Washover Deposits Research Articles

Geologic Effects of Hurricane Alicia on Upper Texas Coast: ABSTRACT

Hurricane Alicia made landfall on August 18, 1983, on the eastern end of Follets Island just west of San Luis Pass and Galveston Island. The maximum winds (198 km/hour), storm surge (390 cm), and wave energy were concentrated just east of landfall, along the western half of Galveston Island and the easternmost tip of Follets Island. In this area, the mean high waterline typically eroded 30-50 m (100-165 ft), whereas the vegetation line typically eroded 20-30 m (65-130 ft). Maximum erosion of the mean high waterline (up to 300 m) and the vegetation line (up to 100 m) occurred adjacent to San Luis Pass. A narrow band of washover sands, typically extending 20 m (60 ft) inland, was deposited along the nondeveloped portions of west Galveston Island. Because of the modification of the foredune system, these washover sediments typically extended 60-80 m (200-250 ft) inland along the developed portions of the shoreline. Erosion of the shoreline along the eastern part of Galveston Island was minor, although some washover channels formed where beach access roads cut through the dunes. In contrast, the area west of landfall on Follett's Island experienced little shoreline erosion and virtually no washovers. This area was dominated by offshore-directed winds during the passage of the hurricane; thus, the main effect was numerous storm-surge ebb channels formed by the draining of West Bay. These channels are, on the average, 20 m (65 ft) wide, 55 m (180 ft) long, and up to 3 m (10 ft) deep; the largest channel is 40 m (130 ft) wide and 110 m (360 ft) long. These channels were initiated along low spots in the dunes (typically beach access roads) and grew landward by headward erosion until they encountered the highway, where they were partially deflected. Much of the sand transported offshore during the storm returned to the beach within the following 6 months by progressive welding of swash bars to the foreshore to form a broad, flat berm. Since then, the patterns of shoreline accretion and erosion appear to reflect seasonal rather than long-term changes. The vegetation line had not shown any significant recovery 20 months after the storm, and complete recovery of the beach to its prehurricane position seems unlikely. End_of_Article - Last_Page 1419------------

Geomorphic evolution and stratigraphy of Price and Capers Inlets, South Carolina

ABSTRACTA three‐dimensional model for a tidal inlet‐barrier island depositional system was constructed through examination of 37 vibracores and 10 auger drill holes on Capers and Dewees Islands, South Carolina. Two cycles of southerly inlet migration and subsequent abandonment resulted in beach ridge truncation on the northern ends of both barriers. Historical evidence indicates that these tidal inlets migrated 1.5 km to the south owing to a dominant north‐south longshore transport direction. The hydraulic inefficiency of these over‐extended inlet channels caused shorter, more northerly‐oriented channels to breach through the ebbtidal deltas. After inlet reorientation, large wave‐formed swash bars migrated landward closing former inlet channels. Weakened tidal currents through the abandoned channels permitted clay plugs to form thick impermeable seals over active channel‐fill sand and shell.Price and Capers Inlets formed during the onset of the Holocene transgression following submergence of the ancestral Plio‐Pleistocene Santee River drainage system. Coarse, poorly sorted inlet‐deposited sand disconformably overlies Pleistocene estuarine clay and is capped by a dense clay plug. Shoreline reorientation and landward retreat of a primary barrier island chain occurred between the first and second cycles of inlet‐channel migration and abandonment. Beach ridges prograded seaward over the first inlet sequence. A second cycle of inlet migration truncated the northernmost portion of these beach ridges and scoured into the clay plug of the earlier inlet deposit. Abandonment of this channel resulted in deposition of a second abandoned inlet‐channel clay plug. Abandoned tidal inlet channels exhibit U‐shaped strike and crescentic‐ to wedge‐shaped dip geometries. Basal, poorly sorted inlet sands are sealed beneath impermeable, abandoned‐channel silt and clay, washover deposits, and salt marsh. Multiple episodes of inlet migration and abandonment during a rising sea‐level deposited stacked inlet‐fill sequences within the barrier islands.The resultant stratigraphy consists of interlayered, fining‐upward, active inlet‐fill sand overlain by thicker abandoned inlet‐fill clay plugs. These clay plugs form impermeable zones between adjacent barrier island sand bodies. Shoreline transgression would remove the uppermost barrier island deposits, sealing the inlet‐fill sequences between Pleistocene estuarine clay and shoreface to shelf silt and clay.

