLOMITE

clasts are incipiently brecciated (Fig. 13A). Matrix is medium to coarse dolomite or coarse equant calcite or dolomite cement. Origin. These breccias were originally thought to be tectonic breccias related to the late Paleozoic orogeny, but most workers now consider them to be collapse features associated with the dissolution of subsurface limestone beds during the formation of the unconformity and possibly later (Harris, 1969). Such features, termed intrastratal karst (Quinlan, 1972; Sweeting, 1972, p. 298), form by subsurface dissolution of carbonate rock. Dissolution of limestone beds in the subsurface of northwestern belts probably was caused by surface drainage from the unconformity. This is suggested by the close association of sinkholes on the unconformity surface with intraformational breccias (Harris, 1971). Also, postunconformity Middle Ordovician Blackford clastics occur in collapse breccias down to 280 m below the unconformity (Harris, 1969,1971; Kyle, 1976). There is, however, little correspondence between paleokarst features and collapse breccias in southeastern belts. Harris (1971) suggested dissolution here was caused by the movement of fresh water through a paleoaquifer (Kingsport Formation of Knox Group) with recharge from the northwest. Grover and Read (1983) suggested that during the Middle Ordovician orogeny, Knox carbonates also were exposed in tectonic highlands to the southeast, which may have provided recharge areas for meteoric waters that moved down- dip toward the northwest (cratonward). The intraformational breccias also may have formed in a zone of mixing between meteoric waters and marine pore waters. Grainstone beds in the host carbonates are tightly cemented by nonluminescent cement deposited from oxidizing meteoric waters (Mussman and others, 1985) fed from the unconformity surface. Localization of the breccias deep below the unconformity and their lesser development high in the section suggest that intense dissolution of limestone and concomitant brecciation of the dolomite interbeds may have been controlled by the zone of mixing, where the waters became undersaturated with respect to calcite. Alternatively, this localization could reflect the position of a deep ground-water table. Reten- sion of much porosity in the Ordovician breccias until the late Paleozoic is suggested by base-metal deposits, and possibly hydrocarbons, deposited from basinal fluids that probably migrated during the Late Devonian to Mississippian (compare Kyle, 1976; Grover and Read, 1983).

Detrital Carbonates Veneering the Unconformity

Elongate northeast-trending arches developed on the unconformity surface during the Middle Ordovician. These separated nonmarine facies (northwestern belts) from marine facies (southeastern belts) during Middle Ordovician transgression. They include the Tazewell Arch, Virginia (Read, 1980), and arches in Tennessee (Benedict and Walker, 1978), Georgia (Chowns and McKinney, 1980), and Alabama (Birmingham An- ticlinorium; Thomas and others, 1980). The unconformity surface in Virginia is covered by as much as 2 m of breccia or conglomerate that thickens into unconformity lows. These occur as lenses and sheets composed of lithoclasts of Knox dolomite, limestone, and chert. Those at the base of the Blackford Formation (Fig. 2) are tan to red, with a nonmarine detrital carbonate matrix (Figs. 14A, 14B). These are regoliths that locally were reworked to form fluvial- channel conglomerates. Coarse, detrital carbonates at the base of the Mosheim-New Market (Fig. 2) have a dolomite matrix of dark gray skeletal or fenestral limestone Figure 13. Intraformational breccias. Scales in cen- (Figs. 14C, 14D). Many clasts are bored, and some conglomerates have timetres. A. Outcrop photograph showing upward transi- infiltrated mud, hard grounds, and beachrock cements with meniscus and tion from incipiently brecciated fabric into host dolomite, pendant, fibrous habits. These sediments are regoliths that were reworked Leaksville, Virginia. B. Outcrop photograph showing tran- largely by marine processes during transgression, when they were admixed sition from breccia (B) to incipiently brecciated-unaltered with marine sediment. host dolomite (H), Leaksville, Virginia. Contact high- Lows on the unconformity in northwestern belts are filled by as much lighted with felt marker. C. Intraformational breccia in as 70 m of Blackford Formation detrital carbonates (Iocs. 3,12,13 in Fig. Young Mine, east Tennessee. 1). The unit consists of red and gray, sandy dolomite; quartz siltstone;

