Modern Shoreline Research Articles

Clastic Sequences Developed During Glacio-Eustatic Sea Level Fluctuations on a Sandy Barrier Coastline: ABSTRACT

Seismic reflection (Geopulse) profiles from a 125 km{sup 2} grid on the New Jersey low-mesotidal sandy barrier coastline reveal a 30-m sediment thickness composed of three unconformity bounded parasequences. Vibracores from 12 sites reveal a variety of depositional environments that existed during the most recent glacio-eustatic sea level fluctuations on this stable passive margin. Environments include fluvial, during glacial lowstands; barrier islands with back-barrier lagoons and pro-barrier ebb tidal deltas and shore-attached ridges; and below storm-wave base shelves during interglacial highstands. Following the last major interglacial (Sangamon, {approx} 125KA), sea level fell and rivers eroded a planar seaward dipping surface by the early Wisconsinan time ({approx}70KA). As sea level rose during the mid-Wisconsinan ({approx} 50KA), a barrier island shoreline migrated to within 0.2-1.7 km of the modern shoreline. Maximum highstand of mid Wisconsinan sea level was 20 m below present. Sea level fell again during the late Wisconsinan ({approx}20KA). A barrier island/lagoonal system developed under rising sea level at least by the early middle Holocene based on peats (8,800 BP) in lagoonal muds cored from {minus}12 m. The root of the mid-Wisconsinan barrier system was preserved under early Holocene transgressive sediments. The modern coastal and inner continental shelf deposits aremore » relatively thin (3-4 m) pebbly sands overlying a prominent ravinement surface unconformity formed during the Holocene sea level rise. Isopach maps constructed for this surface indicate up to 7 m of relief eroded into muds beneath the ebb tidal delta and the shore-attached ridges. The ridges and delta shielded the underlying fine sediments during sea level rise. Based on a reasonable Atlantic shoreline transgression rate of 2 m/yr, the modern Barnegat Inlet ebb tidal delta has existed in its general location for at least 800 years.« less

The Description and Classification of Grain Size Data from Ancient and Modern Shoreline Sands at Lepcis Magna using Log Skew Laplace Distributions

AbstractThe description and analysis of particle size distributions using log skew Laplace distributions is a new technique designed to overcome various mathematical and computational problems associated with other approaches. This paper presents an application of the method. In particular, it describes fitting log skew Laplace distributions to modern and ancient shoreline sands from Lepcis Magna (near Horns), Tripolitania, with the purpose of discriminating between modern environments and so classifying the ancient samples. Satisfactory discrimination was not always achieved between some of the modern ‘calibration’ shoreline sand samples of known provenance — possibly as a result of the presence of multimodal distributions. One layer in the harbour-infill sequence, previously of unknown provenance, was shown to have collected at, or close to, an ancient shoreline which developed within the ancient harbour. The majority of the ‘ancient’ samples of unknown depositional environments which were excavated from exposures in the Romano-Libyan harbour-infill sands, were shown by this analysis to be of neither beach nor aeolian origin. This conclusion supports field observations which suggested that the greater part of the harbour-infill sequence represented reworked dune palaeosols, developing dune soils and fluvial and lagoonal facies.

Biometric Studies of Morphological Variation in the Intertidal Gastropod Nucella lapillus (L): Environmental and Palaeoeconomic Significance

The objectives of this paper are to establish by shellform biometry and statistical analysis: (1) the sea state/exposure conditions, and (2) the areas from which shell fish might have been harvested, during the late Mesolithic occupation. This information is of archaeological, palaeoenvironmental, and palaeoeconomic significance but is often very difficult to obtain. The shell-shape-exposure relationship frequently reported from the coasts of north-western Europe was firmly verified for Oronsay. Statistical comparisons between the dogwhelks of modern shorelines and from the middens suggested that the sea state during the period of Mesolithic exploitation was calmer than today. This must have been of considerable impor- tance in an economy and life style so closely tied to the sea. Potential shell fish gathering zones from which dogwhelks might have been harvested were identified around each midden. The precision of the reconstruc- tion obtained varied according to the quality and sample size of archaeological and palaeogeographic data. The analysis shows that notable areas were also not significantly exploited for shellfish. The palaeoenvironmental and palaeoeconomic reconstructions are unusual in that they depend entirely upon palaeontological and geological investigations, and not upon seeking ethnographic parallels or the extrapolation of anthropo- logical, geographic, or economic models.

