Showing papers in "Journal of Sedimentary Research in 1968"

Morphology and origin of ripple-drift cross-lamination, with examples from the Pleistocene of Massachusetts

Abstract: Many different types of ripple-drift cross-lamination occur in juxtaposition on the gently sloping foresets of a late Wisconsin kame delta near Concord, Massachusetts. A morphological classification is proposed, based on the two end members of a complete sequence of forms. Type A ripple-drift cross-lamination is characterized by non-preservation of stoss-side laminae, and hence consists only of climbing sets of lee side laminae. Sinusoidal ripple lamination (new term) consists of a series of ripples with symmetrical, sine-wave profiles and continuous laminae across the ripple system. One intermediate type (B) is defined between these two end members; the type is characterized by climbing sets of lee side laminae and by preservation of sandy stoss side laminae. The morphological classi ication is extended into a genetic classification, based upon the ratio of sediment fallout from suspension to sediment moved as bed load. Type A represents dominant bed-load movement, and sinusoidal ripple lamination represents dominant fallout from suspension. The cross-lamination described in the paper originated from density underflows of sediment-laden meltwater flowing into a glacial lake. The different types of ripple-drift cross-lamination are attributed here to small fluctuations in the current velocity, and to variations in the composition and concentration of suspended sediment. It is believed that at times of high fallout from suspension, the presence of fines stabilized the lake floor, and sinusoidal ripple lamination was formed. Flume experiments on the formation of sinusoidal ripple lamination tend to confirm this conclusion.

Textural Parameters: AN EVALUATION

Abstract: Analysis of textural parameters (mean diameter, standard deviation, skewness, kurtosis) calculated from a total of 120 samples of modern beach, coastal dune, inland dune, and river sands demonstrates that several combinations of parameters are environmentally sensitive. Parameters calculated from quarter phi sieve data are the most sensitive, but in some cases, half or whole phi parameters differentiate as effectively. The combination of mean diameter vs. skewness is most effective in differentiating between beach and inland dune sands and coastal dune sands; the combination of mean diameter vs. standard deviation is most effective in differentiating between beach and river sands and river and coastal dune sands. Skewness vs. kurtosis is useful only in certain cases and only with quarter phi data. It is, however, never the most effective combination of parameters.

Practical Significance of Birdseye Structures in Carbonate Rocks

Abstract: Two kinds of birdseye structures are recognized in Recent and ancient carbonate rocks: (1) planar isolated vugs 1 to 3 mm high by several millimeters in width and (2) isolated more or less bubblelike vugs 1 to 3 mm in diameter. Both types are generally filled with calcite or anhydrite. Study of more than 100 cores and samples of Recent carbonate sediments showed that birdseye voids are preserved in supratidal sediments (sediments deposited above normal high tide level), sometimes in intertidal sediments (sediments deposited between normal high and normal low tide), and never in subtidal sediments (sediments permanently below water). Both kinds of birdseyes are particularly abundant in supratidal dolomitic sediments. Laboratory experiments and observations in limestone quarry tailings suggest that bubblelike vugs were made by gas bubbles and that planar vugs were made by shrinkage resulting from desiccation of exposed sediments. Birdseye vugs may be valuable indicators of depositional environment, especially when they occur in carbonate rocks containing no depositional structures.

Stylolites; their nature and origin

Abstract: Stylolites are defined in terms of rock grain fabrics; aggregate stylolites and intergranular stylolites are differentiated. A classification of stylolites is given on the basis of the pure geometry of the stylolites themselves and on their relationship to the bedding plane. Megascopic and microscopic observations on stylolite features from the Joppa Limestone Member of the Ste. Genevieve Limestone Formation (Upper Mississippian), the Downeys Bluff Limestone Formation (Upper Mississippian), from Cave-In-Rock fluorspar mines in southern Illinois, and from the Jefferson City Formation (Lower Ordovician) near Rolla, Missouri, are described and their interpretation discussed. Under the microscope, bituminous material, clay minerals, silica, dolomite, sulfides, and fluorite are observed near or within the stylolite seams. Dissolution, recrystallization (neomorphism), grain growth, grain orientation, pressure-twinning, pressure-shadows, micro-faulting and shearing, cavity filling, and geopetal features have been observed along stylolite seams. Difficulties entailed in the mechanism of stylolitization by the pressure-solution theory and contraction-pressure theory are brought out. This study indicates that stylolites form during the various stages of diagenesis. Most frequently, stylolitization in tectonically and metamorphically non-affected carbonates is initiated during the early burial stage, that is, the stage of radiaxial and "dog-tooth" type cementation, and probably ends concordantly with the complete elimination of pore space by drusy mosaic carbonate. It appears that stylolites in tectonites are more the result of pressure-solution and recrystallization phenomena due to the physical factors involved rather than to the chemical factors. It is proposed that in solving the stylolite problem, various diagenetic fabric and chemical changes should be considered.

