STRATIGRAPHIC EVOLUTION OF THE INNER CONTINENTAL-SHELF IN RESPONSE TOLATE QUATERNARY RELATIVE SEA-LEVEL CHANGE, NORTHWESTERN GULF-OF-MAINE

Accumulations of deltaic and littoral sediments on the inner continent al shelf of Maine, Gulf of Maine, preserve a record of postglacial sea -level changes and shoreline migrations, The depositional response of coastal environments to a cycle of regression, lowstand, and transgres sion was examined with seismic-reflection profiles, vibracores, and ra diocarbon dates collected from sediments at the mouths of the Kennebec and Penobscot Rivers, Sequence-stratigraphic analysis of these data r eveals two distinctly different successions of late Quaternary deposit s that represent end members in an evolutionary model for this glaciat ed coast. Seaward of the Kennebec River, coarse-grained shorelines wit h foreset beds occur at depths of 20-60 m and outline the lobate margi n of the Kennebec River paleodelta, a complex, rock-framed accumulatio n of glaciomarine and deltaic sediments capped by estuarine and marine deposits, Sand derived from this system today supports large barrier spits and extensive salt marshes, In contrast, the mouth of the Penobs cot River is characterized by thick deposits of glaciomarine mud overl ain by marine mud of Holocene age, including gas-charged zones that ha ve locally evolved into fields of pockmarks, The distinct lark of sand and gravel seaward of the Penobscot River and its abundance seaward o f the Kennebec River probably reflect differences in sediment som ces and the physiography of the two watersheds, The contrasting stratigrap hic framework of the se systems demonstrates the importance of underst anding local and regional differences in sediment supply, sea-level ch ange, bedrock structure, and exposure to waves and tides in older to m odel river-mouth deposition on glaciated coasts. Evolution of shelf de posits was largely controlled by relative sea level, which locally fel l from a highstand (+60 to +70 m at 14 ka) contemporary with deglaciat ion to a lowstand (-55 m at 10.8 ka), The sea-level lowering was accom panied by fluvial incision of older deposits, producing a regressive, basal unconformity, Major rivers deposited abundant sediment over this surface, Sea level then rose at varying rates, extensively reworking formerly emergent parts of the shelf and producing a shoreface ravinem ent surface in areas exposed to waves, A tidal ravinement surface has developed in sheltered embayments where erosion is due mainly to tidal currents. Incised valleys in both settings preserve transgressive est uarine deposits that contain lagoonal bivalves and salt-marsh foramini fera at depths of 15-30 m, These deposits accumulated ca, 9.2-7.3 ka, locally a period of relatively slow sea-level rise.