Holocene sedimentation of a wave-dominated barrierisland shoreline: Cape Lookout, North Carolina

The sedimentary record of 130 km of microtidal (0.9 m tidal range) high wave energy (1.5 m average wave height) barrier island shoreline of the Cape Lookout cuspate foreland has been evaluated through examination of 3136 m of subsurface samples from closely spaced drill holes. Holocene sedimentation and coastal evolution has been a function of five major depositional processes: (1) eustatic sea-level rise and barrier-shoreline transgression; (2) lateral tidal inlet migration and reworking of barrier island deposits; (3) shoreface sedimentation and local barrier progradation; (4) storm washover deposition with infilling of shallow lagoons; and (5) flood-tidal delta sedimentation in back-barrier environments. Twenty-five radiocarbon dates of subsurface peat and shell material from the Cape Lookout area are the basis for a late Holocene sea-level curve. From 9000 to 4000 B.P. eustatic sea level rose rapidly, resulting in landward migration of both barrier limbs of the cuspate foreland. A decline in the rate of sea-level rise since 4000 B.P. resulted in relative shoreline stabilization and deposition of contrasting coastal sedimentary sequences. The higher energy, storm-dominated northeast barrier limb (Core and Portsmouth Banks) has migrated landward producing a transgressive sequence of coarse-grained, horizontally bedded washover sands overlying burrowed to laminated back-barrier and lagoonal silty sands. Locally, ephemeral tidal inlets have reworked the transgressive barrier sequence depositing fining-upward spit platform and channel-fill sequences of cross-bedded, pebble gravel to fine sand and shell. Shoreface sedimentation along a portion of the lower energy, northwest barrier limb (Bogue Banks) has resulted in shoreline progradation and deposition of a coarsening-up sequence of burrowed to cross-bedded and laminated, fine-grained shoreface and foreshore sands. In contrast, the adjacent barrier island (Shackleford Banks) consists almost totally of inlet-fill sediments deposited by lateral tidal inlet migration. Holocene sediments in the shallow lagoons behind the barriers are 5–8 m thick fining-up sequences of interbedded burrowed, rooted and laminated flood-tidal delta, salt marsh, and washover sands, silts and clays. While barrier island sequences are generally 10 m in thickness, inlet-fill sequences may be as much as 25 m thick and comprise an average of 35% of the Holocene sedimentary deposits. Tidal inlet-fill, back-barrier (including flood-tidal delta) and shoreface deposits are the most highly preservable facies in the wave-dominated barrier-shoreline setting. In the Cape Lookout cuspate foreland, these three facies account for over 80% of the sedimentary deposits preserved beneath the barriers. Foreshore, spit platform and overwash facies account for the remaining 20%.

The Upper Jurassic ‘Boulder Beds’ and related deposits: a fault-controlled submarine slope, NE Scotland

During the late Jurassic (Kimmeridgian) rifting in the North Sea, large-scale normal faults developed to form relatively deep marine basins. The north-western margin of one of these basins, defined by the Helmsdale Fault, is described to show the nature of sedimentation associated with a submarine fault scarp. Subsidence to the SE of the Helmsdale Fault led to a fault scarp down which various sediment mass flows moved, including debris flow, rockfall, high and low concentration turbidity currents and possible storm surge wash-over deposits. Redeposition by soft-sediment sliding was important as a slope accretion process. Deep channels were incised into the slope and infilled by coarse-grained elastics: contemporaneous, small-scale submarine fans, located farther off in the present North Sea, may have developed as a result of slope-bypassing sediment flows down such channels. The succession is characterized by extreme sediment heterogeneity compared to many deep water environments and it may be part of an ancient fan delta. The main influences upon slope accretion were the synsedimentary movements on the Helmsdale Fault and the nature of the available sediment input, such that the succession represents structurally controlled and sedimentologically modified slope development.

Influence of Depositional Environment on Type and Probability of Encountering Coal-Bed Discontinuities: ABSTRACT

Predictive models that characterize coal bed discontinuities enable mine operators to be better judges of the size, geometry, and influence of these features in unmined portions of the coal bed. An analysis of depositional environments in initial geologic studies of prospective mine properties will indicate the specific types of coal bed discontinuities that can be anticipated. The type of coal bed discontinuity and the frequency of occurrence are functions of the depositional environment. Peat, the precursor of coal, accumulated in swamps that may form on fluvial plains, on delta plains, and in littoral areas. Using generalized depositional models for these environments, the types of coal bed discontinuities that can be expected and an estimate of the likelihood of their occurrence can be determined. Coal beds deposited on fluvial plains generally are thin, erratic, and discontinuous because of the highly oxidizing character of this environment. Discontinuities due to irregular topography and fluvial channel activity are common, as displayed by Upper Pennsylvanian and Permian coal beds of the Dunkard basin. Coal beds formed in deltaic settings are generally thick and laterally extensive, as they commonly infill broad interdistributary areas. These coal beds are typically plagued by discontinuities associated with distributary channels (e.g., avulsion, splays). The Freeport and Kittanning coal beds in west-central Pennsylvania demonstrate features characteristic of delta-plain coals and the discontinuities that beset them. Coal beds deposited landward of barrier bar sequences generally are irregular and are interrupted by tidal channels and washover deposits. The Pocahontas 3 coal bed of southern West Virginia exhibits many of the characteristics of this paralic setting. Although some clastic dikes may be depositionally related, such discontinuities, as well as faulting, may be overprinted by tectonic activity on coal beds from any depositio al environment. End_of_Article - Last_Page 1922------------