Recent algal mats of a Persian Gulf lagoon

Abstract: Extensive laminated mats of algae are forming on the protected intertidal and supratidal flats of a highly saline lagoon, the Khor al Bazam, Abu Dhabi, southwest Persian Gulf. At the east end of the lagoon, the largest algal flat parallels the coast for 42 kilometers, and to the west another smaller one parallels the coast for nine kilometers. These flats, part of the seaward edge of a prograding coastal flat, have an average width of approximately two kilometers and a thickness of at least thirty centimeters. In some areas they extend landward in the subsurface for more than two kilometers, beneath a thin cover of evaporites and wind blown and storm washover sediments. Smaller flats occur in the shelter of islands, headlands, and swash bars. The larger algal flats are divided on the basis of surface morphology, into four geographical belts. From the high-water mark seaward these are: (1) Flat zone--firm, smooth algal mat, with no topographic relief, overlying quartz-rich carbonate sand and evaporites; (2) Crinkle zone--leathery algal skin forming a blistered surface over gypsum and carbonate mush; (3) Polygonal zone--algal mat separated into desiccation polygons a few centimeters to two meters in diameter, which cover laminated algal peat; carbonate sand and mud fills the cracks between the polygons. (4) Cinder zone--a warty black algal surface, the color and size of the raised bumps resembling a weathered volcanic cinder layer. These bumps, shaped like small pustules two to three centimeters in diameter, cap an unlaminated algal and sediment peat. The algal growth and structures appear to be determined by the frequency and duration of subaerial exposure and the salinity of the tidal waters; they are only related to wave energy when limited by wave and tidal scour at the edge of the Cinder zone and along ebb channels.

The formation of fine particles in sandy deserts and the nature of "desert' loess

Abstract: Loess consists chiefly of quartz particles with diameters of about 20-50µ. It is commonly thought to have formed in periglacial areas and in deserts. Smalley (1966b) has suggested that direct glacial grinding could produce suitable particles. The existence of dust storms suggests that fine particles also form in hot, sandy deserts; these could be the result of interparticle contacts where a certain critical kinetic energy is involved. There are no loess deposits within sandy desert areas, but they may occur at the desert margins. The major loess provinces to which a desert origin has been ascribed either are not true loess or are explicable in terms of glacial origin. Of the minor deposits, only that of the Negev seems to have a well established desert origin. The quartz particles in the Negev deposit probably formed by simple impact. This mechanism does not seem adequate for the Chinese loess, and it is suggested that the widespread Pleistocene glaciation of China was responsible for the bulk of the fine particles. Thus although loess particles can form mechanically in deserts, they have not done so on a scale sufficient to produce major loess deposits. Desert loess deposits should be distinguishable from glacial oess deposits by their size and their mineralogical nature. Stresses produced by temperature changes can break rocks provided that the rock particles are not below a certain critical size. Most quartz is introduced into the sedimentary system as sand-size particles (about 500µ); if the critical thermal break size is above 500µ, then the thermal breakage process will not produce loess-size particles.

Multiple Parallel-Truncation Bedding Planes--A Feature of Wind-Deposited Sandstone Formations

Abstract: All eolian origin for most of the cross-bedding seen in the blanket sandstone formations of the Colorado Plateau is generally accepted. There are, however, in the cross-bedded formations many extensive, smooth, parallel surfaces which suggest water action. These planes sharply truncate cross-bedding and obviously cannot be of depositional origin; much material has been removed in forming them. For surfaces of this type, the name multiple parallel-truncation bedding planes is proposed. Examples are seen in the Entrada, Navajo, Wingate, De Chelly, and Coconino Sandstones. From the fact that wind has little effect upon water-saturated material it is supposed that conditions were frequently such that previously accumulated sand was removed down to the water table, thus creating the very smooth, horizontal surfaces which now separate intervals of cross-bedded sandstone. This process is to be expected in interior basins with abundant sand, strong wind action, and a water table which rises with sedimentation and occasionally reaches the surface. Windswept, saturated surfaces such as the Great Salt Lake Desert show similar leveling effects, but here the water table is falling so a stratigraphic record is not preserved.

