Showing papers in "Journal of Sedimentary Research in 1976"

Variations of Density and Porosity with Depth in Deep-sea Sediments

Abstract: Reduction of sediment porosity and increase in density under overburden pressure in the sea floor are important subjects in earth sciences. Data and samples from the Deep Sea Drilling Project allow a new look at these subjects, and are used to establish profiles of laboratory values of density and porosity versus depth in the sea floor. To construct in situ profiles, the results of consolidation tests are used to estimate the amount of elastic rebound (increase in volume) which has occurred after removal of the samples from overburden pressure in the boreholes. In situ profiles of porosity and density versus depth are constructed for some important sediment types: calcareous ooze, siliceous oozes (diatomaceous and radiolarian oozes), pelagic clay, and terrigenous sediments. There is l ss reduction of porosity with depth in the first 100 m in these deep-water sediments than previously supposed: 8 to 9% in pelagic clay, calcareous and terrigenous sediments, and only 4 to 5% in the siliceous sediments. From depths of 300 m the most rebound is in pelagic clay (about 7%), and the least in diatomaceous ooze (about 2%); calcareous ooze and terrigenous sediment should rebound from 300 m about 4 to 5%. Terrigenous sediment, from the surface to 1,000 m depth, probably rebounds a maximum of about 9%. Methods are described and illustrated to predict density and porosity gradients in the sea floor, and to compute the amounts of original sediments necessary to have been compressed to present thicknesses. Slightly over 2,000 m of original sediments would have been required for compr ssion to a present-day thickness of 1,000 m of terrigenous sediments.

Grain Flow and Grain Flow Deposits

Abstract: The term grain flow is restricted to sediment gravity flows in which a dispersion of cohesionless grains is maintained against gravity by grain dispersive pressure and in which the fluid interstitial to the grains is the same as the ambient fluid above the flow. Modified flows include those in which a dense interstitial fluid, current, or escaping pure fluid aids in maintaining the dispersion. Conclusions regarding the dynamics of grain flows have been based largely on the analysis of Bagnold (1954) of fully confined, gravity-free dispersions. Natural flows are neither fully confined nor gravity-free, and their characteristics and dynamics differ significantly from his experimental systems. Velocity equations are developed and used to analyze natural grain flows. Unmodified subaerial and subaqueous, steady and uniform flows of sand occur only on slopes at or near the angle of repose, are generally less than 5 cm thick, and cannot individually account for the formation of thick sedimentation units. On slopes inclined at less than the angle of repose, grain flows collapse and freeze; on higher slopes, they accelerate, dilate, and become increasingly influenced by fluid forces. Thick grain flows of gravel-sized debris and thick flows modified by the presence of dense, plastic mud interstitial to the clasts (debris flows), polymodal coarse-sediment size distribution (density-modified grain flows), or concurrent sediment liquefaction or fluidization (liquefied or fluidized sediment flows) can move over relatively low slopes and accumulate as thick sedimentation units.

Depositional Model of Point Bars in the Lower Wabash River

Abstract: A process-response model of point-bar sediments in the lower Wabash River of Illinois and Indiana was constructed by integration of (1) results (reported elsewhere) of repetitive hydraulic surveys of meander bends and (2) distributions of sedimentary structures and sediment size in Holocene alluvium. Vertical sequences of alluvium exposed in cutbanks and in point-bar trenches demonstrated preservation of bedform-texture patterns recognized from the hydraulic program. The depositional model is defined by distributions of three depositional facies around meander bends of disparate relative curvature. A fully developed facies assumed fully developed zones for current velocity and for bed-material size (velocity magnitude and mean size increasing from the inner bank toward the outer bank) A transitional facies incorporated transitional zones for both parameters (neither increasing outward from the inner bank). An intermediate depositional facies contained a fully developed velocity zone but a transitional size zone and was of significant extent only in gently curved bends. The latter two depositional facies differ greatly from standard facies models of meandering streams, constitute the bulk of surficial point-bar sediment in very sharply curved bends and in gently curved bends, and can be preserved preferentially by common types of bend migration. Preservation of the depositional facies of each meander bend depends upon the type of deformation (migration) the bend underwent. Limited observations from other freely meandering streams suggest that the Wabash depositional model may be widely applicable.

Facies and Fabric Specificity of Early Subsea Cements in Shallow Belize (British Honduras) Reefs

Abstract: Early subsea cement and cemented sediment are restricted to marginal facies of shallow Belize barrier and atoll reefs. In these facies cementation is not ubiquitous, but is sporadic within seaward-facing spurs and within the upper meter of the reef pavement. The presence of internal sediment, composed of well-sorted lime silt, is critical to the precipitation of rock-making cement. This lime silt is made up of Mg-calcite peloids of unknown origin and angular chips of aragonite excavated from coral and mollusk skeletons by boring sponges. In Belize the reef flat pavement, a subtidal bed of lithified coral conglomerate 1 m thick and 10 to 100 m wide, occurs in the lee of all marginal reefs. Acropora palmata fronds and logs (radiocarbon age ca. 450 years) are cemented together by a mortar of lime packstone to wackestone. In seaward-facing spurs, cavities between in situ coral and Millepora are locally filled with cemented skeletal packstone. Cemented packstone and wackestone also partially fill intraskeletal pores and borings in coral fronds lying loose on the sea floor. The predominant cement of internal sediments is Mg-calcite (14.5-18.6 mole % Mg) as micrite and bladed spar. Aragonite, although common, is found only in coral pores. The crystal form of the cement and the degree of lithification can be related to variations in the texture of the internal sediments. Mg-calcite micrite, the first cement, develops preferentially in the small pores, less than 15 µ, between lime silt grains. Mixed lime sand and lime silts, with a wider range of pore sizes, are cemented after the lime silts by thin rinds of micrite that grade outward into bladed spar. In the larger pores of grainstones, bladed spar follows thorough cementation of lime silts in the same sample.

