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Mineral Resources > Online Spatial Data > Geology > by state > Pennsylvania Geology

Geologic units in Delaware county, Pennsylvania

Mafic gneiss (Probably lower Paleozoic) at surface, covers 5 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to pCAmgp in places.

Lithology: mafic gneiss; paragneiss

Wissahickon Formation (Probably lower Paleozoic) at surface, covers 26 % of this area

Wissahickon Formation - Includes oligoclase-mica schist, some hornblende gneiss, some augen gneiss, and some quartz-rich and feldspar-rich members due to various degrees of granitization.

Lithology: mica schist; mafic gneiss; augen gneiss; granitoid

Mafic gneiss (Precambrian) at surface, covers 2 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgh" in places.

Lithology: mafic gneiss; paragneiss

Granitic gneiss and granite (Probably lower Paleozoic) at surface, covers 2 % of this area

Granitic gneiss and granite - Includes Springfield Granodiorite (granitized Wissahickon) in Philadelphia area.

Lithology: granitic gneiss; granite; granodiorite

Chickies Formation (Cambrian) at surface, covers < 0.1 % of this area

Chickies Formation - Light-gray, hard, massive, Scolithus-bearing quartzite and quartz schist; thin, interbedded dark slate at top; conglomerate (Hellam Member) at base.

Lithology: quartzite; schist; slate; conglomerate

Octoraro Formation (Probably lower Paleozoic) at surface, covers 0.6 % of this area

Octoraro Formation - Includes albite-chlorite schist, phyllite, some hornblende gneiss, and granitized members.

Lithology: schist; phyllite; gneiss; granitoid

Potomac Formation (Upper Cretaceous, lower Cenomanian) at surface, covers < 0.1 % of this area

Potomac Formation - Predominantly clay to clay-silt, thinly laminated to thick-bedded, mottled red, white, and orange-brown, less commonly dark-gray and woody; interbedded with thin beds and lenses of very fine to medium-grained, massive, white to orange-brown, micaceous sand. Lithologies are typical of the shallow subsurface. Down dip, these lithologies interfinger with thin to thick beds of marine clay-silt, commonly glauconitic and locally shelly. Marine beds are most prevalent in the southernmost part of the southern sheet. Unit 3 was cored in its entirety at Freehold where it is approximately 75 m (246 ft) thick. In the core, the basal 6 m (20 ft) consists of red or mottled red and white clay interbedded with gravel and fine- to coarse-grained sand. The clay is pervaded by reddish-brown siderite. Most of the overlying beds consist of interbedded dark-colored clay, locally weathered to pale yellow or white, and fine- to medium-grained, light-colored sand. Layers that contain fine black carbonaceous material to large lignitized wood pieces are common in unit 3 in this core. At Toms River, the unit is about 60 m (197 ft) thick and consists of dark- to pale-gray clay, locally weathering to white or yellowish gray, and light-colored, micaceous sand. In general, the darker colored clay is more common in the upper part of the section. Locally, the sand has very small amounts of glauconite which may indicate some local marine influence during sedimentation. The age of unit 3 was determined from pollen in the nonmarine deposits and foraminifera in the marine sections. Typical forms found in Zone III in New Jersey are Ajatipollis sp. A, Tricolpites nemejci, T. vulgaris, Tricolporoidites bohemicus, Tricolporoidites sp. A, T. sp. B, and Tricolporopollenites sp. B (Doyle and Robbins, 1977). In the marine facies, Petters (1976) reports a planktic foraminiferal suite containing Praeglobotruncana delrioensis and Rotalipora greenhornensis. Both the pollen and foraminiferal assemblages suggest an early Cenomanian age.

Lithology: clay or mud; silt; sand

Pegmatite (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Pegmatite - Coarse to medium grained, granitic; contacts range from sharp to narrowly gradational; some zoning in places.

Lithology: pegmatite

Felsic gneiss (Precambrian) at surface, covers 8 % of this area

Felsic gneiss - Light, medium grained; includes rocks of probable sedimentary origin.

Lithology: felsic gneiss; paragneiss

"Glenarm Wissahickon" formation (Probably lower Paleozoic) at surface, covers 7 % of this area

"Glenarm Wissahickon" formation - Lithologically similar to oligoclase-mica schist of the Wissahickon Formation (PZw), but also includes lenticular amphibolite bodies having ocean-floor basalt chemistry.

Lithology: mica schist; mafic gneiss; amphibolite;

Conestoga Formation (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Conestoga Formation - Light-gray, thin-bedded, impure, contorted limestone having shale partings; conglomeratic at base; in Chester Valley, includes micaceous limestone in upper part, phyllite in middle, and alternating dolomite and limestone in lower part.

Lithology: limestone; phyllite; conglomerate; dolostone (dolomite); shale

Felsic and intermediate gneiss (Precambrian) at surface, covers 18 % of this area

Felsic and intermediate gneiss - Light, medium grained; includes rocks of probable sedimentary origin.

Lithology: felsic gneiss; mafic gneiss; paragneiss

Trenton Gravel (Quaternary) at surface, covers 10 % of this area

Trenton Gravel - Gray or pale-reddish-brown, very gravelly sand interstratified with crossbedded sand and clay-silt beds; includes areas of Holocene alluvium and swamp deposits.

Lithology: sand; clay or mud; silt; gravel; alluvium

Anorthosite (Probably lower Paleozoic) at surface, covers 2 % of this area

Anorthosite - Medium to coarse grained, light to dark bluish gray, predominantly plagioclase; local alteration minerals.

Lithology: anorthosite

Ultramafic rocks (Probably lower Paleozoic) at surface, covers 3 % of this area

Ultramafic rocks - Includes serpentine, steatite, and other products of alteration of peridotites and pyroxenites.

Lithology: serpentinite; pyroxenite; peridotite

Mafic gneiss (Precambrian) at surface, covers 2 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgp" in places.

Lithology: mafic gneiss; paragneiss

Mafic gneiss (Probably lower Paleozoic) at surface, covers 2 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to pCAmgh in places.

Lithology: mafic gneiss; paragneiss

Bryn Mawr Formation (Tertiary) at surface, covers 3 % of this area

Bryn Mawr Formation - High-level terrace deposits; reddish-brown gravelly sand and some silt. Age uncertain.

Lithology: gravel; sand; silt

Pensauken and Bridgeton Formations, undifferentiated (Tertiary) at surface, covers 9 % of this area

Pensauken and Bridgeton Formations, undifferentiated - Dark-reddish-brown, cross-stratified, feldspathic quartz sand and some thin beds of fine gravel and rare layers of clay or silt.

Lithology: sand; gravel; clay or mud; silt