Geology and Geochemistry of Reefs, Carbonate Sediments, and Waters, Gulf of Aqaba (Elat), Red Sea

Abstract: The Gulf of Aqaba, a slightly hypersaline body of partially enclosed marine water, forms the northern segment of the Red Sea. The Gulf occupies a narrow tectonic valley, about 10 to 26 km wide, bounded by fault planes which form submarine slopes that are virtual precipices and by shores which are equally steep. Near the reef communities, on the shelf of the northern gulf, salinities, which vary seasonally, range from 41.5 to 43.0 per thousand . The climate is hot and dry. Evaporation exceeds rainfall by 157 mm per year; the deficit is made up by inflow of waters from the Red Sea (normal salinity 38 to 40 per thousand ). North and northeast winds blow steadily along the Gulf ("chimney effect"), and waves and currents on the northwestern coast studied are predominantly from these directions. Around the margin of the Gulf of Aqaba is a narrow shelf(1/4 to 1 or 2 km wide) on which the water is 1 per thousand is termed high-magnesium aragonite. Molluscan shells in the area studied are composed of low-magnesium aragonite and skeletons of corals of high-magnesium aragonite. The magnesium content of the carbonate sands reflects the relative abundance of corals, coralline algae, and molluscs. Approximately linear relationships can be shown between barium and magnesium, barium and manganese, magnesium and manganese, iron and manganese, and iron and barium for coral, coralline algal, and molluscan skeletons, and carbonate sands derived from them. By comparison with molluscs, the skeletons of corals tend to be slightly enriched in barium, manganese, and iron, and those of coralline algae are enriched even more. The carbon and oxygen isotopic composition of calcareous skeletons and carbonate sands overlaps with that of comparable Quaternary material from other areas; slight differences from those described from other areas are, however, noted in 16 O enrichment of corals and 18 O enrichment of a barnacle, reflecting the effect of taxonomic position on isotopic fractionation.

Sandstone Petrology, Evidence from Luminescence Petrography

Abstract: Using luminescence petrography equipment previously described in the literature, and modifications of this equipment reported here, one can usually distinguish secondary from primary quartz in sandstones. It is thus possible to distinguish solution and overgrowth phenomena even when usual petrographic criteria fail. By luminescence, examples of concavo-convex boundaries, long boundaries, and sutured contacts, all criteria of pressure solution, are revealed to result from optically continuous quartz overgrowths. The luminescence not only prevents ambiguity regarding mechanisms active in the sandstones, but reveals the detrital fabric so that size, shape, and degree of rounding of the detrital grains may be clearly seen. Features relating to prior cycles of sedimentation are also freque tly seen. Fractures and crushing events, long since healed by growth of secondary quartz, are also distinguishable by the luminescence technique; in some immature sandstones such events seem to be very common. Often, the crystal regions revealed by crossed nicols are related to, but are by no means identical with, the fractured fragments, giving evidence of recrystallization with mobility of grain boundaries. Crenulated boundaries also form in the fracturing-healing-recrystallization process resulting in polycrystalline grains showing metamorphic quartzite texture. It is suggested that many of the polycrystalline quartz grains found in sandstones originate in this way either in place or in precursor source rocks. All these features are well documented with photomicrographs.

