INNER SHELF AND SHOREFACE OF SOUTH-CENTRAL MAINE – RAVINEMENT SURFACE AND PRESERVATION OF HOLOCENE TRANSGRESSIVE STRATIGRAPHY

The inner shelf of the northern Gulf of Maine was profoundly influenced by post-glacial sea-level changes from a highstand 15 cal. ka 60-80 m a.s.l. to a lowstand 12.5 cal. ka –60 m and the subsequent rise and transgression. The postglacial phases were strongly dominated by isostasy. Fall to lowstand created a landscape reworked by littoral processes and incised by fluvial systems, with localized regressive deltaic deposits. During Holocene transgression, early rapid rates of sea-level rise (up to 8 m/thousand years) resulted in minimal reworking of moraines and other shelf features, but also preserved only a limited transgressive sequence or allowed only minimal sediment accumulation. Stabilization of sea level at a “slowstand” ca. –20 m from 11.5 to 8 cal. ka resulted in local progradation, but primarily extensive reworking of nearshore sediments. Renewed rise at rates of 5 m/thousand years 8 cal. ka slowed progressively to the late Holocene 0.5 m/thousand years. Coastal systems in sandy embayments in southern and south-central Maine developed transgressive stratigraphies and localized regressive sequences, while the inner shelf ravinement unconformity progressively erased the record. The modern shoreface records estuarine, lagoon, barrier and nearshore wedges that pinch out down to 15-20 m below present sea level in Wells Bay, Saco Bay, Popham-Reed Beaches, and Pemaquid Beach. These pocket beach systems in more or less headland-enclosed bays efficiently recycle sediments from the ravinement surface back into the nearshore. Other sediment sources include erosion of glacial sediments (moraines, drumlins) and fluvial input. There is no connection to sandy lowstand terrace sources in Wells or Saco Bay, but the extensive Kennebec Paleodelta may continue to supply Popham and Reed Beach systems. Preservation potential of the Holocene transgressive stratigraphy is dependent on depth of shoreface erosion, sediment supply, and principally on paleotopography in incised valley fill, paleo-inlets, and areas sheltered by bedrock and till.