George F. Oertel

The Role of Waves and Tidal Currents in the Development of Tidal-inlet Sedimentary Structures and Sand Body Geometry: Examples from North Carolina, South Carolina, and Georgia

ABSTRACT Morphologic variability in tidal inlets along the southeastern coast of the United States has been considered with respect to the distribution of large-scale sand bodies, intertidal and subtidal bedforms and internal sedimentary structures. Data indicate that the morphologic variability in these inlets can be largely explained as a response to waves and tides. Other factors (tidal prism, inlet cross-sectional area and shape, the nature of the back-barrier bay, the degree of flood or ebb dominance, fresh water input, relative changes in sea level and sediment supply) exhibit lesser controls and their effects are less easily quantified. Three types of inlets are identified: tide-dominated, wave-dominated and transitional. 1) Tide-dominated inlets are characterized by a deep, ebb-dominant main channel flanked by long, linear channel-margin bars. Flood-tidal deltas are poorly developed or non-existent. Sand bodies landward of the inlet throat are confined to tidal point bars further landward in the marsh creek system. 2) Wave-dominated inlets are characterized by large, lobate flood-tidal deltas building into wide, open lagoons. The ebb-tidal delta is small and extends only a short distance from the beach. Tidal channels are generally shallow (less then 6 m) and often bifurcate landward and seaward of the throat. 3) In transitional inlets, major sand bodies are typically concentrated in the inlet throat. These inlets v ry widely in morphology and sand body geometry. Logically, this variability should be expressed in the rock record. In a vertical section through a tide-dominated inlet channel, a coarse base, overlain by bidirectional trough cross stratification from the deep channel and ebb-oriented, planar and trough cross stratification from the shallower channel should be expected. Swash-generated, horizontal plane laminations or slightly inclined accretion beds formed along the channel margins are less likely to be preserved. In contrast, a wave-dominated inlet sequence would contain primarily landward-oriented, planar and cross stratification from the shallower channel bottom, overlain by dominantly horizontal or slightly inclined plane laminations from the shallow channel sides. Transitional inlets would produce a variety of sequences, the xact nature of which would reflect the relative importance of waves and tides.

The barrier island system

Abstract Barrier islands are part of a major coastal system composed of six interactive sedimentary environments. The six environments are also elements needed to impose the designation “barrier island” on littoral sand bodies. The elements are: (1) mainland; (2) backbarrier lagoon; (3) inlet and inlet deltas; (4) barrier island; (5) barrier platform; and (6) shoreface. The morphodynamic and sedimentary evolution of each element affects adjacent environments, as well as the entire system. The interactive elements are defined and described by their morphologic, sedimentologic and stratigraphic relationships to the barrier island and the barrier island system. The stratigraphic relationships of the various elements of the system may be a valuable tool for paleogeographic reconstruction and sand body mapping.

Transgressive systems tract development and incised-valley fills within a Quaternary estuary-shelf system: Virginia inner shelf, USA

