Holocene Barrier Island Research Articles

Influence of early/mid-Holocene climate change and sea level rise on a coast with barrier islands, Rio de Janeiro State, SE Brazil

AbstractThis work aims to identify the influence of climate change on sedimentary processes associated with the usual regression process during the rise of sea level in the early/middle Holocene in southeastern Brazil. The studied area is on the eastern side of Marambaia Barrier Island, which borders the eastern side of Sepetiba Bay (Rio de Janeiro State, SE Brazil). Nowadays, the Marambaia Barrier Island, a ≈ 40 km long and ≈ 5 km wide sandy ridge, shields Sepetiba Bay from the direct influence of the Atlantic Ocean. To achieve this goal, grain size, geochemical data (including elemental and stable isotopes), and radiocarbon dating data from sediment core SP10 (spanning depths from 8.2 to 45.5 m), collected in the eastern sector of the Marambaia Barrier Island were analyzed. Core SP10 predominantly consists of sandy sediments with some textural variations. However, Sr/Ba ratios suggest that brackish waters primarily influenced the depositional environment, which was also subject to cyclical marine incursions. The essentially felsic sediments of the interval between 45 and 41 m (≈ 10.0–8.5 ka BP) were probably deposited in a protected tidal plain estuary with mangroves. They were possibly mainly sourced from the nearby Pedra Branca Complex. Subsequently, between ≈ 41 and 11 m, the marine influence and hydrodynamics increased, and the sediments exhibited a more mafic mineralogical composition resulting probably from the erosion of the Rio Negro Complex, which is mainly found in the northern region of Sepetiba Bay. The mafic component likely reached the study area through coastal drift connected with Sepetiba Bay. During the drought period, recorded between ~ 7.5 and 7.0 ka cal BP (section 25–21 m), the contribution of the mafic component from the Rio Negro Complex decreased and less weathered sediments were accumulated. On the other hand, marine incursions into the study area became more prominent. During the drier climate phase recorded in the interval ≈ 11–9 m (after ≈ 4.0 ka BP), the La/Sc values indicate that the sediment included a higher proportion of felsic particles, probably due to more significant restrictions on the connection of the study area with Sepetiba Bay due to the development of the Marambaia Barrier Island. These findings are significant as they demonstrate the influence of geomorphology, climate change, sea level, and the development of Holocene barrier islands on the sedimentation in coastal regions.

Tracking barrier island response to early Holocene sea-level rise: High resolution study of estuarine sediments in the Trinity River Paleovalley

Understanding how barrier islands respond to factors such as variations in sediment supply, relative sea-level rise, and accommodation is valuable for preparing coastal communities for future impacts of climate change. Increasingly, the underlying antecedent topography has been observed to have a significant control on the evolution of the barrier island system by providing increased elevation, decreased accommodation, and sediment supply for the barrier to rework and anchor upon. However, less attention has been focused on how back barrier sediments respond to this decreased accommodation, and how this may affect barrier island evolution. Additionally, the control in which the geometry of the underlying antecedent topography has on the initiation of barrier islands is poorly understood. Here we examine the stratigraphic framework of the Trinity River incised valley offshore Galveston, Texas to investigate the role of antecedent topography in the evolution of an early Holocene barrier island system. We present high-resolution imaging of the Trinity incised valley fill using over 1200 km2 of 3D seismic, ~700 km of 2D envelope and full waveform chirp data, along with 2 piston cores, 4 gravity cores, and 1 platform boring with associated grain size, foraminiferal, and radiocarbon data. We find that the geometry and elevation of the underlying antecedent topography plays a central role in the evolution of the barrier island system, promoting both initiation and stabilization. This study provides a methodology to investigate the evolution of a relict barrier island system where little to none of the barrier is preserved. With this methodology, we revise the established Holocene paleoshoreline model for the Trinity incised valley.

