bluuebunny/crossref_metadata_2025 huggingface dataset:
Depositional environments of Upper Miocene (Messinian) evaporite deposits of the Sicilian Basin*
Authors: SCHREIBER B. CHARLOTTE , FRIEDMAN GERALD M. , DECIMA ARVEDO , SCHREIBER EDWARD
Published: June 2006

In Sicily, Messinian evaporitic sedimentary deposits are developed under a wide variety of hypersaline conditions and in environments ranging from continental margin (subaerial), to basin‐margin supratidal, to intertidal, to subtidal and out into the hypersaline basin proper. The actual water depth at the time of deposition is indeterminate; however, relative terms such as ‘wave base’ and ‘photic zone’ are utilized. The inter‐fingering relationships of specific evaporitic facies having clear and recognizable physical characteristics are presented. These include sub‐aerial deposits of nodular calcium sulphate formed displacively within clastic sediments; gypsiferous rudites, arenites and arenitic marls, all of which are reworked sediments and are mixed in varying degrees with other clastic materials (subaerial, supratidal, and intertidal to deep basinal deposits). Laminated calcium sulphate alternating with very thin carbonate interlaminae and having two different aspects; one being even and continuous and the other of a wavy, irregular appearance (subtidal, intertidal, and supratidal deposits). Nodular calcium sulphate beds, usually associated with wavy, irregular laminated beds (supratidal, sabkha deposits); very coarsely crystalline gypsum beds (selenite), associated with more even, laminated beds (subaqueous, intertidal to subtidal deposits); wavy anastomozing gypsum beds, composed of very fine, often broken crystals (subaqueous, current‐swept deposits); halite having hopper and chevron structures (supratidal to intertidal); and halite, potash salts, etc. having continuous laminated structure (subaqueous, possibly basinal).

Evidence for diagenetic changes is observed in the calcium sulphate deposits which apparently formed by tectonic stress and also by migrating hypersaline waters. These observations suggest that the common, massive form of alabastrine gypsum (or anhydrite, in the subsurface) may not always be ascribed to original depositional features, to syndiagenesis or to early diagenesis but may be the result of late diagenesis.
Energy relations of density‐current flows: an experimental investigation*
Authors: KERSEY DAVID G. , HSÜ KENNETH J.
Published: June 2006

Three series of density‐current experiments were performed in a 5.76 m flume. In the first series, the flume was horizontal, and in the second and third, it was inclined with a positive slope and negative slope, respectively. Energy relations during successive stages of density‐current movement were computed from observed data, which showed an appreciable frictional energy dissipation. The computed friction factors of our experimental density‐flows were compared to the friction factors for pipe flows (Moody diagram), and while the calculated friction factor increases with increasing Reynold's number within the range of our experiments (Re 2 × 103−2 × 104), it is concluded that with increasing Reynold's number above about 5 × 104 the friction factor decreases. For natural turbidity currents, the Moody diagram gives a reasonable estimate of the friction factor between the current and sediment bed. The value of the friction factor for the interface between the current and overlying water was found to be about 0.2 times the friction factor for the current and flume. However, due to errors inherent in measuring the depth of the current, a value of 0.4 would be more reasonable for density‐currents in our range of Reynold's number. Friction tends to decrease the value of the dimensionless coefficient in Keulegan's law of saline front and to decrease the thickness of the flow. In contrast, the presence of a slope in the direction of flow tends to compensate the effect of friction. The angle θc that provides the potential energy to exactly offset the energy losses incurred during movement by the density‐currents in our experiments has a calculated value of 31′. An empirical formula φ= 0.935θ—0·57 relating friction, in terms of the hydraulic gradient φ, to the slope angle θ was obtained. Since the thickness of the current can be computed from the relationship between φ and θ, we estimated the thickness of naturally occurring density‐currents in Swiss lakes. The results suggest the applicability of our experimental results to small turbidity currents in nature. Our analysis further indicates that large turbidity currents have a small φ and can be expected to flow very long distances on a flat abyssal plain.
An automated rapid sediment analyser (ARSA)
Authors: NELSEN TERRY A.
Published: June 2006