Petrology of Holocene fluvial sand derived from plutonic source rocks; implications to paleoclimatic interpretation

Abstract: Despite the long and continued interest of sedimentary petrologists in the provenance of ancient sediments, little is known about the detrital modes of Holocene sands of known parentage. As a result it has been impossible to use the "present as a key to the past" for purposes of provenance interpretation. In order to fill this information gap and to aid in further understanding of the provenance problem, first-cycle fluvial sands derived exclusively from granitic plutons in the relatively humid Appalachian Mountains and in the relatively arid Rocky Mountains in the United States have been studied. Characterization of the climatic influence on these sands is based on the results of detailed petrographic modal-analyses of three or five size fractions of 87 sand samples. Distinctive size-dependent compositional trends exist in the modal percents of the four principal constituents of the sands; these are potentially useful in paleoclimatic interpretation. In all size fractions rock fragments are more abundant in the arid west than in the humid east. Also, though rock fragments decrease in abundance with decreasing grain size in both the west and east, the rate of decrease is much higher in the west than in the east. Feldspars increase in abundance with decreasing grain size in both regions, but the quartz/ feldspar ratio is >1 in the east and <1 in the west. This relation holds for all size classes. Accessory, minerals are much less abundant than the feldspars, but show similar trends. Quartz generally decreases in abundance with decreasing grain size in the east. In the west quartz is more abundant in medium sand than in other size fractions. A schematic model is proposed to depict evolution of sand in arid and humid climates from polymineralic coarse-grained source rocks. The model predicts the relative abundances of the four principal detrital ingredients of first-cycle sands in different size fractions as a function of their relative resistance to weathering. This study also establishes the original mineralogic maturity of first-cycle sands. "Tie-lines" are drawn between coexisting sand compositions with similar maturity using a modification of the traditional QFR triangular diagram. In addition, trends toward mineralogic maturity within the framework of such a QFR diagram are defined.

Common Provenance for Lithic Grains in Carboniferous Sandstones from Ouachita Mountains and Black Warrior Basin

Abstract: To test the influence that carboniferous graywacke sandstones of the Ouachita Mountains and Black Warrior basin represent deposition in a linked dispersal system analogous in tectonic setting to the modern Ganges delta-Bengal fan system, sandstones from the two areas were compared petrographically. Two point counts, a standard QFL count and a special count of lithic grains, made for each of 24 selected samples, half from the Ouachitas and half from the Black Warrior basin, reveal that sandstones from both areas are rich in quartz and metasedimentary lithic grains, but poor in feldspar and volcanic lithic grains. QFL plots show that Ouachita rocks have quartz percentages consistently higher than coeval Black Warrior basin sandstones because of transport-related attrition of lithic grai s and/or contributions of craton-derived, quartz-rich detritus to the Ouachita sandstones. However, Ouachita and Black Warrior basin samples cluster tightly together on triangular plots of polycrystalline quartz grain (Qp), aphanitic sedimentary/metasedimentary grains (Ls), and volcanic/metavolcanic grains (Lv). Moreover, the lithic populations of the two sample suites are indistinguishable in detail. Eight lithic grain types, mostly metamorphic, can be recognized in point-counting, although care must be taken to discriminate between true matrix and pseudomatrix consisting of deformed, fine-grained lithic fragments. We conclude that graywackes of the Ouachita Mountains and the Black Warrior basin had a common sedimentary/metasedimentary provenance, to the exclusion of significant igneous sources. This conclusion is compatible with the continental-collision model by which a sedimentary/metasedimentary terrane uplifted along a suture belt supplied sediment dispersed longitudinally through alluvial and deltaic systems to depositional sites in a remnant ocean basin. When plotted on QFL, QpLvLs and other diagrams, Ouachita-Black Warrior sandstones and more arkosic sandstones from known arc-related settings form two distinct fields that reflect basic differences in provenance. The data thus appear to underscore the importance of collisional settings for the development f thick lithic-rich graywacke sequences of non-volcanic derivation.