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Figure 14. Post-unconformity breccia and conglomerate. Scales in centimetres. A. Dolomite megiibrec- cia (clasts outlined in ink) in fine dolomite matrix (loc. 7 in Fig. 1). B. Thin section of chert and dolomite breccia in fine dolomite matrix, basal Blackford Formation. C. Slab of chert and dolomite breccia in fenestral limestone matrix (due south of loc. 13 in Fig. 1). Note shardlike dolomite clasts. D. Slab showing angular unconformity relations between Beekman- town dolomite and overlying New Market Limestone (loc. 27 in Fig. 1). Contact lacks breccia, and fractures are filled by limestone.

lithoclastic dolomite; and bentonitic shale (Fig. 15). Fenestral limestones Lithoclast-bearing sheets also occur in southeastern belts in the 0- to are common in upper parts of the unit, where it interfingers with transgres- 80-m-thick, cyclic peritidal New Market-Mosheim and Elway-Five Oaks sive, peritidal Elway-Five Oaks carbonates (Fig. 2). Blackford facies also Formations (Fig. 2). The lithoclastic sheets cap individual, upward- occur rarely beneath New Market-Mosheim carbonates in southeastern shallowing units and are composed of chert and dolomite lithoclasts in a belts (Iocs. 16, 20 in Fig. 1). matrix of dolomitic, skeletal, and pellet-intraclast packstone. The sheets Blackford detrital facies are fine to coarse lithoclastic dolomites and commonly rest on erosional surfaces on fenestral carbonates (Read and minor shale-siltstone (Fig. 16). Coarse lithoclastic dolomites are as much Grover, 1977), and some are reworked into basal beds of the nexi: cycle. as 15 cm thick and generally have erosional bases, and many are graded These lithoclastic sheets are supratidal deposits of intraclasts reworked (Fig. 16A). They commonly have a basal clast- or mud-supported breccia from subjacent fenestral beds, detritus transported from Knox highs, and layer composed o f angular chert and dolomite lithoclasts, grading up into a marine sediment carried across flats during storms. The sheets indicate the detrital dolomite sand (locally with faint cross-lamination) and mud- presence of Knox highs rising as much as 120 m above the local level of supported, fine dolomite caps (Figs. 16B, 16C). Clasts of chert and dolo- the unconformity during Middle Ordovician sedimentation. They can be mite generally parallel bedding, but some beds contain clasts with vertical used to determine minimum local erosional relief on the unconformity and imbricate orientations. Fine dolomite units are light tan to red and surface, in the absence of exposed paleohighs. contain floating quartz grains, lithoclasts, abundant fine detrital dolomite, faint cryptalgal lamination, and thick lamination (locally mud-cracked). EUSTATIC VERSUS TECTONIC CONTROLS ON Red to gray quartz siltstones (as much as 60 cm thick) and bentonitic KNOX UNCONFORMITY shales are interbedded with the detrital dolomites. Blackford facies are restricted nonmai ine to tidal-flat sediments that formed in lows between Data presented in Figure 17A (compiled from Ross and others, exposed Knox beds to the northwest and tidal settings to the southeast. 1982) show early Middle Ordovician time-stratigraphic relationships in Coarse lithoclastic dolomites appear to have formed as subaerial debris the United States. The Knox unconformity is well developed in southwest flows from Knox highs. Fine detrital dolomites probably accumulated on Virginia, Tennessee, and much of Alabama, where Canadian-age Knox subaerial mud flats. Siltstones are the insoluble residue from eroded Knox carbonates are unconformably overlain by Middle Ordovician formations beds, which accumulated along with wind-blown silt, whereas the benton- of latest Whiterockian or Chazyan age (Ross, 1970; Bergstrom, 1973; itic shales are volcanic ash falls. Repetski, 1982). The unconformity decreases in magnitude into northern

Figure 15. Partial columnar section of Blackford Formation (loc. 12 in Fig. 1). Section A rests on Knox Group; section B joins top of section A.