The automated rapid sediment analyser (ARSA) is a pressure‐transducer grain‐size analysis system. This basic Woods Hole‐type fall tube was automated by the addition of a digital voltmeter, Hewlett‐Packard 9810A calculator, and an x‐y plotter. Eight min after sample introduction, the system automatically produces size distribution data in 0·25‐φ intervals, distribution statistics, and a plotted frequency histogram.
The origin of horizontal laminae in ephemeral stream channel‐fill
Authors: FROSTICK LYNNE E. , REID IAN
Published: June 2006

The number of paired horizontal laminae in an ephemeral channel‐fill increases systematically down‐catchment from stream headwaters, and is shown to be functionally related to tributary confluence. Under moving storm conditions, each tributary sub‐catchment delivers a sediment‐laden discharge pulse that finds sedimentary expression in paired laminae of light and heavy minerals. Lamina differentiation is attributed to a congregational sorting mechanism. The direct relationship (r, = 0.903) between paired laminae and confluencing tributaries permits palaeoenvironmental inference of drainage‐net character, and palaeoclimatic variability.
The plan shape of current ripples in relation to flow conditions
Authors: ALLEN J. R. L.
Published: June 2006

Two sets of empirical data on the relationship of the plan shape of current ripples to flow conditions previously thought incompatible are reconciled by taking explicit and necessary account of the effects of flow width relative to flow depth. The aspects of plan shape considered are the mean streamwise ripple wavelength and the wavelength, measured across the flow, of longitudinal features due to centrifugal instability formed on the backs of the ripples. A regression analysis connecting these attributes with flow conditions including the width‐effect shows that the amount of unexplained variability is of the same order as the accumulated experimental errors, and suggests that the two sets of data are in excellent agreement. The relationship of the plan geometry to flow conditions is less simple than has sometimes been supposed, but ways are suggested in which the experimental results can be used to establish the hydraulics of past environments.
Structural state of detrital alkali feldspars
Authors: SUTTNER LEE J. , BASU ABHIJIT
Published: June 2006

The structural state of an alkali feldspar is determined by the nature of the distribution of Al and Si in the tetrahedral sites of the feldspar structure. This state is a function of a number of genetic controls including temperature, cooling rate, deformation, crystal size, and several chemical factors. Together these controls constitute a genetic regime. Identification of the structures of detrital feldspars may enable recognition of genetic regimes and be useful in provenance interpretation and mineral province definition.

A 2‐peak method of X‐ray diffraction (XRD) determination as a variant of an earlier proposed 3‐peak method of structural state determination is used in this study. Total analytical time for each determination is about 50–55 min per grain. Results of structural state identification of 126 detrital feldspars from Holocene stream sands derived from volcanic, plutonic and metamorphic rocks are presented in order to illustrate the application and potential of the technique. Detrital feldspars from the metamorphic rocks are all maximum microcline; those from the plutons range from orthoclase to microcline depending on the age of the pluton, whereas those from the volcanic rocks are sanidines.
Flow regimes in debris flow
Authors: ENOS PAUL
Published: June 2006

The Bingham model has been suggested for the rheological behaviour of debris flow. Velocity plotted against yield strength uniquely defines turbulent and laminar flow in published experimental data on flow of Bingham materials. These regimes are not defined by a unique value of the Bingham number even though it includes both of the critical parameters, yield strength and velocity. Laminar flow in at least the final stages natural debris flows is suggested by sediments with parallel‐to‐flow or inclined clast fabric, preservation of delicate clasts, blocks projecting from the top of the deposit(?), and an absence of flutes.
The origin of bounding surfaces in ancient aeolian sandstones
Authors: BROOKFIELD M. E.
Published: June 2006

Three orders of aeolian bounding surface are arranged in a hierarchy based on their extent and regularity.

First order surfaces are the most extensive. They are flat‐lying bedding planes cutting across all other aeolian structures and are attributed to the passage of the largest aeolian bedforms—draas—across an area. First order surfaces cut across second order surfaces, which are gentle to moderately dipping surfaces bounding sets of cross‐strata. Second order surfaces are attributed to the passage of dunes across draas, or to longitudinal dunes migrating across the lower ice slopes of draas. Third order surfaces bound bundles of laminae within coscts of cross laminae and are due either to local fluctuations in wind direction and velocity or to changes in airflow patterns caused by configurational changes in dune patterns. All these bounding surfaces could be explained by wind variations and dune migration, but the rates of dune migration relative to probable sediment deposition rates are incompatible with this general explanation of the form and spacing of the bounding surfaces. The concept of climbing bedforms of different hierarchical order together with subsidence provides a better explanation. Analogous bounding surfaces in aqueous bedforms have already been attributed to climbing bedforms of differing hierarchical order.
Origin of subaerial Holocene calcareous crusts: role of algae, fungi and sparmicritisation
Authors: KAHLE CHARLES F.
Published: June 2006

