CHIRP SEISMIC DATA SUGGESTS GLACIAL DEPOSITIONAL SETTING FOR LAKE SYSTEM IN CENTRAL MAINE

The extent of the last major sea level transgression in central Maine is not fully understood. The border of the White Mountains and the Penobscot Lowlands is thought to represent the maximum extent of the most recent sea level transgression as these terrains have been interpreted to record glacial and marine stratigraphies, respectively. Along this transition, marine delta sediments have been found at elevations up to 115 m (Thompson et al., 1989) above present day mean sea level. However, a system of lakes at higher elevations to the west have an uncertain depositional origin. If they contain marine sediments, then mean sea level could have been even higher than predicted.

We compared CHIRP seismic data from the Ambejejus (AL), Pemadumcook (PL), and Millinocket (ML) lakes from this lake system. We collected profiles in a grid pattern from each of these lakes using a Stratobox 3510 during the spring and summer of 2020. AL is fed by the west branch of the Millinocket River and feeds into the PL, whereas ML is fed by the smaller Sandy Stream system. We characterized the seismic units from each lake and compared the results. The lakes each expressed three similar seismic units; a dark gray unit (lowest, unit 1), a unit containing parallel seismic reflectors (middle, unit 2), and a light gray unit with no internal reflectors (upper, unit 3). We then mapped the three units from each lake to compare their sediment distributions.

Our maps show sediment thicknesses of 0 – 5m in each unit and similar sediment distribution patterns across the three lakes. Unit 1 covers the entire extent of each study area. Unit 2 appears to overlie unit 1, but never unit 3, and was only absent in areas of elevated topography. Unit 3 covers units 1 and 2, but only appears in areas distal from shore and in areas of depressed topography. We interpret unit 1 as glacially deposited sediments, unit 2 as older stratified lake sediments, and unit 3 as recently deposited lake sediments. PL and ML both express NE-SW trending ribbed moraines but this feature is absent in AL. We think these lakes formed under the same depositional environment, possibly a glacial front, and will use the mapping in this study to inform the collection of lake cores inorder to further characterize these sediments and test our interpretation.