Michael E. Field

Geometry, Lateral Variation, and Preservation of Downlapping Regressive Shelf Deposits: Eastern Tyrrhenian Sea Margin, Italy

ABSTRACT The shelf of the eastern Tyrrhenian Sea margin changes substantially in width, shelf-break depth, and sea-floor steepness over relatively short distances, largely owing to pronounced lateral changes in subsurface geologic structure. Remnants of late Pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of the shelf. We recognize two major controls on the distribution, lateral extent, thickness, and preservation potential of these prograded deposits. First, prograded (downlapping) deposits formed only where the physiographic shelf break was deeper than the lowstand shoreline, thus providing accommodation space for the lowstand deposits. Second, the proximity and relative size of sediment sources and the local coastal dispersal system apparently infl enced the geometry of the deposit. Midshelf and shelf-margin bodies composed of seaward-steepening downlapping reflectors were deposited as thin to thick continuous prograding sheets over an irregular eroded shelf surface and onto the shelf edge during the latest fall and lowstand of sea level. Decreased sedimentation at the end of this lowstand resulted in net erosion. During sea-level rise, shoreface erosion created a major marine erosional (ravinement) surface landward of the 120-m isobath, and much of the downlapping deposits were removed. Preservation of these downlapping deposits is largely a function of their thickness. Thick continuous deposits are common on the shelf edge, whereas on the midshelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface.

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.

Offshore Transport and Sand-Body Formation: Evidence from a Steep, High-energy Shoreface, Southeastern Australia

ABSTRACT Elongate, shore-parallel sand bodies lie on the lower shoreface of the embayed and cliffed coast of central New South Wales, Australia. These sand bodies are 10 to 30 m thick, extend almost continuously for 40 km parallel to the coast, and display a pronounced convexity on the sea floor in profile. Sediment sampling and detailed seismic reflection and side-scan sonar profiling provide evidence that the upper part of these large lobate sand bodies consists chiefly of sand transported downslope from the upper shoreface and surf zone. The sand bodies consist entirely of marine sand probably derived locally from retreating embayed beaches and in part from sandstone cliffs. Channels and sediment lobes oriented normal to the shoreline and internal structure indicate growth by seaward progradation, and surface morphology indicates coalescing of lobes and damming of bed-load sediment against bed-rock ridges in water depths of 70 to 80 m. The precise mechanism, or mechanisms, of seaward transport on the shoreface is unresolved. We surmise that during storm events, disturbance by large waves and seaward transport by downwelling bottom currents result in sand movement onto and across the sand-body surface at depths of 40 to 80 m. Concurrent modification of the sand bodies by shore-parallel processes is suggested by textural trends, bed forms, and the overall alongshore continuity of the sand bodies.

Upper Quaternary peat deposits on the Atlantic inner shelf of the United States

Twenty-one upper Quaternary peat samples have been obtained from vibracores collected along the inner continental shelf of the Atlantic coast of the United States. Radiocarbon ages and pollen identifications from the peats, coupled with those from onshore borings and published data, provide additional information on the latest history of the Atlantic shelf. The radiocarbon ages cluster in two groups: early and middle Holocene time (10,000 to 5,000 B.P.) and late Pleistocene time (35,000 to 20,000 B.P.). Although ages and depths of the upper Pleistocene peats show some agreement with published graphs of changes in sea level, pollen data indicate that most of the peats formed in terrestrial environments and therefore may be unreliable as indicators of sea level. The Holocene peats were deposited in both marine and terrestrial environments. Correlation of the stratigraphic sequences in the cores containing peat with high-resolution seismic reflection profiles indicates a history of transgressive erosion on the inner shelf. Only remnants of carbonaceous sediments originally deposited in bogs, ponds, estuaries, and salt-marsh lagoons are present in the shallow subsurface on the inner shelf. These remnants, which are discontinuous and usually associated with erosional unconformities, provide evidence of regional marine planation of the shelf by the rising Holocene sea.