The Pleistocene Miami Limestone that crops out on the lower Florida Keys is overlain by thin (16 cm or less), discontinuous, Holocene calcareous crusts (caliche) that are usually laminated, composed dominantly of calcite micrite and may or may not incorporate part of the underlying limestone. Both allochems and sparry calcite cement in the former unit contain endolithic algae and fungi, borings and unicellular algae. Biogenic structures identical to those in the Miami Limestone also occur in the calcareous crusts but are somewhat less abundant in the latter unit versus the former unit.

The calcareous crusts were formed in the vadose diagenetic environment. Some of the CaCO3 necessary for the micrite that comprises the bulk of the crusts was probably derived from solution of carbonate from a soil cover and some from wind blown salt spray. Most of the micrite, however, was formed by replacement of the uppermost portions of the Miami Limestone. Replacement involved micritisation of allochems and a previously unreported process, sparmicritisation, the degrading recrystallization of sparry calcite to micrite. Minor sparmicritisation was caused by micrite calcification of endolithic fungi or algae within sparry calcite cement or by micrite precipitation in empty borings within such cement. Most sparmicritisation took place by dissolution of sparry calcite and concomitant precipitation of micrite in the space occupied previously by the dissolved spar. Such sparmicritisation is interpreted to be caused by chemical reactions involving the crystals, pore water which is moving slowly but steadily and organic compounds released during bacterial decomposition of fungi, algae or both.

It is recognized that sparmicritisation occurs in the marine diagenetic environment and is not, therefore, necessarily indicative of vadose diagenesis. Incomplete sparmicritisation is responsible for some of the clotted textures typically found within calcareous crusts and may explain such textures in many other carbonate rock types. A combination of sparmicritisation and micritisation has probably greatly influenced the porosity of many reefs and, in some cases, led to the formation of ‘micritic reefs’.
Nature, origin and classification of peritidal tepee structures and related breccias
Authors: ASSERETO RICCARDO L. A. M. , ST C. KENDALL CHRISTOPHER G.
Published: June 2006

Distinctive peritidal tepee antiform structures, buckled margins of saucer‐like megapolygons are common in marine vadose fenestral and pisolitic limestones and/or dolomites of carbonate platform sequences and occur in intertidal and supratidal carbonates ranging in age from Silurian to Holocene. These megapolygons commonly form and are sometimes truncated before the deposition of the next sedimentary layer. The megapolygons result from the expansion of surface sediments by as much as 15%. The expansion is caused by the following continuously repeated sequence of processes: (1) Desiccation and thermal contraction causing small fractures; (2) phases of wetting causing enlargement of fractures; (3) phases of crystallization of calcium carbonate and other minerals causing the enlargement, fill and cementation of the fractures. Precipitation is from brines and meteoric waters; (4) hydration of minerals, thermal expansion, breaking waves and faulting may add to this disruption.

The development of the tepee fabric can be traced from an initially cemented subaerial fenestral crust, exhibiting expansion and compressional structures, to a completely disrupted and brecciated sediment riddled by a labyrinth of fractures and solution cavities. These spaces are filled by numerous phases of internal marine and fresh‐water cement and sediment, the latter containing penecontemporaneous or younger marine faunas.

Peritidal tepees are useful tools for geologic reconstruction and provide evidence of subaerial exposure; a tropical to subtropical climate; and back‐beach or back‐barrier deposition. Proper identification of tepees is of economic importance, because they provide good early porosity and permeability for petroleum entrapment and a site for mineralization. Aesthetically, tepee rocks are a fine kaleidoscopic decorative stone.
Algal crusts, autochthonous and clastic gypsum in a cannibalistic evaporite basin: a case history from the Messinian of Northern Apennines
Authors: VAI GIAN BATTISTA , LUCCHI FRANCO RICCI
Published: June 2006

The Messinian Vena del Gesso Basin in the Northern Apennines is filled by very thick (up to 35 m) beds of coarse crystalline gypsum (selenite) associated with thinner carbonate and shaly (euxinic) intercalations. The conventional Usiglio model of salt fractionation does not apply to this evaporitic sequence for the following reasons: carbonate which underlies gypsum is not evaporitic but algal in origin; most gypsum did not precipitate from surface brines but at and below a sediment‐water interface occupied by algal mats; a significant portion (10–80%) of gypsum beds is composed of redeposited selenite which was removed from the margins and transported toward the centre of the basin by slope‐controlled currents and gravity flows (debris flows).