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Virginia (Suter and Tillman, 1973; Tillman, 1976; Read and Tillman, 1977), Pennsylvania, and Maryland, where the Canadian-Whiterockian to Chazyan boundary is largely conformable (Boger and Bergstrom, 1976; Savoy and others, 1981) or marked by a minor erosional break. Smith (1980) suggested that there may be an earlier unconformity lower in the Beekmantown Formation in northern Virginia, but this has not been con- firmed. From Pennsylvania to New York, the unconformity is well developed, typically separating Canadian beds from Chazyan or younger formations. In the Ozarks and Arbuckles, unconformities occur at the Canadian-Whiterockian and Whiterockian-Chazyan boundaries (Derby, 1973; Bergstrom, 1971; Ross, 1976). In New Mexico and Texas, an unconformity is developed between Canadian and post-Whiterockian beds, but sections are conformable in the Marathon region. Throughout much of western North America, there is a major unconformity between the Cana- dian and later Ordovician, with the Whiterockian interval being largely absent. The Canadian to Whiterockian, however, appears to be conformable in parts of Utah, California, and Nevada (Harris and others, 1979). From the Appalachians onto the craton, the Knox unconformity progressively bevels older beds, removing much or all of the Canadian (and Whiterockian) sequence and locally cutting into Cambrian beds (Fig. 4B) (McGuire and Howell, 1963). Overlying Chazyan beds also become younger to the west. On the craton, the unconformity is overlain by the transgressive St. Peter Sandstone of early Middle-Late Ordovician age (Sloss, 1963; Ross, 1976) and underlain by rocks of Lower Ordovician to

Figure 16. Blackford lithoclastic dolomite, Lebanon (loc. 3 in Fig. 1). A. O.WHr • fl Upward-fining lithoclastic dolomite. Note mud cracks (?) at base, containing /yjP^'*^ infiltrated detrital carbonate sediment from above. Scale in centimetres. B. •Jo. 25 MM I Thin section of lithoclastic dolomite, plane-polarized light. Note dolomite crys- ^ AJ tal with turbid core and clear overgrowth (arrow). This is characteristic of most dolomite crystals throughout Blackford Formation. C. Same area of thin section as B above, under cathodoluminescence. Detrital dolomite cores of dolomite are nonluminescent (black) and corroded and are overgrown by dull orange luminescent (Mn-rich) overgrowths. Note nonluminescent chert (C) and dolomite (D) lithoclasts.

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Figure 17. A. Time-stratigraphic columns showing distribution of Canadian, Whiterockian, and Chazyan units in the United States (data from Ross and others, 1982). Note widespread development of unconformity during Whiterockian. B. Histograms constructed by plotting time-rock units (vertical axis) from representative stratigraphic sections on various continents against frequency of occurrence (horizontal axis). Nondepositional events are shown by low frequency of occurrence of time-rock units. Sources of data mentioned in text.

