GLACIOMARINE DELTAS IN SOUTHWESTERN MAINE: INDICATORS OF LATE-GLACIAL RELATIVE SEA LEVEL AND ISOSTATIC UPLIFT

Elevations of contacts between topset (T) and foreset (F) beds in Maine's glaciomarine deltas indicate positions of relative sea level (RSL) to which the deltas were graded. Recently improved exposures of T/F contacts enabled us to precisely survey their elevations and refine our earlier profiles of the isostatically tilted marine-limit plane. Both definite and tentative T/F contacts of 41 deltas in southwestern Maine were projected onto a profile trending S30°E, parallel to the average slope of the marine-limit plane. From Berwick NNW to Sanford, the deltas lie on a line that slopes seaward at 0.91 m/km. Between Sanford and the inland marine limit in the upper Kennebec Valley, the 15 highest deltas (mostly ice-contact and including many of the best new exposures) are within 1 m of a straight line sloping 0.50 m/km over a profile distance of 127 km. This reduced gradient is attributed to accelerated isostatic uplift as the ice margin receded north from Sanford and deltas were graded to lowering RSL. Two small valley-side deltas formerly considered as marine (Baldwin and Waterboro) lie above the profile line and are now believed to be glaciolacustrine. Other marine deltas plot below the line, but all of them are in a parallel envelope within 5-7 m of the marine-limit profile. The lower sea levels indicated by the latter deltas are attributed to: (1) having surveyed pseudo T/F contacts produced when the original delta plains were downcut by meltwater or meteoric streams as uplift occurred; (2) pseudo T/F contacts produced by marine erosion of the topsets and unconformable deposition of nearshore gravels on eroded foresets; and (3) artifacts of the profile orientation. The lower levels are believed to record at least one stillstand of RSL during marine regression. This study has also generated a set of criteria for identifying fluvial/marine erosion of the deltas based on sedimentary structures, sorting characteristics, and other properties.