RESPONSE OF THE OUTER CAPE COD COAST TO HOLOCENE SEA LEVEL RISE: PART 1, BLUFF SECTION

The eastward-facing unconsolidated Wisconsinan glacial deposits of outer Cape Cod, Massachusetts, have been subjected to wave action throughout the Holocene transgression. Eroding coastal bluffs and nearshore deposits have supplied sediment for construction of spits and barrier beaches to their north and south. However, the construction of those landforms has not been contemporaneous because the rates and direction of sediment transport have varied with sea level.

Our research focuses on the response of this coastal system to relative sea level changes, most importantly the submergence of Georges Bank, during the past approximately 6,000 years. Previous geological reconstructions have suggested that between 6,000 and 1,000 B.P., the bluff section of outer Cape Cod retreated at rates of about 0.3 m/yr at the north end of the section and 0.6 m/yr at the south end. In contrast, published reports of contemporary bluff retreat suggest a century-scale retreat equivalent to approximately 0.8 m/yr for the entire section without significant variation alongshore.

We resurveyed the bluff section during 2003-2004 by means of differential GPS. Using the bluff toe as a datum, we compared our data with those of a similar survey carried out during 1887-1889. The results confirm an average century-scale bluff retreat rate of approximately 0.8 m/yr., but they also indicate that the retreat rate increases linearly from about 0.5 m/yr at the north end of the section to about 1.0 m/yr at the south end. Thus the new results suggest that the pattern of contemporary change is compatible with the change between 6,000 and 1,000 B.P., but they also suggest that the contemporary change is proceeding at approximately twice the earlier rate.

We plan to extend our survey in the near future to include the submarine portion of the bluff section from the beach foreshore seaward to depths of approximately 10 m in order to determine its volumetric retreat rate. These results will be combined with similar data from the depositional landforms north and south of the section to produce a century-scale sediment budget for outer Cape Cod.