A distinctive assemblage of upper greenschist to lower amphibolite facies metasedimentary rocks with intercalated mafic and ultramafic rocks forms a major tectonostratigraphic unit, the Liberty Complex, within the central Maryland Piedmont. The Liberty Complex is composed of two lithotectonic units, the Morgan Run Formation and the Sykesville Formation. The Morgan Run Formation is a thick sequence of pelitic schist with intercalated quartzite, amphibolite, and detrital ultramafite; metagraywacke and intercalated amphibolite; serpentinite and associated ultramafic rock. The Sykesville Formation is a thick, poorly layered sequence of uniform, medium-grained quartzofeldspathic granulite and metadiamictite. Metamorphosed clasts in the diamictite are everywhere matrix supported, range in size from granules to slabs at least tens of meters in length, and commonly display internal metamorphic foliations that are truncated by the clast margins. Petrographic study of clast lithologies from exposures around Liberty Lake in Carroll and Baltimore Counties indicates that all clasts are represented by Morgan Run Formation bedrock lithologies in the immediate area. The Morgan Run and Sykesville Formations are both polydeformed; however, the Morgan Run displays several phases of deformation that are not expressed in the Sykesville Formation or adjacent Piedmont terranes. Metagraywacke sequences in the Morgan Run commonly exhibit relict soft-sediment structures, and pelitic schist-quartzite sequences contain stratiform zones of block-in-matrix structure, which we interpret to be tectonic mélange. The Sykesville Formation was deposited after initial metamorphism of the Morgan Run Formation as a sequence of mass-flow and slide deposits characteristic of sedimentary mélanges. The lithologic assemblage and the internal structural features of the Morgan Run Formation are strongly suggestive of an origin within a subduction complex. We interpret the Morgan Run as having been assembled in an accretionary wedge fronting a continent-facing volcanic arc in Cambrian time. The Morgan Run may represent underplated oceanic and trench-fill material that was deformed into tectonic mélange and broken formation during subduction and then rapidly uplifted as the continental margin of North America collided with the accretionary wedge. The Morgan Run Formation supplied coarse metamorphic detritus to the Sykesville deposits during collision and uplift. Both units were strongly deformed, locally interleaved, and regionally metamorphosed during final suturing of the arc to the continental margin. This final stage of development represents the classical Taconic orogeny. It resulted in the assembly of the Liberty Complex, a polygenetic mélange, from the Morgan Run and Sykesville Formations.

U-Pb zircon geochronology and field relations provide insights into metavolcanic and associated rocks in the Central Appalachian Piedmont of Maryland and northern Virginia. Ordovician ages were determined for volcanic-arc rocks of the James Run Formation (Churchville Gneiss Member, 458 ± 4 Ma; Carroll Gneiss Member, 462 ± 4 Ma), Relay Felsite (458 ± 4 Ma), Chopawamsic Formation (453 ± 4 Ma), and a Quantico Formation volcaniclastic layer (448 ± 4 Ma). A previously dated first phase of volcanism in the Chopawamsic Formation was followed by the second phase dated here. The latter suggests a possible source for contemporaneous volcanic-ash beds throughout eastern North America. Dates from the Chopawamsic and Quantico Formations constrain the transition from arc volcanism to successor-basin sedimentation. Ordovician metatonalites of the Franklinville (462 ± 5 Ma) and Perry Hall (461 ± 5 Ma) plutons are contemporaneous with the James Run Formation, whereas granitoids of the Bynum Run (434 ± 4 Ma) and Prince William Forest (434 ± 8 Ma) plutons indicate an Early Silurian plutonic event. The Popes Head Formation yielded Mesoproterozoic (1.0–1.25 Ga, 1.5–1.8 Ga) detrital zircons, and metamorphosed sedimentary mélange of the Sykesville Formation yielded Mesoproterozoic (1.0–1.8 Ga) detrital zircons plus a minor Archean (2.6 Ga) component. A few euhedral zircons (ca. 479 Ma) in the Sykesville Formation may be from granitic seams related to the Dalecarlia Intrusive Suite. A Potomac orogeny in the Central Appalachian Piedmont is not required, but the earliest Taconic orogenesis remains poorly constrained.

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.