A Sedimentologic Model for a Microtidal Flood-Tidal Delta--San Luis Pass, Texas: ABSTRACT

San Luis Pass is a microtidal inlet located at the southwest end of Galveston Island, Texas. Continuous cores taken with a portable vibracoring rig, and surface grab samples provide data for developing a 3-dimensional sedimentologic model of the flood-tidal delta complex located landward of the tidal inlet. This model is based on the type, and vertical and lateral distribution of lithologic units, sedimentary structures, textures, and trace fossils. A complete bayward vertical sequence in the flood-tidal delta complex consists (from base to top) of highly bioturbated bay clays and associated oyster reefs, highly bioturbated clayey sands/sandy clays of the delta margin, variably burrowed sand to shelly sand of the delta, and rooted or burrowed muds of the marsh or mud flat. Washover shell-hash deposits may occur at random intervals throughout this sequence. A more seaward sequence in the vicinity of San Luis Pass consists of a basal tidal inlet deposit of graded layers of sand and shell overlain by burrowed to shelly sand of the barrier spit. This model for microtidal flood-tidal deltas differs significantly from models presented for mesotidal flood-tidal delta systems in the general lack of large-scale, high-angle sedimentary structures; the presence of intense bioturbation; the presence of washover deposits; and the general upward-coarsening nature of the vertical sequence. Mesotidal flood-tidal deltas with clean, coarse to medium-grained sands may make good petroleum End_Page 490------------------------------ reservoirs. Microtidal flood-tidal deltas with highly bioturbated clayey sands to sandy clays would undoubtedly prove to be poor petroleum reservoirs. End_of_Article - Last_Page 491------------

Upward-Shoaling Origin of an Eastern Gulf Coast Barrier Island: ABSTRACT

North Bunces Key is a narrow barrier island which has formed on the northern portion of the ebb-tidal platform of Egmont Channel which services Tampa Bay. The island first became supratidal in 1960. Since that time, it has grown to a length of 2 km (1.2 mi) with widespread vegetation and dunes which rise about 2 m (6 ft) above mean sea level. Most investigators discount upward shoaling as a mechanism for barrier formation in favor of drowned beach ridges. A closely spaced sequence of aerial photos documents the origin of North Bunces Key from a subtidal shoal area without benefit of significant storm activity. Requirements for formation of the barrier appear to be (1) abundant sediment, (2) shallow, very gently sloping platform, and (3) low wave energy. Vibracores taken from several traverses across the barrier show a general stratigraphic sequence of nearshore sediments successively overlain by foreshore, backbeach, and dune sediments. Cores immediately landward of the barrier contain muddy lagoonal sands containing fining-upward washover deposits. Absence of typical nearshore sediments landward of the barrier may result from low energy conditions which permitted mud to accumulate even prior to emergence of the shoal or reworking and addition of mud as pellets by benthic organisms. End_of_Article - Last_Page 467------------

Regressive and Transgressive Sand Bodies Associated with Lafourche Delta in South Louisiana: ABSTRACT

