GEOLOGIC EVOLUTION OF NORTHERN NEW ENGLAND SALT MARSHES

Salt marshes within the western Gulf of Maine (WGOM) have similar morphologies, ecosystems, and geologic histories. They occur in cool-temperate climates, meso-to-macro tidal ranges, and variable degrees of ice impact. They form on glaciated coasts of complex morphology, ranging from rocky headlands and embayments in Maine to broad sandy outwash plains on Cape Cod. In Maine, Late Holocene sea-level rise is recorded in fossil salt marsh peats over the past 7300 cal. years and 15 m of sea-level rise. Rates of rise were as high as 4.5 mm/year 6-7 ka, fell to 0.5 mm/yr 1-2 ka, and rose to 1-2 mm/yr over the past 200-300 years. Salt marshes in WGOM appear to be able to keep up with these documented rates of rise. There are several categories of salt marsh in WGOM: 1) Broad back-barrier marshes, characterized by low slopes, abundant high (HM) and higher-high marsh (HHM) and common pools, 2) Tidal river, elongated and dominated by changes in stream positions, with steeper slopes and narrower ecozones, 3) Estuarine fringe, with steeper slopes, narrower zones, and commonly bordering broad tidal flats and/or low marsh (LM) colonizing areas, 4) Pocket back-barrier, characteristically smaller and more subject to variations in salinity due to inlet closures, openings, and overwash. Within most of the four categories, the leading edge of the transgression is an intersection with upland and/or wetland ecosystems, that is a sensitive ecotone experiencing short- and long-term changes, and is critical to understanding the development of the sea-level record. The stratigraphy of these marshes was first characterized by Mudge (1858) as (what we would today call) a transgressive overlap. Shaler (1885) envisioned a prograding, regressive succession. Redfield (1972) saw the Barnstable marsh as both landward transgression and bay-ward colonization. In Maine we find the Redfield model holds in broad systems, with the additional understanding of larger distance transgression and a deeper stratigraphy. Gehrels (1994) refined this further by recognizing the migrating tidal river system with stratigraphic preservation dependent on meandering. The stratigraphic succession may have important influences on the marsh systems due to thickness of peat, autocompaction, groundwater input, stability of creeks, and the development and dynamics of pools.