William J. Cleary

Influence of inherited geologic framework on barrier shoreface morphology and dynamics

Abstract Passive margin coastlines with limited sand supplies, such as much of the U.S. Atlantic margin, are significantly influenced by the geologic framework of older stratigraphic units that occur beneath and seaward of the shoreface. Many U.S. east coast barrier islands are perched barriers in which the underlying, pre-modern sediments determine the morphology of the shoreface and strongly influence modern beach dynamics and composition. Perched barriers consist of varible layers of beach sand on top of older, eroding stratigraphic units with highly variable compositions and geometries. Along many parts of the coastal system, stratigraphically-controlled bathymetric features on the inner shelf modify waves and currents and thereby effect patterns of sediment erosion, transport, and deposition on the adjacent shoreface. It is essential to understand this geologic framework before attempting to model the large-scale behavior of these types of coastal systems. In North Carolina, most shoreline features are controlled by the pre-Holocene stratigraphic framework of the shoreface; the beaches are perched on top of pre-existing Pleistocene, Tertiary, and Cretaceous sediments. The surficial geology of the coastal zone is subdivided into two distinct provinces resulting in different stratigraphic controls of the shoreface. North of Cape Lookout the geological framework consists of a Quaternary sequence that fills a regional depositional basin called the Albemarle Embayment. The coastal zone south of Cape Lookout is dominated by Tertiary and Cretaceous units that crop out across the coastal plain and continental shelf, with very thin Quaternary units only locally preserved. Superimposed upon this regional stratigraphy is an ancient drainage system resulting in a series of fluvial valleys filled with younger coastal sediments separated by large interfluve areas of older stratigraphic units. This results in a coastal system in which the shoreface is either nonheadland or headland dominated, respectively. Headland dominated shorefaces are further divided into subaerial and submarine categories. Nonheadland dominated shorefaces are further divided into those influenced primarily by transgressive or regressive processes, or channel-dominated depositional processes (i.e., inlet migration or stream valley fill). Examples of each of these six types of shorefaces are presented to demonstrate the control that the geologic framework exhibits on shoreface morphologies and processes.

Geology of the Wrightsville Beach, North Carolina shoreface: Implications for the concept of shoreface profile of equilibrium

Abstract Nearly 300 km of 3.5 kHz subbottom profile and 100 kHz sidescan-sonar data, a suite of over 100 short (~2 m) percussion cores and vibracores have been collected on the shoreface and inner continental shelf off Wrightsville Beach, North Carolina. Sidescan-sonar images were analyzed for acoustic backscatter to delineate the surface sediment distribution. Groundtruth data for the sidescan-sonar interpretations were provided by surface grab samples. Cross-shore sediment transport by combined waves and currents is the predominant sedimentologic signature on this shoreface. The shoreface is dominated by a shore-normal system of rippled scour depressions that begin in 3–4 m water depth and extend to the base of the shoreface about 1 km offshore, at 10 m depth. The depressions are 40–100 m wide, and up to 1 m deep. They are floored by coarse, rippled shell hash and gravel; some are separated by rock-underlain fine sand ridges. On the inner shelf, the bathymetric and sedimentary fabrics become shore-oblique, due to a series of relict ridges with 1–2 m of relief. The ridges are coarse on their landward sides and covered on their seaward flanks by thin veneers of fine sand. Field evidence from the Wrightsville Beach shoreface demonstrates that a shoreface equilibrium profile as defined by Dean (1991) and others does not exist here. For example: (1) the grain size varies widely and inconsistently over the profile; (2) shoreface profile shape is controlled predominantly by underlying geology, including Tertiary limestone outcrops and Oligocene silts; and (3) sediment transport patterns cannot be explained by simple diffusion due to wave energy gradients, and that transport occurs seaward of the assumed engineering “closure depth” of 8.5 m. This has several implications for the application of equilibrium profile-based numerical models used to investigate coastal processes and design coastal engineering projects at Wrightsville Beach. The most important practical implication is that a number of assumptions required by existing analytical and numerical models (e.g., Dean, 1991; genesis ; sbeach ) used for the design of shore protection projects and large-scale coastal modeling over decadal time scales cannot be met.