The beach ridge plains and coastal barrier systems along the seaward margin of the wave-dominated Lafourche delta represent regressive and transgressive events associated with multiple delta lobe progradations and abandonments over the last 1,500 years. Two major delta lobes, the Early Lafourche and the Late Lafourche, can be recognized by the subaerial expression of the abandoned deltaic plain. Associated with each delta lobe is a regressive beach ridge plain deposited during distributary progradation and coastal barrier system deposited during distributary transgression. The Cheniere Caminada beach ridge plain is associated with the Late Lafourche delta lobe. It consists of more than 70 subparallel beach ridges in an arcuate fan-shaped configuration which flare seaward along the eastern levees of Bayous Lafourche and Moreau. Radiocarbon dates indicate beach ridge building began approximately 600 years ago when Bayou Lafourche built seaward of the older Bayou Blue shoreline and started intercepting westward longshore sediment transport, resulting in the formation of Cheniere Caminada. Near the fan apex, beach ridges are 7 to 8 m (23 to 26 ft) thick and thin westward to 2 to 3 m (6.5 to 10 ft) thick. A typical beach ridge stratigraphic sequence coarsens upward with shoreface silty sands overlain by a cap of washover and aeolian sands. Beach ridge growth ceased approximately 300 years ago when Bayou Lafourche was abandoned. Distributary abandonment initiated the transgression at Bayou Lafourche and the development of an erosional deltaic headland, the Caminada-Moreau coast, with a set of symmetrical, flanking barriers, the Timbalier Islands to the west and the Caminada Pass spit and Grand Isle to the east. Reworking of distributary and beach ridge sand bodies by shoreface retreat supplies the sand source required for coastal barrier generation. Shore-parallel transport distributes sand from the headland source into downdrift marginal spits, tidal deltas, and flanking barrier islands. Cores show that the flanking barriers (Timbalier Island and Grand Isle) increase in thickness from 2 to 3 m (6.5 to 10 ft) near the headland to 5 to 6 m (16 to 20 ft) at their downdrift ends. A typical flanking barrier strat graphic sequence shows a coarse-grained tidal inlet fill overlying a finer grained interdistributary bay fill and underlying a thin cap of washover and aeolian sands. In the Early Lafourche delta, only remnants of an older regressive beach ridge plain, Chenier Caillou, can be recognized owing to its advanced stage of transgression. A series of relict, partially submerged beach ridges associated with the Caillou headland can be seen spreading seaward on their western margin in the central Isles Dernieres indicating the dominant longshore transport direction was westward. Cores show a regressive sequence 7 to 8 m (23 to 26 ft) thick, similar to Cheniere Caminada, where shoreface silty sands underlie washover and aeolian deposits. The Isles Dernieres represent the transgressive barrier system of the Early Lafourche delta lobe. Abandonment of the Caillou distributaries occurred approximately 600 to 800 years ago. Due to long-term subsidence, the Caillou headland is now submerged below sea level and the Early Lafourche barrier system has evolved into a transgressive barrier island arc separated from the mainland by an intradeltaic lagoon. Sediment dispersal consists of seaward transport into an inner-shelf sand sheet, landward transport into washover deposits, and shore-parallel transport into tidal deltas and marginal recurved spits. Cores show the central Isles Dernieres consist of a thin washover sand sheet 1 to 2 m (3 to 6.5 ft) thick transgressing over delta plain and beach ridge deposits. The downdrift ends of the Isles Dernieres islands are thicker, up to 4 to 5 m (13 to 16 ft), and overlie fine-grained interdistributary bay fills. In the subsurface of Grand Isle and Cheniere Caminada, lies the transgressive barrier system of the Bayou Blue distributary abandoned 1,200 years ago. The sand source for barrier generation is two Bayou Blue distributaries which lie - 10 m (-33 ft) below sea level adjacent to Barataria Pass. The transgressive sand body strikes northwest and is overlain by a sequence of regressive interdistributary clays and silts, regressive beach ridge sands (Cheniere Caminada), and a transgressive flanking barrier sand (Grand Isle). Radiocarbon dates indicate Bayou Blue Barrier was actively transgressing landward 920 years ago. The identification of regressive beach ridge plains along the Lafourche delta suggests that these wave-dominated delta plain components may be stratigraphically more significant in the Mississippi delta than recognized before. Sequential delta lobe abandonments have led to the development of imbricating regressive End_Page 530------------------------------ and transgressive sand bodies in the Lafourche delta complex. End_of_Article - Last_Page 531------------

Regional Variability of Washover Deposits on South Texas Coast: ABSTRACT

Overwash processes play an important role in determining the stratigraphy of microtidal barrier islands. Along the microtidal coast of south Texas, regional variability in barrier island geometry produces a spectrum of washover types. South Padre Island is a low profile, transgressive feature with a discontinuous to nonexistent foredune ridge. It displays sheet overwash, coalescing washover terraces, and washover fans fed by large hurricane channels. North Padre and Mustang Islands are high-profile barrier islands with continuous foredune ridges, and thus only small, discrete interdune washovers occur. The relict tidal inlets between Mustang and Padre Islands are the sites of the largest washovers in the system, termed reactivated tidal deltas. This last type, although re atively rare in modern washover deposits, is probably similar in mode of origin to the large lobate back barrier features found on many high profile barriers on the Texas coast. Internal structures of the washovers were studied by trenching, box coring, and vibracoring. The dominant stratification type is plane beds with extensive heavy mineral laminae. Washover margins display some landward dipping foreset bedding. Hurricane channels show cut and fill structures with shell lags, plane beds, and some trough cross-beds. Directional features indicate that the washovers are formed by storm surge flood, and modified by surge ebb. This is supported by the existence of separate flanking ebb channels on some washovers, as well as current velocity simulations for the storm surge of Hurricane Allen, which show that flood currents are both stronger and longer lived than the ebb. In the ancient record, low profile barriers should be relatively thin deposits consisting of numerous coalescing washovers. Thicker sand bodies corresponding to high profile barriers should be found along strike. Since back-barrier facies have a higher preservation potential than other barrier environments, such a deposit might be dominated by reactivated tidal deltas. End_of_Article - Last_Page 1473------------

