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 uncon- formity 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, sub- aerial 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 be- tween 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 over- lain 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/282/3444953/i0016-7606-97-3-282.pdf by guest on 25 September 2021 ORDO VICIAN KNOX UNCONFORMITY, VIRGINIA APPALACHIANS 283 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) . ) N SHIRE age and are overlain by latest Whiterockian-earliest Chazyan limestone N LINCOLNSHIRE LINCOLN LST (Suter and Tillman, 1973; Tillman, 1976; A. Harris, personal commun.). PART R LENOIR No break is evident here on the basis of paleontologic data, yet sedimento- FIVE OAKS ORO logical evidence suggests that subaerial emergence, even of short duration D TUMBEZ CHAZYA NEW MARKET o — JIOSHEIM (LOWE -J MI (few tens to hundreds of thousands of years), formed distinctive karstic ) H J= 1 features in northern Virginia. ORDOVICIA E SUBUNCONFORMITY KNOX-BEEKMANTOWN LITHOFACIES MIDDL WHITEROCKIAN | N Upper Knox-Beekmantown beds consist of cyclic carbonates. The N 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 ORDOVICIA Figure 2. Simplified stratigraphic chart, Early and early Middle R Ordovician, Virginia. Note that "Whiterockian" is used in the re- stricted sense of Sweet and others (1971) and includes beds between IBEXIAN/CANADIA LOWE the Canadian and Chazyan. | Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/282/3444953/i0016-7606-97-3-282.pdf by guest on 25 September 2021 284 MUSSMAN AND READ 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