Abstract High-frequency Quaternary glacioeustasy resulted in the incision of six moderate-to high-relief fluvial erosion surfaces beneath the Virginia inner shelf and coastal zone along the updip edges of the Atlantic continental margin. Fluvial valleys up to 5 km wide, with up to 37 m of relief and thalweg depths of up to 72 m below modern mean sea level, cut through underlying Pleistocene and Mio-Pliocene strata in response to drops in baselevel on the order of 100 m. Fluvially incised valleys were significantly modified during subsequent marine transgressions as fluvial drainage basins evolved into estuarine embayments (ancestral generations of the Chesapeake Bay). Complex incised-valley fill successions are bounded by, or contain, up to four stacked erosional surfaces (basal fluvial erosion surface, bay ravinement, tidal ravinement, and ebb-flood channel-base diastem) in vertical succession. These surfaces, combined with the transgressive oceanic ravinement that generally caps incised-valley fills, control the lateral and vertical development of intervening seismic facies (depositional systems). Transgressive stratigraphy characterizes the Quaternary section beneath the Virginia inner shelf where six depositional sequences (Sequences I–VI) are identified. Depositional sequences consist primarily of estuarine depositional systems (subjacent to the transgressive oceanic ravinement) and shoreface-shelf depositional systems; highstand systems tract coastal systems are thinly developed. The Quaternary section can be broadly subdivided into two parts. The upper part contains sequences consisting predominantly of inner shelf facies, whereas sequences in the lower part of the section consist predominantly of estuarine facies. Three styles of sequence preservation are identified. Style 1, represented by Sequences VI and V, is characterized by large estuarine systems (ancestral generations of the Chesapeake Bay) that are up to 40 m thick, have hemicylindrical wedge geometries, and occur within large, coast-oblique trending depressions (paleo-estuaries). Style 1 is dominated by fluvial through estuary-mouth depositional systems (Seismic Facies 1–4). Style 2 sequence preservation, represented by Sequences III and II, is dominantly an inner shelf and shoreface succession with a seaward-thickening tabular wedge geometry that does not exceed 15 m in thickness. These shoreface and inner shelf depositional systems of the upper transgressive systems tract (Seismic Facies 9) and highstand systems tract (Seismic Facies 7 and 11) are not associated with paleo-estuaries. Style 3 sequence preservation is represented by Sequence I, the Holocene Sequence. It consists of lower transgressive systems tract fluvial-estuarine, lagoonal, and tidal-inlet fill deposits (Seismic Facies 1–6, and 8) overlain by upper transgressive systems tract shelf and shoreface sands (Seismic Facies 9). Style 3 has a crenulated wedge geometry, and is thickest beneath and seaward of the modern Chesapeake Bay mouth. It thins northward and landward onto Late Pleistocene interfluvial highs on the basinward side of the southern Delmarva Peninsula.

Geomorphic cycles in ebb deltas and related patterns of shore erosion and accretion

ABSTRACT The interrelationship between inlet flow and delta morphology has an important effect upon the bypassing of sediment across inlets. Along the Sea Island Section of the Coastal Plain Physiographic Province the nature of inlet drainage and sediment bypassing profoundly influences patterns of beach erosion and accretion. Small coastal plain inlets generally have arcuate ebb deltas transected by a radially distributed pattern of channels. This arrangement permits an efficient flow of sediment across the inlet with little disturbance to the sediment budget of the adjacent shorelines. Several small inlets also have spits parallel to the shore that divert the flow of river water into the downdrift shore causing erosion. Accretion at the distal ends of these spits takes place at the expense of the proximal end of the spit where the shoreline erodes. Erosion eventually reopens a new channel across the proximal end of the spit and produces a second drainage reentrant. The major inlets along the coastal plain shoreline exhibit shoals with pronounced shore-normal orientations. The proximal ends of these spits may be attached or separated from the shore. When spits are attached to the shore, erosion is observed along the inlet margin and deposition is apparent at the distal end of the spit. When spits are separated from the shore, accretion is observed at shores adjacent to the inlet.

Suspended-sediment distribution and certain aspects of phytoplankton production off Georgia, U.S.A.

Abstract Coastal water of the central Georgia embayment is composed of two distinct zones — the turbid zone and the boundary zone — that are clearly different from the shelf water. The turbid zone is a band of lagoonal and nearshore water that has relatively high concentrations of dissolved and particulate substances. During ebbing tides, contacts between the turbid zone and the boundary zone are well-defined by frontal systems of inlet plumes. In the boundary zone, a gradational transition exists between the turbid zone and the relatively constant but much lower concentrations of dissolved and particulate substances in the shelf water zone. Dispersion of suspended particulates in the lagoonal portion of the turbid zone is primarily influenced by tidal hydraulics. Inlet plumes and wave resuspension are major factors affecting suspended particulate concentrations in the nearshore portion of the turbid zone. The seaward attenuation of particulate and dissolved matter in the boundary zone is primarily the result of dilution by settling and mixing with shelf water. Based on preliminary measurements, the turbid zone appears to be an area of high primary production in the central Georgia embayment. Although photosynthetic processes of phytoplankton are limited to the upper meter of water in this zone, the rapid uptake of relatively high concentrations of nutrients that are available, result in high biomass production. At high tides, production is particularly maximized in the lagoonal portion of the turbid zone, when nutrient-rich water “spills” out of tidal channels and spreads across the extensive marsh surface. Whereas phytoplanktons are dispersed seaward of inlets, resuspended benthic microalgae do not make up a large portion of the organic material ebbing through inlets.

