U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP MF-2173 U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF THE LOUDOUN COUNTY, VIRGINIA, PART OF THE HARPERS FERRY QUADRANGLE By C. Scott Southworth INTRODUCTION Antietam Quartzite constituted the Chilhowee Group of Early Cambrian age. Bedrock and surficial deposits were mapped in 1989-90 as part For this report, the Loudoun Formation is not recognized. As of a cooperative agreement with the Loudoun County Department used by Nickelsen (1956), the Loudoun consisted primarily of of Environmental Resources. This map is one of a series of geologic phyllite and a local uppermost coarse pebble conglomerate. Phyl- field investigations (Southworth, 1990; Jacobson and others, lite, of probable tuffaceous origin, is largely indistinguishable in the 1990) in Loudoun County, Va. This report, which includes Swift Run, Catoctin, and Loudoun Formations. Phyl'ite, previously geochemical and structural data, provides a framework for future mapped as Loudoun Formation, cannot be reliably separated from derivative studies such as soil and ground-water analyses, which phyllite in the Catoctin; thus it is here mapped as Catoctin are important because of the increasing demand for water and Formation. The coarse pebble conglomerate previously included in because of potential contamination problems resulting from the the Loudoun is here mapped as a basal unit of the overlying lower recent change in land use from rural-agricultural to high-density member of the Weverton Formation. The contact of the conglom­ suburban development. erate and the phyllite is sharp and is interpreted to represent a Mapping was done on 5-ft-contour interval topographic maps major change in depositional environment. Further studies are (Loudoun Co., Va., unpub. maps, 1985) at a scale of 1:12,000, needed to determine if use of the Loudoun Formation should be except in areas of complex structure on Blue Ridge where mapping restricted. was done at a scale of 1:6,000. The geology was then compiled on a U.S. Geological Survey base at 1:24,000 scale; some contacts, GEOLOGIC SETTING therefore, may not match topography as shown. Some linear features, such as fracture traces and faults, were interpreted from Rocks in this area that are interpreted to have fluvial, volcanic, or stereoscopic infrared aerial photographs. volcaniclastic {tuffaceous metasediments) origin are both hetero­ The map covers the Loudoun County, Va., part of the Harpers geneous and discontinuous along strike. Some rock units abruptly Feiry 7.5' quadrangle. The area is underlain by rocks of the pinch and swell in thickness, and unconformities are common. For twrifieast-pkingpig Blue Ridge South Mountain anticlinorium, a example, the lower member of the Weverton Formation is in late Paleozoic ADeghanian structure (fig. 1). Middle Proterozoic contact with quartz syenite at Purcell Knob. Units that are absent granitoids intruded by Late Proterozoic metabasalt and metarhyo- here increase greatly in thickness to the north where unique rock Irte dikes and sills and Jurassic diabase dikes form the core of the types, such as sericitic phyllite and arkose of the Swift Run anticlinorium and underlie the area's broad valleys. Outcrops of Formation, are abundant. Field studies have not determined Middle to Late Proterozoic rocks are best exposed along Dutchman whether this is the result of postdepositional erosion (Jonas and Creek and in the bluffs of the Potomae River. Late Proterozoic Stose, 1939) or deposition on a paleotopograpl ;c surface of metasedimentary and metavolcanic rocks and Lower Cambrian moderate relief (Nickelsen, 1956). Unit thicknesses are estimates metasedimentary rocks underlie the high ground of Blue Ridge and based on outcrop width of highly cleaved and often folded rock Short HiD Mountain where topographic reMef ranges from 100 to and are only approximate. Where possible, thicknesses were 300 m (300 to 1,000 ft). Alluvium is found along Piney Run and measured in areas of minimal deformation. along Dutchman Creek in the valley east of Short Hill Mountain. The Swift Run Formation lies unconformably on basement rocks Colluvium is abundant in the mountainous areas. Numerous and grades conformably up into tuffaceous metasedimentary rocks springs in the map area indicate that Blue Ridge and Short Hill of the Catoctin Formation. In the Catoctin, tuffaceous metasedi­ Mountain are important recharge areas for ground water. mentary rocks lie both above and below extrusive metabasalt and grade up into phyllite. A probable hiatus and period of erosion STRATIGRAPHIC NOMENCLATURE proceeded the onset of the Chilhowee Group deposition. Keith (1894) established the stratigraphy of the area, which The rock types are interpreted to reflect changes in depositional