SURFICIAL GEOLOGY OF THE BRUNSWICK 7.5' QUADRANGLE, MAINE

A surficial geologic map of the Brunswick 7.5-minute quadrangle was produced as part of the 2000 STATEMAP component of the National Cooperative Geologic Mapping Program. Data sources include new field data from detailed mapping augmented by data from the Maine Geological Survey Significant Sand and Gravel Aquifer Maps and inventory of bedrock water wells, student theses, subsurface borings and geophysical logs from the Topsham-Brunswick Bypass Project, the Brunswick Naval Air Station Superfund site, water-supply company wells, and private construction sites. Eighty detailed subsurface logs at the air station were correlated using GMS, a 3-D interpretive software program.

The surficial geology in the Brunswick quadrangle reflects the advance and retreat of the late Wisconsinan Laurentide ice sheet, and subsequent Holocene events and deposits. The marine transgression and regression, which occurred during and following deglaciation, is especially well documented in borings from the bypass and superfund sites. The shallow geologic sequence records the transition by late-glacial isostatic emergence from marine to near-shore conditions and the deposition of the Brunswick sand plain. GMS modeling incorporates test-boring logs from the central part of the sand plain and represents the subsurface stratigraphy. Surficial sand of the plain overlies a sandy silt zone that includes discrete, correlated sand units. Beneath the sandy deposits, thick glaciomarine mud overlies sand and till lying on top of bedrock.

The data support the interpretation that the sand plain is a coastal braid-plain delta formed during a period of relative sea-level stability as emergence continued. Radiocarbon ages from the region constrain the time of formation of the sand plain to between 12.5 and 12 ka. When corrected for marine-reservoir effect, the age of the sand plain overlaps with meltwater pulse-1A, an interval of rapid eustatic sea-level rise.