Abstract Recent detrital zircon results in both the central Appalachians and New England demonstrate that middle Ordovician, ‘Taconic’ island arcs, long considered to be peri-Laurentian, are built upon or associated with rock of Gondwanan affinity. This trip will visit granulite-facies orthogneiss of the Wilmington Complex, a 475–480 Ma magmatic arc, and the adjacent Wissahickon Formation. The Wissahickon Formation is intruded by and interlayered with meta-igneous rocks with arc affinity and contains detrital zircon populations characteristic of both Gondwanan and Laurentian sources. The Chester Park Gneiss, now known to have detrital zircon age spectra which match the Gondwana-derived Moretown Terrane in New England, is also featured. The trip will examine contact relationships between arc and Laurentian rocks and a newly discovered location where metapelitic rock contains garnet with crystallographically oriented rutile inclusions, possibly indicative of ultrahigh-temperature or ultrahigh-pressure metamorphism. We will discuss similarities between rocks of the central and northern Appalachians and evaluate a new model wherein the central Appalachian rocks were originally part of the Taconic arc in New England and were translated by strike-slip deformation to their present position in the orogen.

Abstract For safety and environmental reasons, removal of aging dams is an increasingly common practice, but it also can lead to channel incision, bank erosion, and increased sediment loads downstream. The morphological and sedimentological effects of dam removal are not well understood, and few studies have tracked a reservoir for more than a year or two after dam breaching. Breaching and removal of obsolete milldams over the last century have caused widespread channel entrenchment and stream bank erosion in the Mid-Atlantic region, even along un-urbanized, forested stream reaches. We document here that rates of stream bank erosion in breached millponds remain relatively high for at least several decades after dam breaching. Cohesive, fine-grained banks remain near vertical and retreat laterally across the coarse-grained pre- reservoir substrate, leading to an increased channel width-to-depth ratio for high-stage flow in the stream corridor with time. Bank erosion rates in breached reservoirs decelerate with time, similar to recent observations of sediment flushing after the Marmot Dam removal in Oregon. Whereas mass movement plays an important role in bank failure, particularly immediately after dam breaching, we find that freeze-thaw processes play a major role in bank retreat during winter months for decades after dam removal. The implication of these findings is that this newly recognized source of sediment stored behind breached historic dams is sufficient to account for much of the high loads of fine-grained sediment carried in suspension in Mid-Atlantic Piedmont streams and contributed to the Chesapeake Bay.

Foreland basin rocks of the northern Appalachian basin in New York and adjacent areas contain a significant Upper Silurian to Devonian record of Acadian orogenesis. Sediment composition, stratal geometry, stratigraphic anomalies, and distribution of volcanic air-fall tephras through time and space provide insights into patterns of tectonism and quiescence, uplift and unroofing, tectonically induced basin flexure, and explosive volcanism in the orogenic belt. Herein, I combine a literature review and new data to examine several aspects of the foreland basin fill and their implications. Established models of Acadian-related impacts on the foreland, including tectophase development, are tested against a more refined high-resolution stratigraphy. Some sedimentary patterns are cyclic; others evolve through time. Initial study of synorogenic conglomerates across 40 m.y. of sedimentation sketches an unroofing history of the orogen. Stratigraphic anomalies delineate a flexural history interpreted directly from the rock record: topographic features in the foredeep migrate toward the craton in tectonically active intervals and toward the orogen during quiescent intervals. In addition, the forebulge undergoes cyclic uplift and leveling. These results differ from predictions in existing models of foreland basin kinematics. Preserved air-fall tephras reflect a history of explosive volcanism along the orogen. Comparisons of igneous rocks from the foreland and orogen portray a larger picture of Lower Emsian magmatism. Finally, I summarize the chronology of foreland basin signatures of orogenesis. Data and interpretations presented here should be compared with the record of Acadian orogenesis from the mountain belt in order to better determine causation and outline a more detailed synthesis of the Acadian orogeny.