We call this process cannibalistic because of its intraformational character (connected with evaporative fall of water level) and volumetric importance.

A recurrent vertical pattern of six main facies (euxinic to gypsum fanglo‐merates) is interpreted as a bathymetric, regressive cycle controlled by both sedi‐mentological and tectonic‐eustatic factors. The inferred environmental setting is a residual turbidite trough (Marnoso‐arenacea) evolving abruptly toward lagoonal conditions and filled up to sea level by evaporitic and mechanical (mostly fluvial) processes. Repeated inundations of restricted‐marine water started the depositional cycle thirteen or fourteen times.
Gravelly flood deposits of Irvine Creek, Ontario, Canada
Authors: MARTINI I. P.
Published: June 2006

In May 1974 a powerful flood flushed the Grand River basin, Ontario. The effects on the bedload were drastic in a narrow (30 m) and deep (40 m) rock walled and floored gorge near Elora, Ontario. Along Irvine Creek, the tributary occupying the gorge, the gravel cover was reworked in several types of bars, predominantly transverse and point bars. The bars formed very rapidly in response to essentially steady, non‐uniform flow that developed during a brief period of high flood.

Superimposed on major bars are several minor sedimentary features such as coarse transverse ribs, chute channels and bars, longitudinal ribs, imbrication clusters, backsets with well developed imbrication, that were formed under very high stream discharge. Structures like imbrication clusters, transverse ribs and small riffle bars require a ‘live bed’ situation to form, and they develop when stones come to a stop either because they cluster during transport, or because keystone effects occur along shallow channels. In Irvine Creek, very few sedimentary features were formed during waning and low flood stages: only some shadow deposits and a few Ostler lenses. The few fines that were available were lost downstream or filled in lower parts of gravel beds.

This study confirms that in streams that experience strong seasonal fluctuations in discharge, bedforms that develop during high floods have a high probability of preservation. In gravelly deposits, foreset structures and plane beds are most commonly preserved, although they may be difficult to recognize in old deposits, which may appear massive, particularly if the gravel has been infilled with finer pebbles and sand. In the case of Irvine Creek, all deposits are organized, and lateral and vertical variations in textures, particularly imbrication and packing, are very useful in the recognition of sedimentary structures.
Range, velocity, and material flux of compaction flow in growing sedimentary sequences
Authors: EINSELE GERHARD
Published: June 2006

In growing sedimentary sequences range and velocity of vertically ascending pore water (advection) can be determined quantitatively by using a simple graphical method. During continuing deposition and maintenance of compaction equilibrium, the pore water from deeper layers cannot reach the sediment‐water interface. Range and velocity of advective flow decrease from top to bottom of a sequence and depend on the thickness affected by compaction. The velocity of pore water flow as well as the transport of solutes by advective flux are proportional to the sedimentation rate. Compaction flow can also result from under‐con‐solidation and diagenetic reduction of pore space. At the sediment‐water interface, a direct contact between the ascending pore water and the overlying water body is feasible only under special conditions (interrupted sedimentation over under‐consolidated material, erosion). These results are demonstrated on geometric models of sedimentary sequences, on experimentally deposited clay, and on selected examples of the Deep Sea Drilling Project.
Significance of CM diagrams of sediments deposited by suspensions
Authors: PASSEGA R.
Published: June 2006

CM grain‐size diagrams of clastic deposits were introduced in 1957. Since that time a large number of diagrams representing most environments were constructed. Discussions of parameters C and M by the writer and by others showed that these parameters are indicators of hydraulic conditions under which sediments were deposited. In a 1975 paper Vandenberghe discussed the CM diagram of the Boom Clay, Belgium, and excluded that CM diagrams gave indications on the mechanism of sediment deposition.

In order to answer Vandenberghe's assertions, the writer systematically compared the numerous CM diagrams representing suspension deposits with the characteristics of the environments. Result of this comparison is that CM diagrams give coherent indications on deposition which agree with the known environment characteristics. The Boom Clay diagram is no exception.

