PROGRESS TOWARD A NEW BEDROCK MAP FOR THE AUGUSTA 1:100,000 QUADRANGLE, SOUTH-CENTRAL MAINE

Bedrock mapping efforts supported by the Maine Geological Survey over the last nine years have been focused primarily on the Augusta 1:100,000 scale quadrangle in south-central Maine. Despite this being a relatively populated area in the state, few detailed geologic maps from this region had been published prior to the current work supported through the National Cooperative Geologic Mapping Program. This paucity of detailed information, coupled with the complexity of the high-grade metamorphic terrain, has necessitated significant new 1:24,000 scale bedrock mapping in key areas within the quadrangle. An important byproduct of this detailed mapping has been the training of numerous undergraduate students, and the spawning of detailed structural, petrologic and geochemical investigations.

Stratified rocks in the Augusta 1:100,000 quadrangle can be divided into several distinct northeast-trending lithotectonic belts, predominantly of Lower Paleozoic age, that are separated from one another by steeply dipping faults or shear zones. Metamorphism is complex with intensities ranging from biotite to sillimanite + K-feldspar zone, and evidence for polymetamorphism is widespread. The stratified rocks preserve evidence for multiple phases of ductile deformation. The quadrangle also contains a diverse range of intrusive rocks, ranging in age from Ordovician to Mesozoic, with varying relationships to deformational and metamorphic events. Finally, a complex system of post-metamorphic faults and shear zones associated with the regionally extensive Norumbega fault system bisects the quadrangle.

Our recent 1:24,000 scale mapping has revealed several previously unrecognized map units, a better understanding of the spatial distribution of all lithologic units, more accurately located contacts between these units, and a better understanding of local bedrock structure. Tying into the theme of this session, in addition to the publication of numerous new 7.5’ quadrangle maps, this recent mapping has spawned detailed structural, petrologic, and geochemical studies that have led to significant external funding and journal publications. In addition, a new generation of geologists (students) has received valuable field experience and an appreciation of the importance of detailed bedrock mapping.