Estuarine and Fluvial Systems, Lower Mesaverde Group (Campanian), Northwestern Colorado: ABSTRACT

Stacked shoreline sandstones near the base of the Mesaverde Group are well exposed along the southern flank of Rangely dome, northwestern Colorado. Overlying these marine deposits is a thick sequence of carbonaceous siltstones that encase elongate lenticular sandstones. This sequence records the evolution from estuarine to fluvial channels formed along the western margin of the Cretaceous Western Interior seaway. Sections 180 to 250 m (590 to 820 ft) long, were measured along a trend perpendicular to paleoshoreline. Several individual sandstones were studied in detail to develop depositional models. Data from over 300 well logs provided information regarding regional distribution and stratigraphic relationships of the systems. Two major stratigraphic successions were reco nized. Thin (0.5 to 2 m, 20 in. to 6.6 ft) bioturbated and root-mottled fine sandstones, interbedded with pervasive siltstones occur immediately over the marine shoreline deposits in both sequences. The sandstones are interpreted as storm washover deposits. At Gillam Draw in the eastern portion of the outcrop study area, the washover sandstones are overlain by 50 to 60 m (165 to 195 ft) of bioturbated shales and siltstones. Ripplestratified, upward-fining, fine to very fine sandstone lenses occur in this interval. These lenses are 4 to 8 m (13 to 26 ft) thick, have erosional bases, and have well-developed lateral-accretion bounding surfaces. Ophiomorpha and other trace fossils suggest an estuarine influence. The sandstone lenses are point bar deposits formed along meandering tidal creeks. Si tstones, coals, and 8 to 12 m (26 to 39 ft) thick lenticular sandstones overlie the tidally influenced interval. The sandstone lenses change significantly in geometry, bounding surface relationships, and textural trends within this succession. Stratigraphically lower sandstones form broad (100 to 200 m, 330 to 660 ft wide) belts. Individual sandstone bodies within the lenses have erosional bases and prominent lateral-accretion surfaces. Trough cross-bedding near the base is overlain by ripple stratification. These broad, lenticular sandstones represent fluvial meander-belt deposits. Sandstone lenses become narrower and lack accretionary surfaces higher in the section. These younger sandstones are multi-storied, rather than multilateral, channel deposits and are flanked by extensive crevasse splay facies. They are interpreted as confined anastomosing fluvial channels. West of Gillam Draw, the anastomosing fluvial system directly overlies shoreface and storm washover deposits. The multistoried lenticular sandstones are thicker (20 to 30 m, 66 to 100 ft). Sedimentation kept pace with subsidence in the eastern, basinward sections. Lower reaches of streams were tidally influenced even though the area was not inundated by marine waters. The western succession represents aggraded fluvial systems formed inland from the coast. Both sequences are characteristic of areas of rapid subsidence. End_of_Article - Last_Page 525------------

Genesis and Preservation of Antidune Stratification in Modern and Ancient Washover Deposits: ABSTRACT

End_Page 419------------------------------Antidunes are nearly symmetrical, sinusoidal bed forms formed by near-critical flows, and comprise all bed profiles in phase with surface gravity waves in the overlying flow. This definition applies regardless of bed-form propagation direction or mode of formation. They occur in a wide range of depositional environments but are ubiquitous in the swash zones of fine-grained beaches. Antidunes are a common but little-recognized feature of washover fans, where they generate distinct sedimentary structures that are well preserved in the stratigraphic record. On washovers at Seabrook Island, South Carolina, antidune trains form at the fan apex, in phase with an undular hydraulic jump. This jump marks the transition within the overwash surge from supercritical flow of the berm crest to subcritical flow on the fan itself. Bed-form distribution on the fan is controlled not only by this transition but also by downfan energy dissipation within subcritical portions of the flow. From berm crest to fan terminus (i.e., downstream) the lateral sequence of bed geometries is: flat, erosional bed; antidunes; flat bed with primary current lineation and rhomboid rill marks; flat, featureless bed; rhomboid to cuspate ripples; undulatory, linear ripples; washed-out ripples; flat bed. Heavy minerals are concentrated near the fan apex owing to selective down an transport of quartz. Landward fan migration results in a genetic sequence of sedimentary structures in which all of these features are preserved. Streamwise boundaries of the antidune field are abrupt. Within the field itself, both crest amplitude and crest spacing decrease downfan with decreasing velocity. Bedding geometry is controlled by bed-form scale, by the pulsing nature of the flow (which tends to wash out bed forms at the end of each rock event), and by eolian deflation during neap tides and fair weather. Upstream antidunes produce thin lenses of backset laminae which are commonly truncated by flat beds or superposed backsets. These lenses grade downstream into thinner lenses with lower angle backsets, and finally into zones of very low-angle truncation surfaces. Orthoquartzitic washovers in the Carboniferous of Kentucky and the Ordovician of South Africa display well-preserved antidune formset geometries. Although the very well-sorted, mono-mineralic nature of these sandstones renders internal stratification nearly invisible, backset laminae are apparent. Pleistocene washovers in South Africa exhibit antidune bedding types influenced by grain size. Fine to medium-grained sand is organized in en echelon lenses of backset laminae interbedded with flat beds. Coarse sand and granules occur as flow-perpendicular ribbons with lenticular cross sections and spacing similar to antidune spacings on modern fans. Many published photographs of flat beds contain unrecognized examples of antidune stratification. In many places, the antidune cross-laminae would be impossible to detect without heavy minerals or other markers to accentuate bedding. However, in apparent massive beds, x-ray radiographic techniques have revealed cross-laminae with antidunal affinities. Moreover, antidunes may be responsible for much of the low-angle truncation normally associated with flat-bedded facies. End_of_Article - Last_Page 420------------

