Abstract Over 75 percent of the United States ocean shoreline is eroding retreating landward Shoreline progradation where occurring is generally assumed to be a temporary phenomenon When affecting a developed area shoreline retreat is usually termed erosion but considerable confusion remains over the use of this term Retreat and progradation refer to achange in shoreline position whereas erosion and accretion refer to volumetric changes in the subaerial beach As used in this paper however erosion refers to any form of shoreline retreat consistent with common usage. Coastal erosion is a fundamental and widespread process on U S and world shorelines Evidence particularly on barrier island coasts indicates that in the past few decades or millenia erosion may have become a more widespread process Possible causes of this change include the effect of humans shoreface steepening or an increase in the rate of eustatic sea level rise. Mechanisms responsible for shoreline erosion are highly variable both temporally and geographically. In addition, our understanding of shoreline sediment transport dynamics is incomplete. Consequently, we are presently unable to predict accurately future shoreline-retreat rates related to continued sea-level rise. The Bruun Rule, for example, predicts little shoreline retreat relative to using, as a predictive tool, the slope of the land surface over which sea level is expected to rise.

Abstract The Holocene began about 10.7 ka following the glacial readvance of the Younger Dryas (Valders) interval that terminated the Pleistocene. World sea level had fallen to about -54 m. Nearly half the present United States continental shelf was then a coasta] plain with vegetation ranging in nature from subarctic tundra to coniferous woodland. A fluctuating transgression ("Flandrian" stage) followed, accompanied by rising temperatures. Reaching the present coastline about 6 ka, the transgressing seas drowned river valleys, creating estuaries and dendritic embayments. As barrier spits and islands developed, the estuaries and embayments became lagoons. During the later Holocene, world sea level was modulated by numerous negative fluctuations, signa Jing cool intervals that are indicated by pollen analyses and neoglacial advances. Extra-warm cycles were characterized by higher storm frequency; in areas favorable for preservation, these conditions are recorded by distinctive sets of beach ridges. Local paleogeography was dominated in most places by a tectonic “groundswell,” such as is presently found throughout the collapsing forebulge regions along subducting plate margins of southern Alaska and on the taphrogenic coast of California. More or less stable platforms are found only in northwest Alaska and Florida. Present coasts are widely affected by the post-Little Ice Age warming, which has led to steric and glacio-eustatic sea-level rise. The warming has also slowed the velocity of the Gulf Stream and other geostrophic currents, reducing the Coriolis-controlled dynamic tilt and causing further sea-level rise along the mainland coasts.

Abstract Sea-level changes have had an important influence on the distribution of late Quaternary inner continental-shelf sediment in the western Gulf of Maine. Previous stratigraphic models of sea-level change in the region were based on terrestrial observations and a large quantity of offshore high-resolution seismic-reflection data. These models, however, were not constrained by core data. Integration of new vibracore data with earlier observations indicates that nearshore regions were (1) probably deglaciated and subjected to glacio-marine conditions around 13.5 ka, (2) subaerially exposed by a fall in sea level sometime after 11 ka, and (3) flooded by a transgressing sea following an inferred lowstand of sea level between 11 and 9 ka. The greatest amount of sediment accumulated on the shelf during the initial transgression, under glacio-marine conditions. Sandy fluvial sediment accumulated in large quantities during the following regression and early Holocene transgression. Sediment influx from eroding bluffs of glacial origin was significant throughout the Holocene transgression, especially in regions lacking a fluvial source.