Precambrian age. Locally, as in Missouri and Illinois, unconformities (Figs. 18C, 18D). Regional relations in the Appalachians thus cannot be occur in the lower Whiterockian and upper Whiterockian-Chazyan. explained by eustatic sea-level fall alone but also require some tectonic Figure 17B is a frequency plot of Ordovician measured sections influences. plotted against time for various countries (Webby and others, 1981; Dean, The Knox unconformity marked the change from a passive-margin to 1980; Barnes and others, 1981; Ross and others, 1982). The United States foreland-basin deposition behind a convergent margin (Fig. 19) (Newman, data show a major hiatus in the Whiterockian, with more pronounced 1976; Shanmugam and Walker, 1978,1980; Read, 1980). The unconfor- erosion or nondeposition in the basal and upper Whiterockian. In Canada, mity may have formed when the shelf passed over a peripheral bulge with the unconformity is developed mainly in the lower Whiterockian. In Aus- a calculated uplift of as much as 180 m and which formed during eastward tralia and the Near and Middle East, unconformity development progres- underthrusting (Jacobi, 1981; Quinlan and Beaumont, 1984). In the cen- sively increases from the Whiterockian into the Chazyan. The unconform- tral and southern Appalachians, collision appears to have been between a ity also is developed on the Russian Platform (Sloss, 1963). magmatic arc and the North American continental margin, during east- The widespread distribution of the unconformity suggests eustatic ward subduction (Hatcher, 1978; Slaymaker and Watkins, 1978; Shan- sea-level fall (compare Sloss, 1963). Tectonics also appear to have influ- mugam and Walker, 1980). This event is evidenced by a metamorphic enced its development, however. In the United States, although the uncon- peak in the Virginia and North Carolina Piedmont between 438 and 475 formity spans much or all of the Whiterockian, age relations adjacent to Ma, coincident plutonic activity, premetamorphic faulting in the Blue the unconformity vary greatly. In part, this could reflect unconformity Ridge (Hatcher, 1972, 1978; Tull, 1980), and the presence of bentonites development during simple offlap-onlap. Figures 18A and 18B show age on the Knox unconformity and throughout the Middle Ordovician se- relations that characterize the Knox unconformity in the Appalachians. quence (Laurence, 1944; Cooper and Cooper, 1946; Heyman, 1970). Note that successively more complete sections are preserved as basinal During initial collision, the Cambro-Ordovician shelf was warped, areas are approached (for example, continental margins) where subsidence producing open folds (Lowry, 1957; Woodward, 1961; McGuire and was greatest. The unconformity also passes along strike into conformable Howell, 1963; Finlayson and Swingle, 1962; Thomas and others, 1980). sequences, as in Pennsylvania, the site of an actively subsiding depocenter In the northern Appalachians, the shelf appears to have been block-

ACROSS-STRIKE SECTIONS Figure 18. Schematic diagrams showing effects of sub- • 500 KMS - sidence and uplift on unconformity formation and sedimentation. Relative ages of beds shown by letters a to i. T = transgression; R = regression. A. Regression to southeast (across strike), followed by transgression onto craton to northwest. Note how various stratigraphic relationships can characterize a single unconformity (columns 1, 2, 3). Note that in the northwest, Chazyan rocks rest on Canadian strata, whereas in the southeast, Chazyan beds may rest conformably or unconformably on Whiterockian beds. B. Similar to above, except that subsidence or transgression during the Whiterockian has caused the unconformity to be underlain and overlain by Whiterock beds. C, D. Schematic diagrams showing development of the relationships along strike in Virginia. C. Regression of the sea into northern Virginia, where continued subsidence was occurring, al- lowed a thick early Middle Ordovician sequence to accumu- SUBSIDENCE late. In southwest Virginia, however, uplift and deformation STRIKE SECTIONS resulted in deep erosion and unconformity development. D. SOUTHWEST Later, the carbonate platform in southwest Virginia was VIRGINIA rapidly downwarped, whereas northern Virginia remained at about sea level, and thus transgression occurred from south to north.

EMERGENCE

Figure 19. Schematic diagram - PASSIVE SHELF showing tectonic evolution of the Virginia Appalachians, Late Cam- brian-Middle Ordovician. Smaller diagrams at right place evolution- ary sequence in probable regional tectonic setting. A. Late Cambrian- Early Ordovician: widespread deposition of Knox Group cyclic carbonates on passive-margin carbonate shelf facing closing marginal basin. B. Early Middle Ordovician: closure of marginal basin results in leading edge of shelf becoming incorporated into accretionary prism (embryonic Blue Ridge). Warping of the shelf results in widespread emergence and unconformity development in the southern Appa- lachians and formation of a broad arch (Tazewell Arch) landward of the foredeep. This is followed by MIDDLE rapid downwarping in the south- ORDOVICIAN east and initiation of Middle Ordo- vician carbonate deposition. C. Middle Ordovician (Chazyan): continued underthrusting of North American shelf beneath accretionary prism causes rapid foundering of carbonate shelf and deposition of deeper-water basin and submarine-fan facies over earlier shallow-water carbonates. Arch remains as positive area during initial drowning of platform, ponding detrital carbonates and clastics (Blackford Formation) and possibly localizing later build-up formation on the shallow ramp.