Bedform and stratification characteristics of some modern small‐scale washover sand bodies

ABSTRACTWashover sand bodies commonly develop along microtidal coastlines in beach/barrier island or spit settings. Wave runup, usually in conjunction with an abnormally high water level, may overtop the most landward berm of the beach and the foredune crest, if one exists, to produce overwash and subsequent runoff across the more landward subaerial surface. Two main elements of the resulting deposit are the washover fan and runoff channel. Newly formed, small‐scale washover deposits were examined along the Outer Banks, North Carolina, near Pt Mugu, California, and at Presque Isle (Lake Erie), Pennsylvania. The fans were formed in response to unidirectional landward transport, and the runoff channels in response to unidirectional flow usually in a landward direction, but sometimes in shore‐parallel then seaward direction. Where overwash carried across the fan surface and entered a pond or lagoon, a small‐scale delta (microdelta) developed. In this case, the washover fan consisted of two subfacies, the wetted, but ‘subaerial’ part of the fan and the subaqueous washover delta. Flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification.High velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low‐angle (landward dipping) planar stratification which comprised the major part of the fan. Similarly, rhomboid forms were produced by high velocity sheet flow across the fan surface. Where flow carried into a standing body of water, delta‐type foreset strata developed. For this case, the lateral structural sequence was subhorizontal, planar stratification merging landward into landward dipping, delta (tabular) foreset strata. In the runoff setting, where flow became channelized and continuous, both upper‐flow and lower‐flow regime currents were typical. Upper‐flow regime bedforms included antidunes, standing waves, and plane beds. The most commonly observed lower‐flow regime bedforms included microdelta‐like bars, low‐amplitude bars, linguoid ripples, and sinuous‐crested current ripple trains.The sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment.

A PROCESS-RESPONSE MODEL FOR HURRICANE WASHOVERS

The passage of Hurricane Allen over Padre Island in August 1980 presented an excellent opportunity to study the effects and controls of coastal morphology on hurricane deposits. In the Caribbean Sea, Hurricane Allen achieved a central pressure of 899 mb, making it the second strongest Atlantic hurricane ever recorded. Once in the Gulf of Mexico, the storm approached the Texas coast from the east-southeast, building a significant storm surge. Near Brownsville, the storm stalled, spending much of its energy offshore before making landfall early on August 10th near Brazos Santiago Pass at the southern tip of Padre Island. Surge gauges show that peak recorded storm tides of about 3 m occurred at Port Mansfield, some 35 km north of landfall. Analysis of tide data indicates a time lag of some 14 hours on the rising storm tide between the Gulf and south Laguna Madre. This is due both to the limited tidal exchange across Padre Island and to set-down in the lagoon due to southward-directed cyclonic winds. By contrast, measurements taken in Corpus Christi Bay, some 180 km north of landfall, show that at that point Gulf and bay tides were in phase. The cross-barrier water level differential at South Padre Island (up to 1.5 m) greatly facilitated hurricane breaching of the island. Oblique and vertical aerial photography show that Padre Island was breached in many places, with about 40 major hurricane channels still open several days after the storm. Surge heights were sufficient to inundate all of South Padre Island except for isolated "dune islands" resulting in broad and often coalescing washover deposits. The more continuous dune ridge on North Padre Island resulted in smaller, discrete washovers. Intensity, distribution, and morphology of washovers are functions of storm tide elevation, its phase relationships, island topography, and lagoonal water depth. The relationship is complex, yet precise enough to permit prediction of the island's response to the impact of a given storm.

