Showing papers in "Journal of Sedimentary Research in 1974"

Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition; comparison with other methods

Abstract: A modified ignition loss method is described for determining organic and carbonate carbon in calcareous sedimentary materials using equipment found in most laboratories. The method has been found to equal or excel the accuracy and precision of other methods tested and has the advantage of being considerably faster if large numbers of samples are to be analyzed.

An inexpensive titration method for the determination of organic carbon in recent sediments

Abstract: Thirty-three samples of lacustrine and shallow marine sediments have been analyzed for organic carbon content by a modified Walkley-Black titration method and by the LECO(FOOTNOTE 3) combustion method. Comparison of the two sets of data yields a regression line with a slope of 0.992, intercept of 0.00 and a correlation coefficient of 0.989. Thus results obtained with the two methods agree precisely. Because the titration method involves little equipment or operator training it can be used at small laboratories and schools that lack sufficient funds or volume of samples to purchase or justify the faster but more expensive LECO apparatus.

The Natural History Of Crystalline Calcium Carbonate: Effect of Magnesium Content And Salinity

Abstract: Morphology of calcium carbonate crystals is controlled mainly by rate of crystallization, and Mg and Na content of the precipitating waters. Together, these factors integrate to provide important indicators of environment. Magnesium selectively poisons sideward growth of calcite; thus CaCO 3 prefers to crystallize as aragonite, or as minute fibers or steep rhombs of magnesian-calcite, whose sidewise growth is generally stopped at widths of a few microns. Thus in Mg-rich environments, such as beaches or marine bottoms, micritic or fibrous aragonite and magnesian calcite cements form. As seawater is buried, Na usually remains high but Mg is selectively lost so that the Mg/Ca ratio drops from 3:1 to about 1:3. Thus, in the absence of Mg-poisoning, coarse sparry calcite cement can form in the subsurface, and crystallizes as irregular polyhedra. In meteoric waters, both Mg and Na are very low. If precipitation is very rapid, calcite micrite may form (caliche). Fresh-water calcite can also occur as euhedral rhombs in very dilute solutions. In the phreatic-meteoric zone, sparry calcite develops. Carbonate ooze initially contains much Mg. Upon lithification, it is proposed that much of the Mg is retained as a sort of "cage" around each polyhedron of calcite, preventing growth beyond a few microns. Fresh-water flushing removes this Mg-cage, and allows recrystallization to coarser microspar.

The Characteristics and Origins of Dish and Pillar Structures

Abstract: Dish structure is defined by the presence of thin, subhorizontal, flat to concave-upward, argillaceous laminations in siltstone and sandstone units. It is commonly associated with vertical or nearly vertical cross-cutting columns and sheets of massive sand termed pillars. Both form commonly in sediment ranging in grain size from coarse-grained siltstone to coarse-grained, conglomeratic sandstone. In sedimentation units greater than about 0.5 m thick, dish structure is faint and neither cuts across nor is cross-cut by other sedimentary structures. In thinner units dish structures commonly cut across primary flat laminations, climbing-ripple cross-laminations, and convolute laminations. Dish and pillar structures form during the consolidation of rapidly deposited, underconsolidated or quick beds. During gradual compaction and dewatering, semi-permeable laminations act as partial barriers to upward-moving fluidized sediment-water slurries, forcing horizontal flow beneath the laminations to points where continued vertical escape is possible. As water seeps upward through the confining laminations, fine sediment, planar, and low-density grains are filtered out and concentrated in the sediment pore spaces. The resulting clay- and organic-enriched laminations are flat dishes that may be later deformed by the upward pressure of flow around their margins and central subsidence as underlying sediment and water escape. Pillars form during forceful, explosive water escape. It s suggested that the shapes of dishes and pillars within an individual bed can be related to its original water content, thickness, and grain size; to the rate and magnitude of dewatering including consideration of water entering the bed from underlying consolidating sediments; and to the types and distribution of earlier-formed sedimentary structures. Dish structures cannot be used directly to infer transport or depositional processes. Where dishes are associated with or cut across primary sedimentary structures, the latter indicate deposition from currents. The study, indicates that coarse-grained terrigenous sediments often have pronounced and complex consolidation histories. Many rapidly deposited beds undergo partial liquefaction and fluidization during consolidation but retain sufficient strength to resist wholesale downslope flowage in response to gravity.

