Abstract A phenomenon unique to fine-grained sediment is its ability to alter the physical characteristics of the overlying water column. Although the present state of research recognizes many aspects of fine-grained seabed and water column interactions, this study documents how an energetic sandy, shallow marine system can autogenically transition to a system capable of accumulating fine-grained bedforms to clinoforms. To understand these transitional processes this study examines the lithostratigraphy and depositional history of the Suriname portion of the Guiana Coast (French Guiana, Suriname, and Guyana). Four major lithologic facies (Pre-Holocene silty clay; peat-rich silty clay; sandy mud; silty clay with cheniers) were derived from the Late Holocene sea level rise and influx of sediments emitted from the Amazon River. Since approximately 6000 BP, ~10 to 20% of Amazon-derived sediments bypass the Amazon shelf and are transported northwestward toward the study area. Along the Suriname coast (~900 km from the Amazon), however, significant mud accumulation did not commence until 3000 to 3500 BP. Suspended sediments can travel this distance in less than 1 month. A migrating (1.5 km/yr) mud bank could travel the 900 km from the Amazon mouth in 600 years or, after formation of a 400-km-wide subaqueous Amazon delta (by 1200 BP), migrate the remaining 500 km by 4500 BP, still 1000 to 1500 years prior to the time period during which radiocarbon dates indicate significant mud accumulation began. Consequently, either there was a major hiatus in sediment transport and accumulation between 6000 BP to 3000 BP or some other transport process other than suspension or mud-bank migration-controlled initial mud accumulation. Assuming steady-state conditions, lateral accretion rates from 6000 BP to 3000 BP equate to 0.3 to 0.4 km/yr. These rates, which are similar to migration rates cited by previous studies for the trailing edges of mud banks in French Guiana, may reflect postmigration erosion of the initial mud banks. Whether there is an erosional overprint or some other process, this lateral accretion rate is an indicator of the amount of fluid mud necessary for ‘mud to beget mud.’ More general prerequisites necessary for a shallow marine setting to autogenically form fine-grained clinoform-scale accumulations are, first, a single large source of muddy sediments (a major river) and, secondly, unidirectional transport processes to concentrate and continuously supply mud sediments to the system.

Abstract Marine environments include anything seaward of the shoreline, the dividing line between land and water. All environments can be considered as either terrestrial (on land), marine (under water), or transitional (transitional between land and water). The transition zone between terrestrial and marine environments includes such environments as tidal flats, estuaries, dunes, and beaches and barrier islands. Beach features and processes are covered in another chapter of this volume (Bush and Young, this volume). Part of the beach system also belongs to the nearshore zone, so it is impossible to completely disassociate the beach/ transitional zone from the nearshore marine system. Standard beach terminology delineating the environmental zones of the nearshore system is shown in Figure 1 . To define the limits of the beach and the limits of the nearshore marine system, begin with the above water part of the beach, the part landward of the shoreline. This is known as the backshore, but is also often referred to as the dry beach, the recreational beach, or the subaerial beach. Looking seaward, the shallowest part of the beach is the intertidal zone, the portion between high tide and low tide. This is also known as the foreshore. Seaward of the foreshore is the inshore, commonly called the shoreface, the deeper water extent of the beach system. The shoreface is the zone where waves approaching land first start to interact with the seafloor. Another way to look at it is that the shoreface is the seaward extent of sediment movement

This paper is a case history of coastal development at Camp Ellis, Saco, Maine. It begins in 1867 with dredging and jetty construction at the mouth of the Saco River to facilitate commercial navigation. Beach accretion, resulting from tidal delta collapse, was followed by residential development before the ephemeral nature of the shoreline was recognized. A misunderstanding of the riverine source of beach sand and the net, northward direction of longshore transport confounded U.S. Army Corps of Engineers (USACOE) efforts to maintain navigation and the adjacent beach. Beach erosion at Camp Ellis claimed dozens of properties before the role of the north jetty at the mouth of the Saco River became apparent to state and university scientists. Erosion also led to sand migration to the north and to the closure of the Little River inlet and growth of Pine Point spit. This spit was later developed and a jetty was placed at its tip to preclude continued accretion into the Scarborough River inlet. Despite numerous studies, the USACOE failed to recognize the connection between beach erosion at Camp Ellis and beach accretion at Pine Point. Under political pressure, the USACOE recently conducted detailed modeling studies and has proposed construction of breakwaters seaward of Camp Ellis to solve the problem. A discussion of the pros and cons of this proposal is presented in light of the long history of development at Camp Ellis.

Duxbury Beach, Massachusetts, is a retreating, transgressive barrier that is effectively managed to meet a range of competing land uses. While the barrier is heralded as a natural coastal setting, the entire landform is methodically engineered on an ongoing basis to best accomplish the goals established for the beach within a context of natural processes. Historical and geological data indicate that the natural barrier form includes numerous ephemeral tidal inlets (some of which have migrated) and overwash channels, and low discontinuous dunes. At present, the managed barrier has a continuous vegetated foredune and broad backdune. Management techniques have evolved over the past several decades based on growing experience and understanding of the coastal processes and of wildlife habitats. Although the foredune crest is reconstructed each spring, the entire beach is gradually being allowed to retreat to remain in equilibrium with rising sea level. The lagoonal shore is being widened through beach nourishment and through proposed creation of back-barrier salt marshes using silty dredge spoil. Uses of the barrier include town and public recreational beaches, off-road vehicle access, a right-of-way to isolated communities, flood protection of landward areas, and shorebird nesting habitat.

Late Quaternary turbidite and related gravity-flow deposits have accumulated in basins of the California Borderland under a variety of conditions of sediment supply and sea-level stand. The northern basins (Santa Barbara, Santa Monica, and San Pedro) are closed and thus trap virtually all sediment supplied through submarine canyons and smaller gulley systems along the basin margins. The southern basins (Gulf of Santa Catalina and San Diego Trough) are open, and, under some conditions, turbidity currents flow from one basin to another. Seismic-reflection profiles at a variety of resolutions are used to determine the distribution of late Quaternary turbidites. Patterns of turbidite-dominated deposition during lowstand conditions of oxygen isotope stages 2 and 6 are similar within each of the basins. Chronology is provided by radiocarbon dating of sediment from two Ocean Drilling Program sites, the Mohole test-drill site, and large numbers of piston cores. High-resolution, seismic-stratigraphic frameworks developed for Santa Monica Basin and the open southern basins show rapid lateral shifts in sediment accumulation on scales that range from individual lobe elements to entire fan complexes. More than half of the submarine fans in the Borderland remain active at any given position of relative sea level. Where the continental shelf is narrow, canyons are able to cut headward during sea-level transgression and maintain sediment supply to the basins from rivers and longshore currents during highstands. Rivers with high bedload discharge transfer sediment to submarine fans during both highstand and lowstand conditions.

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 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.