Black Shell turbidite, Hatteras Abyssal Plain, western Atlantic Ocean

An upper Pleistocene turbidite with a volume of at least 1011 m3 (100 km3) has been traced in 35 piston cores over a 44,000 km2 area of the Hatteras Abyssal Plain. Entering the plain from the Hatteras Canyon System, the flow traveled uninterrupted for at least 500 km in a southerly direction and resulted in a tongue-shaped turbidite with a width between 100 and 140 km and a thickness of as much as 400 cm. This turbidite appears to be one of the largest, if not the largest, single turbidite yet to be correlated between deep-sea cores and to be mapped on an abyssal-plain floor. Correlation of the unit is based on grain size, mineralogy, relative thickness, and similarity of sequences in the cores. Because correlation between cores is based largely on sand-layer characteristics, the turbidite cannot be traced beyond the last occurrence of a distinct sand layer in the distal (southerly) direction. The turbidite is characterized by its high percentage (2% to 50%) of blackened mollusk shell fragments, which led to the informal name Black Shell turbidite, and by a coarser grain size than other turbidites in the same cores. The maximum thickness of the turbidites sand part is in the center of the abyssal-plain basin, whereas maximum thicknesses of lutite and of the total turbidite are displaced east of the center line of the depositional basin. Depending on lateral position in the flow, the sands texturally comprise a wide range of graywackes, and mineralogically they constitute a suite ranging from lithic arkoses to quartzarenites. Sand petrology indicates that the fluvially derived terrigenous fraction came from United States mid-Atlantic coast rivers, whereas molluscan and foraminiferal bioclastic components indicate an initial shallow-shelf origin from the vicinity of Cape Hatteras, North Carolina. It appears that the turbidity current began as a massive shelf-edge slump. Sand came from the shelf edge; mud was picked up on the upper continental slope. The flow apparently evolved from a slump into a high-concentration flow as it moved down-slope. Characteristics in the axial center zone, such as poor sand sorting, high mud content, Bouma AE sequences, and discontinuous vertical grading, suggest deposition from a high-concentration flow. The nature of the characteristics changed (better sorting, more continuous vertical grading, more complete Bouma sequences, and lower mud content) as the flow spread, reflecting deposition from a low-concentration turbidity current.

Cyclic geomorphic patterns of washover on a barrier Island in southeastern North Carolina

A detailed analysis of historic aerial photographs provided the data for determining the magnitude and importance of oceanic overwash on Masonboro Island, southeastern North Carolina. Overwash, which is both temporally and spatially distributed, produces a suite of physiographic features on the subaerial portion of the island. Four physiographic types are recognized including: A) small coalescing loosely vegetated dunes, B) intact, well-vegetated dunes and terraces, C) individual, isolated washover fans, and D) washover terraces. Vegetation patterns, including shrub thickets and black needle rush marshes are related to old fan sites. Sites dominated by saltmeadow cordgrass and goldenrod are associated with recent overwashes. A process-response model, which synthesizes the physiographic types and vegetation patterns, provides input for a management program for the island. Five sections on Masonboro Island are delineated on the basis of washover history and potential for future washovers. The response of Masonboro Island to overwash is similar to that observed on Core Banks, North Carolina; however there are several differences, probably because of the rapidity with which dune ridges redevelop after washovers.

Embayed Quartz Grains in Soils and Their Significance

ABSTRACT Studies of vertical profiles of quartz-rich Ultisols of the Carolina piedmont and coastal plain reveal patterns of grain dissolution which may be used as an indicator of weathering intensity. The formation of sand-sized material begins with the dismemberment and dissolution of the saprolite and parent rock and forms grains with solution embayments. Further dismemberment occurs until grains show very irregular and skeleton-like outlines. Skeletal grains characterize the root zone which is probably the zone of greatest solution activity. The repetitive nature of the observed patterns of dissolution and grain production enables one to formulate a model based on the degree and the amount of embayed quartz in the soil. The model proposes that after the initial release of the fragment, it i further modified by the soil water and as time proceeds and dismembering of the fragments occurs, dissolution at the grain contacts continues. Eventually, as the zone of greatest activity develops, it is possible that a soil profile consisting dominantly of embayed monocrystalline quartz would develop.

Mica: Its use in determining shelf-depositional regimes

Abstract The fine sand-size mica content was determined in over 450 beach, dune, river, estuary, continental shelf and continental slope sediments from the southern United States Atlantic continental margin. Because of hydraulic equivalence considerations, the mica content can be used to delineate depositional and non-depositional regimes of fine-grained sediments. At the present time deposition of fines in the study area is restricted to a narrow nearshore band and the upper continental slope. The central and outer shelf are areas of winnowing.

