ABSTRACT Accumulations of deltaic and littoral sedi- ments on the inner continental 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 trans- gression was examined with seismic-reflection profiles, vibracores, and radiocarbon dates col- lected from sediments at the mouths of the Kennebec and Penobscot Rivers. Sequence- stratigraphic analysis of these data reveals two distinctly different successions of late Quater- nary deposits that represent end members in an evolutionary model for this glaciated coast. Seaward of the Kennebec River, coarse- grained shorelines with foreset beds occur at depths of 20–60 m and outline the lobate mar- gin of the Kennebec River paleodelta, a com- plex, rock-framed accumulation of glacioma- rine 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 Penobscot River is characterized by thick deposits of glaciomarine mud overlain by ma- rine mud of Holocene age, including gas- charged zones that have locally evolved into fields of pockmarks. The distinct lack of sand and gravel seaward of the Penobscot River and its abundance seaward of the Kennebec River probably reflect differences in sediment sources and the physiography of the two water- sheds. The contrasting stratigraphic frame- work of these systems demonstrates the impor- tance of understanding local and regional differences in sediment supply, sea-level change, bedrock structure, and exposure to waves and tides in order to model river-mouth deposition on glaciated coasts. Evolution of shelf deposits was largely con- trolled by relative sea level, which locally fell from a highstand (+60 to +70 m at 14 ka) con- temporary with deglaciation to a lowstand (–55 m at 10.8 ka). The sea-level lowering was accompanied by fluvial incision of older de- posits, producing a regressive, basal unconfor- mity. Major rivers deposited abundant sedi- ment over this surface. Sea level then rose at varying rates, extensively reworking formerly emergent parts of the shelf and producing a shoreface ravinement surface in areas exposed to waves. A tidal ravinement surface has de- veloped in sheltered embayments where ero- sion is due mainly to tidal currents. Incised valleys in both settings preserve transgressive estuarine deposits that contain lagoonal bi- valves and salt-marsh foraminifera at depths of 15–30 m. These deposits accumulated ca. 9.2–7.3 ka, locally a period of relatively slow sea-level rise. INTRODUCTION Large excursions in relative sea level, a combi- nation of eustatic and local isostatic effects, oc- curred in the northwestern Gulf of Maine during the latest Pleistocene and Holocene, and exerted great influence on the stratigraphy and morphol- ogy of the inner continental shelf. Determinations of former sea-level positions are largely based on seismic observations of submarine terraces, in- ferred to be relict shorelines and river deltas now submerged at various depths across the region (Oldale et al., 1983; Belknap et al., 1987; Fader, 1989; Shipp et al., 1991; Kelley et al., 1992; Stea et al., 1994; Loncarevic et al., 1994; Barnhardt et al., 1995; Forbes et al., 1995). Some of these fea- tures were originally interpreted as submarine “scarps” of structural (fault traces), not marine origin (Johnson, 1925), and many are found within distinct depth ranges. One hypothesis is that some may represent Gilbert-type deltas for- merly graded to lower sea level; a second is that they are submerged shorelines (Schnitker, 1974; Shipp et al., 1991). Alternatively, these features may have a more complex stratigraphic architec- ture composed of glacial and glaciomarine sedi- ments that are merely capped by a thin marine se- quence (Shipp et al., 1991). Arguments that the shorelines are older features, such as ice-contact glaciomarine deltas or subaqueous fans, with no real significance to postglacial relative sea-level fluctuations, cannot be rejected without direct evi- dence from coring. This paper synthesizes the postglacial evolution of the Maine inner continental shelf on the basis of new seismic-reflection and core data from the mouths of Maine’s two largest rivers, the Ken- nebec and Penobscot (Fig. 1). Despite sharing a common history of glaciation and relative sea- level change, and drainages with similar bedrock (Osberg et al., 1985), shelf deposits in these two areas exhibit striking differences in morphology, composition, and internal structure. The Kennebec River deposited a large sandy lithosome, or “pa- leodelta” (Belknap et al., 1986). The Penobscot River, in contrast, lacks sandy deposits at its mouth and is characterized by thick, gas-charged depos- its of marine mud and numerous pockmarks eroded into the surface (Scanlon and Knebel, 1988; Kelley et al., 1994). Buried estuarine depos- its are found in both locations, however, and pro- vide important clues to the interpretation of Holo- cene relative sea-level and shoreline changes. Previous models for the transgressive evolu- tion of shelf sand bodies, developed for the United States mid-Atlantic continental shelf (Fischer, 1961; Swift, 1975; Sanders and Kumar, 1975; Belknap and Kraft, 1985; Ashley et al., 1991) and the Mississippi River delta (Penland et al., 1988), differ from the Maine examples pri- marily in scale (much larger), geographic setting (unglaciated coastal plain), and rates of relative sea-level rise (at least eight times slower). The overall setting in Maine compares more favor- 612 Stratigraphic evolution of the inner continental shelf in response to late Quaternary relative sea-level change, northwestern Gulf of Maine Walter A. Barnhardt* Department of Geological Sciences, University of Maine, Orono, Maine 04469-5711 Daniel F. Belknap } Joseph T. Kelley Maine Geological Survey, 22 State House Station, Augusta, Maine 04333-0022 GSA Bulletin; May 1997; v. 109; no. 5; p. 612–630; 14 figures; 1 table. *Present address: U.S. Geological Survey, M.S. 999, 345 Middlefield Road, Menlo Park, California 94025. E-mail: wbarnhardt@usgs.gov