Geologic units in Fairfax City, Virginia

Popes Head Formation - Old Mill Branch Metasiltstone Member (Cambrian-Ordovician) at surface, covers 45 % of this area

Light-greenish-gray, pale-greenish-yellow- or yellowish-gray-weathering, medium to fine-grained graded micaceous metasiltstone and lesser fine grained micaceous metasandstone; fine-grained beds are phyllite and micaceous metasiltstone. Graded beds are 2 to 24 cm thick. Mineralogy: quartz + muscovite + biotite + plagioclase + chlorite + magnetite + epidote. The unit contains felsic metatuff (mineral assemblage quartz + plagioclase + epidote + muscovite + biotite + chlorite + green amphibole + magnetite) and mafic metatuff (mineral assemblage blue-green amphibole + plagioclase + titanite) layers up to 180 cm thick. Maximum thickness of unit is about 2300 feet; unit overlies with apparent unconformity the Mather Gorge, Sykesville, and Yorkshire Formations, the Annandale Group, and the Piney Branch Complex (Drake and Lyttle, 1981). The unit is intruded by Occoquan Granite.

Mather Gorge Formation - Schist (Proterozoic Z-Cambrian) at surface, covers 19 % of this area

Schist, greenish-gray to gray, reddish-brown-weathering, fine- to coarse-grained, lustrous, quartz-rich; and much lesser mica gneiss; contains interbedded metagraywacke and some calc-silicate rock; also contains abundant mafic and ultramafic rock debris. Typical mineral assemblages from west to east and from low to high metamorphic grade are: (1) quartz + muscovite + chlorite + plagioclase + epidote + magnetite-hematite; (2) quartz + muscovite + biotite + garnet + staurolite + plagioclase + magnetite ± andalusite; (3) quartz + muscovite + garnet + kyanite + plagioclase + staurolite + magnetite; and (4) quartz + biotite + plagioclase + sillimanite ± microcline + magnetite. Higher-grade schists are migmatitic, and in many places show effects of a retrograde metamorphic over print.

Popes Head Formation - Station Hills Phyllite Member (Cambrian-Ordovician) at surface, covers 18 % of this area

Light-greenish-gray, dusky yellow-weathering phyllite and lesser amounts of very-fine grained metasiltstone. Beds are 2 to 12 cm thick; many have thin basal intervals of graded siltstone. Mineralogy: muscovite + quartz + biotite + chlorite + plagioclase + magnetite + epidote. The top of the unit is nowhere exposed; maximum known thickness is 300 m. Some chlorite-rich phyllite is probably mafic metatuff; unit grades down into Old Mill Branch Metasiltstone.

Piney Branch Complex (Proterozoic Z-Cambrian) at surface, covers 10 % of this area

Piney Branch Complex (Drake and Morgan, 1981). Heterogeneous assemblage of metamorphosed peridotite, pyroxenite, and gabbro; dominant rock-types include serpentinite, soapstone, and actinolite schist. The unit contains dikes and sheets of plagiogranite. Secondary unit decription from USGS Geologic Names lexicon (ref. VA004): It is a tectonic melange resulting from the deformation of a layered complex that contained repetitive cycles of ultramafic and mafic rocks that are now metamorphosed to serpentinite, soapstone, actinolite schist, and metagabbro, and intruded by dikes and sheet of plagiogranite. It has a discontinuous underlying border of precursory ophiolitic melange, the Yorkshire Formation. The allochthon consisting of the Piney Branch and Yorkshire was thrust upon the Peters Creek Schist, and with it form a composite allochthon that was emplaced on the Sykesville Formation perhaps by gravity sliding. It unconformably underlies the Popes Head Formation. Movement of the Piney Branch began in the Late Proterozoic after the metamorphism of the Peters Creek, and ended during the Taconic Orogeny. Age is Late Proterozoic and (or) Early Cambrian.

Sykesville Formation (Cambrian) at surface, covers 9 % of this area

Sykesville Formation (Hopson, 1964; Drake, 1985). Light- to medium-gray, medium-grained metasedimentary melange consisting of a quartzofeldspathic matrix that contains quartz "eyes"and a heterogeneous suite of pebble- to boulder and larger-size olistoliths. These include: Mather Gorge Formation migmatite, phyllonite, and metagraywacke; also, ultramafic, metagabbroic, and felsic and mafic metavolcanic rocks, plagiogranite, and quartzite. The Sykesville is intruded by Occoquan Granite.