Abstract Data from seismic-reflection profiles, sidescan-sonar images, sediment cores, and surface samples outline the shallow stratigraphy and surficial sedimentary environments of the Boston Harbor estuary. Bedrock and till of pre-Wisconsinan age form an irregular acoustic basement surface that variably crops out or is buried as much as 35 m below the harbor floor. Where buried, this surface is overlain by discontinuous ice-proximal glacial deposits and by glacio-marine muds. These accumulated in late Wisconsinan time when ice retreat and marine submergence occurred simultaneously owing to crustal depression. During the immediate postglacial period, sea-level regressed due to crustal rebound, and texturally diverse fluvial and estuarine sediments were deposited in small channels that were cut into the subaerially exposed upper Wisconsinan drift. As the harbor was resubmerged in Holocene time in response to the eustatic rise of sea level, waves beveled the substrata, and localized deposits of marine sands and muds accumulated atop the trans-gressive unconformity. The modern wave and current regime in the harbor area has produced three surficial sedimentary environments characterized by erosion, deposition, and sediment reworking, respectively. Areas of erosion are characterized by bedrock outcrops and coarse lag deposits that have been scoured by tidal currents and waves; they are common along mainland and insular shorelines, within large channels having strong tidal currents, and across much of the harbor entrance. Areas of deposition consist of sandy and clayey silts and are found on shallow subtidal flats and in broad bathymetric lows where tidal currents are relatively weak. Environments of sediment reworking are characterized by intermittent erosion and deposition and have diverse grain sizes; they are associated with many different bathymetric features and are indicative of highly variable bottom currents.

Abstract Plymouth, Kingston, and Duxbury Bays (herein referred to as Plymouth Bay) form a large reentrant (area = 46 km 2 ) along the south shore of Massachusetts approximately 57 km south of Boston. The location of the barriers, which front the bay, is controlled by bedrock, drumlins, and other glacial deposits that provide the sediment sources and pinning points for sand accumulation and the development of barriers and spits. A continual sediment supply derived from the reworking of glacial deposits, combined with a barrier alignment that funnels sediment into the bay, created an ideal sink during the past 6,000 years that resulted in accumulation of up to 35 m of sediment. The sediment distribution of the bay fill is controlled, in part, by the location of the inlet, major channels, and degree of sheltering within the bay. Analyses of 42 km of high-resolution seismic and sidescan-sonar profiles, 18.5 km of ground-penetrating radar transects, 336 bottom samples, and 15 vibracores indicate that the present configuration of the embayment and position of the major channels are closely tied to the paleotopography of the region. The existence of a major drainage valley, formed during the late Tertiary and operative during deglaciation, is also recognized. The back barrier is comprised of extensive intertidal flats (62 percent of the back barrier is exposed at mean low water [MLW]), shallow bays and channels, and intertidal and supratidal marsh. Modification of Plymouth Bay and its barrier system during the Holocene has been a product of cyclic barrier progradation followed by destruction and subsequent landward translation of the shoreline. The variety of the back-barrier stratigraphy reflects not only the cyclic barrier transgression, but also the distance from the main inlet channel. The thin nature of the barrier spits and the existence of numerous washovers and flood-tidal-delta deposits associated with the many historical inlets that occurred along the spits are evidence that the barriers are in a transgressive phase. Radiocarbon dates of basal peats in the northern part of the study area indicate that relative sea level has been rising at a rate of about 1.1 mm/yr over the past 3,700 years. A radiocarbon date obtained 200 m seaward of the foredune ridge at an elevation of the beach face indicates that the barriers have been migrating landward at an average rate of 0.27 ± 0.05 m/year.

Abstract The Provincelands in the Cape Cod National Seashore developed about 5 ka from wind- and waterborne Outer Cape Cod outwash sands as sea level rose to submerge offshore banks. A late Holocene chronology of sand dunes in the Provincelands is established from radiocarbon-age determinations on the basal organics from interdunal bogs and ponds. The Provincelands ponds, thought to be lakes trapped behind hooked spits, are shown here to have developed from bogs similar to those in the extant parabolic-dune field. An older, stabilized dune field in the location of the ponds is hypothesized. Episodes of alternating dune movement and interdunal-wetland formation in the Provincelands correlate with climatic changes in the North Atlantic and elsewhere during the last 1.2 ka, as interpreted from ice core, glacier movement, sea-surface temperature, tree-ring, pollen, and historical data. The dune field data suggest a periodicity of change of 0.2 ka. Before about 1.2 ka, dunes in the Provincelands were active. Between about 1.15 and 0.9 ka, bogs formed which, at least in the west-central area of the Provincelands, developed into ponds having continuous organic sedimentation until today. This evidence indicates that the west-central dune field stabilized after 1.1 ka. Between 0.9 and 0.7 ka, dunes were active in the north and east-central Provincelands, but from about 0.7 to 0.5 ka, bogs formed within that dune field, and at least one bog in the west-central area became a pond, both events indicating a warm and wet climate. During the Little Ice Age (0.5 to ∼0.2-0.1 ka), the Provincelands dunes were active, suggesting a cold, windy, and dry climate. Because of a warmer and wetter climate during the last 0.1 ka, bogs have formed again within the dunes.

