POST-GLACIAL STRATIGRAPHY DERIVED FROM 2-D CHIRP DATA AT AMBAJEJUS LAKE, NORTH-CENTRAL MAINE

Ambajejus Lake lies in an area of central Maine that was glaciated most recently by the Wisconsinan Laurentide ice sheet. Present-day central Maine is dominated by glacial deposits including Rogen moraines and bedrock-cored hills and mountains. Ambajejus Lake sits within the Pemadumcook chain of lakes, which is a set of lakes that have been connected into one main water body by damming. Presently, the West Branch Penobscot River feeds into Ambajejus. However, evolution of the stratigraphy within Ambajejus remains unclear. Therefore, the purpose of this study is to describe the stratigraphic evolution as elucidated by 2-D CHIRP acoustic and core data.

To determine the stratigraphic evolution of Ambajejus Lake we interpreted 12 2-D CHIRP acoustic data lines collected in a grid. Three CHIRP facies were mapped on each of the lines, though biogenic gas locally obscured contacts. CHIRP facies thicknesses were used to build isochore maps. Additionally, one 9.5 m core that penetrated the upper two seismic facies was analyzed. The sediments in the core are organic-rich silt. However, no discernable changes in the core sediment are tied to CHIRP reflectors. The changes in acoustic properties of sediment that result in the reflections are still under investigation.

The basal unit is consistent with glacial deposition whereas the upper two units are post-glacial. The basal unit is present throughout the CHIRP lines, with no to locally weak internal reflectivity. The unit occurs both as elevated areas of bathymetry and below accumulations of the upper units. Elevated areas commonly have m-scale bumps interpreted as boulders. This unit is interpreted as glacial till and the elevated areas are submerged Rogen moraines, similar to those surrounding the lake. The middle unit is 2-5 m thick on average and has internal wavy and disjointed CHIRP character. This unit pinches out against the elevated areas of the basal unit. The upper unit has highly variable thickness; thinning toward the edge of the lake and thickening to ~10 m near the elevated parts of the basal unit. A radiocarbon sample from the base of the middle unit is 11.1±0.2 ka (OS-170366), suggesting deposition rates of up to ~1 m/ka from the West Branch into Ambajejus. The sedimentation rate recorded in Ambajejus appears to be one of the highest for the large lakes in Maine.