Calc Schist
Jacinda Saleeby
The information contained on this page was adapted from Maryland Geological Survey's Geologic Map of Maryland (1968). This information reflects geologic interpretations from over 20 years ago and do not necessarily represent an accurate interpretation of currently accepted geologic theory. We present this information for historic purposes only. Do not use this information for anything other than illustrative purposes. When a corrected and updated geologic map of Maryland is available you will see a notification on our web site.
(1) The term "Wissahickon" was first used by Bascom (1902) in Pennsylvania, then Mathews (1904) in Maryland. Southwick and Fisher (1967) recognized five formations within the Wissahickon: Lower Pelitic Schist, Boulder Gneiss, Metaconglomerate, Metagraywacke, and Upper Pelitic Schist. Crowley (1976) recognized six formations within the Wissahickon: Loch Raven Schist, Oella Formation, Piney Run Formation, Sykesville Formation, Pleasant Grove Schist, and Prettyboy Schist. The Pleasant Grove Formation, in particular, had previously been included in the Peters Creek Schist (Knopf and Jonas, 1923) or Peters Creek Formation (Knopf and Jonas, 1929b). Hopson (1964) considered the Peters Creek Formation to be a discontinuous turbidite facies within his Western Sequence of the Wissahickon (comparable to the upper pelitic schist and metagraywacke of Southwick and Fisher (1967).
Crowley, W.P., 1976, The Geology of the Crystalline Rocks Near Baltimore and Its Bearing on the Evolution of the Eastern Maryland Piedmont: Maryland Geological Survey Report of Investigations No. 27, 39 p.
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.
The subduction zone interface is a shear zone of varying thickness that defines the boundary between the subducting slab and overriding plate. The rheology of this shear zone controls several important aspects of subduction dynamics, but accurately estimating its rheology can be complex due to the wide range of subduction materials and their varying rheological properties. Of particular importance is the relative strengths of metasedimentary and metabasic rocks at various temperature and pressure conditions. To better understand these rheological contrasts in naturally deformed rocks, we are conducting field and microstructural work in the Eclogite Zone in the Tauern Window, Austria. The eclogite zone preserves intercalated metamafic (metabasalt and metagabbro) and metasedimentary (quartzite, garnet mica schist, marble and calc-schist) rocks that were subducted and exhumed to the surface as a single structural unit. Using high resolution drone imaging, 2D structural mapping, and 3D structural modeling, we have documented map-scale relationships between metamafic and metasedimentary rocks in the Eissee region near Matrei. Our mapping demonstrates that the mafic eclogites consistently define slabs, lenses and boudins of up to 2 km in along-strike length and 0.2 km in thickness, embedded within the metasedimentary units, all of which are relatively uniformly deformed to very high strain. This suggests that eclogitized metamafic rocks persisted as rheological heterogeneities within the subduction channel through both the subduction and exhumation paths. Additionally, we are using microstructural observations to document the deformation mechanisms of individual rock units and to understand the weakening mechanisms that allowed some of the eclogites to break down from boudins to strongly foliated layers intercalated with the metasediments. At the interface between select metasedimentary and eclogite units there is a marked rheological change in eclogite rheology, likely due to fluids leached from the metasedimentary rocks, resulting in strain localization and increased foliation development within eclogite layers from meter to micron length scales. Integration of our mapping, outcrop, and microstructural observations will provide insights into the length scales of rheological heterogeneity on the deep interface and large-scale geodynamics of subduction through influencing the bulk viscosity of the interface.
Rankin, D.W., Espenshade, G.H., and Shaw, K.W., 1973, Stratigraphy and structure of the metamorphic belt in northwestern North Carolina and southwestern Virginia; a study from the Blue Ridge across the Brevard fault zone to the Sauratown Mountains anticlinorium, IN Glover, Lynn, III, and Ribbe, P.H., eds., The Byron N. Cooper volume: American Journal of Science, v. 273-A, p. 1-40.
Occurs in northeast-southwest-trending belt along Blue Ridge in Carroll, Floyd, Franklin, and Patrick Cos., north-central NC, and in Allegheny, Surry, and Wilkes Cos., south-central VA. Equals laminated gneiss and schist unit of companion map I-709-A by Rankin and others (1972). Units abg = pg, aba = pa, and abs = ps of Map I-709-A. Main part of unit typically consists of finely laminated gneiss composed of quartzo-feldspathic layers a few millimeters thick separated by very thin micaceous partings; rock has pinstripe appearance. Thicker schist or phyllite and amphibolite or greenstone layers are common. Some massive gneiss layers and micaceous granule conglomerate present. Gneiss is generally more micaceous than similar units in Ashe Formation. Calcsilicate lenses locally abundant. Several parts of Alligator Back separately mapped. Unit abs consists of mica schist and phyllite typically containing garnet and magnetite, interlayered with minor biotite-muscovite gneiss and amphibolite. Unit aba consists of amphibolite, garnet amphibolite, and greenstone interlayered with biotite-muscovite gneiss and metapelite. Unit abm consists of layered impure marble and calc-schist. Age is Precambrian and (or) Paleozoic.