Carbonate Cement Stratigraphy of the Lake Valley Formation (Mississippian) Sacramento Mountains, New Mexico

Abstract: Calcite cements of the Lake Valley Formation (Osagean) consist of clear syntaxial and granular cements in non-biohermal facies; cements in bioherms are dominated by distinctive cloudy cements, but also include clear cements. The clear cements show compositional zoning that comprises an early iron-free zone and a later iron-poor to iron-rich zone. Cathodoluminescence of the non-ferroan calcite cements reveals as many as four zones containing varying amounts of MnII. Three of these zones are correlative within single measured sections through about 200 ft of stratigraphic interval; and are correlative laterally for a distance of at least 10 mi. Petrography of the non-ferroan cements from samples inmediately adjacent to post-Lake Valley unconformities dates the oldest of these three zones as pre-Meramecian, and the younger two as post-Meramecian and pre-Atokan. Ferroan calcites largely precipitated during post-early Atokan; a minor amount precipitated in pre-Atokan time. Cloudy biohermal cements are older than the clear cements, and precipitated contemporaneonsly with sedimentation of the bioherm facies--they are interpreted by this and previous work to be marine cements. Clear cements are interpreted to have precipitated in the phreatic zone. Evidence includes crystal and zonal geometry, absence of marine hardgrounds, differences from marine cements, and MnII and FeII content. The stratigraphic and geographic distribution of the non-ferroan clear cement zones is interpreted to reflect ancient phreatic lenses established during pre-Meramec and pre-Atokan subaerial exposures of the Lake Valley Formation.

Influence of coatings on quartz cementation

Abstract: Coatings of chlorite, illite, hematite, chert or carbonate on quartz grains may inhibit quartz cementation. Although these coatings do not always prevent formation of overgrowths, they commonly cause the growths to form with unique features. Recognition of these features may furnish an important clue in locating areas where coatings have been the main factor in preserving high porosity.

Strontium as a Tool in Facies Analysis

Abstract: The distribution of Sr in Mesozoic carbonate rocks of the Central Western Carpathians is facially controlled. This facies control is demonstrated on 75 facies types and more than 1,200 single analyses. The distribution of Sr in limestones is bimodal, with high Sr concentrations in hypersaline, dark coloured and deep sea rock types and low Sr concentrations in littoral, neritic and shallow bathyal limestones of organogenic and organodetrital types. The data, although not quite conclusive, indicate that this bimodality is inherited from original sediments. The early diagenetic dolomites contain higher Sr concentrations than the late diagenetic ones. This might be related to a partition of Sr between aragonite vs. dolomite and calcite vs. dolomite for the two dolomitic facies types.

New Red Sandstone Floodplain and Piedmont Sedimentation in the Hebridean Province, Scotland

Abstract: A sequence of coarse clastic sediments of New Red Sandstone age in western Scotland was probably deposited by alluvial fan (mudflow, streamflood and braided stream), playa and floodplain (low and high-sinuosity channels) processes. Within the alluvial fan group the mudflow deposits are usually laterally persistent, flatbedded paraconglomerates. Individual mudflow units only rarely show basal erosion surfaces. The streamflood deposits consist of laterally impersistent orthoconglomerates and coarse sandstones which often fill channels as large or very large-scale cross-stratified sets. The braided stream deposits are usually sandstones and fine-grained, well-sorted orthoconglomerates with an abundance of trough cross stratification and intraformational pebbles. The braided stream conglomerates, unlike the mudflow and streamflood conglomerates, show no significant relationship between maximum particle size and bed thickness. The playa deposits show alternations of thin, mudcracked mudstones and laminated or rippled sandstones. The deposits of ephemeral, low-sinuosity floodplain rivers are identified as relatively thin, coarse-grained fining-upwards cyclothems in which both fine and coarse members are laterally extensive and there is little lateral variation in structure, thickness or lithology. There is usually a caliche profile capping the fine member. In contrast, thicker, finer-grained, structurally and lithologically more complex cyclothems probably accumulated from high-sinuosity floodplain systems. Alluvial fan (piedmont) sequences show a textural and structural evolution from mudflow up through streamflood to braided stream deposits. The time trend through the laterally equivalent floodplain sequences is from low to high-sinuosity and less ephemeral stream systems. The piedmont trend probably reflects a geomorphic maturing of the basin margins while the floodplain trend suggests an additional climatic change towards less arid conditions. Either trend is consistent with conformably overlying marine (Jurassic) deposits.

