Piedmont Rock Unit Descriptions

Piedmont Rock Unit Descriptions johncallahan Thu, 06/24/2010 - 22:56

Ardentown Granitic Suite

Ardentown Granitic Suite johncallahan Mon, 07/27/2009 - 14:47

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Medium- to coarse-grained granitic rocks containing primary orthopyroxene and clinopyroxene; includes quartz norites, quartz monzonorites, opdalites, and charnockites. Feldspar phenocrysts common. Mafic enclaves locally abundant in proximity to gabbronorites.

Subtitle

Piedmont Unit

Formation Type
ardentown-granitic-suite
Geologic Unit Symbol
Sags
Geologic Time Period
Silurian

Baltimore Gneiss

Baltimore Gneiss johncallahan Tue, 07/28/2009 - 11:34

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Granitic gneiss with swirling leucosomes and irregular biotite-rich restite layers is the dominant lithology and constitutes approximately 75 to 80 percent of the exposed rocks. The remaining 20 to 25 percent comprises hornblende-biotite gneiss, amphibolite with or without pyroxene, and pegmatite. Granitic gneiss is composed of quartz, plagioclase, biotite, and microcline. Minor and accessory minerals are garnet, muscovite, magnetite, ilmenite, sphene, apatite, and zircon. The hornblende gneiss contains plagioclase, quartz, hornblende, and biotite with/without orthopyroxene. Accessory minerals are garnet, muscovite, clinozoisite, perthitic orthoclase, iron-titanium oxides, sphene, and apatite. Amphibolites are composed of subequal amounts of hornblende and plagioclase with minor quartz, biotite, clinopyroxene, and orthopyroxene.

Subtitle

Piedmont Unit

Geologic Unit Symbol
Pbgn
Geologic Time Period
Precambrian

Barley Mill Gneiss

Barley Mill Gneiss johncallahan Tue, 07/28/2009 - 10:01

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Coarse-grained, foliated tonalite gneiss. Major minerals are biotite, hornblende, plagioclase, and quartz. Includes mafic enclaves or layers composed of subequal amounts of hornblende and plagioclase. Also includes a coarse-grained granitic lithology composed of biotite, microcline, plagioclase, and quartz.

Subtitle

Piedmont Unit

Formation Type
barley-mill-gneiss
Geologic Unit Symbol
Obmg
Geologic Time Period
Ordovician

Biotite Tonalite

Biotite Tonalite rockman Wed, 06/02/2010 - 13:05

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Fine- to medium-grained, equigranular biotite tonalite usually occurring as rounded boulders. Tonalites are leucocratic (15 to 25% modal mafic minerals), light gray to buff on fresh surfaces, and locally contain mafic enclaves with reddish rims, the result of iron hydroxide staining. Possibly intrusive into the Perkins Run Gabbronorite Suite.

Subtitle

Piedmont Unit

Formation Type
biotite-tonalite
Geologic Unit Symbol
Sbt
Geologic Time Period
Silurian

Brandywine Blue Gneiss

Brandywine Blue Gneiss johncallahan Mon, 07/27/2009 - 16:37

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Medium to coarse grained granulites and gneisses composed of plagioclase, quartz, orthopyroxene, clinopyroxene, brown-green hornblende, magnetite, and ilmenite. Mafic minerals vary from

Subtitle

Piedmont Unit

Formation Type
brandywine-blue-gneiss
Geologic Unit Symbol
Obbg
Geologic Time Period
Ordovician

Bringhurst Gabbro

Bringhurst Gabbro johncallahan Mon, 07/27/2009 - 14:44

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Coarse- to very coarse-grained gabbronoite with subophitic textures. Primary minerals are plagioclase, olivine, clinopyroxene and orthopyroxene. Olivine, where present, is surrounded by an inner corona of orthopyroxene and an outer corona of pargasitic hornblende, both with spinel symplectites. The gabbronorites locally contain abundant xenoliths of mafic Brandywine Blue Gneiss. (GM13 Geologic Map of New Castle County, Delaware, Ramsey, K.W., 2005)

Subtitle

Piedmont Unit

Formation Type
bringhurst-gabbro
Geologic Unit Symbol
Sbg
Geologic Time Period
Silurian

Christianstead Gneiss

Christianstead Gneiss johncallahan Tue, 07/28/2009 - 10:08

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Coarse-grained, foliated granodioritic gneiss. Major minerals are biotite, microcline, plagioclase, and quartz. Includes thin layers of fine-grained foliated amphibolite plus large pegmatites.