Late Holocene environmental change in Celestun Lagoon, Yucatan, Mexico

Epikarst estuary response to hydroclimate change remains poorly understood, despite the well-studied link between climate and karst groundwater aquifers. The influence of sea-level rise and coastal geomorphic change on these estuaries obscures climate signals, thus requiring careful development of paleoenvironmental histories to interpret the paleoclimate archives. We used foraminifera assemblages, carbon stable isotope ratios (δ13C) and carbon:nitrogen (C:N) mass ratios of organic matter in sediment cores to infer environmental changes over the past 5300 years in Celestun Lagoon, Yucatan, Mexico. Specimens (> 125 µm) from modern core top sediments revealed three assemblages: (1) a brackish mangrove assemblage of agglutinated Miliammina and Ammotium taxa and hyaline Haynesina (2) an inner-shelf marine assemblage of Bolivina, Hanzawaia, and Rosalina, and (3) a brackish assemblage dominated by Ammonia and Elphidium. Assemblages changed along the lagoon channel in response to changes in salinity and vegetation, i.e. seagrass and mangrove. In addition to these three foraminifera assemblages, lagoon sediments deposited since 5300 cal yr BP are comprised of two more assemblages, defined by Archaias and Laevipeneroplis, which indicate marine Thalassia seagrasses, and Trichohyalus, which indicates restricted inland mangrove ponds. Our data suggest that Celestun Lagoon displayed four phases of development: (1) an inland mangrove pond (5300 BP) (2) a shallow unprotected coastline with marine seagrass and barrier island initiation (4900 BP) (3) a protected brackish lagoon (3000 BP), and (4) a protected lagoon surrounded by mangroves (1700 BP). Stratigraphic (temporal) changes in core assemblages resemble spatial differences in communities across the modern lagoon, from the southern marine sector to the northern brackish region. Similar temporal patterns have been reported from other Yucatan Peninsula lagoons and from cenotes (Nichupte, Aktun Ha), suggesting a regional coastal response to sea level rise and climate change, including geomorphic controls (longshore drift) on lagoon salinity, as observed today. Holocene barrier island development progressively protected the northwest Yucatan Peninsula coastline, reducing mixing between seawater and rain-fed submarine groundwater discharge. Superimposed on this geomorphic signal, assemblage changes that are observed reflect the most severe regional wet and dry climate episodes, which coincide with paleoclimate records from lowland lake archives (Chichancanab, Salpeten). Our results emphasize the need to consider coastal geomorphic evolution when using epikarst estuary and lagoon sediment archives for paleoclimate reconstruction and provide evidence of hydroclimate changes on the Yucatan Peninsula.

Holocene evolution of the Ninety Mile Beach sand barrier, Victoria, Australia: The role of sea level, sediment supply and climate.

The Ninety-Mile Beach (NMB) barrier system in southeastern Australia is the largest active barrier island system in Australia. The response of a sandy barrier system to a warming climate is dependent on the boundary conditions of sediment supply and sea level. Deposition during the Holocene can therefore provide an indication of how these barriers may change in the future. In this study airborne LiDAR, ground penetrating radar and subsurface coring were combined with 46 optically-stimulated luminescence and 32 radiocarbon ages to provide a detailed understanding of sedimentation of NMB through the Holocene. The barrier complex formed in three distinct phases dating back to the earliest stabilisation of sea level after the Postglacial Marine Transgression. First, restriction of the flooded open-ocean embayment that had formed around a Last Interglacial beach-barrier sequence occurred with accretion of an island complex at around 8000 years BP, sourced from transgressive sands reworked from the shelf. In the second phase, at around 6000 years BP, a second barrier island formed seaward of the Last Interglacial barrier but disconnected from the earlier Holocene barrier islands by a large tidal inlet. Waning sediment supply from the shelf meant that aggradation of these islands slowed by 4000 years BP. The third and final phase occurred after 3000 years with the initiation of sediment supply at the southwestern end of the barrier. The island system which formed seaward of the Last Interglacial barrier then prograded by several hundred metres and elongated in a northeasterly direction by tens of kilometres infilling the tidal inlet and enclosing the earliest Holocene barrier landward of a newly created lake. Sediment supply appears to be the primary limiting factor in the development of NMB with indications of a change in wave climate and sea level have an influence on barrier evolution.