Carbonate diagenesis; equilibration of sedimentary mineralogy to the subaerial environment; coral cap of Barbados, West Indies

Abstract: It has long been recognized that ancient carbonate rocks are composed of the stable minerals calcite and dolomite, whereas their Recent sediment analogs are predominantly composed of the unstable minerals aragonite and high-magnesium calcite. Further, it has been noted that many subaerially exposed Pleistocene carbonates exhibit mineralogical stability and lithification, thus suggesting that the stabilization process proceeds rather rapidly in fresh water diagenetic environments. The subaerially exposed Pleistocene coral reefs of Barbados provide an excellent opportunity to investigate the stabilization process in greater detail. Owing to its unique history of Pleistocene tectonic uplift, Barbados records a more detailed history of Pleistocene high sea level stands than any other Caribbean shoreline yet documented by radiometric dates. The youngest terrace on the island (82,000 years B.P.) is composed of generally unstable mineralogy throughout its extent. Terraces older than 500,000 years are of stable mineralogy. Terraces of intermediate age exhibit varying proportions of stable and unstable mineralogy. Further, different portions of Barbados have decidedly different climates. Thus reef tracts of equivalent age can be studied in a high rainfaIl-low evaporation terrain and in a low rainfall-high evaporation terrain. Mineralogical stabilization proceeds more slowly in the drier climate. A model is proposed for the stabilization process at or near the outcropping surface of an aragonite-calcite sediment. Kinetics of calcite nucleation, kinetics of calcite crystal growth, and availability and transport of water can each be expected to be the "rate step" in various geologically realistic situations. Finally the model is extended to include mineralogical stabilization in meteoric water in the subsurface.

The development of clast fabric in mudflows

Abstract: Computer simulation of the motion of particles in a viscous fluid in laminar flow shows that strong long-axis fabrics may develop in mudflows in a short period of time. The rate at which the fabric develops in any one situation depends on the velocity gradient. The viscosity of the mudflow one of the main factors controlling the velocity gradient, is increased considerably by the clasts contained in the matrix Fabric development is cyclical and begins as a vague girdle dipping upstream. At the same time a single mode develops, centered on the girdle, and gradually increases in intensity until a maximum is reached when the plunge of the mode is horizontal. The fabric from this point degenerates in reverse order, first to a weak girdle dipping downstream, and then to a fabric similar to the initial fabric when the particles have completed a half cycle of motion. The process then begins again. The strength of the fabric depends on the instant at which the mudflow is arrested. Even though most mudflows travel in a turbulent manner, the fabric is developed in the final stages as the velocity of the flow decreases, and it passes to a laminar phase before coming to rest. Limited experiments and simulations show that a C-axis fabric should also develop with a single vertical mode and girdle transverse to the flow direction. Settling of the clasts under gravity after the flow has come to rest probably leads to a strengthening of the C-axis fabric, as clasts would tend to reorient themselves with their plane of maximum projection (the A, B plane) normal to the force of gravity. The Permian Pagoda Formation in Antarctica consists largely of tillite but contains a small number of beds deposited from mudflows. Four A-axis fabrics were measured from the mudflow units and were found to be similar to long-axis fabrics produced by the simulations. All four have dipping girdles, and two display a central mode. When compared with till fabrics from the same section the mudflow fabrics can be distinguished because of the girdles. Had the mudflows been arrested at a stage of maximum fabric development, it would be impossible to distinguish them from most till fabrics. In this situation the problem may be clarified by measuring several fabrics at different levels in the flow. Because the velocity gradient decreases upward the strength of the mudflow fabrics should be dif erent at different levels. An attempt to exploit this method in the field met with very limited success, probably because the change in velocity gradient upward through the mudflow was too small. C-axis fabrics developed by mudflows have vertical modes while their counterparts in till fabrics plunge steeply in a downstream direction. It is concluded that clast fabric could be a valuable tool in determining the origin of diamictites, but more field data are needed.

Relationship between chlorite coatings on quartz grains and porosity, Spiro Sand, Oklahoma

Abstract: Authigenic chlorite coatings on detrital quartz grains influence the formation of overgrowths and preservation of porosity in the Spiro Sand, Arkoma Basin, Oklahoma. Thick continuous coatings preserve porosity because pressure solution is retarded and quartz overgrowths are nonexistent. If the coatings are sparse, thin, and discontinuous, the sandstone is nonporous because of pressure solution and quartz overgrowths. Intermediate stages between these two extremes also are present.