Quaternary stratigraphic evolution of the southern Delmarva Peninsula coastal zone, Cape Charles, Virginia

The Quaternary evolution of the coastal zone along the southern Delmarva Peninsula of Virginia was investigated by means of a high-resolution seismic survey and supplemental shallow core borings. The seismic stratigraphic framework of the area is composed of three depositional sequences separated by two prominent unconformities. In ascending order, the three seismic sequences represent a late Tertiary substrate, Pleistocene deposits of post-Illinoian(?) age, and Holocene deposits. Stratigraphic analysis indicates that the evolutionary development of the area was controlled mainly by glacio-eustatism and paleotopographic features. The late Tertiary substrate was dissected by southeasterly flowing streams of the ancestral Susquehanna fluvial system, resulting in a highly furrowed surface consisting of valleys and interfluves that exhibit as much as 46 m of local relief. The erosional surface probably represents a mature multicyclic landscape of pre-Sangamonian(?) age. The unconformably overlying Pleistocene deposits attain a maximum thickness of 41 m. Pleistocene sedimentation was closely controlled by the paleotopography, with relatively thicker deposits accumulating within ancestral valleys. In contrast, thinner deposits accumulated over the ancestral interfluves, which did not become active sites of sedimentation until after the topographic relief had been substantially subdued by valley infilling. Pleistocene deposits are characterized by complex fades relationships, multiple generations of fluvial channeling, and the presence of a variety of coastal geomorphic features. Fluvial channel zones largely overlie the thalwegs of major southeasterly trending ancestral valleys, indicating a paleotopographic control of Pleistocene drainage routes. The youngest Pleistocene geomorphic feature, which constitutes a distinctive sand lithosome, is a coastal barrier ridge that appears to have developed during a Sangamonian interglacial or mid-Wisconsinan interstadial high stand of sea level. During the late Wisconsin regression, Pleistocene deposits were eroded, resulting in the development of a relatively low-gradient subdued landscape compared to the highly furrowed pre-Sangamonian landscape. Holocene deposits unconformably overlie the Pleistocene sequence and attain a maximum thickness of 19 m. Sedimentation during the Holocene transgression was primarily influenced by late Wisconsinan paleotopography, with relatively thicker deposits accumulating within paleochannels and along the modern shoreface. Holocene deposits also exhibit complex facies relationships, extensive channeling, and coastal geomorphic features. The Holocene sediments consist of two distinct lithosomes that are representative of modern barrier-island and lagoon–marginal-lagoon subenvironments. Microfossil assemblages indicate paleosalinities ranging from upper estuarine to marginal marine–inlet conditions. Vertical environmental transitions during the late Holocene transgression reflect a progressive restriction of the southern Delmarva coastal area by barrier-island development, with subsequent lagoonal infilling. The modern barrier islands are retrograding, and lagoonal infilling has progressed to varying degrees, having been influenced by both back-barrier sedimentation rates and pre-Holocene topography.

Anatomy of a barrier platform: outer barrier lagoon, southern Delmarva Peninsula, Virginia

Abstract A 7–9m thick prism of fine-grained sediment occurs below the floor of barrier lagoons of the southern Delmarva Peninsula. These fine-grained sediments provide a platform for retreating barriers to migrate across. Initially, the complete fine-grained sedimentary column was believed to represent the infilling of the barrier lagoon during the Holocene transgression. Marine microfauna confirm that deposition of mud occurred in a coastal/estuarine environment. However, X-ray radiographic and palynological analyses suggest shallow-water deposition associated with a cool-climate boreal forest. Much of the lagoonal mud behind the barriers is apparently pre-Holocene, and primordial Holocene lagoons were apparently very shallow. Hence, along the southern Delmarva Peninsula, landward-migrating barrier islands retreated across topographic highs composed of silt and clay.

Examination of Textures and Structures of Mud in Layered Sediments at the Entrance of a Georgia Tidal Inlet

ABSTRACT Radiographic examination of cores taken at the entrance of a Georgia tidal inlet revealed three texturally and structurally different types of mud layers. Type I mud layers are composed of laminations of day-size detritus that occur as flaser, wavy and lenticular bedding. Type II mud layers are composed of foresets and sand-size fecal and organic detritus that also occur as flaser, wavy and lenticular bedding. Type III mud layers are composed of foresets of mud pebbles and occur as wavy lenticular bedding. Mud-layer types form in response to hydraulically different depositional processes combined with the local availability of mud-grain sizes. Examination of textures and structures in these mud layers indicates that grains which constitute mud layers are transported and deposited from the traction load as well as the suspension load. In some areas, deposition from the bedload (traction load)is considerably more pronounced than deposition from suspension load. Type II and III mud layers result from bedload deposition which does not necessitate a period of slack-water in the tidal cycle or a specific set of conditions of wave activity, suspended-matter concentration, and current velocity.