The Origin of Quartz Geodes and Cauliflower Cherts Through the Silicification of Anhydrite Nodules

Abstract: Quartz geodes from the dolostones of the Fort Payne and "Warsaw" formations near Woodbury, Tennessee have proved to be pseudomorphs after early diagenetic anhydrite nodules. Their lithological association suggests that the anhydrite developed in an arid tidal-flat environment by a process similar to that currently operating in the sabkhas of the Persian Gulf. Silicification took place prior to compaction and lithification of the sediments, the most likely source of silica solutions being the abundant sponge spicules which characterize the peritidal dolostones. Similar geodes from other Mississippian localities share the same lithological association and mode of origin. They help to define a recurrent sabkha facies and serve as important shoreline indicators which may be used in the reconstruction of regional patterns of marine transgression and regression.

Supercritical flow in density currents

Abstract: Experiments with saline currents show that most flow phenomena and sedimentary structures characteristic of supercritical flow in open channels can be duplicated with density underflows Antidunes, breaking antidunes, and chutes-and-pools formed spontaneously on an erodible bed at densiometric Froude numbers greater than 10 Runs using fixed model antidunes show that equations derived for antidunes and hydraulic jumps in open channels are equally valid for density currents, provided a term is introduced to take account of the reduced density contrast between fluid layers Antidune wavelength (L) is related to flow velocity (U) by the equation for internal waves: \[\ U\ =\ \sqrt{gL/2{\pi}\ ({\rho}\ -\ {\rho}{^\prime})/{\rho}\ +\ {\rho}{^\prime})}\ \] where ρ and ρ′ are densities of the current and overlying fluid Critical slope for large scale density flows should be about 0001 If so, most natural turbidity currents are supercritical Wavelength of antidunes formed by density currents must be > or =126 times the effective flow thickness For turbidity currents, implied wavelengths are on the order of tens or hundreds of meters Antidunes of this scale might be manifest as long period pinch-and-swell of individual turbidites, but internal lamination should be essentially parallel to bedding Under some circumstances, it may be that many successive turbidites comprise the "laminae" of large, composite antidunes Density currents provide a range of flow conditions where "upper flow regime" bedforms can coexist with "lower flow regime" forms, pointing up a defect in such classification Ripple lamination does not in itself imply subcritical conditions

Sedimentation rates and recent sediment history of Lakes Ontario, Erie and Huron

Abstract: Sedimentation rates and changes in organic carbon, nitrogen, phosphorus and mercury concentrations were determined for 14 core locations, representing basins of fine-grained sediment in Lakes Ontario, Erie and Huron. Sedimentation rates were estimated by averaging the weight of sediment deposited above the Castanea (chestnut) pollen decline dated at 1930 for Lake Ontario and 1935 for Lake Erie, and above the Ambrosia (ragweed) pollen rise, dated at 1850. Present-day sedimentation rates were high in Lake Erie, ranging from 847 to 5,049 g/m2yr, low to intermediate in Lake Ontario, ranging from 366 to 1,156 g/m2yr and low in Lake Huron ranging from 147 to 325 g/m2yr. There has been a threefold increase in sedimentation rate in Lake Erie sinc 1935 and the Kingston basin of Lake Ontario since 1930. The nutrient and Hg concentrations are enriched at the sediment surface in all the cores from Lakes Ontario and Erie, while the Huron cores show little change at the surface from their background concentrations. The enrichments are attributed to increased nutrient and Hg loading to the Ontario and Erie sediments, with the major increases after about 1950. The present-day loading of nutrients and Hg to the sediments parallels the rates of sedimentation at each location, being greatest in Lake Erie. Early-colonial loading of nutrients and Hg to Lakes Ontario and Erie are generally similar to the modern loading of Lake Huron. The total loading of sediment, nutrients and Hg was estimated for each lake. Present-day sediment accumulation of 4,600 103, 23,400 103, and 3,900 l03 metric tons was estimated for Lakes Ontario, Erie and Huron respectively.

Significance of the boring sponge Cliona for the origin of fine grained material of carbonate sediments

Abstract: The marine siliceous boring sponge Cliona boring into carbonate substrates of biogenic or non-biogenic origin produces characteristic particles of 15 to 100 micron in size. These particles were quantitatively determined without difficulties by grain size analysis using a scanning electron microscope (SEM). In the Persian Gulf (Iranian coast) and in the Northern Adriatic Sea (Istrian coast) the portion of the fine grained material produced by Cliona is 2 to 3% of total sediment. In the lagoon of Fanning Island/Line Islands (Pacific) this portion amounts to 30% of the total sediment.