Characteristics of Shoreface Storm Deposits: Modern and Ancient Examples

Abstract: Cores of shoreface sediments off Fire Island, Long Island, New York, in water depths ranging between 5 and 21 m, contain a distinctive three-part sequence; from base upward consisting of: (a) basal lag, up to 50 cm thick, containing coarse gravel; (b) finely laminated sand up to 2 m thick, having median diameters ranging from 0.210 to 0.125 ram, and (c) burrow-mottled sand coarser in size than unit (b), up to 30 cm thick. The boundaries between (a), (b), and (c) are sharp. The relatively coarser, mottled sand composing the upper part of this sequence results from processes operating in the shoreface zone during fair weather. Such processes create either wave-ripple laminae (between wave base and the breaker zone) or burrow-mottled sediment (seaward of wave base). The lower part of the sequence is inferred to result from storms. The basal gravel is interpreted as a lag formed during maximum storm intensity when the coarse material is jostled at the bottom and the finer sediment is kept dispersed or is in turbulent suspension. The fined laminated sand is inferred to have been rapidly deposited under conditions of intense bottom shear as the storm waned. Examples in the geologic record of what we regard as comparable storm-deposited sequences occur at the following locations: Relict Holocene sediments off Fire Island, New York; Pleistocene sediments underlying Fire Island barrier, Long Island, New York; Norfolk Formation (Pleistocene), Benns Church, southeastern Virginia; Eocene sandstones and conglomerates, Tehachapi and San Emigdio Mountains, southern California; and Cambrian sandstones and conglomerates, Parfrey's Glen, Baraboo Range, Wisconsin. In addition, other workers have identified characteristic sequences very similar to the one described here in the following localities: Recent sediments off Virginia; Recent sediments, southern North Sea; upper part of Sundance Formation (Jurassic) of Wyoming and Montana; Upper Cambrian, Ba aboo district, Wisconsin; and "sublittoral sheet sandstones" described from several locations. We speculate that the geologic record of shoreface sediments contains a minimal proportion of sediments deposited during fair-weather conditions and consists largely of storm deposits.

Vadose and Phreatic Diagenesis: Processes, Products and their Recognition in Corals

Abstract: Pleistocene reef corals from Barbados, West Indies, have provided petrographic and chemical criteria for distinguishing the products of mineralogic equilibration (aragonite to calcite transformation) under vadose and phreatic conditions. Petrographically, corals that have equilibrated to subaerial conditions in a phreatic zone contain statistically more calcite cement and secondary porosity than their vadose counterparts. Chemically the phreatic-altered corals contain more magnesium and less strontium than vadose-altered samples. Discriminant function analysis of combined point count and chemical data allows clear distinction of vadose and phreatic populations. Texturally the phreatic calcite mosaic often consists of coarse crystals of calcite (up to several millimeters in maximum dimension) which extend out from the coral skeleton into void filling cement. These mosaics are termed cross-cutting for the manner in which the former walls of filled primary voids do not serve as crystal boundaries. No vadose-altered corals were observed with such cross-cutting mosaics. In contrast the vadose mosaics are termed fabric selective since the boundaries of filled primary voids usually serve as crystal boundaries in the calcite mosaic. These two mosaics result from different modes of aragonite/calcite transformation in the vadose and phreatic zones. Vadose transformation takes place across micron (or less) width films of water, aragonite being dissolved on one side and calcite precipitated on the other. Textural detail is preserved to the level of viewing at low magnification in a light microscope. Phreatic diagenesis often involves the development of zones of chalky aragonite (several millimeters across) between fresh coral skeleton and the growing calcite mosaic. The chalk is a region where extensive dissolution of the coral leaves a very white, friable framework. As the chalk front moves through the coral a calcite product with areas of secondary porosity and calcite cement results. Since water is always present, single calcite crystals may grow to large size and extend beyond the coral skeleton into primary void, yielding the characteristic cross-cutting mosaic. A two water model is proposed in both zones. Dissolution-reprecipitation takes place in a static water held immobile in thin films (vadose) or wider chalk zone (phreatic). In the vadose this water intermittently comes in contact with water percolating through the larger connected pores of the reef sediments. Thus chemical exchange is limited and episodic; strontium values in the calcite build up a la Kinsman (1969) and little magnesium is externally introduced. The immobile water of the chalk areas is constantly in contact with a large reservoir of water passing through the reef; the site of transformation acts as a relatively open chemical system. The resultant calcites have low strontium values due to diffusion from the site and high magnesium values as this ion is constantly introduced from the surrounding high-magnesium calcite sediments. The relative chemical homogeneity of the phreatic zone in time and space is borne out in microprobe analysis of phreatic corals; far more variability is encountered in vadose probe runs.

Petrographic Textures of Detrital Polycrystalline Quartz as an Aid to Interpreting Crystalline Source Rocks

Abstract: The internal textures of detrital polycrystalline quartz can be used to distinguish between polycrystalline quartz populations derived from different crystalline source rocks. This is supported by the petrographic examination of the quartz fraction in 65 metamorphic rocks with known metamorphic histories and the detrital polycrystalline quartz populations in 57 samples of medium sand-size Holocene fluvial sand derived from known crystalline source rocks. Empirical observations by metamorphic petrologists have shown that quartz crystal sizes increase systematically with increase in metamorphic grade of the rocks. Petrographic examination of quartz crystals deformed in controlled laboratory experiments have shown quartz responds to stress via a systematic series of processes: deformation, recovery, primary recrystallization, and secondary recrystallization. Deformation is reflected optically by undulose extinction, deformation lamellae and bands, elongated crystal units, and sutured crystal-crystal boundaries. Recovery is recognized optically be segmented extinction (semi-composite extinction) reflecting polygonization of the crystal. Primary recrystallization occurs when strain-free areas surrounded by dislocation tangles form new crys als, and is recognized optically by the presence of small (50 µm), nonundulose crystals. Secondary recrystallization develops large, nonundulose, strain-free polyhedral crystals with smooth crystal-crystal boundaries. Deformation, recovery, and primary recrystallization features characterize quartz in greenschist-grade metamorphic rocks. Detrital quartz grains with these features are, for the lack of a better term, called "unstable", reflecting the transformation from old crystals to new, recrystallized crystals. Detrital polycrystalline quartz with features of secondary recrystallization are called "stable" grains, and are derived from medium- to high-grade metamorphic rocks. Detrital polycrystalline quartz grains derived from intrusive igneous rocks are also characterized by "stable" features. A plot of a polycrystallinity index (the ratio of detrital polycrystalline quartz with more than 3 crystal units per grain to total detrital polycrystalline quartz) vs. an instability index (the ratio of detrital polycrystalline quartz with "unstable" characteristics to total detrital polycrystalline quartz) discriminates between medium sand-size polycrystalline quartz populations in Holocene sands derived from low-, and medium- to high-grade metamorphic source rocks Detrital polycrystalline quartz populations in Holocene sands derived from granitic plutons partly overlap the medium- to high-grade field.