Subtitle

Piedmont Unit

Formation Type
christianstead-gneiss
Geologic Unit Symbol
Ochg
Geologic Time Period
Ordovician

Cockeysville Marble

Cockeysville Marble johncallahan Tue, 07/28/2009 - 11:18

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

In Delaware, predominantly a pure, coarsely crystalline, blue-white dolomite marble interlayered with calc-schist. Major minerals in the marble include calcite and dolomite with phlogopite, diopside, olivine, and graphite. Major minerals in the calc-schist are calcite with phlogopite, microcline, diopside, tremolite, quartz, plagioclase, scapolite, and clinozoisite. Pegmatites and pure kaolin deposits and quartz occur locally.

Subtitle

Piedmont Unit

Formation Type
cockeysville-marble
Geologic Unit Symbol
Ocm
Geologic Time Period
Cambrian to Ordovician

Faulkland Gneiss

Faulkland Gneiss johncallahan Tue, 07/28/2009 - 10:10

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Predominantly fine- to coarse-grained amphibolites and quartz amphibolites with minor felsic rocks, probably metavolcanic. Major minerals are amphibole and plagioclase with or without pyroxene and/or quartz. Amphibole may be hornblende, cummingtonite, gedrite, and/or anthophyllite. Halos of plagioclase and quartz around porphyroblasts of magnetite, orthopyroxene, and garnet are common features.

Subtitle

Piedmont Unit

Formation Type
faulkland-gneiss
Geologic Unit Symbol
Ofg
Geologic Time Period
Ordovician

Iron Hill Gabbro

Iron Hill Gabbro johncallahan Mon, 07/27/2009 - 14:41

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Black to very dark green, coarse- to very coarse-grained, uralitized olivine-hypersthene gabbronorite and pyroxenite with subophitic textures. Primary minerals are calcic plagioclase, orthopyroxene, clinopyroxene, and olivine. Amphibole is secondary, a pale blue-green actinolite. Olivine, when present, is surrounded by coronas similar to those in the Bringhurst Gabbro. The gabbronorite is deeply weathered leaving a layer of iron oxides, limonite, goethite, and hematite, mixed with ferruginous jasper. The jasper contains thin seams lined with drusy quartz. Contacts with the Christianstead Gneiss are covered with sediments of the Coastal Plain.

Subtitle

Piedmont Unit

Formation Type
iron-hill-gabbro
Geologic Unit Symbol
Sihg
Geologic Time Period
Silurian

Metapyroxenite and metagabbro (undifferentiated)

Metapyroxenite and metagabbro (undifferentiated) johncallahan Tue, 07/28/2009 - 10:57

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Light-colored coarse-grained rocks composed of interlocking grains of light colored, fibrous amphiboles, most likely magnesium-rich cummingtonite and/or anthophyllite with possible clinochlor. These rocks become finer grained and darker as hornblende replaces some of the Mg-rich amphiboles. Associated with the metapyroxenites are coarse-grained metamorphosed gabbros composed of hornblende and plagioclase. The metapyroxenites and metagabbros are probably cumulates.

Subtitle

Piedmont Unit

Formation Type
metapyroxenite-metagabbro
Geologic Unit Symbol
mpg
Geologic Time Period
Cambrian to Ordovician

Mill Creek Metagabbro

Mill Creek Metagabbro johncallahan Tue, 07/28/2009 - 09:56

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary minerals are hornblende and plagioclase.

Subtitle

Piedmont Unit

Formation Type
mill-creek-metagabbro
Geologic Unit Symbol
Omcm
Geologic Time Period
Ordovician

Montchanin Metagabbro

Montchanin Metagabbro johncallahan Tue, 07/28/2009 - 09:58

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary igneous minerals include olivine, clinopyroxene, orthopyroxene, and plagioclase.

