R. Craig Shipp

VARIABILITY IN THE EVOLUTION OF TWO ADJACENT BEDROCK-FRAMED ESTUARIES IN MAINE

Casco and Saco Bays are large, adjacent, bedrock-framed embayments in southern Maine. Bottom samples, underwater vibracores, bridge borings, and 500 km of high resolution seismic reflection profiles were used to compare the Holocene evolution of the bays. Each has experienced glaciation, marine inundation, subaerial emergence, as well as contemporary drowning in the past 13,000 years, but they nonetheless differ profoundly in bedrock composition, shoreline environments, bottom sediment, and bathymetry. Casco Bay is divided by islands of metamorphic rock ridges which parallel the coast. Glacial mud, derived from erosion of the “soft” rocks, filled the bay 12,000 years ago, but was deeply gullied and eroded during later emergence. Recent drowning left ravines partly filled with organic-rich sediment (and gas) and interfluves of bare rock. Bluff erosion of marine clay provides most sediment for todays ubiquitous mudflats. Saco Bay is more exposed to waves than Casco Bay and is bordered by granitic rocks. Sand ultimately derived from glacial erosion of granite, formed barrier spits offshore during emergence. Recent sea level rise has resulted in a smooth sandy seafloor, and sand beaches with salt marshes are the predominant coastal environments.

Sedimentary framework of the southern Maine inner continental shelf: Influence of glaciation and sea-level change

Abstract Although the tidally influenced shoreline of Maine is longer than that of virtually any other state, almost no research on its geology has been published. In order to go some way towards remedying this, 1500 km of high-resolution seismic reflection data and 800 km of sidescan sonar imagery have been collected. On the basis of these data and observations made during ten submersible dives, more than 800 bottom samples were collected and evaluated for texture and composition. The understanding of the sedimentary framework of the southern Maine shelf and the processes that maintain it are summarized, and future research directions to evaluate the strategic mineral potential are indicated. In the past 14,000 years, the Maine shelf has experienced a deglaciation and two marine transgressions separated by a regression. The deglaciation was accompanied by the first transgression and deposited till interbedded with up to 40 m of glaciomarine sediment (the Presumpscot Formation) across the shelf. The first transgression culminated about 12,500 yrs B.P., and its landward limit is marked by large glaciomarine deltas 50–100 km landward of the present-day coast. Sea level fell until about 9500 yrs B.P., when shorelines were cut at about the 65 m depth and some large “lowstand deltas” were deposited. Sea level has risen since then and in the general absence of modern river sediment input marine processes have reworked the older sediment. Five shelf environments have been defined in terms of their surficial sediment and stratigraphy. Nearshore ramps are sandy regions extending to about 30 m deep offshore of sandy beaches. These may be reworked lowstand deltas, and possess the thickest bodies of sand in the region. Nearshore basins are mud-filled troughs seaward of coastal areas lacking significant river input. Slumping glaciomarine deposits provide most of the Holocene mud that floors these basins. Rocky zones are extensive areas of exposed rock most common in the 30–50 m depth range. These are areas of high carbonate productivity and provide shell fragments to adjacent areas. The outer basins are muddy regions that begin at the 65 m shoreline and extend beyond the 100 m isobath to the deep Gulf of Maine. Finally, shelf valleys are former stream courses that connect nearshore ramps and basins to the outer basins. These bedrock-framed valleys are filled with sand and mud and are apparently still active conduits for the offshore movement of sediment. The mineralogy of the shelf sediment is complex, and it is not yet clear whether glacial homogenization has obliterated distinctions between river drainage basins and the adjacent shelf. Studies are continuing into northern Maine and analysis of the heavy mineralogy of the shelf sediment utilizing a 2 m Humphrey Spiral to process large grab samples and cores has begun.