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Uppermost Cambrian and Lowest Ordovician Cowodont and Trilobite Biostratigraphy in Northwestern Virginia

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Uppermost Cambrian and Lowest Ordovician Cowodont and Trilobite Biostratigraphy in Northwestern Virginia May 1988 No. 2 UPPERMOST CAMBRIAN AND LOWEST ORDOVICIAN COWODONT AND TRILOBITE BIOSTRATIGRAPHY IN NORTHWESTERN VIRGINIA Randall C. Orndotffl, John F. Taylor2, and Richard W. Traut2 The Shenandoah Valley in Virginia is part of the ex- tensive Great Valley section of the Valley and Ridge physiographic province. The Great Valley is eroded, folded Cambrian through Middle Ordovician carbonate rocks and Upper Ordovician fine-grained siliciclastic lithologies. Studies by Sando (1957, 1958) established the stratigraphic position of the Cambrian-Ordovician boundary in the Great Valley carbonates of south-cen- tral Pennsylvania and western Maryland, but the po- sition of this systemic boundary in Virginia has not been documented. The preliminary results of a biostrati- graphic study utilizing conodont and trilobite faunas to determine more precisely the position of the Cambrian- Ordovician boundary in the northwestern Virginia area are reported. This is a joint study and researchers are listed alphabetically; Orndorff is responsible for the conodont work, and Taylor and Traut are responsible for the trilobites. Two sections in the Shenandoah Valley were mea- sured and sampled (Figure l).Conodonts and trilobites were recovered from a section along Narrow Passage Creek, 3 km southwest of Woodstock, Shenandoah lDepartment of Geological Sciences, Old Dominion Uni- @ versity, Norfolk, Virginia 23508; present address: U.S. Geological Survey, E-501 U.S. National Museum, Wash- Figure 1. Generalized geologic map of part of north- ington, D.C. 20560. western Virginia showing the location of sections stud- 2Geoscience Department, Indiana University of Penn- ied: NPC-Narrow Passage Creek, T-Tirnberville (mod- sylvania, Indiana, Pennsylvania 15705. itied from Hack, 1965). 14 VIRGINIA DIVISION OF MINERAL RESOURCES Vol. 34 County. A second section along Honey Run, 4 km north- are insufficient to establish firmly whether the occur- west of Timberville, Rockingham County, was sampled rence of the thick-bedded limestones in northern Vir- for conodonts; no trilobites were collected. Structurally, ginia is approximately the same stratigraphic level as the sections lie west of the Massanutten synclinorium the base of the massive limestones in Maryland and and east of the North Mountain fault. The Narrow Pas- Pennsylvania. It is possible, perhaps even likely, that sage Creek section is on the east limb of the Mount the thick-bedded lithology is a biohermal facies that has Jackson anticline. The Timberville section is eastward- its lowest occurrence at significantly different strati- dipping strata on the hanging wall of the North Moun- graphic levels in different areas within the Great Valley. tain fault. Our objectives were to determine the lithostrati- graphic position of the Cambrian-Ordovician boundary BIOSTRATIGRAPHY in northwestern Virginia and to document the succes- sion of conodont and trilobite faunas through the bound- Figure 2 shows the conodont- and trilobite-based ary interval in this region for comparison with the biostratigraphic units recognized in upper most Cam- successions observed elsewhere in North America. brian and Early Ordovician strata in North America. Conodont Conodont Trilobite Trilobite Series zones subzones subzones zones LITHOSTRATIGRAPHY Bellefontia chsrnberlaini I .-5 The strata sampled in this investigation represent the .-m Bellefontia 2 upper part of the Conococheague Formation and the 0 ,o 2 m coiiieana lower part of the Stonehenge Formation. The lithologi- c 2 g a -o X m Xenostegiurn pp cally heterogeneous Conococheague Formation in- Y cludes limestone, dolostone, sandstone, and siltstone. tranklinense I Symphysurina These lithologies occur in 5- to 12-meter-thick cycles z woosteri interpreted to represent progressively shallower-water Q: 0 m Syrnphysurina environments. Subtidal carbonates, including oolitic > m 0 m Upper Part c C butboss .-.- lime grainstone, intraclastic lime grainstone ("edgewise n a a a (O - a conglomerate"), and thrombolitic or stromatolitic lime 0 Y Lower part h C u P boundstone, occur low in the cycles. Laminated dolo- Clavohamulus mite andlor sandstone representing peritidal conditions Syrnphysurina 5 hintzei cn cap the cycles (Demicco and Mitchell, 1982). S a* a Laminated dolomite and oolitic lime grainstone and > Hirsutodontus m brevispicata sandstone common in the Conococheague Formation 2 simplex a are rare to absent in the lower part of the Stonehenge a Clavoharnulus a .-(0 in the Great Valley. A lithology common in the Stone- 'FI elongatus Mis~isquoia 0 0 a - 0 henge, but not present in the Conococheague, is thick- ,, Fryxellodontus 0 typicalis .