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| Merrimack Synclinorium | |
|---|---|
| Name | Merrimack Synclinorium |
| Type | Geological structure |
| Location | New England, United States |
| Region | New Hampshire, Massachusetts |
| Age | Paleozoic |
| Lithology | Metamorphic and sedimentary rocks |
| Namedfor | Merrimack River |
Merrimack Synclinorium is a regional synclinal structural feature in the New England Appalachians that controls distribution of stratified rocks and mineralization across parts of New Hampshire and Massachusetts. The feature has been mapped and described in studies by the United States Geological Survey, the New Hampshire Geological Survey, and academic institutions such as Dartmouth College and the University of Massachusetts. It links exposures in the Merrimack River valley with regional fabrics recognized during Appalachian orogen studies involving the Taconic, Acadian, and Alleghanian events.
The synclinorium lies within the Appalachian Mountain belt and juxtaposes units correlated with the Taconic orogeny, Acadian orogeny, and Alleghenian orogeny in a folded and metamorphosed assemblage; mapping by the United States Geological Survey and the New Hampshire Geological Survey documents its fold hinges, axial planes, and regional strike. Structural studies involving researchers from Dartmouth College, University of New Hampshire, Harvard University, and Massachusetts Institute of Technology show that the feature includes steeply plunging folds, axial-planar schistosity, and regional cleavage tied to crustal shortening recorded also in outcrops studied by field parties from the Smithsonian Institution and the American Geophysical Union. Mesoscopic and map-scale fabrics align with lineations measured near exposures studied by geologists associated with the Geological Society of America and the Society of Economic Geologists. Regional cross sections correlate the synclinorium with basement-controlled faulting recognized in work by the New England Intercollegiate Geological Research Group and surveys by the U.S. Army Corps of Engineers during infrastructure assessments.
The stratigraphic assemblage within the synclinorium includes low- to medium-grade metasedimentary sequences, metavolcanic units, and plutonic intrusions; units have been correlated with formations described in guidebooks from the New England Intercollegiate Geological Conference and mapped by the USGS Professional Paper series. Lithologies include pelitic schists, quartzites, arkoses, and greenstone interpreted as equivalents to formations studied at Mount Monadnock, Mansfield Mountain, and exposures in the Kancamagus Highway corridor; detailed biostratigraphic and radiometric work by teams at Yale University, Brown University, and Colby College refined correlations. Plutonic rocks, including granitic bodies, have been sampled by investigators affiliated with the Geological Society of London and compared to intrusive suites described in the New England Volcanic Arc literature. Contact relationships between units have been characterized in theses from University of Vermont and mapping projects supported by the National Science Foundation.
The tectonic evolution reflects convergence, accretion, and transpressional regimes associated with plate interactions involving terranes addressed in studies of the Iapetus Ocean, the Avalonian terrane, and peri-Gondwanan ribbons; interpretations draw on work published by the Palaeogeography, Palaeoclimatology, Palaeoecology community and reports from the New England Chapter of the Geological Society of America. Timing of deformation has been constrained by geochronology performed at laboratories such as the Woods Hole Oceanographic Institution and isotope labs at Massachusetts Institute of Technology, linking deformation episodes to regional events catalogued in syntheses by the Smithsonian Institution and monographs from the Royal Society. Models invoking continental collision and subsequent exhumation reference analogs in the Caledonides and Variscan Belt described in comparative tectonics literature by authors at Oxford University and Cambridge University.
Economic geologists have documented occurrences of metallic mineralization, industrial aggregates, and dimension stone within the synclinorium; prospecting records filed with the New Hampshire Department of Environmental Services and the Massachusetts Geological Survey list historical occurrences of iron-rich skarns, sulfide mineralization, and quartz-vein hosted gold occurrences evaluated by firms including Kennecott, Rio Tinto, and local mining companies. Aggregate quarries supplying construction projects for the Massachusetts Department of Transportation and municipal works in Manchester, New Hampshire and Lowell, Massachusetts exploit feldspathic and quartzitic units cited in engineering geology reports from the U.S. Army Corps of Engineers. Cement, ceramic, and roofing slate prospects were documented in bulletins by the United States Bureau of Mines and in regional mineral commodity assessments by the U.S. Geological Survey.
The surface expression of the synclinorium influences drainage patterns of the Merrimack River, the Contoocook River, and tributaries feeding the Atlantic Ocean watershed; river incision reveals cross sections used in geomorphology studies published in journals associated with the American Geophysical Union and the International Association of Hydrogeologists. Landforms include rounded ridges near Mount Kearsarge and valley floors hosting urban areas like Concord, New Hampshire and Haverhill, Massachusetts where Quaternary deposits mapped by the USGS National Cooperative Geologic Mapping Program overlie bedrock. Glacial modification linked to the Laurentide Ice Sheet and post-glacial isostatic adjustments studied by teams from the University of Maine produced tills and stratified drift described in regional field guides from the New England Intercollegiate Geological Conference.
Mapping and interpretation have a long history involving state surveys, academic theses, and federal projects; early reconnaissance by geologists at the United States Geological Survey and the New Hampshire Geological Survey was supplemented by doctoral work at Dartmouth College and mapping programs at the University of Massachusetts Amherst. Modern investigations employ geochronology, geophysical surveys by contractors working with the National Science Foundation, and digital mapping initiatives coordinated with the U.S. Geological Survey and the Massachusetts Geological Survey; output appears in field trip guidebooks for meetings of the Geological Society of America, symposia at Harvard University, and technical notes from the New England Intercollegiate Geological Conference.
Fossil occurrences within units correlated to the synclinorium are sparse but include marine faunas and trace fossils recorded in Paleozoic sequences described in micropaleontology reports from laboratories at Yale University and collections housed at the Peabody Museum of Natural History and the Smithsonian Institution. Biostratigraphic indicators used to correlate sequences reference studies of trilobites, brachiopods, and ichnofossils published in journals supported by the Paleontological Society and documented in regional paleontological monographs issued by the New England Intercollegiate Geological Conference.
Category:Geology of New Hampshire Category:Geology of Massachusetts