GEOLOGY of the HARRISONBURG and BRI DGEWATER QUADRANGLES, VI RG in IA Thomas M
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VIRGINIA DIVISION OF MINERAL RESOURCES PUBLICATION 60 GEOLOGY OF THE HARRISONBURG AND BRI DGEWATER QUADRANGLES, VI RG IN IA Thomas M. Gathright, ll and Peter S. Frischmann COMMONWEALTH OF VIRGINIA DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINERAL RESOURCES Robert C. Milici, Commissioner of Mineral Resources and State Geologist CHARLOTTESVI LLE. VI RG INIA 1986 VIRGINIA DIVISION OF MINERAL RESOURCES PUBLICATION 60 GEOLOGY OF THE HARRISONBURG AND BRI DGEWATER QUADRANGLES, VI RG I N IA Thomas M. Gathright, ll and Peter S. Frischmann COMMONWEALTH OF VIRGINIA DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINERAL RESOURCES Robert C. Milici, Commissioner of Mineral Resources and State Geologist CHARLOTTESVI LLE. VI RGI NIA 1 986 FRONT COVER: Crosslaminated dolomite at the top of the Beekmantown Group (Photo locality number 5). VIRGINIA DIVISION OF MINERAL RESOURCES PUBLICATION 60 GEOLOGY OF THE HARRISONBURG AND BRI DGEWATER QUADRANGLES, VI RGINIA Thomas M. Gathright, ll and Peter S. Frischmann COMMONWEALTH OF VIRGI NIA DEPARTMENT OF MINES, MINERALS AND ENERGY DIVISION OF MINERAL RESOURCES Robert C. Milici, Commissioner of Mineral Resources and State Geologist CHARLOTTESVI LLE. VIRGINIA 1 986 DEPARTMENT OF MINES, MINERALS AND ENERGY RICHMOND, VIRGINIA O. GENE DISHNER, Director COMMONWEALTH OF VIRGINIA DEPARTMENT OF PURCHASE AND SUPPLY RICHMOND 1986 CONTENTS Page Middle limestone and dolomite unit ........................ 5 Upper limestone and dolomite unit ........................... 5 Upper Silurian through Middle Devonian rock units (undivided) ...................... I Terraee and alluvia Massanutten synclinorium and Staunton fault........... ....................... 11 Appendix. Geologic faetors affecting land modification ...I7 ILLUSTRATIONS Plate Page Geologic map of the Hamisonburg and Bridgewater quadrangles, Virginia In pocket Figure covER: cross-laminated dolomite at the top of the Beekmantown Group 1. Index map showing the loeation of the Harrisonburg (H) and Bridgewater 2. Diagram of local lithofacies in Ordovician and Cambrian rocks in the Harrisonburg 3. Limestone algal mounds in Elbrook dolomite .................... g 4. Interbedded light-gray straticulate limestone and yellowish-gray weathering dolomite with argillaceous partings of the Elbrook Formation .......................... g 6. Ribbon-banded algal limestones of the upper member of the Conococheague Formation ........................ 4 7. Boulders of residual chert weathered from the lower dolomite unit of the 8. Medium-bedded, yellowish-gray weathering, very light-gray dolomite of the upper limestone and dolomite unit of the Beekmantown Formation .............. ........................... b 9. Collapse breccia with dolomite cement filling interstiees .,................. b 10. Micrite of the New Market Formation with Ma,elarites sp. ............... ..................... 6 11. Bryozoan boundstone with black chert nodules of the Lincolnshire Formation .......................................... 6 12. Black calcareous shale of the lower member of the Edinburg Formation grading upward into interbedded shale and limestone of the Liberty Hall limestone member . 7 13. Very dark-gray to black even-bedded Liberty Hall limestone of the Edinburg Formation .................... ? 14. Zones of chaotic bedding and dispersed and unsorted intraformational limestone clasts within the even-bedded limestone of the Liberty Hall ............ .......................7 15. Cobbly weathering, nodular bedded Oranda limestone over lying the Edinburg Formation ................. 8 16. Very coarse-grained erystalline dolomite vein-like mass .......... .......... g 17. Structural elements of the Harrisonburg and Bridgewater quadrangles ............... .......... ..lz 18. Probable fault zone in Liberty Hall limestone ................ .. 1g 19. Conceptual structure section across Harrisonburg and Bridgewater quadrangles ................................. 14 GEOLOGY OF THE HARRISONBURG AND BRIDGEWATER QUADRANGLES, VIRGINIA Thomas M. Gathright, II and Peter S. Frischmann ABSTRACT The area is in the Valley and Ridge physiographic provinee and entirely within Shenandoah Valley. The Harrisonburg and Bridgewater quadrangles Maximum local relief is 1762 feet between Mas- comprise an area of approximately 117 square sanutten Peak(2922 feet) and Cub Run at the base miles in Rockingham County in northwestern Vir- of Massanutten Mountain (1160 feet), less than 9000 ginia. This area is in the western part of the feet to the south of Massanutten Peak (Harrison- Shenandoah Valley, a portion of the Valley and burg quadrangle). Generally, the area is a rolling Ridge physiographic province, and contains parts upland