Intergranular Pressure Solution and Cementation of the Tuscarora Orthoquartzite

Abstract: Intergranular pressure solution is believed by many sandstone petrologists to be the major source of silica for the cementation of orthoquartzites, although there is no quantitative supporting evidence from thin-section petrology. The white Tuscarora orthoquartzite, often cited as a good example of cementation by pressure solution, has been studied using luminescence petrology to evaluate this hypothesis. Point counts were made to determine the amount of pressure solution and authigenic silica in 183 thin sections of samples of the Tuscarora orthoquartzite from Pennsylvania, Virginia, and West Virginia. Luminoscope measurements show large amounts of pressure solution to be an uncommon phenomenon in the Tuscarora throughout the study area. Pressure solution can account for only 30-35% of the pore-filling quartz cement. Proximity to folds, grain size, sorting and clay content are factors that have been suggested as controls on the amount of pressure solution in orthoquartzites. Of these, only clay appears related to the occurrence of pressure solution in the Tuscarora. Furthermore, the well-cemented samples show less pressure solution than more friable samples, but contain more pore-filling cement. We conclude that initiation of cementation by silica during early diagenesis has prevented widespread development of intergranular pressure solution by equalizing the distribution of stress along grain boundaries. Of the many possible sources of silica other than pressure solution, transport of H4SiO4 in surface-derived ground water seems the most likely source of the majority of the cement found in the Tuscarora.

Comparative Fabrics of Length-Slow and Length-Fast Calcite and Calcitized Aragonite in a Holocene Speleothem, Carlsbad Caverns, New Mexico

Abstract: A specimen of flowstone, three cm thick, contains an important variety of carbonates. Ordinary length-fast calcite forms large, clear columnar crystals. Length-slow calcite forms bundles of fibers, some in bands resembling coconut meat, and also zoned crystals; these crystals appear to be the first type of calcite formed after halts in crystallization. Areas thought to have formerly been fibrous aragonite are now converted to calcite; they are recognized by ghost square-ended rays 0.2-0.7 mm wide and 2-4 mm long, now replaced by a mosaic of equant calcite spar crystals 0.05-0.2 mm across. Some fibrous crystals remaining within this spar probably represent surviving original aragonite. Some solution also took place in this zone, as well as inversion of aragonite to calcite. Inversion probably took place when the bathing solutions had the proper intermediate pH to just dissolve aragonite and precipitate calcite, the ions "hopping across" the liquid film from one lattice to the other. Solution took place at lower pH, where ions left the system. Examples of similar square-ended rays once aragonite but now converted to sparry calcite, are shown from the Triassic of the Italian Alps, thus these fabrics can be fossilized. Presumably all aragonite rays were precipitated by waters with a Mg/Ca ratio greater than 2.

Stratigraphy and Formation of Holocene Dolomitic Carbonate Deposits of the Coorong Area, South Australia

Abstract: Geologically significant and spatially extensive deposits of primary lacustrine dolomite are found over a 90 200 km region of the coastal plain in the vicinity of the Coorong Lagoon, in southeastern South Australia. The microcrystalline dolomites and associated cabronates, which range in age from Pleistocene to Recent, occur beneath interdune flats between stranded barriers. The most active but by no means only region of present-day microcrystalline dolomite formation is the linear zone of shallow. ephemeral alkaline lakes which occupies the regional groundwater discharge area, immediately landward of the Coorong Lagoon and along the southeastern extension of its interdune corridor. Comparative studies between sediment cores from th s region and a range of contemporary sedimentary environments show that it is possible to characterize the sedimentary sequence which is being generated by Holocene sedimentary regression. In its ideal form this sequence passes upward from a basal marine or lagoonal unit, through a protodolomite + Mg-calcite lacustrine unit, to culminate in an uppermost dolomite or dolomite + magnesite unit which may be capped by a soil. Hydrological considerations, which imply the presence of a landward-dipping continental groundwater-marine groundwater interface immediately inland from the coast, show the above sedimentary sequence to be the result of a progressive seaward movement of that interface, caused by sediment accretion and consequent shoreline regression. The position of a particular microcry talline carbonate-forming environment at any point in time, with respect to the interface, defines its carbonate mineralogy. These observations clearly indicate that optimal conditions for Coorong-type dolomite formation are found in ephemeral lakes which are fed solely by evaporitically-modified continental groundwater and which occur inland from the interface. The relatively large amounts of Quaternary primary dolomite in the area render it somewhat of a geological anomaly in terms of present-day dolomite-forming environments. This study implies that the main reasons for this anomaly may be a favourable combination of factors which include an abundant, renewable source of magnesium for the seaward-flowing groundwaters, an active grounwater regime, and a climate which seasonally varies rom humid to semiarid. The most likely source of magnesium is hypothesized to be from weathering of areally extensive Quaternary basic volcanics which occur in groundwater source areas marginal to the coastal plain. From a geological viewpoint it is suggested that many microcrystalline dolomite occurrences in the geological record may be of the Coorong-type, formed in shallow migrating ephemeral lakes in a relatively humid climate on regressive coastal plains, overlying beach deposits. Such dolomites, which contain no obvious traces of evaporite minerals, are contrasted with superficially similar sabkha-type dolomites which, at least in part, are early diagenetic replacement dolomites and which contain abundant evaporitic associates such as gypsum and anhydrite.