Holocene shorelines and tectonic movements in eastern Saudi Arabia

Although Quaternary marine terraces have been reported from eastern Saudi Arabia at heights of up to 150 m, unambiguous shoreline deposits could be identified only within 3 m of present-day High Water and close to the modern shoreline. Unaltered aragonitic mollusc shells from five such deposits yielded 14C ages of 3700–6000 yr B.P. Comparison of their elevations with those of coeval deposits on the Zagros coast of Iran demonstrates the relative stability of the Saudi shore of the Gulf during the late Holocene.

Carbonate Cementation of Gulf Coast Barrier-Island Sands and Formation of a Stratigraphic Trap: ABSTRACT

Field investigations in the area of Grand Isle, Louisiana, indicate the presence of active carbonate precipitation. Modern shoreline facies are being cemented by high-magnesium calcite, ranging from 10 to 50 mole % magnesium carbonate. Cementation occurs between the beach and marsh environments as surface crusts or laminae. The environment is characterized by seasonal high temperature (up to 50°C), high salinity (40 to 90^pmil), supratidal stromatolites and organic decomposition. After burial, lithified crusts undergo chemical alteration, as indicated by a decrease in magnesium carbonate content (20 to 35 mole %). The cement eventually undergoes dissolution and reprecipitation, forming a well-cemented sandstone. Rip-up clasts of these sandstones are found along Gulf Coast beaches where a transgressive sequence occurs. These clasts are typically cemented by high-magnesium calcite with 10 to 15 mole % magnesium carbonate. Carbon isotope analyses of rip-up clasts indicate that dissolution and reprecipitation processes are methane-derived due to organic decomposition. Buried crusts may be preserved in the subsurface and form an effective barrier to fluid migration. Where cementation is extensive, porosity is occluded on the marsh-side of the barrier-island sequence. Cement distribution is, therefore, a primary controlling agent of hydrocarbon migration and subsequent entrapment. Migration of hydrocarbons from organic-rich marsh sediments would be prevented by the early formation of the permeability barrier. Cementation, then, would account for accumulation of hydrocarbons on the marsh side of the barrier-island sequence and the formation of a stratigraphic (or diagenetic) trap. End_of_Article - Last_Page 245------------

Modern deposition of manganese along the Dead Sea shore

Manganese-rich encrustations along the modern shoreline of the Dead Sea are of three kinds: (a) Black laminae, ranging in thickness from 0.1 mm to 4 mm, alternating with aragonite layers which form continous hard crusts on pebbles and boulders (b) Thin black laminae in laminated muds found in very shallow waters. (c) Thin black laminae, alternating with aragonite and gypsum, surrounding partially submerged tree trunks. No identifiable X-ray diffraction pattern was obtained in the black layers. SEM studies revealed discrete, shapeless manganese-rich aggregates embedded in an aragonitic matrix. The black laminae consist of approximately 75% aragonite, 15% insoluble residue (mostly quartz), and about 10% manganese-rich fraction. Chemical analyses of the isolated manganese-rich fraction gave up to 50% manganese, and less than 1% iron. Profiles of dissolved manganese in interstitial water show a two-fold enrichment of manganese as compared with the overlying water, with a particularly high concentration (18 mg/l) at the sediment—water interface. This is interpreted to be due to release of manganese from solid phases in the reduced sediments, and upward migration of dissolved manganese in interstitial water, and oxidation to insoluble tetravalent manganese at the interface with the oxygen-containing water. The manganese-rich crusts were formed below the water table and were deposited no later than 350 years and possibly sooner.

Construction of Process-Response Models for Ancient Epicontinental Seaway Depositional Systems Using Partial Analogs