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faulted, folded, and regionally uplifted (Zen, 1968; Klappa and others, ern United States, where a regional, latest Devonian unconformity caps 1980). Deposition on the shelf apparently was influenced by depocenters the Devonian shelf-carbonate sequence (Poole and Sandberg, 1977; Gut- in Tennessee and Pennsylvania. In the southern Appalachians, collision schick and others, 1979; Wilson and Laule, 1979). In the Persian Gulf probably occurred during or just prior to the Whiterockian. Here, regres- area, a middle Late Cretaceous unconformity separates stable shelf car- sion during the late Canadian or early Whiterockian, when sea-level low- bonates from the overlying platform carbonates and associated foreland- ering exceeded subsidence (or was amplified by uplift), caused the shelf basin clastics (compare Murris, 1980). In the Timor-Australia region, a carbonates to be exposed and eroded (Figs. 4,18A, 19). The sea retreated late Miocene-early Pliocene unconformity separates folded and block- northeastward (abng present strike) into the Pennsylvania depocenter faulted shelf carbonates from the overlying platform carbonates and (Fig. 18C), where successively more complete sequences were deposited. foreland-basin clastics (Veevers, 1971; Crostella and Powell, 1975). As sea level dropped, successively younger beds continued to be deposited Several features characterize these types of unconformities. Such un- in the Pennsylvania depocenter, where the miogeocline was less affected conformities develop on uplifted shelf carbonates of the passive-margin by collision. Here, subsidence approximately equaled sedimentation, and sequence, which may be gently folded or block-faulted. The unconformi- >1,200 m of Canadian and Whiterockian peritidal beds accumulated in ties are characterized by erosional and commonly structural relief in the contrast to only 500 m of Canadian beds preserved in the southern form of paleohighs and elongate anticlinal flexures, which may affect facies Appalachians. distribution, composition, and thickness of overlying formations. The un- After emergence in the Whiterockian, widespread transgression in the conformities are overlain by postunconformity clastics and ramp carbon- Middle Ordoviciai occurred from southwest to northeast, possibly asso- ates deposited marginal to the foreland basin, cratonward of the i:arlier ciated with global sea-level rise (Fig. 18D). In the southern Appalachians (passive) shelf margin. during Middle Ordovician (Chazyan) time, tectonic loading by thrust The unconformities act as surface recharge areas for meteoric waters sheets and rapid aixumulation of synorogenic clastics caused downwarp- that may deeply penetrate the underlying carbonates, causing widespread ing of the previously emergent shelf, causing peritidal conditions to be cementation of lime sands. The unconformity development also may lead rapidly succeeded by deep-water environments that migrated northward to widespread intraformational brecciation of interbedded limestone and (Figs. 18D, 19), possibly accompanied by formation of arches in the dolomite. These breccias are potential hosts for petroleum or base metals foreland (Tazewell Arch and Birmingham Anticlinorium). With westward that may be emplaced during burial diagenesis and expulsion of fluids transgression, successively younger beds were deposited on progressively from basinal sequences. Finally, unconformities may act as permeable older Cambro-Ordovician beds (Figs. 4, 18A). Where transgression was conduits for upland-sourced meteoric waters that may penetrate deeply initiated in earliest Chazyan time, Chazyan beds overlie the unconformity; into the subsurface, causing leaching or cementation and modifying petro- however, where locally rapid subsidence of the margin was occurring, leum reservoirs. these areas were transgressed earlier than were areas undergoing less rapid subsidence, and Whiterockian beds were deposited on the unconformity ACKNOWLEDGMENTS (Fig. 18B). With continued collision, the leading edge of the continental shelf Thanks are