Abstract The sedimentologic development of Horseneck Beach on the northwest coast of Buzzards Bay, Massachusetts, is interpreted from 40 vibracores, 20 boreholes, 9 km of ground-penetrating radar and eight topographic profiles. The beach-ridge barrier is 4 to 10 m thick and consists primarily of fine sands with some coarse sand and gravel layers underlain by a coarsening-upward estuarine sequence (3-8 m thick) composed of silts and clays grading to fine sand. Underlying the estuarine deposits are glacio-fluvial sands and gravel up to 6 m thick, blanketing a Paleozoic bedrock surface. Ground-penetrating-radar records and backhoe trenching indicate that progradation of the barrier occurs sporadically. An inferred-process model is proposed in which the accretionary phase represents a period of abundant sand supply when the beach widens, builds vertically, and is punctuated by a period of low-sediment supply and erosion. At the eastern end of the barrier, the erosional phase coincides with an influx of gravel, steepening of the beach profile, and ridge construction, which is probably controlled by the 100-year, or longer, storm frequency. During these infrequent events, sand and gravel are released from an offshore drumloid. Segregation of the sand and gravel results from differences in longshore sediment-transport rates.

Abstract The development of coastal-barrier lagoons is strongly governed by the primordial character of the lagoon floor, sea-level fluctuation and sediment input. During transgression, rising sea level affects lagoon capacity in several ways. When boundary conditions are laterally stable, flooding of the lagoon increases the capacity of the basin for sediment storage. However, shorelines associated with the mainland and shoreface are quite mobile during transgression. Relative movements of the inner and outer shorelines of a lagoon also affect lagoon capacity. At wave-dominated coasts, the floors of barrier lagoons are initially smooth after lagoon formation, and basin infilling is dependent on changes in capacity relative to sea-level rise and sediment input from siltation, runoff, inlets and cross-island transport. However, lagoons are displaced laterally long before basins are filled. As a result, basin filling by upbuilding is rarely complete and open-water lagoons tend to stay open. At tide-dominated coasts, the lagoon floors are irregular and reflect the antecedent topography of pre-transgressed landscape. The interfluve areas of the topography produced very shallow areas in the lagoon that form tidal flats or are colonized by marshes. Along many sections of coast, sediment supply is insufficient to keep pace with the rate of increasing lagoon capacity. Consequently, many of the lagoons along the middle Atlantic, Gulf and west coast of North America may have initiated as marsh lagoons and have evolved into open-water lagoons. Thus, marsh lagoons (particularly in low-sediment-input areas of the middle Atlantic States) are not the climax stages of a long upfilling sequence, but are the initial stages of inundation in which marsh colonization occurs over shallow topographic surfaces.