Nature of feldspar-grain size relations in some quartz-rich sandstones

Abstract: The abundance of feldspar as related to grain size has been examined in quartz-rich cratonic sandstones of Cambrian, Ordovician, Pennsylvanian-Permian and Jurassic ages, which have traditionally been characterized as supermature quartz arenites. Feldspar contents ranged from less than 1% to more than 50%. In the Cambrian sandstones of the upper Mississippi Valley, detrital feldspar grains are almost invariably < .125 mm. Due to the great abundance of feldspar, fine and very fine-grained sandstones in this sequence are either feldspathic (10-25% feldspar) or highly feldspathic (25-65% feldspar). The percentage of feldspar is nearly linearly related to the volume of the very fine sand fraction. The St. Peter Sandstone (Ordovician) usually contains less than 2% detrital feldspar, but larger amounts occur where it contains significant amounts of very fine sand and coarse silt. Feldspar in the St. Peter is concentrated in the coarse silt size fraction rather than in the very fine sand fraction as in Cambrian sandstones. This may indicate intensive abrasion of initially small feldspar derived from Cambrian sediments. The Weber Sandstone (Pennsylvanian-Permian) contains up to 20% feldspar whose abundance also increases with decreasing mean size, being most abundant in samples with means below < .125 mm. In samples of the Navajo Sandstone (Jurassic), feldspar is relatively evenly distributed among the size fractions < .35 mm, yet the highest feldspar concentration occurs in the .125 to .044 mm size range. The distribution of feldspar within various size fractions of sandstones is significant in making provenance studies, in explaining regional and stratigraphic mineralogical variations, in interpreting depositional environments, and in assessing the origin of compositional maturity or immaturity.

Sediments and Stratigraphic Framework of the Copper River Fan-delta, Alaska

Abstract: The Copper River has prograded a marine dominated fan-delta onto the deep, tectonically active northern shelf of the Gulf of Alaska. The morphology and internal stratigraphy of the delta system are products of the sporadic influx of great volumes of bed load sediment into a basin characterized by a high wave, tide, and current energy flux. Subaerial morphology and processes of the delta are typical of humid region alluvial fans. The fan margin is, however, completely reworked and modified by wave and tidal action, and marine portions are skewed westward by longstore drift and marine currents. Major environments of the delta include (1) the subaerial deltaic plain, consisting of marsh and swamp organic muds, and braided to estuarine distributary channel fills; (2) the tidal lagoon, composed of tidal sand and mud flat sequences interlaced with a complex of tidal channel fills; and (3) the shoreface, which consists of marginal island, breaker bar, and middle shore face sands, lower shoreface sand and mud, and prodelta/shelf mud. Marine influenced tidal lagoon and shoreface facies are volumetrically dominant components of the system. The Copper River fan-delta is a working depositional model for tectonic clastic wedges prograded into an open marine basin.

Evaporites: Relative Humidity Control of Primary Mineral Facies

Abstract: Increasing dissolved salt concentration decreases the thermodynamic or chemical activity of H2O in a solution and as a result lowers the equilibrium water vapor pressure exerted by the solution. The calcium sulfate facies minerals of marine evaporites are precipitated from brines with aH2O 0.93-0.76; halite facies minerals from brines with aH2O 0.76-0.67; potash facies minerals from brines with aH2O less than 0.67. Thus halite can only be precipitated in those areas where the mean relative humidity of the atmosphere is less than 76%. Mean relative humidities of many low latitude coastal and oceanic areas, even in the most arid regions, commonly hover between 70 and 80%. Lower mean relative humidities occur ov r marine areas surrounded by large expanses of land. Thus only marine basins with a high continentality are likely to precipitate potash facies evaporites. It is suggested that some evaporite basins may be restricted to the calcium sulfate mineral facies only, even if the basin becomes entirely cut off from the world ocean, the controlling factor being a high mean relative humidity.

