CHARACTERISTIC FEATURES OF ROCK-FRAMED, GLACIATED ESTUARIES: NORTHERN NEW ENGLAND, USA

To better site expanding aquaculture facilities, manage essential fish habitats and understand the distribution of historic contaminants, better models of estuarine sedimentary deposits and evolution are needed. Existing models, developed in the Coastal Plain, are of little use in rocky, glaciated regions because of differences in processes and materials. High-resolution seismic reflection profiles and side scan sonar mosaics of 6 estuaries in the western Gulf of Maine allow definition of characteristic features of rock-framed, glaciated estuaries. Bedrock structure defines estuarine shape, which controls the spatial distribution of tidal current velocities. In Cobscook Bay, strong currents associated with a 6 m tidal range are confined by a plunging anticline whose limbs create a series of narrow constrictions. Sediment is scoured out of constrictions and modern mud accumulates only where whirlpools form in sub-bays. In rock-framed, linear estuaries like the Saco, Kennebec and Penobscot, high water velocities in the narrow estuaries restrict fine-grained sediment to shallow coves; most of the estuary bottom is floored by rock, sand and gravel. Numerous deep scour holes with up and downstream bedform fields are common. The narrow, confined upper estuaries allow spring freshets to regularly export sediment seaward. Glacial drainage derangement prevents major rivers from entering most embayments, making glacial bluffs and subtidal outcrops primary sources of sediment. In outer areas of the Cobscook, Somes and Damariscotta estuaries, eroding glacial-marine sediment or till crops out extensively and leads to abrupt changes in sediment size and bathymetry. In all bays, random rock outcrops lead to local scour and associated shell and gravel deposits. Late Quaternary sea-level fluctuations formed lakes and bogs during lower sea-level positions. Where modern mud buried these organic-rich sediments, methane evolved and exists beneath the seabed. In muddy estuaries, pockmark fields and sediment mass movements associated with gas permit large-scale sediment reworking. The nearly complete reworking of modern mud has left only small lenses of Holocene sediment in outer estuaries.