Sabellariid worms; builders of a major reef type

Abstract: Sabellariid worms build extensive wave-deforming reefs along tropical and subtropical coasts; they may be, in fact, among the important reef-producing agents in the world. A stretch of worm reefs built by Phragmatapoma lapidosa has been studied in detail along 320 km of the Lower East Florida coast, (U.S.A.), where the reef system is essentially continuous. Related species are reported from other places in Florida. The ability of the worms to thrive under high-energy breaker conditions and to extend their colonial tube masses upward and seaward by extraction and agglutination of littoral drift materials, makes them highly important factors in the development of the coastline. Beachrock, converted from the reefs, and materials impounded on their landward sides, provide for actua progradation of the beach. The activities of these organisms are, therefore, important to the zoologist, oceanographer, coastal engineer, and geologist. The latter may find that the same or closely related forms have also been instrumental in building and protecting beaches of the geological past and in exerting control over the evolution of shorelines.

Composition and mean age of detritus of the Colorado River delta in the Salton Trough, southeastern California

Abstract: The Salton Trough--the northwest landward extension of the Gulf of California structural depression--is filled primarily with fine-grained sandstones and siltstones that constitute the Colorado River delta. These rocks have an aggregate thickness of approximately 20,000 feet and are all of late Cenozoic age. Detritus that makes up the delta was eroded primarily from Mesozoic sedimentary rocks of the upper Colorado River drainage basin. This detritus consists of dominant quartz and calcite, subordinate dolomite, plagioclase, K-feldspar (both monoclinic and triclinic), montmorillonite, illite and kaolinite, and minor magnetite, zircon, leucoxene, clinozoisite, biotite, and chlorite. Wide ranges in both major and minor elements can be correlated with grain size and clay content of the sa ples. Lead isotope data from the acid-insoluble fraction of Colorado River delta samples from the Salton Trough indicate that the mean age of the detritus is 1700 m.y., and strontium isotope data indicate that the mean age is greater than 500 m.y. In view of the small percentage of exposed Precambrian rock in the upper Colorado River drainage basin, the bulk of the acid-insoluble detritus probably consists of reworked detritus of Precambrian age.

Sedimentation in an Arctic lake

Abstract: Stanwell-Fletcher Lake is 400 miles north of the Arctic Circle, covers 131 square miles, and is over 100 m deep. Its size and the severe climate restrict summer melting of ice to the margins. The ice insulates the water, which remains essentially isothermal, warming slightly from 1.3 to 1.6°C. during the summer. Inflowing water is nearer 4°C, and therefore sinks and mixes with the lake water, a process which maintains thermal and chemical homogeneity in the lake and oxygenates the surficial bottom sediment. Sedimentation is very slow because of the short period of stream flow and the low organic activity in the lake. Silty sand derived from river bed loads accumulates on shallow marginal deltas and shelves. The sediment is poorly sorted because ice cover prevents wave action and removal of the fine material to deeper water. Mud, derived largely from the suspended loads of rivers, settles slowly in the central part of the lake. Faint laminae in the shelf sands may be varves, but the deep water mud is structureless. Evidently seasonal sedimentary variations do not affect the center of the lake, probably because ice cover minimizes transporting currents. The oxidation-reduction boundary in the sediment is at 5-20 cm, deeper than in temperate lakes, because of oxidation at the lake bottom and slower reduction within the sediment.

Fluviatile obstacle marks from the wadis of the Negev (southern Israel)

Abstract: Current crescents and several types of obstacle shadows, the result of both scouring and deposition, appear along the pebble-lined beds of the ephemeral streams (wadis) of the Negev. These structures are formed by secondary currents which originate when the main flow is deformed by obstacles within the streampath.

Positions of empty pelecypod valves on the continental shelf

Abstract: The commonest and most stable position of the empty valves of large pelecypods on the wave-washed zone of beaches is concave down; this position is due to the presence of fast currents and the absence or inadequacy of bioturbation. In contrast, most large pelecypod valves on the surface of the continental shelf lie in a concave up position in response to the action of carnivores and scavengers and to powerful bioturbation and the absence of fast currents. These observations of preferred position can be applied to paleoecological studies of ancient strata.