Life History, Growth Habits, And Constructional Roles of Acropora Cervicornis in the Patch Reef Environment

Abstract: In two representative patch reefs within the Florida Reef Tract the majority of Acropora cervicornis colonies appear to have developed from broken fragments of parent colonies rather than directly from settled larvae. Following breakage, both the parent and the fragment regenerate polyps to cover the broken area and apical polyps to continue growth (the parent appears to recover more quickly). Fragments first attach to the substrate by growing laterally, thereby forming a stabilizing base. They then begin to grow upward. Two main morphological growth forms result from this process: (1) Forms which result from branches of the colony fusing with other branches of the colony; and (2) Forms which result from the interaction of the branches of the colony with the surrounding substrate and organisms. Branches of A. cervicornis may grow parallel or cross closely without fusing. However, most branches fuse on contact, with polyps covering the contact area. On the reefs examined, the A. cervicornis colonies were small, relatively scattered, and did not appear to be a major binding agent. However, within the immediate vicinity of the colonies, they actively bind carbonate rubble and other clastic sediment. Thus in reefs where A. cervicornis is more abundant, this coral could be a major factor in fortification and stabilization. Intergrowth and attachment habits of A. cervicornis are compatible with its occurrence and morphology in many horizons of the Key Largo Limestone, generally interpreted to be a Pleistocene equivalent of the present day Florida patch reefs.

Reddening Of Desert Sands: Simpson Desert, N. T., Australia

Abstract: Measurement of each size fraction with Munsell Color Chart shows that the saltating population (2 to 4.5) has reddest, darkest, and most intense color, averaging 2.5 YR 4/7. Red color in these sands is caused by a hematitic clay cutan formed during Pleistocene lateritization, enhanced by aging, and covered over by a thin silica coat. A Holocene limonitic fluff occurs on top of the silica coat. Coarser grains are yellower because they have more yellow fluff and less red cutan; reg sediments are darker because they contain carbonaceous dots. With aging, sands change from yellow to red and become darker and less intensely colored because of a four-stage process: weathering to limonite, dehydration to diffuse hematite, recrystallizatio to hematite crystals, and green-darkening. Thus Holocene sands are orange, Pleistocene brick-red, and early Paleozoic dull liver color.

The Development of Rodriguez Bank, a Holocene Mudbank in the Florida Reef Tract

Abstract: Rodriguez Bank is a Recent mound of unconsolidated calcareous sediments deposited during a relative rise in sea level in the absence of vigorous wave action. This mound, unlike modern coral-algal reefs, has no rigid organic framework and plants are directly and indirectly responsible for the accumulation. The principal sediment contributors to the bank are algae. An embayment in the Pleistocene rock floor localized the initial deposition of sediment that provided the nucleus for bank development. Although the hydrography and biotic assemblages changed, the bank maintained itself as a topographic feature.

Early Precambrian Tidal Sedimentation in the Pongola Supergroup of South Africa

Abstract: Sedimentary sequences which resemble the regressive intertidalite model of Klein (1971, 1972) occur associated with volcanic rocks in the 3,000 m.y. Pongola Supergroup. Two inliers in northern Natal which were selected for the present study include prominent orthoquartzitic sandstones with thinner siltstones and mudstones. The arenite member at the base of the Wit-Mfolozi section is attributed mainly to shallow marine deposition. The upper 5 m display features which are suggestive of a low tidal sandflat environment. These include reactivation structures, herringbone cross-stratification, and ripple forms indicative of decreasing water depth and possible subaerial exposure. Above this is the alternating arenaceous-argillaceous member with a distinctive association of shrinkage cracks, mudclast breccia, and double-crested, flat-topped and interference ripples. Similar features occur in modern mid-tidal flat deposits. A high tidal mudflat environment is envisaged for the uppermost unit, the argillite member. A hematitic (jaspilite) bed is locally present at the top of the sequence and suggests the possibility of biological liberation of oxygen in periodically flooded depressions in the inner mudflat. Three similar upward-fining sequences are present in the Mpongoza inlier. Paleotidal ranges of between 12 and 25 m are inferred. The maximum figure is considerably in excess of any present-day tidal amplitudes, and lends support to the hypothesis of an early Precambrian origin for the earth moon system.

Sediment Transport and Dispersal Across the Deep Fore-Reef and Island Slope (-55 m to -305 m), Discovery Bay, Jamaica

Abstract: Observations and samples taken during submersible operations on the deep fore-reef and island slope (-55 m to -305 m) off Discovery Bay, north Jamaica, revealed a surprising lack of reef-derived sediments on the deeper parts of the island slope as a result of the barrier effect of sill reefs at the top of the deep fore-reef. In those areas where sill reefs are absent such as the area immediately above the Discovery Bay submarine canyon, compositional and textural data, together with bulk carbon-14 age dates that range from 900 to 3,530 years BP, indicate occasional catastrophic movement of discrete sediment masses down the axis of the canyon, separated by long periods of suspended sediment addition and biologic mixing. During the present sedimentologic regimen, the coarse reef-derived debris found in the canyon does not continue down-slope into the adjacent deep basin (Cayman-Bartlett trench), but seems to be restricted to the island slope above 800 m. The green alga, Halimeda, is a major reef-sediment source, particularly in the deep fore-reef where it was found alive and growing in significant quantities to depths as great as 100 m. The various species of Halimeda can be useful sediment tracers, with H. goreauii and H. opuntia being characteristic of fore-reef slope environments and H. cryptica essentially restricted to the deep fore-reef. The present study confirms that boring sponges particularly Cliona sp. generate significant quantities of easily recognizable silt-sized detritus as a result of their boring activity. More than 5% of the total volume of the island slope sediment samples can be attributed to the boring activity of the clionid sponges with a mean of 24% of the silt fraction being composed of clionid debris. Where sponge chips contain microscopic algal borings from the original substrate, clionid debris may serve as a powerful sediment tracer from shallow to deep water. The submersible NEKTON Gamma made possible direct observation and sampling of a hitherto unknown, highly complex environment. These observations, and the data generated from these samples, have significantly reshaped our understanding of fore-reef sediment transport and deposition.