Epicontinental seaways, such as those that existed during the Jurassic through the Cretaceous in North America, do not have modern counterparts. Thus, exact analogs are not available for use as sedimentologic models of ancient epicontinental seaway depositional systems. As a result, well-documented modern shoreline features traditionally have been used as physical analogs to describe most paleodepositional features. During the past decade, marine geologists have accumulated a sufficient amount of data concerning modern continental-shelf sedimentologic processes and resulting features to relate these to similar features recognized in epicontinental deposits. For example, studies along the Atlantic epicontinental margin of North America and the North Sea between Great Britain and Europe reveal the dynamics of sand movement in response to storm-generated and tidal currents. These processes interact with sediment-covered sea bottoms to form transverse and longitudinal bed forms. Transverse bed forms, which include sand waves and ripples, form in response to the opposing current couplets created by tidal circulation systems. Longitudinal bed forms, which include sand ribbons, longitudinal furrows, and large-scale sand ridges, are formed predominantly on storm-dominated sand-covered shelves. The mechanisms which create and maintain these features are poorly understood but sand ridges and related longitudinal features apparently may form in response to cellular flow structure in storm-generated currents. Similar processes probably operated within ancient epicontinental seaways. Using present-day continental-shelf environments as partial analogs, we can construct process-response models to explain the development of some epicontinental paleodepositional systems rather than relying solely upon shoreline physical models. This method of model construction consists of an empirical, or data-gathering phase, followed by an interpretive phase. The objective is to match sedimentary sequences to process elements, and then combine these elements with information concerning the regional geologic setting to make paleoenvironmental interpretations. In this way, interpretations are based upon possible processes within particular geologic settings rather than the physical comparison of stratigraphic sections and well-documented present-day depositional systems. The process- esponse approach to sedimentary models frees us from the constraints imposed by the lack of well-documented sedimentologic studies of offshore depositional systems. This method of model building has been used to recognize and delineate offshore depositional systems in the Jurassic and Cretaceous epicontinental seaways of North America. It is postulated that currents generated by tides, storms, and regional circulatory mechanisms played major roles in creating and distributing continental-shelflike depositional systems within the offshore portions of these and other ancient epicontinental seaways.

Sediment distribution and Quaternary history

The superficial sediments in Start Bay consist of three discrete lithological units, know as barrier, bay and bank deposits. Flint and quartz are the dominant constituents of the barrier deposits with small quantities of rhyolite, felsite and grantie. The bay deposits are mainly fine-grained sediments varying in thickness from about 1 m in the nearshore zone, as a result of winnowing by tidal currents and waves, to a maximum of 28 m in the middle of the Bay. The bank deposits consist of coarse shelly sand, are up to 18 m thick and are located within the general vicinity of the Skerries Bank. Sedimentological and foraminiferal studies, together with C– 14 dates, show that barrier-estuarine-lagoonal environments existed seaward of the modern shoreline in the past and that barriers migrated landward in response to oscillations of sea level during the later stages of the Flandrian transgression. Available evidence indicates that Start Bay is a closed system under present-day conditions. The influence of the Skerries Bank on wave refraction and energy dissipation along the coast has been assessed. The Hallsands disaster can be attributed to the legacy of dredging shingle from the inter-tidal zone, combined with a critical combination of storm waves, gales and spring tides.

Sedimentary Processes Operative Along Western Louisiana Shoreline: ABSTRACT

The western Louisiana shoreline represents the complex interaction of low-energy wave turbulence along a broad, shallow shelf, a small tidal range, and a varying of very fine sand and mud. These are in turn modified by shoreline orientation and complex offshore bathymetry, proximity to brackish estuary tidal efflux, and varying nearshore salinity. Three dominant types of strand-line sedimentation have been chosen as typical of the modern shoreline in the study area. Mud flats, both tidal and subaqueous, form one type. Another dominant type of strand-line deposit is described as a sand-rich, normal beach. Facies relations of the normal beach are similar to the shoreface sequence described for the tidal mud-flat setting. Intermediate between mud flats and normal beaches is a transitional type of strand line consisting of thin swash-zone and breaker-bar deposits resting on eroded, earlier-formed mud-flat and marsh sediments. Mud-flat progradation is initiated by establishment of a shallow-water breaker-bar system, providing a protected site for subaqueous deposition of mud. As sedimentation raises the level of the mud flat to an intertidal level, sand-rich breaker-bars of low amplitude slowly migrate shoreward across the flat. The flat ultimately becomes intertidal. Thin washover-fan sands associated with this setting constitute one form of strand-line ridge found in the Recent chenier plain. Shoreface sedimentation in the normal beach differs from that of the mud flat in having fewer but considerably larger breaker-bars and a rapid shoreward transition into swash-zone, berm, and thick washover-fan sands. The pronounced increase in thickness of washover-fan sands in the normal setting provides a second distinct type of strand-line ridge or chenier. The intermediate nature of the transitional beach setting is emphasized as a temporal, highly dynamic phase in strand-line development. Although apparently retrogradational, it nevertheless represents a dominantly progradational strand line which builds seaward at an intermediate rate compared with quickly prograding mud flats and relatively slowly prograding normal, sand-rich beaches. A classification of progradational shoreline types based on energy of coastal versus sediment supply (e.g. Bernard, 1965) can be used to typify rates of progradation in the study area. Energy of coastal processes include (1) deep-water wave period, (2) orientation of shoreline with respect to prevailing wave front, (3) width of inner shelf, and (4) tidal range. Sediment includes (1) fresh-water efflux and salinity variation, (2) suspended mud supply, and (3) sand supply. End_of_Article - Last_Page 454------------

