PRELIMINARY BEDROCK GEOLOGIC MAP OF NEW ENGLAND, USA

State geologic maps of New England, published from 1983 to 2011, contain multiple cross-border edge-match inconsistencies. Scales of the maps range from 1:100,000 to 1:500,000 and the maps are not seamless. The map content changes across state lines with units variably defined by formation, protolith, age, and lithology. The greatest edge-match challenges exist in places underlain by complex geology such as migmatites and areas with no detailed 1:24,000-scale mapping. A preliminary 1:500,000-scale bedrock geologic map of New England is an attempt to reconcile the major edge-match inconsistencies in the region. The new map depicts map units and faults, and future versions will incorporate radiometric ages and depict metamorphic zones.

The bedrock of New England consists of rocks that span the last 1.4 billion years of Earth’s history. Crystalline metamorphic rocks are the most widespread and occur in several north-northeast trending tectonic belts that transect state borders and form the core of the Appalachian Mountain chain which extends from Alabama to Newfoundland. The belts of crystalline rock are subdivided by age, protolith, provenance, and paleo-tectonic setting. The oldest bedrock is basement of the Grenville Province of Laurentia exposed in massifs from the Adirondacks to the Hudson Highlands. Neoproterozoic to lower Paleozoic rocks of Laurentian affinity were transported as thrust sheets of the Taconic allochthons over the Laurentian Cambrian to Ordovician carbonate platform. Silurian to Devonian ­­­clastic rocks form widespread sedimentary basins such as the Connecticut Valley – Gaspé trough and Central Maine trough. Peri-Gondwanan Ganderian and Avalonian arc-related rocks occur in gneiss domes in central New England and along coastal exposures.

Paleozoic and older rocks were variably deformed and metamorphosed during multiple orogenic phases. Paleozoic metamorphism to amphibolite facies was widespread and locally reached lower granulite facies. Middle to late Paleozoic plutonic rocks are widespread. The Mesozoic Hartford Basin and magmatic rocks are related to the opening of the Atlantic Basin and mantle plumes. The Appalachian Highlands disappear beneath the Cenozoic sedimentary rocks and unconsolidated deposits of the Atlantic Coastal Plain in coastal southern New England.