A Tidal Flat Evaporitic Facies in the Visean of Ireland

Abstract: A sequence of thinly bedded limestones, dolomites, shales, and sandstones contains evaporitic beds in Counties Leitrim and Cavan, Republic of Ireland. This sequence constitutes the Aghagrania Formation (new name) of B2-P1c age (Upper Visean), with a type section east of Drumshanbo, County Leitrim. The evaporitic beds, which have not previously been recorded from this horizon or locality, are mostly unfossiliferous laminated limestones and dolomites with macrocells and with pseudomorphs after gypsum, anhydrite, and halite. This facies of the Aghagrania Formation also includes evaporitic breccias, and celestite and carbonate replacements of calcium sulphate. Blocks of gypsum in boulder clay, on the shore of Lough Allen, are probably derived from these beds. The evaporitic strata alternate with shales and limestones containing marine faunas, and with unfossiliferous sandstones. All the facies show evidence of shallow water deposition and were probably formed in an area of low relief subjected to transgressions and regressions of a shallow sea. The evaporitic beds may be compared t the dolomite and gypsum deposits of present day tidal flats and associated shallow lagoons. They also resemble certain other occurrences of ancient laminated dolomite and limestone beds which have been recently described and attributed to a tidal flat origin.

An Occurrence of Submarine Cementation of Carbonate Sediments Off the West Coast of Barbados, W.I.

Abstract: Subsea cementation of carbonate sediments was observed at depths of about 50 feet on a submerged barrier reef off the west coast of Barbados, W. I. The calcite cement is attributed to the burrowing of the pelecypod Gastrochaena hinds through carbonate sands trapped in a sponge mat. Because of the difficulty in recognizing the cemented sediment on the sea floor, its areal distribution could not be estimated.

Dynamic diversion; influence of longshore current-tidal flow interaction on chenier and barrier island plains

Abstract: Chenier and barrier island plains are characteristic products of post-Pleistocene sedimentation along the northern and western coasts of the Gulf of Mexico, but similar features may be recognized throughout the world. These plains have been formed by net progradational displacement of the sea throughout the latest 3000 to 5000 year period of sea level stability. They consist of low, narrow, well-sorted shell and sand ridges that are elongate parallel with the former shoreline, and which are separated by tidal mud flats or zones of poorly sorted, fine sand. A correlation has been established in the vicinity of the Mississippi River delta, by means of radiocarbon dating, between low rate of fine sediment discharge into westward moving longshore currents, stability or minor recession of the downdrift shoreline, and slow accretion of well-sorted chenier beach ridge sand. High rate of sediment discharge into nearshore currents, however, coincides with rapid progradation of sandy, silty mudflat deposits along the downdrift shoreline. It is suggested that the presence of a high content of fine sediment in longshore currents changes their fluid dynamic characteristics in such a way as to enhance traction transport, inhibit separation of differing grain size modes, and thus promote deposition of unsorted detritus. Light sediment loads halt rapid progradation and induce reworking of previously accumulated sediment. Thus no change in general wave regime is required to account for alternation of chenier sand accumulation with interchenier mudflat deposition. Barrier island plains closely resemble chenier plains, except the barrier island beach ridges are separated by clayey, silty, poorly-sorted fine sand, but cheniers are separated by unsorted, sandy, shoreface mud deposits. Comparison of sublittoral beach profiles show that, rising from equivalent depths of water, the barrier island profile has a seaward gradient that is approximately twice that of the non-barred shore. This steeper profile is related to the coarser average grain size of barrier island shoreface sediment compared to chenier plain shoreface detritus. The greater coarseness is in part the result of tidal jet winnowing of clay from longshore-moving sediment as it sweeps past estuary mouths. As a consequence of the steep barrier Island profile, a higher rate of wave energy issipation takes place in shallow offshore water. This higher rate of energy transfer contributes to the further winnowing of barrier island sand that is initiated by longshore current-tidal flow interaction. The textual differences between barrier island interridge sand and sandy, interridge chenier plain mud appear to mask similarities of origin. A close geographic association invariably exists between chenier deposits and river or estuary mouths. Although major rivers are the ultimate source of sediment composing chenier plains, cheniers are not uncommonly located adjacent to estuaries receiving little or no sediment from the rivers passing through them to the sea. This suggests that sand build-up is related primarily to dynamic interaction between sediment-bearing longshore currents and tidal flow from estuary months, and is not necessarily dependent upon the rivers as sediment sources. The interaction, here termed the "dynamic diversion" effect, is particularly important to chenier plain growth because of its influence on deposition of in-transit sediment that approaches an estuary during ebb tide. Both surf suppression by he issuing jet and decrease in velocity of nearshore transport associated with current deflection cause deposition of sediment on the updrift shoreface of mainland or developing spit. Some sediment stranding may also take place during flood tide. A second important site of ebb flow deposition is offshore, and downdrift, from the estuary mouth. Here longshore flow reasserts its dominance over the tidal jet, and energy dissipating gyral flow occurs as the seaward-displaced longshore drift moves back toward the shoreline. Deposition at this site results in barrier island plain development. Exploration for subsurface chenier and barrier plains sands might well be pursued in the vicinity of provable intersection between paleoshorelines and ancient river systems.