Crestal Overtop and Washover Sedimentation on a Fringing Sandy Gravel Barrier Coast, Carnsore Point, Southeast Ireland

ABSTRACT Storm sedimentation on a fringing. sandy gravel barrier system in southeast Ireland is examined. Recent wave overtopping and overwashing during severe onshore storms have resulted in a distinctive pattern of crest and backbarrier sedimentation. This is not easily matched with available models of washover sedimentation drawn from the barrier islands of the eastern United States. Beach crest overtopping occurs at heights up to +6 m MHWS and is related to the development of upper beach swash ramps that are plastered against storm-wave-eroded dune scarps located at the barrier crest. Ramp sediments comprise coarse basal beach gravels overlain by coarsening-upward units and finally topped by distinctive seawarddipping fine grit/coarse sand couplets. In places where ramps reach the dune cre t, overtopping swash has deposited a fine marine gravel veneer (

Depositional Environments and Porosity Evolution, El Abra Limestone (Cretaceous), Mexico: ABSTRACT

Reservoir quality in the middle Cretaceous Golden Lane oil field, Mexico, was enhanced by freshwater dissolution during subaerial exposure in the Late Cretaceous and early Tertiary. Study of the outcropping equivalent El Abra Limestone in the type area demonstrates the presence of repeated emergence and submergence and the formation of subaerial discontinuity surfaces. Physical correlation has been established between El Abra quarry sections. This dovetails with a corresponding correlation of discontinuity surfaces to provide a series of reliable horizons on which to construct a shelf-edge model. Near back-reef rocks are characterized by thin (3 m) shoaling-up sequences usually capped by thick (to 10 m) supratidal and island sequences. Storm washover deposits with strong calcrete overprint are interbedded with penecontemporaneously dolomitized supratidal rocks. Freshwater dissolution during emergence produced voluminous moldic and vuggy porosity, subsequently reduced by interlayered marine internal sediment and a radiaxial fibrous cement mosaic. Platform-interior rocks are characterized by thick subtidal deposits and 3 to 5 m shoaling-upward sequences attributable to tidal flat progradation in the lee of shelf-edge islands or the migration of tidal channels. Paleosols and karstic surfaces are present at the tops of several cycles. Moldic and vuggy porosity developed during sub erial exposure was reduced by vadose and marine internal sediment. Radiaxial fibrous cement mosaics are not present. Porosity development in the El Abra type area is a result of synsedimentary emergence. Porosity was of local extent and largely occluded by processes active during succeeding sedimentary episodes. Post-El Abra freshwater leaching appears to have been minor. Contrasts in reservoir development between the Golden Lane and the comparatively tight El Abra type area may reflect differences in later exposure. End_of_Article - Last_Page 607------------

Holocene depositional history of a microtidal cuspate foreland cape: Cape Lookout, North Carolina

The Holocene stratigraphy of Cape Lookout, North Carolina, and the connecting barrier limb (Core Banks) displays sharply contrasting depositional histories. Beneath the Core Banks barrier chain a transgressive sequence of overwash storm deposits rests on back-barrier silty sands and salt marsh peats, evidence of the island's erosive landward migration. However, core holes drilled at Cape Lookout reveal a regressive vertical stacking of sediments that marks a major Holocene facies change. Here, fine-grained, burrowed shoreface deposits are overlain by coarse-grained, horizontally bedded, barrier-washover sands. Cape Lookout has evolved during the late Holocene as a prograding cuspate spit type of headland. This accretion is a result of the combined effects of oceanic washover deposition with sand accumulated from a dominant southerly longshore transport. During the past 4000 years the rate of sea-level rise slowed markedly in the area resulting in a decreased rate of landward migration for Core Banks. As the island became relatively stable with respect to sea-level rise, large volumes of sand were being transported along-shore to Cape Lookout and offshore to its subaqueous shoals. The slower landward retreat of the island probably allowed for a rapid southerly extension of Core Banks along with a buildup of the Cape Lookout shoals. The sedimentation and growth of Cape Lookout over the past 4000 years probably reflects a similar history of development for other capes and shoals along the southeastern coast of the United States. The large-scale cuspate morphology present today may therefore be a relatively recent phenomenon.

