BEDROCK AND RADIONUCLIDE MAPPING IN THE COLCHESTER QUADRANGLE, VERMONT

In response to the identification of significantly elevated radionuclide levels in numerous domestic wells in the towns of Colchester and Milton, the Vermont Geological Survey undertook a detailed bedrock and radionuclide mapping project in the Colchester 7.5’ Quadrangle during 2001. In addition to standard bedrock mapping techniques, gamma ray spectrometer readings were taken at every outcrop as well as in detailed grid areas. This presentation will summarize the significant lithological, structural, and geophysical findings.

The Clarendon Springs Formation (CSF) is a massive, gray, poorly bedded, crystalline, sparsely fossiliferous, dolostone of Cambro-Ordovician age with abundant breccia horizons that are composed of fragments of black and gray chert and/or dolostone. Locally, sandy dolostone and breccia unconformably fill paleokarst surfaces in the CSF dolostone.

The CSF dips gently east beneath black calcareous shales of the Skeels Corners Formation. The eastern contact of the CSF is an east-dipping Taconian thrust fault of regional significance (Muddy Brook Thrust(MBT)); this thrust is decorated along its length with slivers of highly cleaved limestone. The western contact of the CSF interfingers with cross-bedded dolomitic sandstones of the underlying Danby Formation. East-northeast trending steeply dipping normal faults of presumed Mesozoic age offset the CSF and MBT.

All seven well groupings with elevated radioactivity levels in the Colchester Quadrangle are in CSF dolostones. Anomalously radioactive horizons are directly associated with some breccia horizons and open fractures in the CSF but have no spatial relationship to the MBT or later cross faults. No radioactive anomalies were found in the neighboring Dunham, Monkton, Winooski, Danby, or Skeels Corners formations. Detailed radiometric grid surveys showed irregularly shaped “hotspots” that may have structural and stratigraphic control.