DEGLACIATION HISTORY OF THE CHINA LAKE AREA, SOUTH-CENTRAL MAINE, BASED ON GLACIOMARINE SEDIMENTS AND NEWLY OBTAINED C-14 AGES

The China Lake 7.5' quadrangle, south-central Maine, was glaciated by the late Wisconsinan Laurentide Ice Sheet. The area experienced late-glacial marine submergence and has marine sediments exposed well above present sea level. The authors mapped the suficial geology of the quadrangle as part of the STATEMAP co-op between the Maine Geological Survey and USGS. Prior to mapping, aerial photographs, topographic maps, well logs, and soils mapping (Faust and LaFlamme, 1978) were used to create a preliminary sediment map. The authors then conducted field work for four weeks in the China Lake area describing sediments in road cuts, gravel pits, auger cores, and collecting in-situ marine shell fossils.

The stratigraphic succession of Pleistocene sediments in the study area includes till, eskers, submarine fans, deltas, glaciomarine silt and clay (Presumpscot Fm.), and subaerial outwash. Several glaciomarine deltas with gravelly foreset beds up to 10 m thick are present south of China Lake, including the Meadow Brook delta. At the top of the Presumpscot Fm., in-situ shell fossils were collected. Two new radiocarbon ages (uncorrected for the marine-reservoir effect) have been obtained: Palermo site, 11,980+/-230 yrs BP, GX# 31328, Mytilus edulis, elev. 73 to 76 m, UTM 460,593mE 4,926,820mN; and E. Vassalboro site, 13,400+/-70 yrs BP, GX# 31329, Portlandia arctica, elev. 58 to 63 m, UTM 450,750mE 4,923,230mN.

Marine sediment is present at elevations up to at least 90 m a.s.l. in the quadrangle, and this represents a minimum elevation for late-glacial sea level in the China Lake area. The Meadow Brook delta has a surface elevation of approximately 96 to 98 m, but the topset-forset contact was not visible. Deglaciation of the study area occurred by approx. 13,400 C-14 yrs BP (E. Vassalboro site). This age correlates well with published deglaciation isochrons (Retelle and Weddle, 2001; Borns and others, 2004). The Palermo radiocarbon age is much younger than expected based on an analysis of relative sea-level change and postglacial uplift by Retelle and Weddle (2001). By 11,980 C-14 years ago relative sea level should have been much lower in the area. This large age discrepancy is poorly understood at this time, but the large error bar in this date might be an indicator of poor accuracy.