Pelagic Chalks and Calciturbidites from the Lower Tertiary of the Troodos Massif, Cyprus

Abstract: The Troodos Massif of Cyprus is blanketed by a sequence of predominantly calcareous pelagic sediments ranging in age from Upper Cretaceous to Middle Miocene. Upper Paleocene and Lower Eocene chalks and marls vary laterally from thick sequences of pelagic calciturbidites in south Cyprus, to thinner in situ deposits on former topographic highs located in southwest Cyprus, and highly condensed ferruginous chalks situated along the north margins of the Troodos Massif. Identification of the calciturbidites is based on grading, lateral continuity of beds and diagnostic sedimentary structures. The calciturbidite beds are chalks composed of nannoplankton, planktonic foraminifera, and Radiolaria, with occasional redeposited benthonic foraminifera. The basal zones of individual calciturbidite beds contain silt derived both from erosion of the Troodos volcanic basement and from contemporaneous acid volcanism. Features of the interturbidite chalks and marls, including the absence of benthonic foraminifera, demonstrate slow deposition in relatively deep water, whereas the stratigraphically condensed chalks of North Troodos accumulated slowly in shallower water. General field relationships and paleocurrent measurements show that the calciturbidites flowed from the north-east down a regional paleoslope which was previously developed by deformation of the south margin of the Troodos Massif in the Maastrichtian.

Formation, Maintenance and Geomorphological Significance of an Aeolian Shell Pavement

Abstract: The veneer of shell material on a beach berm represents a lag deposit created by aeolian deflation processes. This feature, termed an aeolian shell pavement, is analogous to the stone pavements of desert regions. The presence of the pavement protects both the beach berm and the foredunes from marine erosion, although it also restricts sediment availability to the dunes. During the intervals when the pavement is exposed, the roughness characteristics of air flow across the beach are altered, causing changes in the movement of sediment. The pavement also acts as a sand reservoir during relatively calm periods. Monitoring of weekly changes in extent of the pavement suggests that it is a persistent feature except during extreme storms, when it is liable to be buried, reworked or eroded. R establishment of the feature following storms is rapid, between 14 and 40 days in these examples. The existence of the feature has an important bearing on shoreline progradation.

Early Dissolution of Sponge Spicules from Reef Sediments, North Jamaica

Abstract: Opaline sponge spicules are being preferentially removed from reef as opposed to back-reef sediments from North Jamaica. In contrast to presumed chemical solution of silica in the deep sea, rapid removal of opal from reef sediments may be taking place as a result of biologic agents.

Sedimentology of a Melange: Franciscan of Trinidad, California

Abstract: A study of layer features within coarse proximal-exotic sediment gravity-flow deposits of the Jurassic(?) Franciscan Formation and Patricks Point metasedimentary unit of the Trinidad quadrangle reveals the following common layer sequence: I--massive fine to medium-grained or inversely graded pebbly sandstone, II--pebbly sandstone or conglomerate with or without preferred clast orientation, III--coarse tail and/or content normally graded pebbly sandstone, IV--diffusely laminated and/or cross-stratified pebbly sandstone, V--massive fine to medium-grained sandstone, and VI--diffuse to sharply laminated fine to medium-grained sandstone which may contain dish and pillar structures. This sequence is developed vertically and laterally during a single flow event. It reflects transitions from rain support by dispersive pressure to fluid turbulence which occur as a sediment gravity-flow progresses downslope and the entrained grain population becomes finer. The sequence may therefore be used to recognize individual flow events in a vertical sequence and to determine proximality of deposition on the inner submarine fan or suprafan. Franciscan Formation sedimentary rocks were deposited chiefly on an inner fan, whereas those of the Patricks Point unit were deposited on an outer fan downfan from a suprafan lobe. The Patricks Point unit is either allochthonous and of a different age than the Franciscan Formation, or was severed, metamorphosed and emplaced east of the Trinidad Franciscan during melange formation.

Recent High Magnesian Calcite/Aragonite Cementation of Beach and Submarine Sediments from Denmark

Abstract: Carbonate cementation of recent submarine and littoral sediments is recorded from several locations in the Kattegat Sea and the adjoining coasts. Two different types of cement related in mode of formation are described from the localities: 1) Aragonite cement restricted to sediment bodies of accumulated shell material mixed with quartz sand and 2) cement of magnesian calcite associated with sediment bodies of almost pure quartz sand. Both types of cement are present at all localities studied. The magnesian calcite shows a variable magnesium content in the range of 10-25 wt% MgCO3. Several larger concretions of almost pure gypsum were found at a single locality, but minor amounts of gypsum associated with the cement of magnesian calcite have been detected by X-rays in several samples analysed. The carbon which forms part of both the aragonite and the magnesian calcite displays extreme negative C13 values in the range of -25 to -55 PDB. The values are of the same order of magnitude as those typical for the carbon of methane, which frequently is recorded in the interval of -40 to -80 PDB. Furthermore, the carbon of the cement is radiocarbon-dated to be approximately 18,000 years old, whereas the cemented skeletal carbonate is recent to about 4,000 years old. Tires it is suggested that the carbon of the cement originates from oxidized methane from Quaternary deposits. The precipitation of calcium carbonate is thought to take place by mixing outflow of bicarbonate-enriched meteoric water with seawater. Magnesian calcite is believed to be the primary phase formed by the precipitating solution and the contemporary formation of aragonite most likely is caused by the higher nucleation rate of that polymorph when proper nuclei are present (i.e., mollusc shells).