Sand Bodies on Coastal Plain Shelves: Holocene Record of the U.S. Atlantic Inner Shelf Off Maryland

ABSTRACT The inner continental shelf of Maryland, Delaware, and northern Virginia has been examined with high-resolution seismic reflection equipment and vibracores to develop an understanding of Quaternary processes and history on a coastal plain shelf. Morphology of the study region is dominated by the large shelf valley of the ancestral Delaware River and estuary and a linear-ridge field. Shallow subsurface strata consist of gently seaward-dipping Neogene sedimentary units showing no evidence of tectonic deformation. Eleven major acoustic surfaces, including the presumed Tertiary-Quaternary nonconformity at about -30 m to -60 m, are present within the upper 120 m of the shelf subbottom. Buried channels are common to the seafloor of the entire region; in the Delaware Bay entrance, most are c t to 45 m below sea level and were filled laterally by split plalform progradation from both the New Jersey and Delaware shelves. Many small channels on the Maryland shelf are aligned with existing onshore drainage or historical inlet sites and display a linear relationship between maximum thalweg depth and distance from shore. The upper 6 m of the sedimentary sequence of the inner shelf consists of terrigenous sand and silt derived from the adjacent Coastal Plain and Piedmont Provinces. Four major sediment types are recognized: three of these are subarkosic arenites varying only in modal grain size and sorting; the fourth is a slightly sandy mud. Environments of deposition preserved on the present shallow shelf are: modern marine, back-barrier, lagoonal, and fluvial. Gray-brown, fine to coarse, well-sorted quartz sand is the dominant type of surface sediment and its relative abundance decreases in the subsurface. Increases in sand thickness occur locally in ridge areas and correspond directly to topographic relief. The ridge sand unconformably overlies poorly sorted fine sand and mud remnant from Holocene b ckbarrier and lagoonal deposition; coarse constituents of the unit are commonly incorporated into the base of the ridge sand. Linear ridges are a dominant topographic feature of the U.S. mid-Atlantic shelf, and they are particularly well-defined on the Maryland shelf. Marked similarities in geometry and sediment relations of these features provide evidence of their origin on the Holocene shoreface and later segmentation and isolation on the shelf. Individual ridges commonly display a progressive south to north change from a well-defined, narrow, single-crested shape to a poorly defined, broad, multi-crested shape. This axial trend and the variation in coastal intersection angle are inherited from the ridges origin on the shoreface where growth and bifurcation occur along the northeastern tip. Shelf sand bodies off the central Delmarva Peninsula have formed by wave and current processes acting on previously deposited sediments, and these sand bodies are being formed and modified at present. The inner shelf of this region represents the trailing edge of a marine transgression; as such, it is the coastal sedimentary facies most likely to be preserved in the rock record.

Turbidite deposition from multiple sources; Quaternary Paola Basin (eastern Tyrrhenian Sea)

ABSTRACT The western margin of the Calabrian Arc is underlain by a thick (as much as 5 km) post-Miocene sedimentary succession. The uppermost part of this margin sequence is referred to as the Paola Basin System (Quaternary in age) and partially fills a present-day physiographic slope basin. Seismic and high. resolution reflection profiles reveal that a prominent unconformity marks the base of the Paola Basin System (PBS). The unconformity connects upslope to an area of recurrent shelf. edge instability and is overlain by large mass-failure deposits over much of the basin area. Basin-wide sedimentary drape deposits form stratigraphic markers that subdivide the Paola Basin System into three stages, each with unique patterns of slope erosion, including mass wasting, and lower.slope to basin-floo turbidite deposition. Three major sediment pathways elongated perpendicular to the margin developed between the unconformity and the first basin-wide drape. Upsection, sediment input for turbidite sedimentation in Paola Basin has involved many, commonly ephemeral, point sources. Most of the turbidite deposits do not form complete submarine fan sequences but consist of only a few turbidite elements. Changes in the size and type of turbidite elements through time suggest a decrease in the size and/or duration of individual sediment sources in the latest Pleistocene. The Paola Basin System is an example of a complex fill of a tectonically controlled basin where the effects of cyclic eustatic changes in sea level modify the effects of longer-term changes in sedimentation driven by tectonic ctivity in the source region and structural deformation in the depositional area.

