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ABSTRACT: Effects of Sea Level Rise on Rugged Coasts

Daniel F. Belknap

Rugged coasts are usually rock framed, with more or less discontinuous unconsolidated sediment supply. Evolution of rugged coasts with respect to rising sea level is locally variable, involving rapid changes in facies over short distances and times. Glaciated and tectonic coasts are common examples. Three aspects of coastal evolution dominate rugged coasts during rising sea level: (1) drowning of resistant rocky shores, (2) bluff erosion in sediments or cliff erosion in less resistant rocks, involving marine abrasion as well as terrestrial processes and control by rock structures, and (3) larger scale structural control creating indentations and salients. The third aspect results in embayed coasts that may evolve as estuarine, fjord, or barrier-backbarrier systems. Of ove riding concern for stratigraphic interpretation are paleogeography and sediment sources. The Maine coast and nearshore are classic examples of drowned shoreline, and record the effects of glaciation and rapid relative sea level changes over the past 14,000 yr. Offshore at -60 m is a lowstand shoreline (approximately 10,000 yr B.P.), which ranges from cliff to barrier split(?). The inner shelf has evolved through drowning of shelf valleys and peninsulas, bluff erosion, and formation of local transgressive barrier-backbarrier systems. Often, short-term steady state equilibrium is established until a threshold is reached, followed by rapid reorganization at a new location. This occurs in barrier systems, by overtopping of sills in estuaries, and in variable bluff and cliff erosion sites. Pr servation potential is high only in valley axes and at sea level stillstand positions.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990