Oscillatory ripple marks and the evaluation of ancient wave conditions and environments

Abstract: Of special interest to geologists is the possibility of utilizing observations of symmetrical ripples preserved in rock strata to interpret the wave conditions of the ancient environment. This paper reviews the relationship between the wave action and the ripples produced. In shallow water with waves of low period and height, ripple lengths in sand in the 88-177 µ range are approximately a factor 0.8 of the orbital diameter of the water motion near the bottom just above the ripples. Such conditions exist principally in lakes and bays of limited fetch (and in wave flumes). In these environments the ripple lengths will tend to increase onshore in progressively shoalling water. Under oceanic conditions with longer period waves and deeper water, the water orbital diameter near the bottom is generally several times larger than the ripples produced. In this environment the ripples tend to decrease in length onshore. Graphs and computer programs are presented which aid in determining possible combinations of wave dimensions and water depths that could have generated observed oscillatory ripple marks.

Submarine cementation in reefs; example from the Red Sea

Abstract: The present study has three objectives: (1) to show that cements form within centimeters of depth below surfaces of reefs and eliminate pore space almost entirely within less than 60 cm below the surfaces of reefs, (2) to describe and illustrate the fabrics and the kinds of minerals of marine cements in Red Sea reefs, and (3) to infer a possible mechanism of precipitation of carbonate cement which is based on observations of strong etching, corrosion, and partial replacement by calcium-carbonate cement of quartz particles in Red Sea reefs. Although seawater is generally undersaturated with respect to silica corrosion and embayment as a result of dissolution, such as observed in the Red Sea reefs, are not normally prevalent in quartz particles lodged on the sea bottom. Thus a mechanism other than undersaturation of silica must explain the partial dissolution of quartz; as carbonate cement replaced quartz particles, carbonate precipitation and quartz solution must proceed simultaneously. These reactions are under the control of pH: a pH level exceeding 9 and even 10 appears to be a necessary condition for the dissolution of quartz and the precipitation of carbonate. Photosynthesis and respiration of the biomass in the reef cause a shift in the carbonate buffer system of seawater with the uptake of CO2. Although pH values in excess of 9 can be measured in the waters of reefs, microlevels of pH of 10 and even 10.5 may likely be maintained in thin gellike films or monomolecular layers ("skin effect") that cling to the surfaces of the framework builders of reefs. Such high pH levels may trigger the precipitation of carbonate cement in reefs. The crucial evidence in this study is petrographic (the replacement of quartz particles by carbonate cement); the pH control is inferred.

Significance of Color in Red, Green, Purple, Olive, Brown, and Gray Beds of Difunta Group, Northeastern Mexico

Abstract: Varicolored rocks of the Difunta Group (Upper Cretaceous-Paleocene) are composed of detritus derived from a relatively uniform terrane of volcanic rocks and deposited in fluvial, deltaic, and shelf environments. Red, green and purple rocks are restricted to delta-plain facies, whereas the dark colors are present in all facies. The color of claystone is a function of color mixing of red hematite, green illite and chlorite, and black organic matter; and possibly of grain size of hematite (purple color). Red and purple rocks owe their color to pervasive hematite grain coatings and crystals intergrown with clay; brown rocks owe their color to faint or localized iron-oxide grain coatings; and gray rocks to organic matter and authigenic iron sulfide. Green rocks owe their color to chlorite and illite and to the absence of hematite, organic matter and sulfides. Olive and yellow claystone colors are imparted by color mixing of green clay and black organic matter. Field relations and petrographic studies indicate that red and purple colors originated through post-depositional reddening of sediment, in part in soil zones on the delta plain, in a sub-humid to semi-arid climate that had seasonal wet and dry periods. Reddening occurred both by aging of hydrous ferric oxides plus staining of grains by hematite pigment formed by oxidation of detrital iron oxide and mafic grains. Some brown siltstone beds were pigmented in a manner similar to red beds, but other siltstone beds developed brown color upon weathering. Green beds formed by bleaching of red (or proto-red) beds by interstratal percolation of reducing water derived largely from fluvial channels overlying the green beds. Olive and gray claystone are present predominantly in marine facies that contain abundant organic matter and in some delta-plain facies where destruction of organic matter was incomplete. Total Fe content of claystone samples is essentially the same regardless of color, except that gray claystone has significantly less total Fe than other colors; 67% of the samples have total Fe between 3 and 4%. Iron reduced in red beds was not removed in solution but resides in chlorite in green strata, and some iron reduced in gray beds resides in sulfides.

