Reports
Geological interpretations of aeromagnetic maps of the crystalline rocks in the Appalachians, Northern Virginia to New Jersey
1979, Fisher, G.W., Higgins, M.W., and Zietz, I.
Report of Investigations 32
Abstract
Previously published aeromagnetic data have been compiled at a scale of 1:250,000 for the northern Piedmont of the Appalachians by removing the International Geomagnetic Reference Field (E.B. Fabiano and N.W. Peddie, 1969, ESSA, Coast and Geodetic Tech. Rept. 38), selecting 100-λ contours. The resulting aeromagnetic map has been colored to portray field intensities more graphically; it clearly reflects many known geologic features, and provides valuable insight into several long-standing problems of Piedmont geology.
In the areas underlain by upper Precambrian (1 b.y.-old gneisses), the aeromagnetic maps provide information on the three-dimensional form of the contact between the gneisses and younger rocks. In the Reading Prong, the older Precambrian gneisses are associated with strong magnetic highs, and contrast markedly with adjacent Paleozoic sedimentary rocks, which are essentially nonmagnetic. The map patterns and profiles of the anomalies both support the interpretation that the Reading Prong is an extensive nappe that is refolded about northeast-plunging axes, and has its overturned limb exposed in synforms at the southwestern end of the Reading Prong and its upright limb exposed in antiforms in the northeastern Reading Prong.
The older Precambrian gneiss in the cores of the gneiss “domes” near Philadelphia and Baltimore is nonmagnetic and/or reversely magnetized, and is associated with deep magnetic lows. Well-defined anomalies associated with magnetic schists in the Wissahickon Group of W.P. Crowley (1976, Md. Geol. Survey Rept. Inv. 27) continue beneath the areas underlain by the older Precambrian rocks in several places, strongly supporting the interpretations of J.H. Mackin (1962, Geol. Soc. America Bull., v. 73, p 403-410) and Bromery (1968, unpub. Ph.D. dissertation, Johns Hopkins Univ.) that the Woodville and Avondale anticlines near Philadelphia, and the Phoenix anticline near Baltimore are refolded nappes, not simple domes.
The Glenarm Supergroup of Crowley (1976), which occupies a regional synclinorium between the South Mountain Anticlinorium to the northwest and the anticlinorium cored by older Precambrian gneiss at Baltimore and Philadelphia, can be divided into three sequences on the basis of magnetic and lithologic properties: (A) an essentially nonmagnetic group of rocks including the Setters Formation and the Cockeysville Marble; (B) a group of weakly magnetic mica schists and phyllites, including the Ijamsville Phyllite, and the albite-chlorite phyllites and garnet-staurolite-mica schists of the Wissahickon; and (C) a strongly magnetic group of rocks including the diamictites, metagraywackes, and quartzose schists of the Wissahickon Group. The regional distribution of these three groups of rocks as inferred from our aeromagnetic data, supplemented by limited regional reconnaissance, and by detailed mapping of a few key areas, suggests that the three sequences approximate time-stratigraphic units. Sequence A can be traced almost continuously into the upper Precambrian to lower Paleozoic sequence of basal clastics and carbonate rocks, and appears to be the eastern continuation of these rocks. Sequence B appears to record submergence of the carbonate platform, and widespread deposition of argillaceous muds, presumably marine. Sequence C includes three distinct lithologies. On the southeast, it consists of a coarse, bouldery diamictite facies. The diamictite grades northwestward into a metagraywacke facies, in which bedding thickness and grain size appear to decrease westward. Finally the metagraywacke appears to grade into thinly laminated quartz schists and phyllites. Together with rock compositions and relict sedimentary structures, these relations suggest that the entire sequence represents a single coherent package of flysch sediments, fed into the basin from the southeast. Fragments of volcanic and ultramafic rocks in the diamictite suggest that the source of the sediments was composed of rocks like those presently exposed in the Baltimore Complex (Baltimore Mafic Complex of Crowley (1976), which occurs in a thrust block just southeast of the Wissahickon diamictite. We therefore concur with Crowley’s view that this thrust was emplaced during sedimentation, and we conclude that its emplacement may well have triggered deposition of the flysch sequence. The age of the flysch sequence is constrained in two ways: (a) it must be younger than the Baltimore Complex, which includes volcanic rocks of the James Run Formation, dated at approximately 550 m.y. by G.R. Tilton and others (1970, p. 429-437, in Fisher, G.W. and others, eds., Studies in Appalachian Geology: Central and Southern; New York, Interscience); and (2) it must be older than the Upper Ordovician Arvonia Slate (Middle to Upper Ordovician) of Virginia. Consequently, the entire Wissahickon Group may be in normal stratigraphic position on the Setters Formation and Cockeysville Marble, which appear to correlate with the lower Paleozoic clastic/carbonate section of the Blue Ridge.
The present distribution of carbonate rocks implies that a deep indentation may have existed in the edge of the carbonate bank northwest of Baltimore, as proposed earlier by John Rodgers (1968, p. 141-149, in Zen and others, eds., Studies of Appalachian Geology; Northern and Maritime; New York, Interscience). This configuration, the major bend in the Appalachians between Baltimore and New York, and the regional variation in thickness of the basal Cambrian sedimentary sequence could all reflect the original geometry of the fracture system along which the Paleozoic ocean opened. One possible reconstruction is that the northern and southern Appalachians originated along fracture systems emanating from different triple junctions, and that the central Appalachians represent the area where the two rift systems were joined by a major east-west cross fracture, possibly a transform fault.