with local relief between 100 and 300 feet. of three major geologic structures-the Staunton Important geomorphic features that are character- and North Mountain thrust faults and the Mas- istic of the Shenandoah Valley land surface include sanutten synclinorium. linear hill systems developed on chert-and-sand- The roeks within the two quadrangles are folded bearing limestones and dolomites of the Conoeo- and faulted limestones, dolomites, shales, and sand- cheague Formation, Beekmantown Group, and Lin- stones of Paleozoic age. These rocks have been colnshire Limestone. Local conical hills such as divided into groups, formations, and members that Round Hill west of Bridgewater have developed include the Elbrook and Conococheague formations because of underlying eoncentrations of bedded (Cambrian), the Stonehenge Formation, the Beek- chert in the silieeous dolomites of the Beekmantown mantown Group, the New Market Limestone, and Group. An exception is Mole Hill, a high conical the Lineolnshire, Edinburg, and Martinsburg for- hill underlain by the basalt of an ancient voleanic mations (Ordovieian), the Massanutten Sandstone pipe. Dry River and North River flow across sand- and the Bloomsburg Formation (Silurian), and and-gravel-filled deposits in broad, shallow, steep- probably Upper Silurian and Iower and Middle sided valleys northwest of Bridgewater. Massanut- Devonian limestones and shales that are not ex- ten Mountain, part of the Massanutten mountain posed but are judged to be on Massanutten Moun- range lying along the southeast side of Shenandoah tain. Triassic diabase dikes and Eocene volcanic Valley, is underlain by the Massanutten Sandstone pipes intrude the Paleozoic sequence locally. Much and dominates the landscape. Ridge slopes gener- of the sequenee is eovered by alluvium or talus of ally have less than 20 percent grades exeept on Quaternary age. Massanutten Mountain, Mole Hill, and in the bluffs Seleeted strata of the Cambrian and Ordovician along North River and Dry River. More than 75 carbonate roek sequence are sources for road ag- percent of the area has slopes with less than 10 gregate, agricultural lime, concrete aggregate, di- percent grade and, except for urban areas, this mension stone, tettazzo, portland cement, flux portion is almost entirely agricultural land. Deep stone, and pellet lime. Some of the carbonate rocks may be potential sources of lead and zinc minerals. a-. ,'6t"o*ooon / '. Ordovician shale formations have potential for structural clay products. INTRODUCTION The Harrisonburg and Bridgewater 7.5-minute quadrangles are loeated in Rockingham County in northwestern Virginia and include the city of Har- risonburg and the towns of Dayton and Bridge- water (Figure 1). They comprise 117 square miles Figure 1. Index map showing the loeation of the and are bounded by 78o45'and 79o00'west lon- Harrisonburg (H) and Bridgewater (B) quadran- gitudes and 38o22'30"and 38o30' north latitudes. gles. VIRGINIA DIVISION OF MINERAL RESOURCES soils developed on many of the limestone and dol- omite rock units and generations of good farming practices in the Bridgewater and Harrisonburg area have made some of most produetive the ag- F ricultural land in Virginia. The regional geology of Shenandoah Valley and zo Rockingham County is deseribed in Butts, 1988, 1940; Brent, 1960; and Haek, 1965. Geologie reports containing regional data related to economic de- posits in the area include: Parrott, 1954; Gooch and others, 1960; Edmundson, Lg45: Herbert and Young, 1956; and Calver and others, 1961. Geologic factors affecting land modification are related to general land units derived from bedrock geology and/or surficial deposits. The land unit characteristies and their suitability or sensitivity to modification is evaluated with regard to slope stability, erodibility, and response to excavation or other influences. Karst lands are commonly poten- tial areas for surface subsidence and ground-water pollution. The writers wish to acknowledge the encourage- ment, concepts, and local information provided by the entire staff of the Geology Department of James Madison University. Numbers preceded by "R" in parentheses (R- 7145) correspond to rock or mineral sample local- ities; those preceded by "F" (F-1019) eorrespond to fossil sample localities (Plate). These mineral and fossil samples are on file in the repository of the Virginia Division of Mineral Resourees where they are available for examination. Norlh Mounl Fou ll STRATIGRAPHY Figure 2. Diagram of local lithofacies in Ordovi- The Paleozoic-age formations present in the quad- cian and Cambrian rocks in the Harrisonburg and rangle have been described with respect to a facies Bridgewater quadrangle areas. distribution diagram (Figure 2) lhat infers the migration of laterally and vertieally contiguous dolomite. These rocks