Impact of drainage and soil hydrology on sources and degradation of organic matter in tropical coastal podzols

In podzols important environmental issues converge, including dissolved organic matter (DOM) transport, DOM-metal binding, and carbon storage in the subsoil. Therefore, it is important to understand the formation and degradation of podzols in relation to (changes in) environmental conditions. For this purpose a Holocene barrier island with coastal sand dunes (Ilha Comprida, SE Brazil) was chosen as study area. The island is build-up of five geomorphic units that have different age (from >5000 to 325 y BP), vegetation (restinga ecosystems), soil hydrology (flat units or units with ridges and swales) and drainage (poorly drained, well-drained, and improved drainage). Representative profiles were studied for each geomorphic unit, resulting in 100 samples from A, E, B, and C horizons, from which soil organic matter (SOM) was isolated by alkaline extraction. To better understand carbon sources and dynamics, we additionally sampled litter from different vegetation types and DOM from various sources. The molecular composition of SOM, DOM and litter was analyzed with pyrolysis gas chromatography mass spectrometry (pyrolysis-GC/MS). Comparing the pyrolysates of all samples (DOM, SOM and litter) factor analysis demonstrated that the major difference in molecular composition (factor 1) was related to the contribution from DOM (phenol, acetic acid, benzofurans, pyridine, benzene and naphthalene) or in-situ root material (straight chain aliphatics and methoxyphenols from the biomacromolecules suberan and lignin, respectively). The contribution from DOM or roots was characteristic for a profile i.e. without much change with depth. Factor 2 reflected decomposition processes, and showed that the predominantly DOM-derived B horizons were relatively enriched in aromatics compared to DOM, indicating selective decay and/or selective precipitation. In geomorphic units with swales and ridges that received DOM via groundwater flow from the catchment area, the B horizon was predominantly DOM-derived; these profiles showed large differences in the contribution from black carbon (BC; (poly)aromatics), with the younger profiles showing a larger contribution from BC in precipitated DOM. B horizons with a relatively large contribution from in-situ root materials were found in well-drained soils and in some of the poorly drained soils without lateral groundwater flow from the catchment area, i.e., the flat geomorphic unit without ridges and swales. Microbial material (N-containing compounds and sugars) was associated with relatively recent SOM and with well-drained conditions at present. i.e., well-drained profiles, and profiles with improved drainage.

American coastal defence Third System forts: how geomorphology and geology dictated placement and influenced history

Abstract In total, 42 Third System forts were built along the east coast of the USA between 1816 and 1867. Because of their purpose, to defend key locations against potential British amphibious assault, their sites were limited to islands, shoals, shorelines, riverbanks or hilltops. Sited along the Atlantic and Gulf of Mexico coastlines, many are constructed upon distinctive geomorphological landforms that are related more to features influenced by Quaternary glaciation or sea-level changes than by pre-Quaternary bedrock. A selection of forts is described to illustrate the principal geological and geomorphological features that influenced their construction: at Boston Harbor in Massachusetts on drumlins, near New York City on moraine and at Fort Pulaski protecting Savannah Harbor in Georgia on saltmarsh. Further south, Fort Clinch in northern Florida, forts guarding Pensacola Bay in western Florida and the entrance to Mobile Bay in the Gulf region, and Fort Massachusetts on Ship Island, Mississippi, were all primarily situated on Holocene barrier islands. Construction, use and 1861–65 Civil War threats to these forts can be directly related to their geomorphological context. Although all are now redundant, most remain as a remarkable and highly visible aspect of the military history of the USA.

