INTRODUCTION TO THE NEW 1:100,000 BEDROCK GEOLOGIC MAP OF VERMONT

The Bedrock Geologic Map of Vermont is the culmination of over 20 years of geologic mapping supported by the U.S. Geological Survey and the State of Vermont. Approximately 60% of the State was mapped at 1:24,000 scale using plate tectonic concepts not yet developed for the 1961 map. It presents detailed lithologic and structural information as well a new U-Pb ages (Aleinikoff,Rankin this session). Cross sections from the Adirondacks eastward show the general east dipping major lithotectonic units of the Taconian cycle that consists of: 1. Laurentian margin Mesoproterozoic (0.95 to 1.39Ga) basement overlain by westward thinning transgressive Neoproterozoic to Upper Ordovician rift, shelf/ basin sequence. Slices of Green Mountain basement and pre-shelf cover capped by Middle Ordovician West Bridgewater Formation over thrust by the Pinney Hollow Formation (573 Ma). The Neoproterozoic to Lower Ordovician Taconic allochthons are inferred to have been deposited west of and/or above the restored Pinney Hollow 2.Transitional margin tectonically thinned rocks of the Rowe zone(Ottauquechee and Stowe Formations). 3.Taconian accreted terranes Moretown Formation-North River Igneous Suite(NRIS) (485 to 500 Ma) and Albee Group and igneous rocks of the Bronson Hill arc(443-469 Ma).

Taconian imbricate(out of sequence) faults marked by distinctive rodded phyllonites, as in the Rowe-Hawley zone, extend west through the Green Mountain massif to the Taconic frontal faults in the south. Taconic thrusts such as the Prospect Rock fault to the north (Gale, Thompson this session) and sole of the Taconic allochthon predate imbrication.

Siluran and Devonian rocks of the Connecticut Valley Trough (CVT) and of the Bronson Hill rest on the rocks of the Taconian orogen and are markedly time transgressive consistent with development of the regional Taconic unconformity. A thin unresolved section at the top of the Moretown and Cram Hill Formations consisting of reworked NRIS,volcanic agglomerates and quartzites that may be transitional deposits to the CVT following intrusion of 419-423 Ma diorite-trondhjemite- granite at Newport, Braintree and east of the CVT. Plutons and dikes of Devonian granite (370- 390 Ma) and Jurassic- Cretaceous age are shown. Lower Miocene Brandon Lignite is the youngest unit of bedrock.