Washover of Deltaic Barriers on Louisiana Coast: ABSTRACT

Washover processes are an important factor in the transgressive development of barrier coastlines. Sea level elevation necessary for overwash is composed of a storm surge component, a wave setup component, a wave runup component, and an astronomical tide component. The nature and severity of overwash are a function of overwash elevation, its frequency of occurrence, and regional barrier geometry. Washover deposits on Louisiana barriers commonly account for over 50% of total sediment storage. Louisiana barriers have evolved by deltaic distributary abandonment; continuing sequential evolution of Mississippi delta complexes has generated a corresponding sequence of transgressive barriers. The resulting spectrum of barrier geometries has provided an ideal field site for the examination of washover form variability and its controlling processes. End_of_Article - Last_Page 1682------------

Louisiana Chenier System--Some Preliminary Reinterpretations and Refinements: ABSTRACT

End_Page 1560------------------------------The Louisiana chenier system is the product of a complex interaction of coastal, riverine, biologic, and storm processes. An important component of the chenier system, the chenier ridge, has been described as an accumulation of sand and/or shell material winnowed from existing marsh and tidal flat deposits. According to most interpretations, these ridges are correlative with changes in the flow direction of the Mississippi River from west to east. Most workers believe that the extensive mudflat and marsh sediments that separate the chenier ridges represent periods of progradation, influenced by a high sediment load from the Mississippi during periods of westerly discharge. These alternating periods of easterly and westerly flows represent the classical model that has been presented fo genesis of the chenier plain during the past 3,000 years. While it is generally agreed that Mississippi River sedimentation has been a major factor in the development of coastal geomorphology of southwestern Louisiana, the model presented above is over-simplified. Furthermore, the term, chenier ridge, has been indiscriminately applied to almost any morphologic unit within the system that shows noticeable relief as reflected in vegetative development. Therefore, little genetic significance should be ascribed to the term. Multiple beach ridges, recurved spits, overwash deposits, storm berms, strandline ridges, perched beaches, and ancient oyster reefs all quality as chenier ridges under previous guidelines. A more serious problem that appears with the Mississippi River flip/flop model is that relict shoreline dates are not directly correlative with the Mississippi delta-lobe dates, and a growth rate curve of the chenier plain does not support this model. If the modern southwestern Louisiana coastal zone is examined, several distinctly different environments are encountered: shelly sand beaches, exposed tidal mudflats, and eroding marsh deposits. The conceptual model that has been implied in previous studies of a strandline consisting either of chenier ridges or a marsh/tidal flat environment, depending on the Mississippi River discharge direction, appears to be unjustified on the basis of modern day conditions and C14 dating. Refinements in the model based on current field work and further dating should yield a more detailed and integrated picture of the development of the chenier system through time. End_of_Article - Last_Page 1561------------

Determination of Storm Overwash Periodicity from Stratigraphic Relations in Delaware Coastal Barriers: ABSTRACT

Atlantic coastal barriers of Delaware are characterized by relatively thick (> 1 m) sandy washover units interbedded with thin silts and clays. The coarse-grained sediments represent overwash deposition during storm conditions, and are separated by lagoonal and marsh muds deposited during quiescent periods between storms. Washover deposits consist of fine-grained to coarse-grained, moderately sorted, negatively skewed sands. Internal sedimentary structures, consisting of lower units of small- and large-scale trough cross-bedded laminations and upper units of slightly landward-dipping parallel laminae, reflect the transition from lower to upper flow regime as storm intensity peaked. The fine-grained materials represent lagoonal and back-barrier marsh deposition, and con ist of silts and clays containing characteristic faunal and floral components. The sand unit forms a sharp contact with underlying muds, whereas the upper boundary is a gradational sand-to-mud contact as back-barrier marsh deposition reestablished on the washover-fan surface. Thickness and lateral extent of washover deposits vary depending on magnitude of wave parameters and storm tide. Large fans may extend as far as 1 km landward, and exceed 1 m in thickness. Radiometric dating of organic material in marsh and lagoon deposits permits establishment of up to four major depositional events during the past 2,700 years. Back-barrier marsh mud sequences suggest a time of stability and low storm periodicity, whereas the greater part of the barrier washover sequences suggests a higher frequenc of major storm overwash and upbuilding of the back barrier. Thus, storm-overwash periodicity may be the major determinant in establishment of lateral facies interrelations between coastal-barrier and lagoonal sequences in the stratigraphic record. End_of_Article - Last_Page 492------------

Carbonate shelf environments and faunal communities in the Upper Bringewood Beds of the British Silurian

The Upper Bringewood Beds of the type Ludlow Series preserve a lateral suite of carbonate shelf facies. From west to east, the shelf edge is represented by three parallel belts of sediments: thin-bedded biocalcarenites (downslope deposits), biosparrudites (bank deposits), and thin-bedded biocalcarenites (overwash deposits). Further east, the inner shelf is represented by highly bioturbated calcisiltites, calcilutites, and mud-supported biocalcarenites. The shelf edge was a high-energy environment with high species diversity and dense populations of crinoids and the pentamerid brachiopod Kirkidium. Low-energy mud environments of the inner shelf contained a low-diversity community of tetracorals, ostracords, Bryozoa, and the brachiopod Atrypa. The composition and distribution of benthic communities across the shelf was determined by substrate, palaeogeographic relations, and the historical factor of previous communities in the area.