A new look at the geological history of Great Duck Island and Mount Desert Rock through high resolution aerial imagery, geological mapping, geochemistry, and geochronology

Gray, Spencer; Hall, Sarah; Michalak, Melanie; Bailey, David

From the abstract: "Great Duck Island and Mount Desert Rock are two remote islands off the southern coast of Mount Desert Island (MDI), Maine. Both islands are working field stations maintained by College of the Atlantic. Each island has been host to many ecological research projects during the last decade, although none have specifically focused on the bedrock and surficial geology. Maine Geological Survey maps show both islands as composed dominantly of Devonian granite, similar to nearby islands including MDI. The goal of this project is to map the dominant fractures, dikes, bedrock types, glacial features, and sediments of both islands and create geologic maps in ArcGIS software, as well as to obtain geochemical and chronological data for each island. We will be reporting new geochemical data for both Great Duck Island and Mount Desert Rock. In addition, Mount Desert Rock lacks a topographic dataset and is beyond the range of available high-resolution aerial satellite and LiDAR data. By processing new drone imagery using Agisoft Photoscan, an aerial orthomosiac was created of the entire island. Preliminary results show fracture sets and minor fault zones on both Mount Desert Rock and Great Duck Island follow dominant north-south and east-west regional trends. The entire ~7 acre island of Mt. Desert Rock is composed of a fine to medium-grained light gray, phaneritic granite with less than ~5% mafic minerals. Conversely, Great Duck Island is underlain by a fine-grained, felsic, bright pink granite, also quite low in a mafic minerals, a welded flow banded rhyolite, as well as a zone of highly altered metasedimentary rocks similar to the Bar Harbor Fm of MDI. Preliminary U238-Pb206 zircon ages of the granites from both islands suggest that they are significantly younger than the Silurian and Devonian granites of nearby MDI and Swans Island with Great Duck Island, yielding ages of 313.3 +/- 3.0 Ma (Carboniferous) and Mount Desert Rock, 296.5 +/- 2.9 Ma (Permian). Taken together, these new geochronologic, geochemical and field and remote-based spatial data enable us to refine the volcanic and tectonic history of this portion of the easternmost edge of North America."