Abstract Stage 5 deposits (unit Q2) have been identified along the inner shelf of Maryland. Peak sea-level substage 5e deposits (lower Q2) consist of (buried) shoal-forming sands on the inner shelf, analogous to modern shelf sands, and relict barrier facies onshore. Substages 5d-5a, however, are represented by geographically widespread, thick, fossiliferous muds (upper Q2), whose age range was determined by amino-acid racemization. The time frame of mud deposition was further subdivided into stage 5 substages. Ostracode assemblage zones in unit Q2 record a consistent, repetitive sequence of four rapid and distinct climatic fluctuations that follow the isotopic excursions in stage 5. Sea levels remained high for the duration of stage 5, fluctuating within a maximum range of 30 m. The first physical evidence of substage 5d sea levels being higher than -23 m mean seal level (MSL) is contained within ostracode zone 2 in unit Q2. An anomalous lithofacies such as a thick mud deposited in a relatively high sea level requires alternative sediment sources and shelf sedimentation processes than those operating on the present and substage 5e transgressive sandy shelves. Sea level during stage 5 along the Maryland coast consisted of a prolonged period (=45,000 yrs) of water covering the shelf at less than maximum sea levels, following the substage 5e peak highstand (+ 6 m MSL) spanning 11,000 yrs. Stage 5 may be modeled as follows: Early stage 5 transgressive facies migrated across the shelf and were deposited at their peak position during substage 5e (at 125 ka), when sea level reached +6 m MSL. Late stage 5 sea levels maintained a range of —23 (extreme minimum) to +0.1 (present) m MSL and deposited thick, laminated, unbioturbated muds containing numerous radiations of Mulinia lateralis (Say). Onshore facies of late stage 5 include a barrier system (substage 5c at +0.1 m MSL) and tidal flats and/or marshes (of substages 5d, 5b, and 5a). It is inferred that at slightly depressed late stage 5 sea levels the Chesapeake and Delaware estuaries became fluvially dominated, as the estuarine portion of the drowned river valley was expelled, or shifted seaward out of the basin. Muds were thereby exported from these major fluvial systems onto the inner shelf and redistributed by shelf currents. Sources of excess fine sediment could include pre-stage 5 glacial sediment, as well as sediment eroded from exposed coastal areas at lowered sea levels. Deposition appears to have been exceedingly high, as evidenced by intact laminae of fine sand-silt in the muds and by the in situ fossil assemblages and their relation to sedimentary characteristics of the unit. The 45,000-yr-long period of relatively high sea level during stage 5 was unique in Pleistocene history, and was the controlling factor in establishing sea-level-related processes resulting in the muddy shelf environment, which characterizes only stage 5.

Abstract Holocene sediment thicknesses measured from seismic-reflection profiles, together with long-term rates of sediment accumulation calculated from these thicknesses and the history of relative sea level, indicate that the Chesapeake Bay has filled rapidly with sediment during Holocene submergence of the bay. Sediment-accumulation patterns indicate that both the Susquehanna River system and the continental shelf are important sources of sediment; averaged over Holocene time, sediment transported from the continental shelf through the mouth of the bay may be volumetrically more important than sediment derived from rivers. Average Holocene rates of sediment accumulation show considerable spatial variability, presumably related to local variations in sediment sources, wave energy, and tidal currents. Nonetheless, these rates show several clear trends. Rates on the shallow marginal shelves of the bay tend to be low (0 to 2 mm/yr) and to increase only slightly toward the bay mouth. Rates in the deep channels are higher (1 to 5 mm/yr), have local maxima, and increase distinctly toward the bay mouth. At any given position in the bay, sediment-accumulation rates increase with depth to the base of the Holocene section. Our estimates of average Holocene rates of sediment accumulation are clearly higher in many places than previous estimates, but they are somewhat less than short-term rates previously measured by a variety of methods. Short-term rates may be affected by anthropogenic changes in the basin and by recent acceleration of relative sea-level rise. In addition, most short-term rates are site specific, biased in their distribution, and fail to account for the large spatial variability observed in long-term rates. Maximum long-term rates of sediment accumulation are limited by the rate of submergence; many existing short-term measurements clearly cannot be extrapolated back in time. Long-term rates of sediment accumulation confirm the ephemeral nature of estuaries and the close tie between sea level and estuarine history. These observations are important considerations for studies of the evolution of estuaries and the record of estuaries in the geologic record.