extended to W. D. Lowry (an early proponent of Middle was deformed, uplifted, and incorporated into the accretionary prism to Ordovician folding in the southern Appalachians) and C. G. T:.llman form the embryonx Blue Ridge (Fig. 19), which by this time had as much (deceased) for helpful discussions concerning biostratigraphy; Fred Webb, as 3 km of structural relief relative to the foreland basin (Lowry and Jr., for numerous field discussions; and colleagues George Grover, Jr., others, 1972). Knox beds probably were increasingly beveled toward the John Bova, and William Koerschner for their time and suggestions. De- embryonic Blue Ridge. This is indicated by reworked Knox detritus in tailed mapping of the unconformity at Leakesville was done with Hank later Middle Ordovician conglomerates derived from the southeast (Kell- Ross. Technical assistance was provided by R. Harris, C. Ross, S. 'W alker, berg and Grant, 1956; Lowry and others, 1972; Karpa, 1974). Similar and M. Ostrand (field and laboratory work). Financial assistance was relationships occur in New Jersey, where Middle Ordovician beds above provided largely by Grants EAR 7911213 and EAR 8108577 from the the unconformity ixmtain lithoclasts derived from all of the Lower Ordo- National Science Foundation, Earth Sciences Section, to J. F. Read and by vician units of the Beekmantown Group, suggesting a source that exposed a Grant-in-Aid from Sigma Xi, the Research Society of North America. a complete sectior. of these rocks, possibly to the east (Savoy and others, 1981). Also in Alabama, Shaw and Rodgers (1963) suggested that the unconformity truncates progressively older Cambro-Ordovician carbonates to the southeast, although Carrington (1973) argued that this contact REFERENCES CITED is a fault. In later Ordovician time, subsidence in the southern depocenter Bally, A. W., andSnelson, S., 1980, Realms of subsidence: Canadian Society of Petroleum Geologists Memoir'», p. 1-94. Barnes, C. R., Norford, B. S., and Skevington, D., 1981, The Ordovician System in Canada—Correlation chart and slowed following cessation of collision, whereas increasing convergence in explanatory notes: International Union of Geological Sciences Publication 8,27 p. the central Appala chians caused rapid downwarping of the Pennsylvania Benedict, G. L., [II, and Walker, K. R., 1978, Paleobathymetric analysis in Paleozoic sequences and its geodynamic significance: American Journal of Science, v. 278, p. 579-607. depocenter, which filled with 2,000 m of Ordovician synorogenic clastics Bergstrom, S. M., 1971, Conodont biostratigraphy of the Middle and Upper Ordovician of Europe and eastern North America, in Sweet, W. C., and Bergstrom, S. M., eds.. Symposium on conodont biostratigraphy: Geological (McBride, 1962). Society of America Memoir 127, p. 83-157. 1973, Biostratigraphy and facies relations in the lower Middle Ordovician of easternmost Tennessee: American Journal of Science, v. 273-A, p. 261-293. Bird, J. M., and Dewey, J. F., 1970, Lithosphere plate-continental margin tectonics and the evolution of the Af palachian UNCONFORMITIES AT PASSIVE- TO CONVERGENT- orogen: Geological Society of America Bulletin, v. 81, p. 1031-1060. MARGIN TRANSITIONS Boger, J. L., and Bergstrom, S. M., 1976, Conodont biostratigraphy of the upper Beekmantown Group and ti e St. Paul Group (Early and Middle Ordovician) of Maryland and West Virginia: Geological Society of Ameriia Annual Meeting Abstracts with Programs, v. 8, p. 465. Campbell, J. K., 1975, Beekmantown Formation—Middle Ordovician limestone unconformity on the northwest limb of Formation of regional unconformities on passive-margin sequences the Green Ridge anticline near Fincastle, Virginia [M.S. thesis]: Blacksburg, Virginia, Virginia Polytechni: Institute, as they become convergent is common in developing thrust-fold belts 56 p. Carrington, T. J., 1973, Metamorphosed Paleozoic sedimentary rocks in Chilton, Shelby and Talladega cou ities, Ala- (Bally and Snelsori, 1980). They occur in the Antler Orogenic Belt, west- bama: Alabama Geological Society Annual Field Trip Guidebook 11, p. 22-38.