Mineralogy of the Recent sediments of Tigris and Euphrates rivers and some of the older detrital deposits

Abstract: Mineralogical study of the light and heavy fractions of the recent sediments of Tigris and Euphrates Rivers and sonic of the older deposits revealed the presence of seven minerals in the light fraction and 32 mineral species represented by 41 varieties in the heavy fraction. The recent sediments of the Tigris and Euphrates Rivers as well as the Pleistocene terraces of the Euphrates and the Upper Fats deposits (Late Miocene) contained the same mineral species in about the same relative frequencies, characterized by an abundance of iron ores, epidotes, amphiboles and pyroxenes that make more than 80 percent of the heavy fraction. Shatt el Arab sediments contained a similar assemblage derived mainly from the disintigration of metamorphic and basic igneous rocks. The Aqsu and Adhaim River showed an assemblage characterized by an increase in the relative frequencies of epidotes.

Recent sediments and diagenesis of south Bonaire, Netherlands Antilles

Abstract: The Recent sediments of south Bonaire are filling a depression in the Pleistocene surface. The general sedimentation and diagenetic history are as follows: (a) deposition of a Halimeda grainstone; (b) deposition of a lithoclastic sediment with clay beds and cementation of this sediment to form a lacy carbonate crust; (c) deposition of an intertidal pelleted lime mud; and (d) bedded gypsum deposition and incipient dolomization of the pelleted lime mud. Based on the similarity between the texture of Recent gypsum and anhydrite and subsurface anhydrite, it is hypothesized that subsurface anhydrite represents anhydrite formed near the surface or anhydrite replacing gypsum formed near the surface.

Selective dolomitization of Recent sedimentary structures

Abstract: Relatively non-dolomitic limestone nodules which float in lithified dolomitic sediment are forming on a supratidal mud flat in the lower Florida Keys. Storm tides periodically deposit layers of lime mud above normal high tide level which dry and crack to form typical mud-crack polygons. These polygons erode into flattened nodules that subsequently become buried in relatively more porous and permeable sediment. Magnesium-enriched brines concentrated through evaporation are more readily transmitted through the more permeable sediment than are the less permeable nodules. Selective dolomitization of the more permeable sediment produces features bearing a strong similarity to sedimentary structures commonly found in ancient dolomitic rocks.

A Florida Sabellariidae reef and its effect on sediment distribution

Abstract: Heavy minerals and size distribution of bottom sediments and reef materials have been examined in a Sabellariidae reef at Seminole Shores, Florida, approximately 100 miles northeast of Miami. Heavy mineral and shell fragment percentages in the reef material are up to ten times greater than in the adjacent bottom sediment. This distribution results from the tendency of the worm Phragmatopoma lapidosa to utilize the finer mineral grain sizes and platy shell fragments in tube construction. Accordingly, these worms improve the sorting of the beach sediments to a significant degree.

The generation of scour marks near obstacles

Abstract: Local depressions in a sediment bed, caused by local nonuniformity of the flow, are often found in the vicinity of obstacles lying on or embedded in the sediment. It is concluded that current crescent scour marks can be explained as effects of the stretching and accumulation of the vorticity in the flow approaching an obstacle, the fluid-mechanical aspects of vorticity transport being described in some detail. The observed differences between scour-remnant ridges and scour-deposit ridges may be explained in terms of differences in the flow configuration geometry. Sediment transport is a functional with respect to some of the hydraulic variables--that is to say, the transport depends not only on the range of values of fluid velocity, for example, but also upon the sequence in which the values in the range are attained. This aspect of transport phenomena is emphasized and it is suggested that experiments should be performed to determine the sensitivity of the functionals to the flow functions.