Environmental Sensitivity of Submicroscopic Surface Textures on Quartz Sand Grains--A Statistical Evaluation

Abstract: A statistical evaluation of the occurrence of selected surface textures on quartz sand grains from eolian and subaqueous environments was made to determine their environmental sensitivity. Samples collected at Padre Island, Texas, and the adjacent inland dune fields were used to identify the limits of sensitivity on a local scale ( i.e. , beach, dune, etc.). In comparing means and standard deviations, only the lagoonal facies was found to be distinctive. It could be identified easily by the abundance of chemically etched, oriented triangular-shaped patterns. The other features characteristic of subaqueous or eolian processes are more or less equally distributed throughout the coastal area. Analysis of scatter plots and linear regression revealed weak trends in which subaqueous features are replaced by eolian features moving inland from the beach. Comparison of statistics for the samples from the Texas coastal region with statistics for samples from four interior deserts suggests that if over half of the grains in a sample have chemically etched, oriented triangular-shaped patterns or mechanically formed V-shaped pits and straight or slightly curved grooves and scratches, then a coastal province might be deduced. Irregular or polygonal cracks were present on over half of the desert grains examined and were much less abundant on the grains from the coastal region. None of the other six features examined were statistically distinctive for environmental determinations.

The Morphology, Magnitude and Regime of a Carboniferous Fluvial-Distributary Channel

Abstract: A meandering fluvial-distributary was initiated into the interdistributary bay of a Carboniferous (Yoredale) delta lobe by avulsion. Initiation immediately preceded abandonment of the lobe, and lateral migration of the channel produced a meander belt sandstone in the short period of channel activity. The channel fill comprises an 8 m fining upwards sequence with lateral accretion surfaces in the coarse member which permit calculation of palaeochannel parameters. The coarse member also exhibits inclined, erosive discontinuities which reflect temporal variations in flow. Paleochannel regime was dominated by frequently recurring normal floods which produced the lateral accretion surfaces, and less frequent exceptional floods which were primarily effective in scouring the bar surface, pro ucing the discontinuities.

Sediment Sources and Sedimentation Rates, Chincoteague Bay, Maryland and Virginia

Abstract: Chincoteague Bay receives an estimated 90,000 m3 of sediment annually, of which roughly half is sand and half mud. Sand is derived principally from the barrier, Assateague Island, with storm overwash being twice as effective as eolian transport in supplying sand to the bay. The mainland of Delmarva Peninsula is the primary source of mud-size sediment for the bay. Annual contribution of mud from shore erosion is approximately eight times that introduced by streams. Sediment transported from the Atlantic Ocean through the two active tidal inlets is important only in the immediate vicinity of the inlets. Present average rate of sedimentation in Chincoteague Bay estimated from annual sediment supply is 0.3 mm/yr. This is far less than the rate of 1.5 mm/yr, obtained from sediment thickness in borings in the bay and believed to represent average conditions over the past 5,000 years. The present anomalously low sedimentation rate and accompanying decline in salt-marsh growth are attributed to a decrease in ocean-derived sediment caused by the closing of former tidal inlets through Assateague Island. If this interpretation is correct, it underscores the wisdom of a recent decision against dune stabilization for prevention of storm overwash on Assateague Island. Overwash is a major means of providing sediment for landward accretion of barrier islands in response to relative sea-level rise. Elimination of storm overwash in the absence of sediment influx from the sea via tidal inlets almost certainly would have resulted in long-term net erosion on the lagoonal shore of Assateague Island.

The influence of upwelling on suspended matter and shelf sediments off southeastern Brazil

Abstract: Surface waters off southeastern Brazil contain very little terrigenous material in suspension, even off the mouths of major rivers. As a result of the low rate of supply of terrigenous sediment, calcarenites and live algal reefs are widespread on the middle and outer continental shelf. Upwelling near the coast and over offshore banks, is associated with elevated amounts of suspensates, which are mainly planktonic and non-skeletal, and reach concentrations of 0.5 to 1.0 mg/l. Beneath the biologically productive upwelled surface waters there is remarkably little sedimentation of this organic matter. Also, although phosphorite deposits are usually associated with upwelling centers, there are no phosphatic sediments off southeastern Brazil. The absence of both organogenic sediment and pho phorite differentiates this coast from the areas of upwelling in the eastern Atlantic, and probably results from the high degree of oxygenation of upwelled water, which contains up to 6 ml/l of dissolved oxygen (more than three times as much as found in upwelling centers off northwest Africa, for example). Phosphorite and organically enriched sediment would be absent from paleo-upwelling centers of similar type, but these centers might be recognized through local enhancements of organic matter in the silt and clay fraction of marine sediments.