Geomorphological control on podzolisation – An example from a tropical barrier island

We investigated how the geomorphology of coastal barrier islands impacts soil hydrology and drainage at the landscape scale. Ilha Comprida is a Holocene barrier island with a 2.5 km-long cliff that is perpendicular to the coastal shore which provides an ideal condition to study the relation between age, relief, hydrology, and podzol morphology. Five geomorphic units were identified that differed in surface morphology and alignment of ridges and swales. Optical stimulated luminescence (OSL) dating showed that these geomorphic units had growth phases that decreased in age from west to east (Units I-V, from 5250 ± 820 to 325 ± 31 years ago, respectively). The geomorphic units were studied in two parallel 3 km transects on the southern part of the island. Along transect A-B, about 1 km from the southern shore, deep augerings were used to study sedimentary sequence and soil development, while on transect C-D on the southern shore, the continuous cliff exposure allowed more detailed morphological investigation. On all geomorphic units excluding the youngest, podzolisation has been the main soil-forming process. Groundwater level was monitored monthly for two years in 14 deep wells along transect A-B. Groundwater level during the formation of the B horizon was ascertained by determination of Fe. Podzol morphology (color of B horizon and its boundary with the E horizon) generally showed correlation to groundwater levels for both transects, except for the podzols in southwestern part of the island (Unit II). The podzols of Unit II showed an extremely thick (3 m) Bhm horizon devoid of Fe, indicating that they were formed under poor drainage conditions. However, soil morphology (undulating EB horizon boundary) and measured groundwater levels (below the B horizons) demonstrated that drainage has been improved. The extremely thick B horizon (3 m) in those podzols, which was formed in approximately 3000 years, and its genesis is explained by concentrated lateral flow of DOM-loaded groundwater due to the converging ridge alignments found in these units, in combination with a gradual uplift of the southwestern part of the island.

Shoreline and Sand Storage Dynamics from Annual Airborne LIDAR Surveys, Texas Gulf Coast

ABSTRACT Paine, J.G.; Caudle, T.L., and Andrews, J.R., 2017. Shoreline and sand storage dynamics from annual airborne LIDAR surveys, Texas Gulf Coast. Annual airborne LIDAR surveys were conducted along the Texas Gulf of Mexico shoreline between 2010 and 2012 to map shoreline position, determine shoreline movement and its historical context, and quantify beach and dune morphology by determining elevation threshold area (ETA) relationships for Holocene barrier islands, strandplains, and fluvial and deltaic headlands and marshes. Historical (1800s to 2007) movement is erosional for all major Texas shoreline segments, averaging 1.3 m/y of retreat. Shorelines retreated between 2007 and 2010 at a higher average rate of 2.8 m/y because of erosion and partial recovery from Hurricanes Ike (2008), Humberto (2007), and Dolly (2008). Despite the erosional context, airborne LIDAR surveys show that the shoreline advanced at 75% of 11,783 monitoring sites between 2010 and 2011 and moved an average of 6.5 m seaward durin...

Micropaleontologic record of Pliocene and Quaternary paleoenvironments in the southern Albemarle Embayment, North Carolina, U.S.A

The Albemarle Embayment, a Cenozoic depositional basin on the Atlantic coast of the USA, is an ideal setting to understand the temporal and spatial variation of eustatic sea-level fluctuations, glacio-hydro-isostasy, tectonics, subsidence, environments and sedimentation patterns of a passive margin Quaternary section. A NE–SW transect of cores and seismic data in the southern Albemarle Embayment were analyzed using micropaleontologic (foraminifera, diatoms, pollen), sedimentologic, stratigraphic, and geochronologic data to reconstruct the paleoenvironmental evolution and paleoclimates in the nearly 90m thick Quaternary section. The study area is a very low gradient Quaternary landscape that is cross-cut by several Pleistocene incised valleys; a Holocene barrier island complex forms its eastern margin. In the subsurface, the Albemarle Embayment is bordered to the north by the Norfolk Arch and to the south by the depositionally-constructed Cape Lookout High, which is positioned on the northern flank of the structural Carolina Platform.The Quaternary section overlies mid-Pliocene carbonates in three cores; the contact rises in elevation towards the Cape Lookout High. Fossils and sediment characteristics suggest a subtropical, shallow, high energy marine environment during the Pliocene. Overlying units include incomplete Pleistocene, clastic, transgressive-regressive (T-R) deposits. These have similar ages and stratigraphic signatures as the T-R cycles in the central and north-central Albemarle Embayment, although mid-Pleistocene deposits may be older in the southern region. The bulk of the early and mid-Pleistocene record consists of inner to mid-shelf sand and muddy sand. In contrast, late Pleistocene sands are of inner shelf origin, reflecting the infilling of the basin. Lowstand paleovalleys, with fluvial, wetland and estuarine fill, dissect the early, mid- and late Pleistocene marine units; their locations reflect antecedent topography. Holocene sediments were deposited in shoreface and barrier island environments. Quaternary foraminiferal assemblages in the southern Albemarle Embayment exhibit greater species richness than those in the central and north-central embayment reflecting the presence of a major biogeographic boundary at the same location as the modern biogeographic boundary at Cape Hatteras.