Carbonate Cementation in Coralline Algal Nodules in the Skagerrak, North Sea: Biochemical Precipitation in Undersaturated Waters

Abstract: North Sea sublittoral carbonate sediments in the Skagerrak are affected by submarine dissolution which attacks skeletal grains of aragonite, Mg-calcite, and low-Mg calcite, and coarse detrital calcite in limestone clasts. All post-mortem changes in skeletal carbonate grains are destructive and intragranular space in hollow grains and microborings remains empty of secondary carbonate after the disappearance of the cavity-forming organisms. As a consequence, the boring--filling micritization mechanism does not operate in these waters. Evidently, the sublittoral marine environment is undersaturated with calcium carbonate. Yet, in this environment, carbonate cement is forming in various kinds of openings within living coralline algal nodules (rhodoliths) consisting mostly of Lithothamnium . The internal cement is composed of aragonite needle cement, aragonite spherulitic cement, Mg-calcite fringe cement, Mg-calcite micrite, and Mg-calcite intraskeletal micro-druses, the latter exclusively in the algal skeleton proper. Cement growth is related to metabolic activities in the living carbonate-secreting red algae. After death of the algae, the rhodoliths and their cement undergo dissolution. Theoretically, precipitation may take place in several steps, involving accessory organisms in the rhodoliths, for example, unicellular green algae, or bacteria. However, all observations indicate that the primary cause for the cementation lies in the life processes of the frame-building algae. The thin films of encrusting algae are exceptionally well suited as instruments for a microenvironmental control. Few passages lead to the interior of rhodoliths, except through the sieve of algal filaments. Furthermore, the red algae may secrete as much as 30-40% of their total photosynthetic product in the form of extracellular metabolites. The data suggest that biochemical influence of frame-building algae may be an important cause for internal cementation of reefs and algal crusts in the marine environment.

Transport and Deposition of Resedimented Conglomerates: The Cap Enrage Formation, Cambro-ordovician, Gaspe, Quebec

Abstract: There is no descriptive model for resedimented conglomerates (akin to the Bouma sequence for turbidites). Hence there is no generalized basis for the interpretation of transport and depositional processes. We have therefore attempted to define facies. in superb coastal exposures of the Cap Enrage Formation. The cobble-boulder, and cobble facies are characterized by a lower zone of inverse grading, followed by massive bedding or normal grading. Clast imbrication is very well developed, long axis both parallel to flow and clipping upstream. Stratification is absent. In the fine-grained facies (coarse- and fine-pebble conglomerates, :red graule sandstones), there is no inverse grading, but stratification is dominant. In the lower parts of beds, the stratification is shown up by concentra ions of quartz granules in layers. Higher in the beds, the stratification marks erosional surfaces, and higher still, there are concentrations of granule-rich and granule-poor sandstones. The inverse-to-normally graded model (characterized by inverse grading) is interpreted in terms of deposition from suspension. The fabric strongly suggests that clasts did not roll on the bed. Clasts were maintained in suspension both by fluid turbulence, and by dispersive pressure between clasts in a highly concentrated zone near the base of the flow. The graded-stratified model (characterized by stratification) is interpreted in terms of deposit/on from suspension for the lower portion, but with bed load movement becoming increasingly important upward. Calculations suggest that granule layers 10 cm thick and 15 m long may have taken about one half to two hours to emplace. The upper granule and sand layers can alternate through several meters of section implying fluctuating flow velocities on a time scale of perhaps 30 to 120 minutes, but with continuing flow for many 1 hours. The generality of these two models can only be established by detailed observations in many other areas.

Ooids from Great Salt Lake, Utah, as an Analogue for the Genesis and Diagenesis of Ooids in Marine Limestones

Abstract: Study of 13 oolite samples from Great Salt Lake, Utah by means of X-ray analysis, staining for aragonite with Feigl's solution, and the use of scanning and petrographic microscopes show that aragonite makes up all or more than 90% of the carbonate mineralogy of the samples. This new evidence, combined with the high degree of allochem recrystallization, indicates that the aragonite in the allochems typically undergoes recrystallization to aragonite and not to calcite as interpreted by Eardley (1938) and Carozzi (1962). Failure to take note that the original grain orientation in the rim of Great Salt Lake ooids is radial (Eardley, 1938), along with confusion about the meaning of the term structure as applied to ooids, has conceivably retarded a better understanding of ooid genesis and diagenesis. Radial grain orientation is demonstrably not developed in many initially aragonitic Pleistocene marine ooids as a result of their partial to complete conversion to calcite. Many ooids in marine limestone probably formed initially with an entirely radial grain orientation in their rims under a variety of environmental conditions. This hypothesis provides an alternative to the popularly accepted hypotheses that all ooids in marine limestone were initially like those that occur today in the Bahamas and Persian Gulf, and that radial grain orientation in ooids is invariably a product of diagenesis.

