Delayed Postglacial Uplift and Synglacial Sea Levels in Coastal Central New England

Abstract

The postglacial uplift pattern indicated by elevations of ice-marginal glaciomarine deltas in coastal New England, deposited between approximately 15,000 and 14,000 yr B.P. during ice retreat from northeastern Massachusetts into southwestern Maine, is very similar to that previously recorded for glaciolacustrine deltas of similar age from inland areas of New England. Multiple regression analyses of elevations from both sets of deltas show an extremely close fit to tilted flat surfaces that rise 0.852 m/km to the N 28.5°W along the coast and 0.889 m/km to the N 20.5°W in western New England. The close similarity of uplift pattern in areas where elevation data are from different base-level media, along with additional shore-line evidence, indicates (1) that both areas are part of the same crustal postglacial uplift block, (2) that postglacial uplift was delayed until after 14,000 yr B.P., and (3) that little or no eustatic sea-level change occurred between 15,000 and 14,000 yr B.P., during which time the margin of the late Wisconsinan Laurentide ice sheet retreated about 100 km from Boston, Massachusetts, into southwestern Maine. Elevation data from even younger glaciomarine deltas in the coastal area indicate that soon after the ice margin reached southwestern Maine and adjacent New Hampshire (ca, 14,000 yr B.P.), eustatic sea level rose rapidly 7-10 m during the time that the ice margin retreated 5-10 km, which may have occurred during an interval of only 50-100 yr, Our new data not only confirm the delayed postglacial uplift model previously described for western New England, but also indicate that little or no eustatic sea-level change occurred during a substantial period of early deglaciation. However, at about 14,000 yr B.P., sea level rose rapidly. Postglacial uplift in the region apparently began between 14,000 and 13,300 yr B.P., before the retreating ice margin reached eastern Maine.

Late Quaternary morphogenesis of a marine-limit delta plain in southwest Maine

“…At some point, glacial unloading triggered initial isostatic recovery of the crust in the Sanford-Kennebunk area. As the rate of rebound approached the rate of eustatic rise, local relative sea level remained relatively stationary for a period of time long enough to allow the growth of a series of highstand ice-contact deltas along the ice-sea boundary (Smith and Hunter, 1989;Thompson et al, 1989;Crossen, 1991;Koteff et al, 1993). As isostatic rebound overcame eustatic sea-level rise, the retreating ice became detached from the shallow-marine water and sea level began to regress rapidly across the area (Fig.…”

“…Subsequent retreat by the calving tidewater glacier occurred in a northwest, parallel to the coast fashion (Smith, 1981) until 13.0-12.5 ka, at which time ongoing glacial unloading and isostatic rebound led to decoupling of the ice and sea (Smith, 1985;Belknap et al, 1987;Barnhardt et al, 1995). Prior to this time, the sea reached its maximum inland extent at ϳ70 m elevation (Koteff et al, 1993), and a series of glaciomarine deltas formed at the marine limit (Belknap et al, 1987;Thompson et al, 1989;Barnhardt et al, 1997). As the ice margin became terrestrial based and glacial retreat shifted to a more northerly direction (Thompson et al, 1995), the sea regressed rapidly due to isostatic uplift (Fig.…”

“…Specific features used to develop sea-level or deglacial chronologies have included the many ice-contact features in the region, such as Merriland Ridge, the Bragdon Road delta, and the Jailhouse delta (Thompson et al, 1989;Crossen, 1991;Koteff et al, 1993), and numerous end moraines (Smith, 1982(Smith, , 1984Smith and Hunter, 1989). Relatively few studies provide absolute dates for landform morphogenesis due to the scarcity of radiocarbon-datable materials.…”

“…Neil, 1997, personal commun. ), and on glaciomarine delta topset-foreset contacts, some reasonable deglacial chronologies have been developed (Smith, 1985;Belknap et al, 1987;Smith and Hunter, 1989;Thompson et al, 1989;Koteff et al, 1993).…”

The Early Lake Ontario barrier beach: evidence for sea level about 12.8–12.5 cal. ka BP beneath western Lake Ontario in eastern North America

“…Rebounded surfaces to the south in New England are tilted but flatter than uplifted shorelines in the Great Lakes region (Koteff & Larsen ; Koteff et al . ; Hooke & Ridge ).…”