Subtitle

Piedmont Unit

Formation Type
montchanin-metagabbro
Geologic Unit Symbol
Omm
Geologic Time Period
Ordovician

Pegmatite

Pegmatite johncallahan Tue, 07/28/2009 - 10:29

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Coarse- to very coarse-grained granitic pegmatite with tourmaline crystals locally. Where outcrop is present, pegmatite is tabular and concordant with the regional trend of the underlying Wissahickon Formation. Lenticular xenoliths of Wissahickon gneisses occur locally in the pegmatite.

Subtitle

Piedmont Unit

Formation Type
pegmatite
Geologic Unit Symbol
p
Geologic Time Period
lower Paleozoic

Perkins Run Gabbronorite Suite

Perkins Run Gabbronorite Suite johncallahan Mon, 07/27/2009 - 16:34

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Fine- to coarse-grained gabbronorite and minor diorite with subophitic to ophitic textures, variably foliated or lineated. Plagioclase, orthopyroxene, clinopyroxene, and hornblende are major minerals; biotite and olivine locally present. Olivine typically surrounded by corona structures as described for the Bringhurst Gabbro. Contemporaneous with the Ardentown Granitic Suite.

Subtitle

Piedmont Unit

Formation Type
perkins-run-gabbronorite-suite
Geologic Unit Symbol
Spgs
Geologic Time Period
Silurian

Rockford Park Gneiss

Rockford Park Gneiss johncallahan Tue, 07/28/2009 - 09:53

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Fine-grained mafic and fine- to medium-grained felsic gneisses interlayered on the decimeter scale. Layers are laterally continuous, but mafic layers commonly show boudinage. Felsic layers are composed of quartz and plagioclase with

Subtitle

Piedmont Unit

Formation Type
rockford-park-gneiss
Geologic Unit Symbol
Orpg
Geologic Time Period
Ordovician

Serpentinite

Serpentinite johncallahan Tue, 07/28/2009 - 11:03

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Massive fine-grained dark to light yellow-green serpentinite. Contacts with the Wissahickon Formation are not exposed.

Subtitle

Piedmont Unit

Formation Type
serpentinite
Geologic Unit Symbol
s
Geologic Time Period
Cambrian to Ordovician

Setters Formation

Setters Formation johncallahan Tue, 07/28/2009 - 11:28

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

In Delaware, predominantly an impure quartzite and garnet-sillimanite-biotite-microcline schist. Major minerals include microcline, quartz, and biotite with minor plagioclase, and garnet. Muscovite and sillimanite vary with metamorphic grade. Accessory minerals are iron-titanium oxides, zircon, sphene, and apatite. Microcline is an essential constituent of the quartzites and schists and serves to distinguish the Setters rocks from the plagioclase-rich schists and gneisses of the Wissahickon Formation.

Subtitle

Piedmont Unit

Formation Type
setters
Geologic Unit Symbol
Osq
Geologic Time Period
Cambrian to Ordovician

Windy Hills Gneiss

Windy Hills Gneiss johncallahan Tue, 07/28/2009 - 10:12

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Thinly interlayered, fine- to medium-grained hornblende-plagioclase amphibolite, biotite gneiss, and felsic gneiss, possibly metavolcanic. Felsic gneisses contain quartz and plagioclase with or without microcline with minor pyroxene and/or hornblende and/or biotite. Metamorphic grade in this unit decreases from granulite facies in the northeast to amphibolite facies toward the southwest. Correlated with the Big Elk Member of the James Run Formation in Cecil County, Maryland.

Subtitle

Piedmont Unit

Formation Type
windy-hill-gneiss
Geologic Unit Symbol
Owhg
Geologic Time Period
Ordovician

Wissahickon Formation

Wissahickon Formation johncallahan Tue, 07/28/2009 - 10:36

The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.

Interlayered psammitic and pelitic gneiss with amphibolite. Psammitic gneiss is a medium- to fine-grained biotite-plagioclase-quartz gneiss with or without small garnets. Contacts with pelitic gneiss are gradational. Pelitic gneiss is medium- to coarse-grained garnet-sillimanite-biotite-plagioclase-quartz gneiss. Unit has a streaked or flasered appearance owing to the segregation of garnet-sillimanite-biotite stringers that surround lenses of quartz and feldspar. Throughout, layers of fine to medium-grained amphibolite composed of plagioclase and hornblende, several inches to

Subtitle

Piedmont Unit

Formation Type
wissahickon
Geologic Unit Symbol
Owf
Geologic Time Period
Cambrian to Ordovician