Cyclic Sedimentary Sequences in Frontier Formation (Upper Cretaceous), Casper Arch Area, Wyoming, and Some Stratigraphic and Possible Paleoenvironmental Implications: ABSTRACT

The Frontier Formation is generally about 1,000 ft. thick in the Casper arch area and consists of a series End_Page 2035------------------------------ of asymmetric, upward-coarsening, depositional sequences referred to here as Four major cycles called sandstone zones by others, have been studied in exposures and mapped in the subsurface in a preliminary manner. Thinner, less conspicuous cycles occur within each major cycle. Where typically developed, a cycle begins with relatively non-sandy gray shale at the base and becomes progressively more sandy upward, terminating where the cycle is thickest with a conspicuous body of fine- to medium-grained, submature to mature, chert-rich orthoquartzite having a relatively sharp upper boundary. Chert-rich pebble conglomerate of uncertain origin occurs in places within the sandier parts of the cycles. Detailed correlations of bentonitic layers reveal that the major cycles pinch and swell laterally in a complex but generally systematic manner, displaying two- to four-fold thickness variations. The relatively thick parts of each major cycle generally lie on flanks of the underlying cycle. Proceeding upward through the sequence, axes of maximum sandstone development are offset progressively eastward away from the central Casper arch. The uppermost cycle also is relatively thick and sandy on the arch. Upward coarsening within the cycles and the complex, but orderly, shifting pattern of the depositional axes of the major cycles suggest that the rate of sediment influx exceeded the rate of subsidence in the area during deposition of the Frontier. The thickness variations are not most plausibly explained by differential compaction or by differential subsidence, at least in the lower three major cycles. Thickness trends suggest that the major source of sand was north and northwest of the Casper arch area. Although the cycles appear to be essentially regressive in nature, sand deposition under transgressive conditions is suggested in places by more glauconitic and (or) calcareous upper parts of the sandstone bodies. West of the Casper arch, coaly beds and channel-like conglomerate lenses directly overlie sandstone bodies in cycles that otherwise are similar. Definite evidence of non-marine deposition has not been observed in the Casper arch study area and general observations suggest that the sequence is essentially marine. As reasoned from stratigraphic observations, the general slope at the top of the lowermost major cycle did not exceed ½° and the maximum depth of water in the area was about 100 ft. Although understanding of the depositional agents must await additional studies specifically aimed at this objective, some analogies are drawn between gross characteristics of the cycles and typical features of deltaic deposits along modern shorelines. More realistic concepts of the internal geometry and genesis of sandstone and shale complexes could be of considerable aid to the petroleum geologist in explaining and predicting oil and gas occurrences. End_of_Article - Last_Page 2036------------

Comparison of Recent Shoreline Sedimentation with Stratigraphy of Upper Cretaceous Oil Fields, Rocky Mountain Area: ABSTRACT

The recognition of the shoreline of a marine basin is one of the most important tasks facing the stratigrapher. Determining the position and trend of the shoreline aids in lithofacies studies and is the basis for the art of paleogeography. An important depositional feature of some modern shorelines is the barrier island on which barrier bar sands are accumulating. The barrier bar deposit is characterized by a narrow linear, well sorted porous and permeable sandstone, beach structures and littoral fauna on one flank, beach ridges, eolian deposits, and other minor features. Sedimentation along the central Georgia coast illustrates the factors which disrupt normal barrier island development and cause abrupt termination of barrier bars. A knowledge of these factors is importa t to the trend-pursuing petroleum geologists. By using lithologic and biologic criteria developed from Recent sediment studies, barrier bar sands can be demonstrated as common shoreline deposits in the Cretaceous of the Rocky Mountains. These sands are important petroleum reservoirs and are prime targets in the search for stratigraphic traps. Surface and subsurface studies indicate that the newly found petroleum accumulations on the Wamsutter arch, southern Wyoming, are in barrier bar sands of Late Cretaceous age. The trend of these shoreline deposits is nearly perpendicular to the axis of the broad east-plunging anticline. The updip permeability seal on the trap is the change from barrier bar sands to impermeable swamp and lagoonal sediments. Because barrier bar sands represent less than 1 per cent of the Cretaceous strata, they are elusive targets in petroleum exploration. End_of_Article - Last_Page 381------------