10.8 Morphodynamics of Barrier Systems: A Synthesis

The morphodynamics of open-ocean barrier systems (barrier islands, barrier spits, and mainland or headland beaches), synthesizing classic studies, current scientific knowledge, and future research directions regarding a number of barrier systems globally are reviewed. Within a coastal tectonic framework, the authors address: (1) Amero-trailing-edge coasts (USAs New England coast, mid-Atlantic Bight coast, North Carolina Outer Banks, Georgia Bight coast, and Florida Atlantic coast; Brazils Santa Catarina coast; German Bight coast; and southern and western Australian coasts); (2) marginal-sea coasts (USAs Florida Gulf Coast; Gulf Coast of Alabama, Mississippi, and Louisiana; Texas Gulf Coast; and eastern Australian coast); and (3) collision coasts (USAs Alaskan Pacific coast and New Zealand). Moreover, the chapter includes a glossary and robust current set of references.

Coastal response to late-stage transgression and sea-level highstand

Coastal morphologic features associated with past shoreline transgressions and sealevel highstands can provide insight into the rates and processes associated with coastal response to the modern global rise in sea level. Along the eastern and southern Brazilian coasts of South America, 6000 years of sea-level fall have preserved late-stage transgressive and sea-level highstand features 1‐4 m above present mean sea level and several kilome ters landward of modern shorelines. GPS with real-time kinematics data, ground-penetrating radar, stratigraphy, and radiocarbon dating within a 2‐3-km-wide river-associated strandplain in central Santa Catarina (southern Brazil) uncovered a diverse set of late-stage transgressive and highstand deposits. Here, the highstand took the forms of (1) an exposed bedrock coast in areas of high wave energy and low sediment supply; (2) a 3.8-m-high transgressive barrier ridge where landward barrier migration was prohibited by the presence of shallow bedrock; and (3) a complete barrier-island complex containing a 5.2-m-high barrier ridge, washover deposits, a paleo-inlet, and a backbarrier lowland, formed in a protected cove with ample sediment supply from small local streams and the erosion of upland sediments. Similar signatures of the mid-Holocene highstand can be traced across all coastal Brazilian states. This study presents the fi rst complete compilation of the diversity of these sedimentary sequences. They are broadly classifi ed here as exposed bedrock coasts (type A), back barrier deposits (type B), transgressive barrier ridges (type C), and barrier-island complexes (type D), according to localized conditions of upland migration potential, wave exposure, and sediment supply. These Brazilian systems present a paradigm for understanding future coastal response to climate change and accelerated sea-level rise: the recognition of a minimum threshold sea-level-rise rate of ~2 mm yr ‐1 above which transgression proceeded too rapidly for the formation of these stable accretionary shoreline features demonstrates the nonlinearity of coastal response to sea-level change, and the site specifi city of conditions associated with the formation of each highstand deposit type, even within a single small embayment, demonstrates the non-uniformity of that response.

Genesis and Significance of Marsh Islands Within Southeastern North Carolina Lagoons

ABSTRACT Fifty-four islands of low relief, varying from 36 m to 1512 m long are present within the narrow, marsh-filled lagoons behind the barrier islands between New River Inlet and Carolina Beach Inlet in Southeastern North Carolina. Information derived from inspection of aerial photographs, historic maps and charts, and field data, indicate that these islands form where sands overtop marshes forming a narrow berm. These features likely form during periods of increased wave swash. The occurrence of these geomorphic features can be used to indicate the migration of a single inlet or several inlets along a zone prone to inlet formation. The islands consistently present a stratigraphic sequence where fine-grained, humic-stained, carbonate-free sands overlie muddy peat. Landward of these islands organic-rich peat sequences are commonly thick (>100 cm) while just seaward, peat sequences are thin (<30 cm). Vegetation composition and species diversity on the islands are dependent upon age and height above mean sea level. Saltmeadow cordgrass (Spartina patens) and sea oxeye (Borrichia frutescens) generally characterize young islands or those islands occasionally flooded by spring tides. On islands with relief greater than 1 meter, tidal flooding is uncommon. A diverse flora, including wax myrtle (Myrica cerifera), red cedar (Juniperus virginiana), yaupon (Ilex vomitoria), and occasional live oak (Quercus virginiana) succeed the colonizers, cordgrass and sea oxeye.

Mud in the surf: Nature at work in a Brazilian bay

Massive discharge of mud from coastal rivers is a well-documented phenomenon. However, in areas with limited historical and instrumental records it is often difficult to assess the nature and history of the process. This article looks at Tijucas Bay, in southern Brazil (Figure 1a) (an area that was the landfall region in March 2004 for South Americas first recorded hurricane [Bossack, 2004]), to examine the time frame for extensive deposition of fluid muds in the nearshore (Figure 1b). The new geological data suggest that whereas recent human activities (e.g., massive sand mining) along the Tijucas River may be important in increasing the suspended sediment discharge, the shift to a mud-dominated regime was part of the natural evolution of this coastal plain.