Late Quaternary development of the Croatan Beach Ridge Complex, Bogue Sound, Bogue Banks, NC, USA and implications for coastal evolution

Foraminiferal, sedimentological, geophysical, and geochronologic data were utilized to elucidate the late Quaternary geologic development of the Croatan Beach Ridge Complex (CBRC), Bogue Sound, and Bogue Banks, North Carolina, USA. The CBRC is a relict beach ridge feature located on the mainland. It is separated from the modern barrier island, Bogue Banks, by Bogue Sound. Seventeen cores along shore-normal and shore-parallel transects provided material for sedimentologic and foraminiferal analysis and resulted in the recognition of seven depositional facies representing a variety of coastal depositional environments.Chronologic and depositional facies data suggest the CBRC was initiated during MIS 5a and rapid southward progradation produced a cape structure. Eolian reactivation of the upper sand of the CBRC occurred during the last glacial maximum (∼18 ka). The age of flood tide delta deposits in Bogue Sound suggests that the Holocene barrier island, Bogue Banks, had formed by ∼6 ka. Shoreface ravinement resulted in a shoreface landward of the present shoreline by ∼3.5 ka. Seaward and westward spit progradation of Bogue Banks began ∼1.7 ka and continued to ∼1.3 ka. Normal marine salinity conditions were present in Bogue Sound ∼1.1 ka, suggesting removal of at least the narrowest parts of the barrier island, coeval with a previously documented segmentation of the southern Outer Banks barrier islands. Previous work has linked this segmentation to climate warming and increased tropical storm activity during the Medieval Climate Anomaly.This study illustrates the complex response of this coastal system to Pleistocene and Holocene sea-level and climate change over two major sea-level cycles. In particular, the regional geomorphology during MIS5a and the Holocene sea-level highstand differ significantly and this, in large part, was controlled by the antecedent geologic framework, resulted in the contrasting more localized coastal geomorphic response.

Visual Assessment and Interpolation of Low Chroma Soil Colors

Color is one of the most obvious properties of soils and is utilized in many soil evaluation strategies. Color data are usually documented using Munsell notation of hue, value, and chroma, with the latter two parameters normally documented to the nearest whole unit. Soil colors with chroma less than or equal to 2 are particularly important in soil classification and soil interpretation, especially with regard to the identification of field indicators of hydric soils. During a field study of dune soils on late Holocene barrier island landscapes, the investigators sought to improve the quality of color data by noting color value and chroma to the nearest half unit by interpolating between chips in the Munsell book. To evaluate whether interpolating color to half units of value and chroma was justifiable, a study was initiated using 23 homogenized and randomized soil samples collected from dune profiles in the study area. Moist color values were generally 6 or less and moist chromas were 3 or less, while hue mostly ranged between 10YR and 5Y. As experienced soil morphologists, we estimated moist and dry colors to the nearest half unit of value and chroma, using the same reference color book and under fixed lighting conditions. A digital colorimeter was used to measure the color of the soil samples to the nearest 0.1 unit of hue, value, and chroma. This study demonstrates that when individuals are forced to select a whole chip rather than interpolating between chips, there is a significantly greater error in the estimation of both the Munsell value and chroma. Therefore, experienced soil morphologists appear to be justified in interpolating Munsell value and chroma to the nearest half unit.