Hydrographic surveys off Plymouth in 1959 and 1960 : Armstrong F.A.J. and E.I. Butler, 1962. J. Mar. Biol. Assoc., U.K., 42 (2): 445–463

Relict (perched) lacustrine deltas around the perennially ice-covered lakes in the Taylor Valley, Antarctica, imply that these lakes were up to 40 times larger in area than at present since the last glacial maximum (LGM). These deltas have been used to constrain ice-margin positions in Taylor Valley, and the boundaries of the proposed LGM ice-damned Glacial Lake Washburn. The timing of these high lake levels has depended on 14C chronologies of algal layers within relict lacustrine deltas. To provide additional geochronometric data for the post-LGM lake-level history, we applied photon-stimulated-luminescence (PSL) sediment dating to polymineral fine silt and sand-size quartz from 7 perched-delta and 3 active-delta sites of different elevations along 3 major meltwater streams entering Lake Fryxell. Our PSL dating of 4 quartz-sand samples from core tops in the seasonal ice-free moat of Lake Fryxell (elevation ∼18 m a.s.l.) and two core-top moat samples from the seasonal moat of Lake Vanda in nearby Wright Valley establish that adequate PSL clock zeroing (by daylight) occurs in regional, modern shoreline deposits. Minimum-age micro-hole PSL results from the moats are consistently near 100 a. Minimum-age micro-hole age estimates for the deltas range from ∼50 to 100 a near the present lake level up to 13.4 ± 1.3 ka at 240 m. These are systematically younger than the comparable, reservoir-uncorrected, 14C ages that range from 7 ka (cal yr BP) to 13 ka (cal yr BP) near lake level up to 20 ka (cal yr BP) at 220–240 m elevation. Our results indicate the occurrence of a dramatic discrepancy between PSL minimum-age and 14C age estimates that is presently unresolved.

North-South-Trending Submarine Ridge Composed of Coarse Sand Off False Cape, Virginia: ABSTRACT

A narrow north-south-trending submarine ridge composed of coarse brown sand lies seaward of False Cape, Virginia. The ridge joins the present coast at the Virginia-North Carolina boundary (36° 33^prime N.) and extends northward approximately 8 miles to a point southeast of Little Island Coast Guard Station (36° 40^prime N.). The ridge is well defined by the 30-foot depth contour. Depths of 16-20 feet are common along the crest of the ridge; the water deepens to 30 feet or more on both its landward and seaward sides. The bottom sediment consists of fine gray sand in the deeper water landward and seaward of the ridge and also north of it. The surface of the coarse-grained sand on the ridge consists of symmetrical ripples with wave length of 50 cm. and height of 10 cm.; these ripples were active on a day when the height of the swell was 2-3 feet. The fine-grained sand landward of the ridge showed symmetrical ripples of 10 cm. wave length and 2-4 cm. height under the same swell conditions where the depth was less than 25-30 feet. In water of greater depth, no ripples were present and the bottom consisted of mounds and pits and other organic structures. The ridge is only one of a series of north-south-trending linear sand bodies that occur both landward and seaward of the modern shoreline. These ridges are tentatively interpreted as ancient coastal dune-beach complex which formed at various Pleistocene stands of the sea but have been truncated by the present shoreline, whose trend is approximately N. 30° W. from Cape Hatteras, North Carolina, to Cape Henry, Virginia. The origin of this change in trend of shoreline orientation is not known at the present time. End_of_Article - Last_Page 278------------

Sample Components Obtained by the Method of Differences

ABSTRACT The method of first and second differences permits, in certain instances, the detachment of major components from zig-zag curves. Because of the overlap between components, and other factors, the method is commonly subjective in part and therefore not rigorous. Three applications are shown: two dealing with modern shoreline marsh samples and one dealing with an Eocene sand. Recombination of the detached components provides curves which, generally, deviate from the original less than two percent. After components have been obtained, it is still not possible to identify the site of deposition of the sample. Additional applications are known from geomorphology, stratigraphy, river hydrology, and other branches of earth science. Because the results in geomorphic problems can be verified i the field, it is thought that the analysis of sediment samples is also meaningful.