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Seafloor Geology of the Inner Continental Shelf of Maine
School of Earth and Climate Sciences
University of Maine
Maine has a very long and geologically complex shoreline that is matched by the extraordinary geology of its inner shelf geology. Glaciated coasts are notorious for their complexity, but in the past 30 years, the University of Maine and Maine Geological Survey have gathered more than 4,000 km of side scan sonar records, 5,000 km of seismic reflection profiles (mostly “boomer records), and mosaicked extensive parts of six embayments with multibeam bathymetry. Groundtruth for these remotely sensed observations were provided by 1,700 bottom samples, all analyzed for grain size and many for organic carbon/nitrogen, carbonate content and heavy mineral abundance, 75 vibracores, and numerous ROV and submersible observations. Despite this, vast areas, especially east of Penobscot Bay, remain completely unexamined.
The inner shelf bedrock consists of a series of terranes accreted during the Paleozoic and (in Maine) only slightly modified by the Mesozoic opening of the Atlantic Ocean. Glaciation was the primary agent to shape the modern seafloor by scouring many deep channels, depositing till (often in the form of long, linear moraines) and glacial-marine mud locally called the Presumpscot Formation. This latter deposit if the most abundant sediment type along the Maine coast. A sea-level lowstand at -60 m 12,500 years ago was associated with the formation of now-drowned deltas and shorelines, and other shorelines and estuaries formed during a period of slow sea-level rise from 10,000-7,000 years ago.
The seafloor is best understood by subdividing the region into six physiographic areas that are internally homogeneous, yet differ from one another in a variety of factors.
1) Outer Basins are generally muddy areas deeper than 40 m and continue without interruption into the deeper water of the Gulf of Maine;
2) Hard-Bottom Plains are concentrated east of Machias Bay where strong tidal currents have eroded sediment down to bedrock and coarse gravel;
3) Rocky Zones occupy a lot of the exposed shelf landward of 30 m depth;
4) Shelf Valleys range from drowned canyons greater than 100 m deep to smaller rock channels and include channels filled with glacial sediment;
5) Nearshore Basins are muddy areas in sheltered embayments. These are notable for their accumulation of methane and the occurrence of pockmarks; and
6) Nearshore Ramps are the sandy regions seaward of large beach systems and include drowned deltas and deltaic remnants.
Dr. Joseph T. Kelley is a marine geologist who enjoys working on basic scientific research problems that have societal implications. He was the Maine State Marine Geologist for 16 years before becoming a professor at the University of Maine. He has worked on sediment provenance problems and used both physical and chemical signatures to track sediment. He has become interested the response of developed and pristine shorelines to sea-level change. This work involves measuring changes in sea level, as well as studying contemporary coastal processes. In addition, he has monitored and mapped coastal hazards that are a response to rising sea level (beach and bluff erosion, landslides) with the intent of providing information to the public. He has mapped the seafloor of the Gulf of Maine, and developed interests in phenomena like gas-escape pockmarks and cold-water carbonates. Recent mapping has been in support of UMaine’s effort to develop an offshore wind farm.
Salt marshes have long been a subject of interest and Dr. Kelley continues to study their geomorphology. He is currently working on funded projects to evaluate sea-level change in the Irish Sea, the burial of Medieval coastal communities in Scotland by dune sand; Peruvian archaeological sites buried by coastal dunes. He hopes to soon begin mapping underwater landslides in Sebago Lake, ME and to map the bathymetry and sediment of several Maine estuaries to support the recent EPSCoR Aquaculture project at the University of Maine.