Foraminiferal and Sedimentary Record of Late Holocene Barrier Island Evolution, Pea Island, North Carolina: The Role of Storm Overwash, Inlet Processes, and Anthropogenic Modification

Abstract Foraminiferal and sedimentary data, supplemented with geochemical dating and ground-penetrating radar transects, show that the barrier island at Pea Island National Wildlife Refuge just north of Rodanthe, North Carolina, has been dominated by a combination of inlet and overwash processes for at least 1000 years. The stratigraphic record of several vibracores does not preserve every, or even many, overwash events but, instead, is characterized by three to four fining-upward sequences. The last three commence with overwash sand or gravel that is overlain by a variety of finer-grained estuarine, inlet, and marsh deposits. The dynamic nature of this segment of the Outer Banks was muted in the late 1930s by construction of artificial barrier dune ridges, extensive planting of grass and shrubs, and construction of Highway 12 in 1953. Subsequently, the road and barrier dune ridge were rebuilt and relocated several times following storm events.

Holocene book reviews : Geology of Holocene barrier island systems

Holocene book reviews : Geology of Holocene barrier island systems

Barrier Island Evolution and Reworking by Inlet Migration along the Mississippi-Alabama Gulf Coast: ABSTRACT

ABSTRACT The five barrier islands along the Mississippi-Alabama coast are located 10 to 14 miles (16 to 23 kilometers) offshore and separate Mississippi Sound from the Gulf of Mexico. The barrier islands in the chain are, from east to west: Dauphin Island, Petit Bois Island, Horn Island, Ship Island, and Cat Island. The islands are low sand bodies situated on a relatively broad Holocene sand platform that extends 70 miles (113 kilometers) from Dauphin Island on the east to Cat Island on the west. The platform varies in thickness from 25 to 75 feet (7.6 to 23 meters) and rests on Holocene marine clays or on Pleistocene sediments. The barrier islands are nourished chiefly by littoral drift from sand sources to the east. The barrier island chain predates the St. Bernard lobe of the Mississippi delta complex, which began to prograde about 3,000 years ago, and continued until it was abandoned approximately 1,500 years ago. In contrast to the other islands, Cat Island at the western down-drift end of the Mississippi-Alabama barrier island chain, is characterized by more than 12 prominent east- west oriented progradational linear ridges. The ridge system of Cat Island is interpreted as a relict of an earlier stage in the life cycle of the barrier platform when there was a more robust littoral drift system and an abundant sediment supply. During the Pre-St. Bernard Delta period of vigorous sedimentation, the islands in the barrier chain probably exhibited progradational ridges similar to those now found only on Cat Island. Presently, only vestigial traces of these progradational features remain on the islands to the east of Cat Island. Unlike Cat Island, which has been protected and preserved by the St. Bernard Delta, the other barrier islands have been modified and reworked during the past 1,500 years by processes of island and tidal inlet migration, accompanied by a general weakening of the littoral drift and a reduction of the available sediment supply. The four tidal passes, or inlets, that separate the barrier islands are broad low areas in the Holocene barrier island sand platform, ranging from 3.5 to 5.8 miles (5.6 to 9.3 kilometers) in width. The water depth in the passes is generally shallow, less than 15 feet (4.6 meters), except in the tidal channels where natural depths of more than 30 feet (9 meters) are often reached. Due to the water depths reached by the tidal channels, the nearly the full thickness of the Holocene island platform east of Cat Island has likely been reworked during the migration of the islands and tidal passes. Additionally, the down-drift islands have become increasingly sediment-starved due to sediment losses in the island passes. Shoals and lobes of sand, commonly found on the north (Mississippi Sound) side of the barrier islands, are interpreted to be the remains of interchannel shoals that mark former tidal inlet positions.