A rapid and accurate method for the analysis of calcium carbonate in small samples

Abstract: A method which permits rapid and accurate analysis of calcium carbonate by weight loss-acid digestion is presented. The method permits analysis where only small weight samples are available, such as deep sea cores, but can also be used with larger samples. The technique makes use of a commercially available bacterial filter apparatus and plastic or nitrocellulose filter discs.

Sedimentation on gravel outwash fans, Malaspina Glacier Foreland, Alaska

Abstract: Three depositional facies occur on the outwash fans of Fountain Stream and Alder Stream, which presently drain portions of the Malaspina Glacier along the northeastern Gulf of Alaska. Deposition of longitudinal bars occurs during high-flow stage and consists of plane-bedded, imbricate, very poorly sorted pebble and cobble gravels. The sediment of the bars decreases in mean size downstream. Some bars terminate in avalanche slip faces of poorly sorted, silty, medium to coarse sand. Paleocurrent directions suggest that stream flow diverges across bar surfaces. Deposition in channels occurs in two morphologically distinct areas, riffles and pools. Sediment deposited in pools consists primarily of ripple cross-laminated, poorly sorted, silty, medium to coarse sand, commonly capped by draped lamination. Ripple crests vary from straight to cuspate. Sediment in riffles consists of gravel laid down under upper flow regime conditions as transverse ribs and stone cells. Both apparently are relict antidune bedforms. During late-stage flow, thin patches of horizontally bedded sand are deposited between transverse ribs and adjacent stone-cell borders. The longitudinal bar facies and channel pool facies are common in cutbank sections. The channel riffle facies was not recognized in cutbank sections. Scour pits, commonly observed on some bar surfaces, are produced by currents scouring around grounded ice blocks. Ice-block trails are produced by ice blocks dragged through soft sediment. Neither of these sedimentary structures have been reported in the literature. If found in ancient sediments, both structures would indicate a nearby source of ice.

Paleocurrent analysis of alluvial sediments; a discussion of directional variance and vector magnitude

Abstract: A tabulation of recent work on current indicators in modern rivers shows that directional variance increases with decreasing structure scale, in fairly close agreement with the structure hierarchy concept of Allen (1966). Fluvial currents are vectors, definable by direction and magnitude, but most paleocurrent studies ignore magnitude. It is proposed that azimuth readings be weighted according to the cube of current structure thickness, this being a volume measure corresponding to the distance in all three dimensions over which a local flow vector might reasonably be assumed to maintain the same direction. It is also a measure of the quantity of sediment moved by the flow vector. Examples are presented in which the proposed weighting factor is applied to data from the fluvial Isachsen Formation (Cretaceous) and deltaic Eureka Sound Formation (Cretaceous-Tertiary) of Banks Island, Arctic Canada. It is shown that the use of the weighting factor can differentiate flow patterns on the basis of sedimentary structure size, leading to interpretations of channel size, sinuosity, and other parameters of sedimentological importance. The weighting factor also provides an important check on calculations of vector mean.

Biogenic sedimentary structures produced by crabs in lagoon margin and salt marsh environments near Beaufort, North Carolina

Abstract: Intertidal burrowing crabs produce distinctive biogenic sedimentary structures in protected lagoon margin and salt marsh environments in the vicinity of Beaufort, North Carolina. Distribution of crabs is determined primarily by substrate characteristics, salinity, and vegetation cover in the intertidal zone. Protected sand beaches and flats are heavily burrowed by the sand fiddler, Uca pugilator, which produces gently curved J-shaped and sharply curved L-shaped burrows 1 to 2 cm in diameter. Larger Y- and U-shaped burrows are formed by the ghost crab, Ocypode quadrata, in the backshore and foredune ridge zones above the high tide line. Salt marshes influenced by normal to near normal marine salinities are burrowed by U. pugilator, U. pugnax, and Sesarma reticulatum. U. pugnax and S. reticulatum burrow in muddy substrates, particularly along tidal creek banks, whereas U. pugilator is restricted to sandy substrate areas. U. pugnax burrows are 1 to 2 cm in diameter and have complex twisting and turning forms. The larger communal burrows of S. reticulatum consist of a complex of interconnected shafts with several oval-shaped surface openings. Panopeus herbstii is often found inhabiting these burrows. In salt marshes where the influence of fresh water is great, U. minax and U. pugnax are the dominant crabs. U. minax forms burrows 2 to 5 cm in diameter with ho ded entrances and long vertical shafts. Intertidal crabs are important agents of bioturbation in the environments that they inhabit. Their burrows have excellent potential for preservation in the sedimentary record. Recognition of the record of biogenic sedimentary structures produced by intertidal crabs would be of significant value in identifying paleoenvironments, fixing ancient lagoon margin and estuarine shorelines, and determining paleosalinities.