Deposition of Deep-Sea Sands: Comparison of Two Areas of the Carolina Continental Rise

ABSTRACT Twenty-eight piston cores have been investigated in a study designed to compare and contrast the sediments and sedimentary processes in two adjacent but topographically distinct portions of the Carolina continental rise. Cores were collected from both the high relief portion of the continental rise encompassing the Hatteras Canyon-Hatteras Outer Ridge system and the adjacent relatively smooth and uncomplicated portion of the continental rise to the south. Sand grains are found concentrated in distinct sand layers (greater than 1 cm) and laminae (less than 1 cm) in the entire study area, but the characteristics of the sand layers are distinctly different in the two regions. Sand layers from the Hatteras Outer Ridge-Hatteras Canyon system are abundant, often graded, poorly sorted, and contain shallow water shells and unique occurrences of pyrite cementation. In contrast, the sand layers and laminae from the continental rise adjacent to the ridge-canyon system are less abundant, not graded, better sorted, and finer in grain size than the ridge-canyon sand layers. Light mineral assemblages from both regions indicate a source north of Cape Hatteras. In addition to grains concentrated in layers and laminae, very fine sand grains are also evenly distributed in minute quantities throughout the sediment column. The relative size, mineralogy, and even dispersion of these grains are constant throughout the study area and display no changes that can be related to local topography, indicating a slow, continuous depositing mechanism. Analyses of both sand layers and dispersed sands suggest that turbidity current transportation is important only in the canyon vicinity. To the south, lateral flowing bottom currents are responsible for secondary transportation of sediments originally introduced via rapid downslope movement and slow settling. This two-step process of sediment transported by turbidity currents through the Hatteras Canyon System and subsequently redistributed by bottom currents has been a major contributor of material for Quaternary mantling of the continental rise south of Cape Hatteras and probably as well for the Blake-Bahama Outer Ridge.

Buried Strandline Deposits on the Central Florida Inner Continental Shelf

Accumulations of the intertidal clam Donax variabilis (Say) have been recovered from the Cape Kennedy inner-shelf subbottom. Depth of the shell deposits correlates with a marked, widespread sonic horizon that underlies the entire central Florida inner shelf. Composition, texture, and radiocarbon ages of the shelf deposits correlate with available data on adjacent onshore beach ridges and with established sea-level curves. The occurrence and location of these deposits are considered significant for four reasons: (1) accumulations of D. variabilis may provide a definitive marker in some areas for relict submerged beach deposits; (2) the restricted depth range of these mollusks makes them suitable for refining portions of the sea-level curve that are based on less well defined criteria; (3) shoals and ridges on the inner shelf have often been interpreted as relict barriers, yet off Cape Kennedy these present-day bottom morphologic features (shoals) appear to be unrelated to the relict beach deposits described herein. This suggests that modern shoal formation must be unrelated to relict strand topography; and (4) the rare occurrence of buried Strandline deposits on the shelf in the vicinity of Canaveral Peninsula, which is an area of well-documented beach-ridge accretion, suggests a large-scale planing or truncating of the surface by the last transgression, which has removed all but trace remnants of the previous Strandline morphology. In other shelf areas where beach ridge accretion has been less dominant, therefore, all traces of former strandlines may have been removed.

Feldspar in Atlantic Continental Margin Sands off the Southeastern United States

Sand-size sediments on the Atlantic southeastern United States continental margin are generally low in feldspar; this reflects the deeply weathered source area. The feldspar content of sediments indicates that shelf sands are derived mainly from Piedmont Province rivers and that contour currents may be important in distributing continental rise sands. There is no clear indication of significant ice-age subaerial weathering of feldspar on the shelf or on present day beaches. However, the size distribution of feldspar in continental shelf sands compared to the size distribution of feldspar in river sands indicates that the shelf sands have undergone a significant abrasional history subsequent to contribution from rivers.