Beaches and beach-related facies, Holocene barrier islands of Georgia

AbstractThe mesotidal coast of Georgia encompasses diverse sedimentary features and depositional environments, most of which typify major facies of the entire Georgia Bight – a broad embayment on the southeastern U.S. shelf. Barrier island strandline environments especially include tidal inlets and shoals, the beach shoreface, foreshore, and backshore, and dunes and washover fans. Low energy beach segments may grade directly into small seaside tidal flats. Relict salt marsh deposits crop out on erosional beaches. All deposits are in dynamic equilibrium with fluctuating coastal conditions and a gradual rise in sea level.Most of these facies are important in interpreting ancient epeiric transgressive/regressive coastal sequences. Pronounced local variations in the overall sequence result from complex latero-vertical relationships between (1) the shoreface, foreshore, and shoals, and (2) the shoals, low energy beaches, and beach-related tidal flats. Washover fans and relict deposits on erosional beaches cause perturbations within present day lateral relationships but are normal in transgressive facies tracts along such coasts. Close stratigraphic control of outcrops or cores would be necessary for detailed reconstructions of, or correlations among, ancient analogs.

Physical and biogenic aspects of backbarrier sedimentary sequences, Georgia Coast, U.S.A.

Configuration of the present Georgia coast, with its broad expanses of salt marsh and intricate tidal drainages behind Pleistocene—Holocene barrier islands, implies the filling of an extensive backbarrier lagoonal system. However, riverine and salt marsh estuaries dominate the modern depositional setting and reconstructed vertical sequences suggest that backbarrier filling was accomplished by estuarine processes. Backbarrier sediments exhibit considerable diversity in grain size and physical and biogenic sedimentary structures. Principal sediment textures include clean to muddy coarse and fine sand, mud and, to a lesser extent, shells, pebbles, and mud and bedrock clasts. Physical sedimentary structures, many modified by bioturbation, include large-scale cross-bedding (trough and planar) with graded sets, small-scale crossbedding (ripple lamination and flaser, wavy and lenticular bedding), laminated to interbedded sand and mud, and rare pebble or shell beds. Ebb-oriented structures predominate over flood-oriented ones. Except in upper reaches of riverine estuaries, biogenic sedimentary structures typically are diverse and abundant. characteristic lebensspuren include polychaete dwelling tubes, decapod and polychaete burrows, ray holes, and bioturbate textures imparted by amphipods, ophiuroids, bivalves and anemones. Patterns of bioturbation are controlled to some extent by sediment type, salinity levels and rates of deposition and physical reworking of sediments. Although intensity of bioturbation generally increases seaward, bioturbate textures are more readily preservable in the lower-energy, middle reaches of estuaries. Because of numerous local Pleistocene sediment sources and local variations in hydrographic regimes, the stratigraphic record of salt marsh estuaries differs relatively little from that of riverine estuaries. The basic sequence, in ascending order, typically consists of: (1) channel-lag and (or) channel-fill deposits; (2) estuarine accretionary beds; (3) point bar, tidal flat and shoal sediments; and (4) channel margin and salt marsh deposits.

Coastal lithosome preservation: Evidence from the shoreface and inner continental shelf off Bogue Banks, North Carolina

Seismic and vibracore data from the continental shelf as well as borehole data from an adjacent barrier island indicate that the migrating shoreface, responding to rising sea level, has nearly completely removed the entire coastal sedimentary record in northern Onslow Bay, North Carolina. This process was active throughout the Quaternary and even the late Tertiary as middle Tertiary sediments and rocks directly crop out on the sea floor in vast areas of this shelf sector. Seaward of Bogue Banks (a Holocene barrier island located along northern Onslow Bay) seismic sequences have been truncated by the modern shoreface which extends to about 12 m depth. Correlations with borehole data on the island indicate that these sequences are Pleistocene and Holocene in age. The Holocene sequences contain numerous channels which are interpreted to be relict tidal inlets. However, none extends vertically lower than the shoreface indicating that landward translation of the shoreface would remove most of even the deepest Holocene coastal lithosomes, parts of the Pleistocene and even a portion of the Tertiary. The middle and inner portions of the continental shelf have been incised by numerous channels. Channel facies are an important component of shelf sedimentary and stratigraphic sequences and comprise about one-third of northern Onslow Bay. Similar seismic infilling facies and vibracore data as well as the poor preservation potential of the tidal inlet throats indicate that the channels were lower coastal plain streams. The infilling sediments are mostly muds with some sands and shells. Dates from shells via the amino acid racemization technique are Pleistocene in age. These channels, with some having multiple infilling events, assume greater importance when one recognizes that essentially they alone hold the vast majority of the Quaternary record of numerous sea-level fluctuations.