Clay mineral mounting techniques for X-ray diffraction analysis; a discussion

Abstract: Suction of slurry onto ceramic tile, smearing on glass slides, and dry powder pressing are considered to be the only acceptable methods of avoiding mineralogic segregation in preparing oriented clay specimens for X-ray analysis.

Sedimentary Cycles in the Green River Formation (Eocene), Uinta Basin, Utah

Abstract: Several cyclic patterns occur in deposits of fluvial, shallow lacustrine, and deep lacustrine environments in the Parachute Creek Member of the Green River Formation (Eocene), northeastern Uinta Basin, Utah. Individual cycles may be traced laterally, and ideal patterns correlated. Changes in the level of Lake Uinta, possibly resulting from climatic fluctuations, are postulated as the direct cause of the cycles. The presence of such correlative cycles in different facies imposes restrictions on possible modes of origin, and aids in the reconstruction and interpretation of Lake Uinta.

The role of high Mg calcite in the preservation of micrite envelopes and textural features of aragonite sediments

Abstract: Algal borings are responsible for the development of micrite envelopes, and high Mg calcite may be present in these borings and other void spaces within aragonite grains. In these cases, the presence of the two minerals furnishes the solubility contrast necessary for the preservation of textural features in the face of massive solution. The earliest and most rapid change in the diagenesis of such a carbonate sediment is the stabilization of high Mg calcite as low Mg calcite. Simultaneously or later, aragonite may undergo solution, leaving behind a fine network of the less soluble low Mg calcite which is pseudomorphic after high Mg calcite. This network is essentially a skeleton of the original texture of the sediment and serves as a mold and nucleation site for later infilling by calcite.

Settling Convection and Grain-Size Analysis

Abstract: Grain-size analysis by settling techniques presupposes free sinking of the particles. There are two phenomena that might have an adverse effect on this basic assumption. The first has been termed "hindered settling" and denotes the obstruction that settling particles experience from the upward flow around neighboring grains. This influence will not be considered here. Another phenomenon, that I have called "settling convection," signifies the current system set up by differences in density of clouds in a settling suspension. The two processes are normally combined. The action of the latter strongly varies according to conditions. Bradley's "vertical density currents" form a border case. In suspensions prepared for pipette analysis it develops lively small-scale currents. Tests are pre ented to show that the influence on the deduced size distribution is fortunately negligible and roughly equivalent to heating by 1 degree centigrade. In salt water-clay suspensions and various other mixtures of solids with liquids, the action of settling convection helps to preserve a uniform suspension below a steadily sinking interface. At the bottom there exists a rising boundary between the suspension and the deposit. This constant composition excludes the possibility of grain-size analysis in sea water by means of the settling velocity of the separate particles. A rough alternative visual method for size analysis of flocculated suspension is described. Sedimentation balances for size analyses of silt and clay employing a shaken suspension suffer the same slight acceleration as pipette analyses. Moreover, the convection around the submerged dish forms a serious drawback to this technique. Settling convection tends to augment the sinking velocity of sand grains released at the surface, especially in samples with a narrow range in size and also for particles of less than 0.1-0.05 mm. Hence, the usefulness of various types of sedimentation analyzers based on sample introduction from above is restricted to sizes above 50 microns. Settling convection also renders ineffective counter-current methods (elutriators) in water. Elutriation in air is inapplicable for lutum samples that have been wet, hence for most natural sediments except eolian deposits

Wheeler Gorge Turbidite-Conglomerate Series, California; Inverse Grading

Abstract: A coarse-grained sequence of turbidite-conglomerates at Wheeler Gorge, California, was deposited by a series of high density, highly fluid underwater flows into deep water. Inverse grading is developed at the base of each conglomerate trait within the sequence, and one unit forcibly intruded shales to form graded sills. One sill is over 100 feet long. The development of reverse grading may in part be explained by Bernoulli's principle, whereby pressures near the boundary of deposition tend to drive the large particles upward. Bernoulli's principle relates the velocity in a fluid system to the pressure in the fluid. The resultant of the pressures is a net upward or inward pressure away from the boundary which operates most effectively on the largest particles. The "Bernoulli boundary layer effect" may explain reverse grading in coarse-grained turbidite deposits, in mudflows, possibly in ignimbrite layers, and in igneous intrusions of the "Muskox type" which have boundaries that are finer-grained than their interior parts.