Two distinct types of mélange deposits, distinguished by their matrix, occur within a collage of thrust slices in the Piedmont of the central Appalachians. They crop out in a northeast-trending belt that extends from at least central Virginia northeastward through most of Maryland. One type is a block-in-phyllite mélange that constitutes the Mine Run Complex (new name) of Virginia. It consists of a variety of metaplutonic, metavolcanic, mafic, and ultramafic blocks enclosed within a matrix of phyllite or schist and metasandstones of feldspathic or quartz metagraywacke. The Mine Run Complex is interpreted to consist of four imbricated thrust slices, each with its own distinctive exotic block content. The blocks in one of these mélange subunits (III) are almost exclusively mafic and ultramafic rocks, such as serpentinite, greenschist, metabasalt, and talc schist. The second mélange type within this Piedmont terrane, a metadiamictite, contains a less extensive variety of exotic blocks, the most common being mafic and ultramafic blocks. Such exotic blocks are enclosed in a micaceous quartzofeldspathic matrix, which has contemporaneously deposited schist and quartz-lump fragments as its characterizing features. The Sykesville Formation of Maryland and Virginia is typical of this type of mélange. Several varieties of metadiamictite that have some lithologic differences from the type locality of the Sykesville have been recognized in Virginia and are described as the Lunga Reservoir and the Purcell Branch Formations (new names). Mélanges of the block-in-phyllite and the metadiamictite types are interpreted as having been formed in a Cambrian-Ordovician back-arc or marginal basin that lay on the continentward side of an island-arc system (central Virginia volcanic-plutonic belt and the James Run Formation of Maryland) that had formed in Cambrian time. This Cambrian-Ordovician back-arc basin is assumed to have been underlain, at least initially, by a transform-segmented spreading ridge. The metadiamictite deposits now occur, for the most part, along the flanks and at the southeastern end of the Baltimore-Washington anticlinorium or antiform. This spatial relation is interpreted to be a consequence of thrusting of the diamictite across, as well as onto, the cover rocks and basement of the anticlinorial core terrane (“Baltimoria”) during the Taconic orogeny. The chaotic-textured diamictite formed as a sedimentary slump or slide apron or aprons somewhere to the east of “Baltimoria.” The source area for this diamictite probably was another crystalline landmass that lay east of “Baltimoria.” The matrix rocks of the block-in-phyllite mélange (Mine Run Complex) may have accumulated contemporaneously, in part with diamictite, probably by along-trough basin sedimentation. The block-in-phyllite mélanges of the Mine Run Complex occur south of the Baltimore-Washington anticlinorium metadiamictite terrane and are not associated directly with metadiamictite mélanges. The metavolcanic and metaplutonic blocks within the eastern parts of the Mine Run Complex probably were shed from the island-arc terrane as it was thrust westward during the progressive tectonic telescoping of the back-arc basin in Cambrian and Ordovician time. The mafic and ultramafic blocks in both types of mélanges are believed to have been derived, in part, from the “oceanic” back-arc basin floor from various sources and different processes. Ultramafic protrusions, for example, may have been emplaced periodically along transform faults. Some of the mafic-ultramafic rock may have formed talus rubble along steep submarine scarps. Others may represent blocks broken and segmented by faults from what once were mafic sills emplaced within the sediments accumulating in the basin.

Roy and Mencher (this volume) presented an interesting summary of the stratigraphy of northeastern Maine. Counterparts to some of the lithotectonic units they described occur along strike to the south, although in detail the stratigraphy shows some marked differences. For comparative purposes, I will briefly discuss some of the similarities and differences in the stratigraphy on the west side of the Aroostook-Matapedia anticlinorium southward from the Presque Isle area. The Carys Mills Formation (of Middle Ordovician to Early Silurian age) extends as a continuous unit throughout northeast Maine within the core of the Aroostook-Matapedia anticlinorium. The lithology and the internal stratigraphy of this formation have been described elsewhere (Pavlides, 1968Pavlides, 1966; Pavlides and others, 1965) and are essentially as described by Roy and Mencher (this volume) except for minor differences; for example, in the Bridgewater area, about 20 mi (32 km) southeast of Presque Isle, the formation contains lenses of slate and of slate and graywacke. Absence of marker beds hinders subdivision of the Carys Mills Formation into subunits, and Roy and Mencher’s subdivision of it into a lower and upper part north of Presque Isle is apparently biostratigraphic rather than lithostratigraphic. The Frenchville Formation, of Llandovery C3 –C5 age, is absent along the west flank of the Aroostook-Matapedia anticlinorium; in the Mars Hill area (about 13 mi [20.8 km] southeast of Presque Isle), the coeval Spragueville Formation overlies the Carys Mills Formation and underlies the early Ludlow upper part of the Perham or Jemtland Formation of Roy and Mencher

Abstract The Oakfield Hills lie within a hornfels zone in a region of folded lower Paleozoic rocks that have been intruded and thermally metamorphosed by felsic plutons. An area of prominent aeromagnetic anomalies correlates closely with the hornfels zone. Ground geophysical measurements made in the vicinity of the largest aeromagnetic anomaly have delineated several steeply dipping tabular rock units striking northeast along the northwest margin of one of the felsic plutons. Geophysical and geological evidence indicates that this pluton is the largest of several in the area. The tabular units have a high apparent susceptibility, high density, and in places are conductive. They apparently are lenticular beds of manganese-bearing iron formation that have been thermally metamorphosed to magnetitebearing rocks by the nearby intrusion. Elsewhere in the area, electromagnetic anomalies are probably caused by sulfide-bearing rocks.