Aragonitic and high-Mg calcite caliche from the Persian Gulf-a modern analog for the Permian of Texas and New Mexico

Abstract: Laminated, lacy, and pisolitic carbonate crusts have been found on and within some outcrops of Pleistocene limestones which rise above the modern sabkha surface in Abu Dhabi. These sediments are similar in many ways to typical caliche but have unusual mineralogies and cement textures. Internally, the pisolites are uniformly aragonitic and are cemented together by fibrous aragonite and micritic and fibrous high-Mg calcite in isopachous, meniscus, and microstalactitic textures. It is suggested that caliche crusts with unstable carbonate mineralogies, together with marine or brine associated isotopic ratios and trace element concentrations, yet showing dominantly vadose textures, can develop in marine influenced areas where vadose and phreatic pore waters have unusually elevated salinities and high concentrations of Sr and Mg. Thus, in strongly evaporitic, low relief settings such as the Persian Gulf sabkhas, supratidal vadose areas are characterized, not by "fresh water," but by hypersaline brines which allow the precipitation of cements of marine chemical character with vadose textures. Similar examples are known from Bonaire, another evaporite locality. Thus, very clear distinctions should be drawn between the terms "vadose" and "fresh water"; ancient vadose textures do not immediately imply fresh-water pore fluids. One can have "hypersaline vadose" as well as "meteoric vadose" conditions. The Abu Dhabi pisolitic crusts with their "unusual" mineralogies (including some dolomite) appear to be quite analogous to the pisolitic facies of the Permian reef complex of Texas and New Mexico and may provide a model for their formation and diagenesis. The initial presence of aragonite and high-Mg calcite in the pisolitic and microstalactic crusts of the Permian back-reef sediments would be more compatible with the observed penecontemporaneous dolomitization than would a stable low Mg calcite caliche.

Palaeokarstic Surfaces in Upper Visean (Carboniferous) Limestones of the Derbyshire Block, England

Abstract: Undulating, hummocky and often deeply pitied discontinuities, commonly up to 1 m in amplitude, occur throughout D1 and D2 subzones in the Carboniferous Limestone sequence of the Derbyshire Block, England. Previously termed mammillated surfaces and potholed surfaces, these horizons usually underlie a thick K-bentonite clay and are shown to be Carboniferous erosion features that formed by subaerial solution beneath a cover of soil derived from volcanic ash. These palaeokarstic surfaces present evidence for contemporaneous lithification of the limestones, and brown laminated coatings are interpreted as caliche-type crusts similar to some modern examples. Surfaces of average development may each represent subaerial exposure of between 30,000 and 100,000 years, and certain karstic morpholo ies may reflect the influence of a near-surface water table.

Bedforms and Structures Associated with Bar Topography in the Shallow-Water Wave Environment, Kouchibouguac Bay, New Brunswick, Canada

Abstract: Bedforms and structures identified on a micro-tidal, low to moderate energy barred coastline, Kouchibouguac Bay, New Brunswick, are controlled by wave-energy, water depth and the resulting wave form and correlate closely with the sequence documented for a high energy, non-barred nearshore by Clifton et al. (1971). The major differences are: (1) steep (up to 25°), landward-dipping, medium-scale cross-bedding produced by migration of the avalanche slope of the bar front; (2) seaward dipping, small and medium-scale cross-bedding produced by megaripples in seaward flowing rip-currents which dissect the bar; (3) repetition of the sequence of bedforms and structures as waves shoal, break on the bar and reform in the trough to finally break at the beach face.

Replacement of fossils by length-slow chalcedony and associated dolomitization

Abstract: The Middle Permian Getaway Limestone Member of the Cherry Canyon Formation was deposited as carbonate turbidites in the Delaware Basin. Invertebrate skeletal components and ooids exhibit selective replacement by length-slow chalcedony and megaquartz within the Getaway. Where silica has partially replaced skeletal components originally consisting of high magnesium calcite (fusulinids, bryozoans and echinoderm fragments) dolomite rhombs occur only within areas of silicification. This suggests that silica replacement occurred before high magnesium calcite stabilized to low magnesium calcite and that magnesium for dolomitization was locally derived. In silica-replaced areas of monocrystalline echinoderm components all dolomite rhombs are aligned in linear patterns and exhibit single cryst l extinction simultaneous with that of the unreplaced calcite monocrystal; thus orientation of dolomite crystals was controlled by that of the monocrystal. It is inferred that dolomite exsolved from high magnesium calcite at centers of magnesium enrichment during ionic mobilization that accompanied solution-precipitation replacement of magnesium calcite by silica. Silica nuclei then expanded, coalesced, engulfed and partially to completely replaced dolomite rhombs. In formerly aragonitic shells, original structure and texture are commonly pseudomorphed by silica replacement; in portions of shells not replaced by silica original texture and structure have been obliterated when aragonite later inverted to calcite forming a coarse anhedral mosiac. Thus, silicification also preceded mineralogic stabilization of aragonite. Where megaquartz partially replaced skeletal material it exhibits highly undulose extinction like that of most calcite which has inverted from aragonite. Silicification is inferred to have occurred during relatively shallow burial when the carbonate was partially lithified. Silica nucleation seems to have been controlled by diagenesis of organic matter in skeletal components and ooids.