contains the type localities of the Catoctin Schist, Loudoun For­ environments related to the change from the rift-to-drift sedimen­ mation, Weverton Sandstone, and Harpers Shale. He considered tary regime. Clastic fluvial sediments of the Swift Pun mark the the Catoctin to be basement and the Loudoun Formation to beginning of rifting of Grenville basement to form the lapetus include all rocks beneath the Weverton. Jonas and Stose (1939) Ocean (Rankin, 1976). Intrusive metabasalt and meterhyolite dikes and Stose and Stose (1946) thought that the Catoctin Metabasalt and sills and extrusive metabasalt were emplaced during the main intruded and overlaid the basement rocks; they subdivided the phase of continental rifting. Conglomerate of the lower member of Loudoun into the Swift Run Tuff, Catoctin Metabasalt, and the Weverton Formation is rift facies, and quartziter of the lower Loudoun Formation. Nickelsen (1956), who first mapped this area member of the Weverton Formation were deposited in a rift-to-drift in detail, adopted this stratigraphy and considered the Swift Run transition. Conglomeratic quartzite of the upper member is another and Catoctin Formations to be of Precambrian age and that the rift facies. The drift regime was established during deposition of the Loudoun Formation, Weverton Quartzite, Harpers Formation, and Harpers Formation. MIDDLE PROTEROZOIC ROCKS Garnet Monzogranite The oldest rocks underlying the study area are granitoids of Garnet monzogranite (Ym) is the most abundant basement unit. probable intrusive origin. Four distinctive units are recognized: Garnet monzogranite is well exposed along the Potomac River hornblende quartz monzonite, biotite granodiorite gneiss, garnet from Potomac Wayside to just west of Dutchman Creek (ref. loc. monzogranite, arid quartz syenite. Most rocks, however, are mon- 4). South of Elvan, the garnet monzogranite has much biotite and zogranite (fig. 2). Primary textures and minerals have been sub­ little garnet. Cataclastic mylonite and augen gneiss are locally stantially altered by deformation and retrograde metamorphism. present, but in general this unit is massive. The massive nature of this unit and the lack of foliation implies that it may be intrusive into Hornblende Quartz Monzonite the hornblende quartz monzonite (Yhm) and biotite granodiorite Hornblende-bearing quartz monzonite (Yhm) is very resistant to gneiss (Yg). Almandine, biotite, and spihene suggest amphibolite- weathering and underlies the south half of Loudoun Valley and the facies metamorphism during the Middle Proterozoic Grenville Purcell Knob synformal anticline (ref. loc. 7). The contact between orogeny. Altered plagioclase and garnet replaced by chlorite and the hornblende quartz monzonite and the garnet monzogranite sericite indicate retrograde metamorphism to greenschist facies (Ym) is not exposed but trends northwestward. Hornblende during the Paleozoic. Blebs of chlorite define a lineation on the crystals characterize the unit, which is generally massive and has a schistosity planes where the unit is most affected by greenschist few zones of mylonitic foliation. Dark hornblende and opaque metamorphism and deformation. At these places the rock has a minerals define the foliation that is cut at a high angle by Paleozoic greenish hue and greasy luster. schistosity, which is characterized by chlorite, epidote, and sericite Light-brown (SYR 5/6) gneiss that has bands of almandine, clots aggregates. Part of the unit is gneissic and contains perthite and of chlorite, and books of graphite is found near Dutchman Creek oligoclase augen. Hornblende, biotite, pyroxene (hypersthene?), and resembles a paragneiss. Outcrops are sparse, so the gneiss is and perthite suggest hornblende-granulite facies metamorphism not mapped separately. during the Middle Proterozoic Grenville orogeny. The unit is Quartz Syenite lithologically correlative with the Pedlar Formation of the Blue Ridge province. The Pedlar intrudes the 1,138-Ma Old Rag Quartz syenite (Ys) is areally restricted to the east limit of the Granite (Gathright, 1976); therefore, if this correlation is correct, Purcell Knob antiformal syncline (ref. loc. 1) in Loudoun Valley the quartz monzonite probably was emplaced during the Grenville and an area west of Mt. Olivet Church (ref. loc. 2). This unit is event (Clarke, 1984). adjacent to hornblende quartz monzonite (Yhm) in Loudoun Valley, but no amphibole or pyroxene and only sparse biotite are Biotite Granodiorite Gneiss present in the quartz syenite. The quartz syenite, which may be Biotite granodiorite gneiss (Yg) is well exposed along the intrusive to both the hornblende quartz monzonite and the garnet Potomac River (ref. loc. 5) where it is intruded by numerous dikes monzogranite (Ym), is restricted