-0 to very thick-bedded gray to dark gray limestone with .- inornatus a 0 a 0 .- Missisquoia crinkly siliceous laminations. Small algal bioherms sep- Hirsutodontus I arated by channels filled with skeletal and intraclastic depressa lime grainstone are common within the thick-bedded hirsutus Eurekia apopsis Carnbrooistodus lithology. We tentatively place the Conococheague- Z Saukiella serotina Stonehenge contact in our sections at the base of the minutus lowest interval of thick-bedded limestone with crinkly Eoconodontus siliceous laminations. This occurs 108 meters above the notchpeakensis Saukiella junia U) base of the Narrow Passage Creek section and 82 me- Proconodontus W 0 ters above the base of the Timberville section. Sando 2 rnuelieri (1957) similarly utilized the base of the "massive algal n P ~roconod~nt~~Rasettia mama beds" as the base of the Stonehenge in mapping the Great Valley in Maryland, although he subsequently posterocostatus (Sando, 1958) lowered this contact, adding approxi- v mately 61 meters of thin-bedded, impure limestone to Figure 2. Conodont and trilobite biostratigraphic zones the base of the formation as the Stoufferstown Member. in the Upper Cambrian and Lower Ordovician of North The limited biostratigraphic data available at present America (after Miller and others, 1982). No. 2 VIRGINIA MINERALS 15 @ Ross (1951) and Hintze (1952) described Lower Ordo- Proconodontus Zone: Conodont collections from the vician trilobite faunas from the Basin and Range Prov- Proconodontus Zone characteristically contain species ince of the western U.S. and placed the Cambrian- of Proconodontus and Cambrooistodus, two genera Ordovician boundary at the base of the Symphysurina that do not range upward into the C. proavus Zone or Zone. Subsequently, Winston and Nicholls (1967) re- younger strata. A possible exception is Proconodontus ported an older Ordovician fauna from central Texas muelleri Miller which, in a few instances, has been re- and lowered the systemic boundary to the base of the ported in association with C. proavus faunas (J. F. newly defined Missisquoia Zone. This horizon pres- Miller, written communication, 1986). Proconodontus ently is used as the Cambrian-Ordovician boundary in serratus Miller and species of Cambrooistodus do not North America (Stitt, 1971, 1977; Taylor and Halley, occur in rocks younger than the Proconodontus Zone. 1974; Miller and others, 1982). Fortey (1983) proposed Conodont collections from the Narrow Passage Creek that Missisquoia Shaw is a junior synonymy of Par- section (NPC) allow recognition of the two youngest akoldinioidia Endo. Westrop (1986), however, consid- subzones of the Proconodontus Zone. Samples from ered this synonymy premature given the limited infor- NPC-5 to NPC-58 (5 to 58 meters above the base of the mation presently available concerning morphologic section) yielded conodonts characteristic of the Eocon- variability within Parakoldinioidia. We share Wes- odontus notchpeakensis Subzone of the Proconodontus trop's opinion that a decision regarding the relationship Zone (Figure 3). The lowest occurrence of Cambroois- of Parakoldinioidia and Missisquoia should be de- todus minutus (Miller) at NPC-58.5 marks the local ferred pending additional study of type material of Par- base of the overlying Cambrooistodus minutus Sub- akoldinioidia. Consequently, we employ the more fa- zone. The C. minutus Subzone is at least 4.5 meters miliar name, Missisquoia, for the genus in question and thick at Narrow Passage Creek. the biostratigraphic units characterized by species of that genus. Cordylodus proavus Zone: The lowest occurrence of As shown in Figure 2, the base of the trilobite Mis- Cordylodus proavus Muller, Hirsutodontus hirsutus sisquoia Zone falls within the lowest subzone of the Miller, or both, defines the base of the C.' proavus Zone. conodont Cordylodus proavus Zone. The base of the C. The base of the C. proavus Zone in the Narrow Passage proavus Zone, which corresponds with the base of the Creek section occurs 64 meters above the base of the Eurekia apopsis trilobite Subzone, provides a good ap- section, 44 meters below the Conococheague-Stone- proximation of the Cambrian-Ordovician boundary in henge contact as defined in this study. Conodonts from sections or intervals where trilobites are scarce or lack- the base of the Timberville section, 75 meters below the ing. Similarly, the base of the Hirsutodontus simplex Conococheague-Stonehenge contact, include H. hirsu- Subzone of the C. proavus Zone is only slightly younger tus indicating that the base of the Cordylodus proavus than the base of the Symphysurina Zone. We use the Zone lies below the base of that section. lowest occurrence of Utahconus utahensis (Miller), a Miller (1978, 1980) recognized five subzones within conodont species that does not occur below the base of the C. proavus Zone. The small size of many collections
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