GEOPHYSICAL ANALYSIS OF GLACIO-LACUSTRINE STRATIGRAPHY WITHIN LAKE BOTTOMS OF THE NORTH AND SOUTH TWIN LAKES NEAR MILLINOCKET, MAINE

Global ice sheet incursions in the Pleistocene were a dominant geologic force with extensive impacts on the dynamics of erosion, deposition, fluvial systems, and isostatic adjustment of Earth’s crust. The record of glaciation in Maine is quite complex. One largely untapped archive lies within lake bottoms. An issue that arises when studying lacustrine stratigraphy is the inability (without cores) to examine the detailed depositional record. One method used to combat this issue is CHIRP (Compressed High-Intensity Radiated Pulses) imaging, a high-resolution seismic reflection technique. Here, we interpret data from 44 2D CHIRP lines collected in North and South Twin Lakes west of Millinocket, Maine. The objective is to examine glacio-lacustrine stratigraphy within lake bottoms of North and South Twin Lakes, the southernmost lakes in the Pemadumcook chain of lakes. CHIRP data reveal three distinct reflectors in both lakes. The basal layer is a dark grey undulatory layer that is present in all CHIRP profiles. It is akin to the ribbed moraine found on the lake shore deposited directly by Pleistocene glaciers. This layer is typically exposed at the lake bottom in shallow parts of the lake indicating a lower lake level at the time of deposition of the layers above. The middle layer is well-structured, with parallel, continuous dark (thin) and light (thick) bands. This layer is typically found in deep depressions of the basal unit in both lakes. The distribution of the middle unit suggests initial lacustrine deposition following glacial melting. The uppermost layer is a lighter package that contains anomalies identified as boulders. This indicates deposition beyond the terminus of the glacier where icebergs could drop boulders into a body of water. This layer is typically exposed at the surface where the basal layer forms a depression in the subsurface. Both lakes contain the same CHIRP facies, but the middle sediment package is more widespread within North Twin, whereas in South Twin the middle package is confined to central portions of the lake. Due to the distribution of the middle and uppermost packages and dam construction in the Pemadumcook lake chain, we think North Twin was once part of the main drainage of the west branch of the Penobscot River and South Twin was once an isolated lake.