Chowns, T. M., and McKinney, F. J., 1980, Depositional fades in Middle-Upper Ordovician and Silurian rocks of Poole, F. G., and Sandberg, C. A., 1977, Mississippian paleogeography and tectonics of the western United States, in Alabama and Georgia, in Frey, R. W., ed., Excursions in southeastern geology: American Geological Institute, Stewart, J. W., Stevens, C. H., and Fritsch, A. E., eds., Paleozoic paleogeography of the western United States: Atlanta, Georgia, p. 323-348. Society of Paleontologists and Mineralogists, Padflc Section, Padfic Coast Sodety of Economic Paleogeography Collins, J. A., and Smith, L., 197S, Zinc deposits related to diagenesis and intrakarstic sedimentation in the Lower Symposium 1, p. 67-85. Ordovician St. George Formation, western Newfoundland: Canadian Petroleum Geology Bulletin, v. 23, Poole, F. G., Sandberg, C. A., and Boucot, A. J., 1977, Silurian and Devonian paleogeography of the western United p. 393-427. States, in Stewart, J. W., Stevens, C. H., and Fritsch, A. E., eds., Paleozoic paleogeography of the Western United Colton, G. W., 1970, The Appalachian Basin; its depositional sequences and their geologic relationships, in Fisher, G. W,, States: Sodety of Economic Paleontologists and Mineralogists, Padfic Section, Padfic Coast Paleogeography and others, eds.. Studies of Appalachian geology: Central and southern: New York, Interscience, p. 5-47. Symposium 1, p. 39-65. Cooper, B. N., and Cooper, G. A., 1946, Lower Middle Ordovician stratigraphy of the Shenandoah Valley, Virginia: Quinlan, G. M., and Beaumont, C., 1984, Appalachian thrusting, lithospheric flexure, and the Paleozoic stratigraphy of the Geological Society of America Bulletin, v. 57, p. 35-113. Eastern Interior of North America: Canadian Journal of Earth Sdences, v. 21, p. 973-996. Cooper, B. N., and Diggs, W. E., 1953, Geology of the iron deposits at the Riverside Mine near Alvarado, Washington Quinlan, J. F., 1972, Karst-related mineral deposits and possible criteria for the recognition of paleokarsts: A review of County, Virginia [abs.]: Virginia Journal of Science, v, 4, p. 265-266. preservable characteristics of Holocene and older karst terranes: International Geological Congress, 24th, v. 6, Crostella, A., and Powell, D. E., 1975, Geology and hydrocarbon prospects of the Timor area: Indonesian Petroleum p. 156-167. Association Proceedings, 4-II, p. 149-171. Rader, E. K., and Biggs, T. H., 1975, Geology of the Strasburg and Toms Brook quadrangles, Virginia: Virginia Division Dawson, T. A., 1967, Knox oil may revive Hoosier hunt: Oil and Gas Journal, v. 65, p. 126-130. of Mineral Resources Report of Investigations 45, 104 p. Dean, W. T., 1980, The Ordovician System in the Near and Middle East—Correlation chart and explanatory notes: Read, J. F., 1980, Carbonate ramp-to-basin transitions and foreland basin evolution. Middle Ordovician, Virginia International Union of Geological Sciences Publication 2. Appalachians: American Association of Petroleum Geologists Bulletin, v. 64, p. 1575-1612. Derby, J. R., 1973, Lower Ordovician-Middle Ordovician boundary in western Arbuckle Mountains, Oklahoma, in 1982, Geometry, fades, and development of Middle Ordovician carbonate buildups, Virginia Appalachians: Rowland, R. T., Regional geology of the Arbuckle Mountains: Geological Society of America Annual Meeting American Assodation of Petroleum Geologists Bulletin, v. 66, no. 2, p. 189-209. Guidebook, Field Trip 5, p. 24-26. Read, J. F., and Grover, G. A., Jr., 1977, Scalloped and planar erosion surfaces, Middle Ordovician limestones, Virginia: Dodd, J. R., and Siemers, C. 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