David B. Duane

Post-Pleistocene history of the United States inner continental shelf: Significance to origin of barrier islands

Segments of the U.S. Atlantic coast differ markedly in shoreline and shelf configuration because of both geologic setting and Quaternary geologic history. Regional trends in shelf structure, sediment sources, and history correlate with regional trends in adjacent shoreline configuration, and the Holocene transgressive history of the shelf has a direct bearing on the evolution of the present coastline. The shelf sedimentary record, particularly in the mid-Atlantic region, contains substantive evidence that precursors to existing barrier islands were common in Holocene time. Nearshore linear shoals are associated with barrier-island coastlines, and their continuity in position on the shelf, coupled with the presence of an underlying lagoonal substrate, implies persistent retreat of a barrier coastline during sea-level rise. Retreat paths of shoals off large capes and estuaries similarly indicate a consistency in shoreline type and orientation through time. Collectively, this information strongly suggests that barrier islands of the U.S. Atlantic coast originated far out on the shelf, and it invalidates certain criteria (coastal linearity, lithology of sediments beneath the modern lagoon) formerly cited as evidence for mode of formation. Understanding how and the processes by which barriers evolved relate directly to present and future behavior of barriers. Sediments eroded from the foreshore and shoreface are transported both along the coast and landward, aggrading as nearshore shoals, spits, tidal deltas, dunes, and overwash fans. Barrier islands lengthen both by sand accumulation (spit extension) and by submergence (deepening and headward extension of the lagoon). Development of barrier islands is influenced by rate of sea-level advance and sediment supply. Once initiated, barrier islands lengthen and retrograde through a combination of processes, including spit elongation, overwash, and flooding of back barrier lagoons.

Significance of skewness in recent sediments, western Pamlico Sound, North Carolina

ABSTRACT Application of the method of moments to size distribution characteristics of Recent sediments in western Pamlico Sound, North Carolina, and vicinity indicate that the sign of skewness is environmentally sensitive. A winnowing action produced by fluid media is the mechanism producing negative skewness. Sediments of beaches, the littoral zone, and tidal inlets are negatively skewed. Sediments from a sheltered lagoon filling with sedimentary material are dominantly positively skewed. Where winnowing action can be shown to be operative intermittently, sediments are characterized by local differences in sign of skewness. The sensitivity of skewness to several environments in the western Pamlico Sound area strongly supports similar conclusions reached by Mason and Folk (1958) and Friedman ( 961) who studied different areas. Preliminary studies of the comparison of results obtained using the same data in different formulae indicate the sign of skewness is reproducible. As a reproducible parameter, skewness sign should be directly applicable to studies of Recent sediments elsewhere, and used with other criteria it should be valuable in the interpretation of paleoenvironments, particularly in those sediments where effects of diagenesis are negligible or non-existent.

Littoral Transport in the Surf Zone Elucidated by an Eulerian Sediment Tracer Experiment

ABSTRACT An Eulerian, or time integration, sand tracer experiment was designed and carried out in the surf zone near Pt. Mugu, California on April 19, 1972. Data indicate that conditions of stationarity and finite boundaries required for proper application of Eulerian tracer theory exist for short time periods in the surf zone. Grain counts suggest time required for tracer sand to attain equilibrium concentration is on the order of 30-60 minutes. Grain counts also indicate transport (discharge) was strongly dependent upon grain size, with the maximum rate occurring in the size 2.5-2.75 (0.18-0.15 mm) decreasing to both finer and coarser sizes. The measured instantaneous transport was at the annual rate of 2.4 106 m3/yr.

Marine Placers: Reconnaissance Exploration Technology

Active placer mines and known deposits provide the stratigraphic clues and paleoenvironmental indictors useful to offshore exploration programs. Existing marine mapping, surveying, and sampling tools are judged capable of providing most of the kinds of information needed to find prospective targets for detailed exploration near coastlines. Beyond the nearshore area, and the relatively facile extension of terrestrial knowledge, exploration problems loom large. The problem becomes less the tools or how to apply them, but rather where to apply them. A few new techniques in the fields of bathymetric mapping, positioning systems, analytical methods, and exploration models hold promise for improving the accuracy and speed of exploration.

Late Pleistocene-Holocene Sedimentation History of Cape Kennedy Inner Continental Shelf: ABSTRACT

Lithologic analyses of 91 cores averaging 10 ft in length, radiocarbon dates of in situ peat deposits from the shelf, and interpretation of 360 mi of high-resolution continuous seismic profiling indicate a complex sedimentation history for the Cape Kennedy inner continental shelf. Two prominent and continuous acoustical horizons in the upper subbottom can be traced throughout the 200 sq mi grid studied. The lower reflector lies at -60 to -110 ft MLW and is characterized by an irregular surface probably resulting from erosive processes during long-term subaerial exposure. The upper horizon lies at depth of -40 to -90 ft MLW and generally 2-15 ft below the existing bottom. This horizon is relatively smooth and dips seaward at a low angle. Internal reflectors between the upper and lower horizons suggest prograding beds. Cores penetrating the upper reflector contain subaerially cemented sands and recrystallized shells. End_Page 337------------------------------ Subsurface sediments in the Cape Kennedy area are widely diversified in sorting, texture, composition, and macrofauna. Major lithologic types are commonly correlative between cores; however, individual cores commonly contain several distinct changes in sediment type. Lithologic characteristics and thickness of strata suggest rapid changes of depositional environment; marsh, lagoon, littoral, and open-shelf facies are represented. Most of the sediments studied were produced by bottom erosion of Pleistocene surfaces and by shoreward migration and mixing of an outer-shelf oolitic sand with an inner-shelf quartzose-molluscan sand during the Holocene transgression. End_of_Article - Last_Page 338------------