New Bedrock Geologic Maps and Bulletin Released
The Maine Geological Survey is pleased to announce the publication of Bulletin 46: Bedrock geology of the Louds Island and New Harbor 7.5' quadrangles, Maine, by Professor David West of Middlebury College in Vermont. The Bulletin accompanies the simultaneously published Louds Island and New Harbor bedrock geologic maps. The work represents on-going efforts to provide detailed bedrock geologic information in the mid-coast region.
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The quadrangles are located on the western side of Muscongus Bay along the southeastern margin of the Fredericton belt, an extensive Silurian-aged depositional basin that extends from Boothbay Harbor northeast into southeastern New Brunswick. In the area of this work, the belt is dominated by fine-grained metamorphosed sedimentary rocks of the Bucksport Formation. Greatly subordinate older (Cambrian-Ordovician?) migmatitic rocks of the Cross River Formation are also present along the western edge of the map area. All of these rocks have been metamorphosed to amphibolite facies conditions, and experienced penetrative ductile deformation during Devonian time. In addition, these stratified metamorphic rocks have been intruded by a wide variety of igneous rocks that span an age range of over 200 million years (Early Devonian to Early Jurassic).
Highlights of the new work include: (1) Recognition of a thin meta-volcanic unit within the Bucksport Formation (Greenland Cove Member). Two new U-Pb zircon ages from the unit indicate deposition at 423 ± 4 Ma and provide a minimum age for the deposition of the Bucksport Formation in mid-coastal Maine. (2) Four new U-Pb zircon ages from intrusive rocks previously included in the Waldoboro Pluton Complex are Early Devonian, more than 30 million years older than the previously published Late Devonian age for the Waldoboro Granite. It is recommended that the term Waldoboro Pluton Complex be abandoned. (3) Numerous diabase dikes of presumably Late Triassic-Early Jurassic age are present, including the larger Christmas Cove and newly recognized Moxie Cove dikes. (4) At least two distinct episodes of folding are present, an early recumbent folding event (F1) followed by later north-northeast-trending upright folding (F2). (5) The north-south-trending contact between the eastern margin of the Bucksport Formation and the Early Devonian South Pond Porphyry is a steeply dipping, polyphase, left-lateral shear zone that includes both mylonite and pseudotachylyte. This supports the earlier work of Mark Swanson and his University of Southern Maine students who labeled the zone the Harbor Island shear zone. (6) Six north to northeast trending post-Devonian high-angle faults have been mapped primarily on the basis of prominent multi-kilometer-long topographic lineaments.
The bulletin and associated geologic maps are a basic information source that can contribute to understanding groundwater quality, groundwater flow and contaminant transport, soil chemistry, distribution of mineral resources, and engineering properties relevant to construction projects. Additionally, in that these maps provide information on the history of the earth, they are important educational tools for both geologists and non-geologists.
 View to the south along the western side of Muscongus Bay in the southern part of the Louds Island quadrangle showing west-dipping Silurian metasedimentary rocks of the Bucksport Formation intruded by Devonian granite.
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