Within the Swift Run Formation, sericitic quartzite and metasandstone (Zsq), quartz sericite schist (Zss), sericitic phyllite (Zsp), and marble (Zsm) are differentiated. Sericitic quartzite and metasandstone occur at the base of the formation. The unconformable basement contact is not exposed. Locally, paleoregolith of weathered basement rock occurs and can be seen north of Purcell Knob (pl. 1, ref. loc. 3). The paleoregolith retains the mineralogy of the parent gneiss, but grains are rounded, and clasts of phyllite are common. Outcrops of sericitic quartzite and metasandstone are as much as 10 m thick and contain cobbles and pebbles of vein quartz, siltstone, and iron-rich sandstone (fig. 5). These rocks are best seen from north of Purcell Knob (pl. 1, ref. loc. 4) to Loudoun Heights and along Maryland State Route 17.
Quartz sericite schist grades laterally into the sericitic metasandstone and often has a protomylonitic texture. The unit is best seen in the area of the Dutchman Creek and Mt. Olivet synclines (pl. 1, ref. loc. 5), where it is distinctively different in color and texture from outcrops at the base of Short Hill Mountain.
Sericitic phyllite interbedded with thin arkose and sandstone is above the sericitic quartzite at the Purcell Knob antiformal syncline (pl. 1, ref. loc. 6). Stratigraphic relations here are obscured by polyphase deformation.
A lens of dolomitic marble less than 2 m thick marks the top of the Swift Run Formation at the Potomac River east of Short Hill Mountain. Volcaniclastic phyllite and metabasalt of the Catoctin Formation overlie the marble. Dolomitic marble is between the sericitic metasandstone and phyllite units on the limbs of the Purcell Knob antiformal syncline. The marble is found as tectonic boudin as much as 3 m thick and locally is mixed with the metasandstone. The marble is discontinuous and is interpreted to have been deposited in freshwater lakes (McDowell and Milton, 1992) rather than during a marine transgression as envisioned by Parker (1968).
The age of the Swift Run Formation is poorly constrained. It rests unconformably on the 1055+-5 Ma biotite gneiss, and it underlies the Catoctin Formation. Metarhyolite of the Catoctin Formation has a U/Pb age of 564+-9 Ma (Aleinikoff and others, 1995) (fig. 3).
Metadiabase dikes (Zmd) intrude the basement gneisses, and some also cut the Swift Run Formation. Because metadiabase dikes make up 50 percent of the basement core along the Potomac River (pl. 1, ref. loc. 7) Keith (1894) and Cloos (1951) incorrectly deduced that the granite intruded the metabasalt. The dike swarm was generated during rifting that led to continental breakup and opening of the Iapetus Ocean (Rankin, 1976). The distribution and density of dikes in this quadrangle indicate a highly extended continental crust.
The metadiabase dikes have long been interpreted as feeder dikes for the extrusive metabasalt of the Catoctin Formation (Stose and Stose, 1946), and similarities in chemical composition (table 1, figs. 6, 7, and 8) support this concept. The metadiabase ranges from aphanitic (chemical sample 14, table 1) to porphyritic (chemical sample 15, table 1) in texture, which may result from both primary cooling history and Paleozoic deformation. Everywhere their strike and dip are parallel to the Paleozoic cleavage within them and to their contacts with the basement gneiss.
A metarhyolite dike (Zrd) as much as 50 m wide and more than 14 km long intrudes the garnet monzogranite (Ygt) as well as the quartz sericite schist (Zss) of the Swift Run Formation east of Short Hill Mountain (pl. 1). This dike (chemical sample 16) can be traced south from the Potomac River into the Purcellville quadrangle (Southworth, 1995). Small metarhyolite dikes occur east of Potomac Wayside and in the extreme southeast corner of the map. The metarhyolite dikes are interpreted to be feeder dikes to rhyolite flows in the Catoctin Formation, and U/Pb data support this (Aleinikoff and others, 1995) (fig. 3). A metarhyolite dike (Zrd) to the east in the Point of Rocks quadrangle (Burton and others, 1995) has a U/Pb zircon age of 571.5+-4.7 Ma (Aleinikoff and others, 1995) (fig. 3).
Tuffaceous metasedimentary rocks (Zct) and metabasalt (Zcm) constitute the Catoctin Formation. Tuffaceous metasedimentary rocks include phyllite, schist, mud-lump (rip-up) breccia (Reed, 1955), and thin-bedded metabasalt. Metabasalt includes massive to schistose metabasalt (greenstone) and agglomeratic metabasalt breccia that contains epidosite. The metabasalt unit is locally interlayered with metasedimentary rocks that were incorporated during deposition (Reed, 1955). Metabasalt of the Catoctin Formation in central Virginia has a Rb-Sr whole-rock and pyroxene age of 570+-36 Ma (Badger and Sinha, 1988).
The Catoctin Formation is best exposed along the Potomac River at Short Hill Mountain. There, the lowermost part consists of tuffaceous phyllite and thin-bedded metabasalt that overlie marble (Zsm) of the Swift Run Formation. A 50-m section of the metabasalt consists of four belts of massive to schistose metabasalt, each ranging in thickness from 6 to 21 m. The metabasalt contains epidosite bodies, as much as 1 m in diameter, which are bounded on either side by mud-lump breccia and tuffaceous phyllite (fig. 9A) (pl. 1, ref. loc. 8). The belts of metabasalt are interpreted as flows (chemical samples 19 and 20, table 1). They form prominent ledges in the Potomac River but are absent north of the river for at least 2.5 km.
As much as 61 m of amygdaloidal, massive, aphanitic to schistose, metabasalt showing flow structures and columnar joints is exposed on Blue Ridge along the Loudoun Heights trail (chemical sample 18, table 1). Light-gray tuffaceous metarhyolite is interbedded with the metabasalt (fig. 9B). Tuffaceous metasedimentary rocks increase in abundance upward and are transitional with the phyllite of the Loudoun Formation-(-CZlp) (pl. 1, ref. loc. 9).
U.S. Geological Survey, U.S. Department of the Interior
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