Comparison of Pleistocene and Holocene barrier island beach-to-offshore sequences, Georgia and northeast Florida coasts, U.S.A.

Well-exposed vertical sequences of Pleistocene shoreline deposits are rare on the coastal plain of the southeastern United States. An important exception is an exposure along the St. Mary's River on the Georgia-Florida state line. This outcrop contains an excellent depositional strike section of a prograding offshore, shoreface, foreshore and backshore sequence with well-preserved physical and biogenic sedimentary structures. Offshore sediments are composed of highly bioturbated, muddy fine sand. Distinct burrows and bedding become progressively more abundant upward through a transition zone into shoreface deposits. The shoreface is dominated by the trace fossil Ophiomorpha nodosa and physical sedimentary structures are poorly preserved. Foreshore sediments contain low-angle seaward dipping beds, high-angle landward dipping beds and ripple laminae. Heavy-mineral accumulations in the backshore accentuate bedding and biogenic structures such as ghost crab and insect burrows and bioturbation by amphipods. Direct correlation of most primary physical and biogenic sedimentary structures and textures can be made between the Pleistocene and Holocene beach-to-offshore facies assemblages. However, our studies of the Pleistocene indicate that the existing Holocene vertical sequence model for the Georgia coast needs to be modified to account for biogenically produced post-depositional effects.

Ingleside Strandplain Sand of Central Texas Coast: GEOLOGIC NOTES

The Ingleside sand is a Pleistocene unit which crops out along the mainland margin of the Texas coast from Pass Cavallo to Baffin Bay. This sand body is a thin blanket of shoreface and eolian sediments, deposited as an extensive strandplain sand sheet, which extends into the subsurface under the Holocene barrier islands and lies directly on an oxidized surface of older Pleistocene deltaic sediments. The Ingleside strandplain sand was deposited during a mid-Wisconsin interstadial.

Sediment distribution and evolution of tidal deltas along a tide-dominated shoreline, Wachapreague, Virginia

Borings from the barrier island/lagoon system of the Eastern Shore of Virginia penetrated an unconformity which separates Pleistocene barrier island and offshore marine sediments from the overlying Holocene tidal delta and barrier island sediments. Offshore marine sediments and deposits within the flood-tidal delta (marsh, tidal flat-bay, inlet-mouth bar complex) are recognized on the basis of sediment color, composition, grain-size changes in the vertical sequence, presence of organic matter, and faunal suite. Subsurface data, historical records, and morphology of lateral accretion on barrier islands suggest that major inlets in the vicinity of Wachapreague have been relatively stable throughout Holocene time; they appear to be located where Pleistocene stream valleys previously existed. Holocene barrier islands apparently developed on drainage divide areas following post-Wisconsin transgression of the sea. The initial phase of tidal delta development was characterized by vertically accreting, fan-shaped, inlet-mouth bars; tidal channels stabilized after bar crests had shoaled sufficiently for marsh to form. With landward progradation across the lagoon, sand-rich deposits graded laterally away from the inlets and vertically into clayey sand and silty clay of the tidal flat-bay and marsh environments. Ebb inlet-mouth bars developed asymmetrically southward in response to littoral drift. Flood tidal deltas also built preferentially toward the south as indicated by: (1) sand distribution of the inlet-mouth bar complex; and (2) greater development of marsh south of the inlets.