Abstract The elemental compositions of relatively unweathered Fe-Ti oxide grains, mostly ilmenite, separated from 83 samples collected from late Pleistocene to modern beach sands in Virginia and North Carolina were compared to those of 72 samples from five potential source rivers, the Roanoke, James, Potomac, Susquehanna, and Hudson Rivers. The composition of the Fe-Ti oxides from the toe of the Suffolk Scarp have a much different provenance than do younger beach deposits to the east. Based on discriminant analysis classification of the Fe-Ti oxide compositions with potential source rivers, the Suffolk Scarp beach is inferred to have been derived primarily from the James River; the younger beaches, including modern beach deposits of the Outer Banks, North Carolina, are inferred to have been primarily from the Susquehanna River with minor input by the Hudson River via longshore transport and reworking of shelf sands. The difference in provenance is due primarily to the origin of the Suffolk Scarp beach by erosion of older estuarine units in a protected-bay beach setting, whereas the younger beach deposits were derived from reworking of shelf sands, probably bay-mouth sand deposits (massifs), in an unprotected or barrier-beach setting. Subtle differences in the Fe-Ti oxide compositions among beach deposits are due to changes in the mix from the different river sources. Discrimination of the differences allows for a clearer understanding of the interrelation among those coastal-plain ridges and scarps that contain the beach sands.

Abstract The middle Atlantic Coastal Plain records the late Pliocene transition from typical marine deposition and coastlines of the Tertiary to that of fluvial-estuarine-marine of the Quaternary. The Early Pliocene Yorktown Formation (4.5-3.0 Ma) and Late Pliocene Chowan River Formation (approx. 2.8 Ma) record dominantly marine deposition in temperate to warm temperate climates with moderate- to low-sedimentation rates in a tectonically controlled basin. The Late Pliocene Bacons Castle Formation in Virginia (2.3-2.0 Ma) and the upper Beaverdam Formation of Delaware (2.3-2.0 Ma?) contain deposits with indicators of high rates of sedimentation, abundant sediment supply, an Appalachian source, and a cool terrestrial climate. Deposition was in response to a colder climate in the source area associated with Northern Hemisphere glaciation at approximately 2.4 Ma. Mechanical weathering and colluviation during a colder period moved sediment into the transport system that was later moved and deposited by high-discharge streams flowing into the coastal plain during subsequent warming. Deposition during the ensuing transgression kept pace with sea-level rise with some progradation of coastline. Later, Quaternary erosion and deposition in the region incised and partially reworked these late Pliocene deposits.

Abstract A total of 68 vibracores and 14 box cores in conjunction with high-resolution seismic records are used to describe the late Quaternary development of a twin-barrier-island complex. Based on the stratigraphy, lithology, radiocarbon dates, and microfossils, both a transgressive outer Holocene and inner Pleistocene barrier-island complex are recognized. The two subaerial subparallel barriers are a result of separate marine transgressions that occurred before and after late Wisconsin glaciation. The present landward migration of the Holocene barrier should put it atop the Pleistocene barrier in approximately 1,400 years. Conformable and unconformable contacts separate the two barrier-island systems. The outer barrier island, Smith Island, is typical of the Holocene Virginia barrier-island chain; its dimensions are approximately 11 km in length, and width and height mostly in the range of 200 m and 1.5 m above mean sea level, respectively. The Pleistocene barrier, Mockhom Island, lies approximately 7 km landward of Smith Island but within the back-barrier lagoon created by the Holocene marine transgression. Although Pleistocene shorelines are recognized on the mainland, these two chronologically distinct barrier systems are in contact, resulting in a geomorphology and upper Quaternary stratigraphy unique to the middle Atlantic coast. Holocene sediments deposited in the back-barrier environment show a general shallowing and fining-upward sequence. A significant portion of these back-barrier deposits, at least, but probably greater than, 3 m thick, should be preserved below 75 to 100-cm-thick nearshore sands. However, inlet-fill deposits likely will not be preserved. With present sea-level rise and wave-base conditions continuing into the future, the lower Holocene and uppermost Pleistocene transgressive sequences have a strong preservation potential. The preserved succession of deposits consists of Pleistocene back-barrier mud and shoreface sand below Holocene back-barrier deposits. The stacking of transgressive barrier deposits, albeit those from different transgressions, might serve as a straligraphic petroleum trap if preserved into the geologic record.