ABSTRACT

Geological controls on the origin and preservation of organic-rich facies in Holocene barrier sequences of northern New England are documented using sedimentological and geophysical databases. In addition to backbarrier marsh interfingering with washover/aeolian deposits, several distinct modes of organic accumulation are recognized in association with clastic barrier facies. These include: 1) basal lake gyttja or wetland peats (thickness: 0.1–2.5 m); 2) intra-barrier saltwater/freshwater horizons (0.01–1.2 m); and 3) foreshore peat exposures (up to 1.2 m). Freshwater peat and gyttja underlying barrier lithosomes below contemporaneous Mid-Holocene sea level suggest extensive backbarrier lake and wetlands, possibly due to a wetter climate. Present freshwater organics accumulate up to several meters below lake level, which is controlled by the elevation of groundwater table and, over the long term, sea level trends. In some areas, the saltwater-freshwater peat transition is attributed to cessation of long-term saltwater input into the backbarrier, commonly as a result of tidal inlet closure, rather than relative sea-level fall. The association of well-preserved tree stumps and saltmarsh peat exposed on the foreshore suggests drowning of the upland fringe by rising sea level around 2.5–3.0 ka BP and subsequent barrier rollover.

Whereas basal ages of high-marsh peats are conventionally used for sea-level reconstruction, radiocarbon dates of the top portions of in-situ organic units may provide near-maximum ages of burial by barrier sediments. This information is independent of compaction and type of dated material and may be used to estimate the timing of barrier emplacement at or near its present position. Age estimates are less constrained when dating allochtonous organic material, but may still provide upper age limits for overlying clastic sequences. Similar to other mid- to high-latitude coastal settings, parts of maritime bogs and ponds in mid-coastal Maine have been buried episodically by storm overwash or dune migration over the past 5,000 years. The chronology of these events varies over a short distance along the coast as a function of barrier exposure and morphology, antecedent geology, and changes in sediment supply and vegetation cover.

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