Source: GNU records (USGS DDS-6; Reston GNULEX).
Pegmatites from the area around Spruce Pine, NC, similar to the type intruding Alligator Back, yield an age of 350 m.y. (Lesure, 1968); therefore, minimum age of Alligator Back could be implied. Authors date gneiss and schist samples of Alligator Back using Rb-Sr methods with the following results. Location 1, sample 906 (Alligator Back Formation in Wilkes Co., NC) and its biotite separate, yields an age of 317 m.y. Location 4, sample 1156, (Alligator Back Formation in Carroll Co., VA) is a gneiss which yields a biotite-whole rock age of 329 m.y. Biotite from location 3, sample 518 (Ashe Formation of Carroll Co., VA) yields a mineral whole rock age of 337 m.y. Of note is that stratigraphically younger Alligator Back Formation does not yield significantly younger whole rock ages than Ashe Formation.
Source: GNU records (USGS DDS-6; Reston GNULEX).
Hack, J.T., 1982, Physiographic divisions and differential uplift in the Piedmont and Blue Ridge: U.S. Geological Survey Professional Paper, 1265, 49 p. [Available online from the USGS PubsWarehouse: ]
In this report, Lynchburg Group consists of typical Lynchburg, Ashe, and Alligator Back Formations. Alligator Back applied to interlayered schist-gneiss sequence containing mafic and ultramafic rocks, graphite schists, and marble that forms top of Lynchburg.
Source: GNU records (USGS DDS-6; Reston GNULEX).
Mapped in narrow belt in Caldwell Co., central NC, in Blue Ridge belt. Age refined to Late Proterozoic and (or) early Paleozoic. Consists of laminated quartzofeldspathic gneiss with thin micaceous partings and local thick lenses of phyllite, schist, and amphibolite; also includes mica schist and phyllite containing garnet and magnetite interlayered with subordinate biotite-muscovite gneiss and amphibolite.
Source: GNU records (USGS DDS-6; Reston GNULEX).
Raymond, L.A., Yurkovich, S.P., and McKinney, Marjorie, 1989, Block-in-matrix structures in the North Carolina Blue Ridge belt and their significance for the tectonic history of the southern Appalachian Orogen, IN Horton, J.W., Jr., and Rast, Nicholas, eds., Melanges and Olistostromes of the U.S. Appalachians: Geological Society of America Special Paper, 228, p. 195-215.
Alligator Back Formation is mapped as part of the Lynchburg Group. It is divided into three unnamed lithologic map units: actinolite schist, banded marble, and feldspathic metagraywacke. The actinolite schist in the Lynchburg quad. was previously mapped as part of the Catoctin Formation or the Slippery Creek Greenstone by Brown (1958). The banded marble includes the Arch Marble of Brown (1958) and the Archer Creek Formation of Espenshade (1954). Metagraywacke units here mapped as Alligator Back Formation were previously mapped as Mount Athos Formation and Pelier Schist of the Evington Group and considered to be younger than the Lynchburg Group. However, recent mapping by Henika (1991) indicates that rocks assigned to Alligator Back by Rankin and others (1973) are continuous with the upper part of the Lynchburg in the type section along the James River. They dip southeast beneath the overlying Candler Formation from the VA-NC border to the Lynchburg. Rock units immediately southeast of the Candler Formation in an outcrop belt from Stapleton on the James River, southwest to Leesville Dam on the Roanoke River, are older than the Candler and although previously mapped as upper Evington Group, these rocks are herein correlated with the Alligator Back Formation of the Lynchburg Group. Rocks southeast of the Bowens Creek fault, originally mapped as Evington are also assigned to the Alligator Back. Age of the Alligator Back is shown here as Late Proterozoic and Cambrian.
Source: GNU records (USGS DDS-6; Reston GNULEX).
The Cockeysville Marble is a Precambrian, Cambrian, or Ordovician marble formation in Baltimore, Carroll, Harford and Howard Counties, Maryland. It is described as a predominantly metadolomite, calc-schist, and calcite marble, with calc-gneiss and calc-silicate marble being widespread but minor.[1]
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