Response To Deglaciation
On the Maine coast, evidence of local relative sea level 12.5 ka is now exposed 60-80 m above present sea level. At that time, eustatic sea level was at least 70 m below present in most parts of the world. The difference is due to isostatic depression of the Maine coast by the weight of glacial ice. During deglaciation, the sea advanced inland in contact with the retreating margin of the marine-based ice sheet. Due to isostatic rebound and the contours of the land, the ice sheet grounded as much as 150 km inland of the present coast, glaciomarine deltas formed, and the transgression reached a stillstand at what is termed the upper marine limit. Due to differential tilting during rebound, this marine limit is now over 132 m in elevation at its farthest inlet extent. As rebound became dominant, sea level reached to 65 m below present at about 9.5 ka. At that time rebound slowed to about the same rate as that of eustatic sea-level rise. Shorelines were cut and deltas were formed at this lower marine stillstand position. Subsequently, eustatic rise became the predominant mode. Radiocarbon dates on fossil marine mollusks provide timing for this onlap and offlap.
From 7.0 ka to the present, radiocarbon dates on wood and salt marsh peats provide a relatively precise sea-level curve. During the period 4.2--1.5 ka, sea level rose at 1.22 m/1,000 yrs. Before that period, it may have risen more than twice as fast. After 1.5 ka, it slowed to half the mid-late Holocene rate. Recent tide-gauge records show an acceleration in rate to 2--3 mm/yr for the past 40 yrs. Releveling, tide gauges, and other evidence (Anderson and others, 1984) suggest that the coast is being warped downward to the east, possibly due to non-glacially induced neotectonics.
Figures & Tables
Sea-Level Fluctuation and Coastal Evolution - This Special Publication is the result of a symposium in honor of W. Armstrong Price held at the first SEPM Midyear Meeting at San Jose, California, on August 12, 1984. The factors controlling relative sea-level change along our shores are varied and, at best, imperfectly understood. Yet, the relative rate of change is what controls shoreline erosion, the arrangement of sedimentary facies of the coastal zone, and the character of deformities within the coastal stratigraphic record. Therefore, these papers address sea-level changes, shoreline responses, and the controls on the three-dimensional geometry of the consequent lithosomes; in short, the architecture of the coastal depositional systems.