There is no completely accepted definition of a mélange, and the papers in this volume reflect this fact. In our regional assessment, the term mélange is principally used for a technically fragmented and mixed body of rock. A different term, olistostrome, is used for a chaotic and mixed rock body that formed by sedimentary processes such as slumping or gravity sliding. The term olistostromal mélange is used here if sedimentary processes and tectonic deformation were both involved in the fragmentation and mixing. In some cases there is evidence that these were effectively concurrent. Four main belts of Paleozoic mélanges and olistostromes have been recorded in the Appalachians of the northeastern United States and mainland Canada. These include: (1) olistostromes and olistostromal mélanges along Logan’s line and the Taconic allochthons, which are related to thrusting during the Taconian orogeny; (2) mélanges associated with ophiolite fragments along the Baie Verte–Brompton line, which are thought to represent a Taconian suture; (3) mélanges containing ophiolite fragments along the Hurricane Mountain mélange belt (Boone and Boudette, this volume), which are thought to represent a Penobscottian terrane boundary; and (4) Acadian mélanges and olistostromes such as the Silurian Deadman Harbour mélange, an olistostromal mélange that probably formed at the front of an Acadian overthrust. In addition, Precambrian olistostromes have been recognized in southeastern New England and in the Green Head Formation of New Brunswick. These may have originated along normal faults on the rifted continental shelf of Gondwanaland, or they may relate to an earlier Pan-African cycle. These Precambrian olistostromes, therefore, accompany the extensional development of the Iapetus Ocean, whereas the Paleozoic olistostromes and mélanges mark its progressive closure. Five main sets of Paleozoic mélanges and olistostromes have been recorded in the central and southern Appalachians. These are: (1) a composite mélange-olistostrome belt in the Piedmont of Maryland and Virginia that includes olistostromes such as the Sykesville Formation (of unknown age) and mélanges such as those of the Morgan Run Formation (of debated age) and Mine Run Complex, some of which contain possible ophiolite fragments; (2) mélanges in the Blue Ridge Province from Virginia to Alabama, which contain possible ophiolite fragments; (3) Silurian or Early Devonian olistostromes of the Lay Dam Formation in the Talladega slate belt of Alabama; (4) mélanges, including the Falls Lake and Juliette mélanges in the Carolinas and Georgia, that border the Carolina terrane as defined by Secor and others (1983); and (5) broken formations and mélanges along major faults such as the Pulaski and Brevard, which are mostly related to Alleghanian thrusting. In addition, there is good evidence in the Ocoee Supergroup of the Blue Ridge Province for Late Proterozoic olistostromes related to the initiation of grabens prior to the opening of the Iapetus Ocean.

ABSTRACT The Baltimore terrane, the Baltimore Mafic Complex (BMC), and the Potomac terrane are telescoped tectonostratigraphic packages of metasedimentary and meta-igneous rocks that record the geologic history of eastern Maryland from 1.2 Ga to 300 Ma. These terranes provide insight into the understanding of the rifting of Rodinia and the initial amalgamation of eastern Laurentia. The oldest of these rocks are exposed as gneiss domes in the Baltimore terrane, with gneissic Grenvillian crust overlain by a metasedimentary cover succession believed to have been deposited during Rodinian rifting and the formation of the Iapetus ocean. These rocks are interpreted to be analogous to the Blue Ridge sequence in western Maryland. Late Cambrian ultramafites and amphibolites of the BMC discordantly overlie the Baltimore terrane to the east and north, and may represent ophiolitic oceanic crust obducted over eastern Laurentia continental rocks as an island-arc collisional event during the Taconian orogeny. To the west, a thick assemblage of schist, graywacke, metadiamictite, and ultramafic bodies comprises the Potomac terrane, a polygenetic mélange that may have formed in an accretionary wedge during Taconian subduction and collision with the Laurentian continental margin. The Pleasant Grove fault zone marks the Taconian suture of these accreted terranes to Laurentian rocks of the central Maryland Piedmont, and preserves evidence of dextral transpression during the Alleghenian orogeny in the Late Pennsylvanian.

Abstract The Appalachian Piedmont in south-central Pennsylvania and north-central Maryland contains metasedimentary siliciclastic rocks (phyllites to quartzites) that were deposited largely offshore of Laurentia, prior to and during the early history of the Iapetan Ocean. The Peach Bottom area is centered on the belt of Peach Bottom Slate and overlying Cardiff Quartzite, which is surrounded by the late Neoproterozoic and early Paleozoic rocks of the Peters Creek and Scott Creek (new name) Formations. Their provenance was the Brandywine and Baltimore microcontinents that lay farther offshore of the Laurentian coast. This area also includes an ophiolitic mélange that formed in front of an advancing island arc in Iapetus. All these rocks lay largely undisturbed throughout much of the Paleozoic, experiencing only chlorite-grade greenschist facies metamorphism through deep burial. Alleghanian thrusting associated with the growth of the Tucquan anticline imparted their present widespread, monocline, steep southeast dip of the bed-parallel foliation.

Abstract This field trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the significance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.