Vertical variation in strength and porosity of calcrete (nari) on chalk, Shefela, Israel and interpretation of its origin

Abstract: Observations and measurements on hardpan calcrete (nari) from chalk indicate that three distinct horizons can be distinguished in the nari profile, (a) a very hard thin laminar crust (500-800 kg/cm2), capping (b) a thick strongly indurated upper nari (100-500 kg/cm2) and (c) a thick soft moderately indurated lower nari (20-120 kg/cm2). Moisture content affects greatly the hardness value. If dry, the underlying chalk is harder than the lower nari. The Schmidt Test Hammer Type N was used for non-destructive measurements of relative hardness (strength) in the field. Calibration against unconfined compression strength on rock cubes in laboratory tests produced a highly significant relationship. Porosity has been shown to be the major controlling factor in determining hardness, while CaCO3 content and its microtexture are contributing factors. This indicates that induration of the nari is due to infilling of CaCO3, partial recrystallization in fissures and cracks, and some displacement of non-carbonate material. It is suggested that the process of infilling and crystal growth of carbonate in the host sediment, with some domains of recrystallization, is characteristic of calcrete formation in consolidated, but porous sediments. In unconsolidated sediments the deposition of carbonates can be accompanied by volume expansion and an increased separation of the non-carbonate grains.

Origin and significance of grapestone, Bahama Islands

Abstract: Bahaman grapestone is produced by cementation of recrystallized ooids in the marine environment. Initial binding of grains is accomplished by growth of algae and encrusting forams in the substrate. Later cementation and infilling of the composite may be due to continued growth of these organisms in the interior of the aggregate and to chemical or biochemical precipitation of cement. Data indicate the recrystallized ooids making up grapestone were formed at an earlier stage in sedimentation as the sea transgressed onto the Bahaman Platform at the end of the last glacial lowstand. Thus, grapestone is produced through a sequence of depositional conditions rather than a single set of conditions. Given a supply of firm grains, uneven water turbulence, high water circulation rates, and very low sedimentation rates favor the formation of grapestone.

Petrography and Paleoecology of Holocene Peats from the Okefenokee Swamp-Marsh Complex of Georgia

Abstract: The Okefenokee Swamp, a paludal region of over 400,000 acres is developed upon a terrace which was formerly a Pleistocene marine lagoon separated from the open ocean by a long barrier island (now Trail Ridge). Peat has developed in this region to a maximum depth in some places of approximately 15 ft. Nine peat cores taken within the Swamp with a piston-coring device were selected for detailed paleobotanical and petrographic analyses. The lack of any salt tolerant plants in the peat at the contact with the sandy substrate suggests that some considerable length of time occurred after withdrawal of the sea and before production of the first peats. Also the fact that the first-formed peat in all cases was a highly oxidized deposit containing residual siliceous material indicates that some thickness of previously deposited peat may have been destroyed before the onset of modern peat deposition. Paleobotanical analyses of cores from the three largest marshes (Chase Prairie, Grand Prairie, and Floyd's Prairie) reveal that these areas have not changed greatly in vegetational and depositional environment from the beginning of peat formation in the Okefenokee. In addition, the present positions of these prairies seems to be controlled by the positions of pre-peat topographic lows. Their vegetational continuity to the present day is probably related to: (1) a continuous, uniform rise in water table; (2) the common occurrence of fires in the Okefenokee; and (3) the consistently greater depths of peat in these regions. In this region, fires were found to have played an important part in the history of peat development. These burns were recorded in the cores as zones of abundant charcoal and residual, siliceous, fresh water sponge spicules.

A settling tube system for sand-size analysis

Abstract: An accurate and precise settling tube is described having an inside diameter of 12 cm and a settling distance of 140 cm or more. It has three options for data handling: (1) recording data on a strip chart recorder; (2) recording data on punched paper tape; and (3) utilizing a dedicated mini-computer to handle the data.