Seismic stratigraphy and coastal drainage patterns in the Quaternary section of the southern Delmarva Peninsula, Virginia, USA

Abstract Seismic-stratigraphic analysis of the coastal zone and inner shelf of Virginias southern Delmarva Peninsula has revealed three geochronologically significant surfaces of post-Tertiary age that impose a relative chronostratigraphic framework on Quaternary marine transgressive and regressive events. Characteristics of these surfaces indicate that two are sequence boundaries, and one is a ravinement surface. Lying at depths of 18–70 m (msl datum), the LPb surface (a late Pleistocene basal unconformity) represents the sequence boundary separating the Tertiary Chesapeake Group from the overlying late Pleistocene Nassawadox Formation. High relief (approximately 50 m) on the LPb surface is associated with large fluvial channels. Higher in the stratigraphic section, the LPr, surface is found at depths of 6–28 m, and corresponds to a late Pleistocene transgressive, or ravinement surface. The surface dips southeastward with a regional dip of 0.04° and has local relief of less than 2 m. The LPr surface may represent a ravinement which extended to the west side of the Chesapeake Bay prior to the development of the Nassawadox barrier spit. However, the LPr surface may steepen between the axis of the southern Delmarva Peninsula and the Holocene lagoon to form a shoreface attached to one of several known late Pleistocene shorelines. Lying at depths of 0–20 m, the Hb surface is a basal unconformity that marks the Holocene sequence boundary. It deepens seaward, with maximum local relief of about 15 m, and has a topographic expression very similar to the present-day lagoonal drainage pattern. Maximum thicknesses of Holocene and Pleistocene sediments (12 and 70 m, respectively) are found above channels on the Hb and LPb surfaces. The Pleistocene channels are large and limited in number and represent high-order channels of a drainage system that drained the Piedmont and Coastal Plain. The greater density of low-order stream channels on the Hb surface suggests a relationship to much smaller drainage basins that were confined to the seaward part of the Coastal Plain east of the Delmarva Peninsula. These late Wisconsinan smaller Hb channels do not re-occupy the former drain paths of the much larger high-order LPb channels.

Sequence morphodynamics at an emergent barrier island, middle Atlantic coast of North America

Abstract The southern Delmarva Peninsula is located along the middle Atlantic Coastal Plain of the United States. The axial highland of the peninsula formed in four stages of Pleistocene spit progradation. The landward shoreline of the peninsula is on the Chesapeake Bay. The seaside shoreline of the peninsula is on the Atlantic Ocean. The coast of the peninsula is composed of five landscape sections described as a headland, a left-hand spit, a right-hand spit, a wave-dominated barrier island, and tide-dominated barrier islands. Fisherman Island is a barrier island located at the southern end of the southern Delmarva Peninsula. The landscape features on Fisherman Island do not illustrate a direct linkage to (1) the sediment dispersion from the Delaware headland or (2) the influence of local antecedent topography. The island has a bipolar progradational history that is normal to the axis of the southerly sediment dispersion pattern from the Delmarva headlands. During the late Holocene, sea-level rise flooded the low-elevation land at the distal end of the southern Delmarva Peninsula. The submerged area formed a shallow platform in the entrance to the Chesapeake Bay. Two sediment dispersion tracts affected the development of this area. On the ocean side of the peninsula, sediment moved southward along the lower shoreface to the Chesapeake Bay entrance. On the west side of the peninsula, southerly moving bay currents also dispersed sediment to the entrance of the bay. The two tracts converged on the northern side of the bay entrance forming a broad sand shoal. Wave diffraction and refraction around the margins of the shoal “swept” sediment into linear sand bars that migrated back toward the peninsula. By the middle of the 19th century, the fusion of sand bars on the shoal surface produced a permanent nucleus for island development. Wave refraction caused wave crests to “wrap around” the island core producing separate easterly and westerly components of shore aggradation. The westerly aggradational history is recorded in closely spaced sets of beach ridges. The easterly aggradational history is recorded in broadly spaced hammocks.