PRELIMINARY BEDROCK GEOLOGIC MAP OF NEW ENGLAND, USA, ROUND 2

A 1:500,000-scale bedrock geologic map of New England reconciles edge-match inconsistencies at a regional scale. Six state geologic maps (published 1983-2011) contain cross-border inconsistencies. Scales of the maps range from 1:100,000 to 1:500,000, and the source maps depict units that change across state lines and are inconsistently defined by formation, protolith, age, and lithology. The greatest edge-match challenges exist in places underlain by complex geology such as migmatites and areas without detailed 1:24,000-scale mapping. The new compilation map depicts reconciled geologic units and faults at state borders. Maps of metamorphic zones are digitized but not yet edge-matched for 3 states: VT, NH, and MA.

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. The belts of crystalline rock are subdivided by age, protolith, provenance, and paleo-tectonic setting. The oldest bedrock lies in the Grenville Province of Laurentia and is exposed in massifs in western New England. Neoproterozoic to lower Paleozoic rocks of Laurentian affinity were obducted as thrust sheets of the Taconic allochthons over the Laurentian carbonate platform in western New England. Silurian to Devonian ­­­clastic rocks form widespread sedimentary basins including the Connecticut Valley – Gaspé trough and Central Maine trough. Neoproterozoic Peri-Gondwanan Ganderian and Avalonian arc-related rocks occur in gneiss domes in central and coastal New England.

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 reflect the formation of the Atlantic Basin and mantle plumes. The bedrock of the Appalachian Highlands disappears beneath the unconsolidated Cenozoic sediments of the Atlantic Coastal Plain, which occur only in coastal southern New England. The new map provides an opportunity to identify future detailed mapping needs.