Another result of the present study is that, in environments as a whole, two types of currents can be recognized, distinguished by the relation between bottom turbulence and grain‐size of sediment in suspension. Deposits of these two current types and distribution of bottom turbulence are indicated by CM diagrams. Characteristics of these currents vary with the environment and help the reconstruction of ancient environments.

Information given by CM diagrams has limits and should be integrated with other evidence on the environment. However, contrarily to the information given by ecology or sedimentary structures, the evidence provided by grain‐size has the advantage of being universal, as it can be obtained in all clastic deposits. This evidence therefore is a means for comparing and classifying clastic deposits as a whole.
Quaternary sediments in the northwestern part of the Barents Sea and their relation to the underlying Mesozoic bedrock
Authors: BJØRLYKKE K. , BUE B. , ELVERHØT A.
Published: June 2006

A sedimentological study of Quaternary sediments from the northwestern part of the Barents Sea shows that their composition is controlled by the underlying Mesozoic bedrock and that very little sediment has been supplied from outside sources.

The Quaternary sediments consist of Pleistocene glacial clays (moraines) and Holocene gravel, sand and mud, derived by erosion of the clay‐rich moraines, which again have been derived from underlying Mesozoic rocks.

On the shallow Spitsbergen Bank (30‐100 m depth) we find a high energy facies of bioclastic carbonate sand and gravel and lag deposits of Mesozoic rock fragments from the underlying moraine. 14C‐datings of the bioclastic carbonates (Molluscs and Barnacles) suggest that soft bottom conditions with Mya truncata prevailed in early Holocene time, succeeded by a hard bottom high energy environment with Barnacles in the last 2000‐3000 years. This may be due to a southward movement of the oceanic polar front into the Spitsbergen Bank due to colder climate in Late Holocene (subatlantic) time, which at present day produces strong bottom currents down to 100 m depth.

On the Spitsbergen Bank carbonate sedimentation has succeeded glacial sedimentation as a result of withdrawal of clastic sediment supply in Holocene time and high organic productivity because of upwelling. A similar mechanism may have been operating during earlier glaciations, i.e. in Late Precambrian time to produce an association of glacial and carbonate sediments although the biological precipitation was different at that time. In Late Precambrian time precipitation or carbonate by algaes may have occurred in colder water on the shelves due to higher saturation of carbonate in the sea water.
Concretions, isotopes, and the diagenetic history of the Oxford Clay (Jurassic) of central England
Authors: HUDSON J. D.
Published: June 2006

In interpreting the results of a petrographic and isotopic study of concretions, a range of subjects is discussed including the original texture of the Oxford Clay sediment, Jurassic palaeotemperatures, the diagenetic history of pore‐waters and the palaeo‐hydrology of central England.

The concretions are all composed predominantly of calcite. They include precompactional, pyrite‐rich concretions that later suffered an eposide of brecciation, and others that only commenced to form after compaction had crushed ammonite shells included in the bituminous clay sediment. Petrographic, chemical, and especially carbon isotope data demonstrate a dominantly organic source for the carbon in the early formed concretions. Oxygen isotopes indicate formation at the same temperatures (13‐16°C) at which benthic molluscs were living. Concretion growth in pelleted, anaerobic mud proceeded concurrently with bacterial sulphate reduction and pyrite precipitation. Cracking of the concretions started at this stage: in a few concretions, the cracks were also partially filled with brown calcite.

During post‐compactional growth, δ13C increased and pyrite content decreased, showing waning organic influence; δ18O decreased. The brecciated concretions were intruded by clay in which baryte crystals grew; finally, most remaining voids were filled with strongly‐ferroan calcite of δ18O about—7 PDB and δ13C about O PDB. This must indicate strong depletion of the pore waters in 18O. Mechanisms that might lead to this are reviewed. It is concluded that the sequence of mineralogical and chemical changes is most readily explained if originally marine porewaters, first modified by bacterial activity, were flushed from the compacting clays by water of ultimately meteoric origin. This had its source in palaeo‐aquifers beneath the Oxford Clay. Speculative attempts are made to relate this history to the geology of the region.
Depositional and diagenetic history of Pleistocene and Holocene oolitic sediments and sabkhas in Kuwait, Persian Gulf
Authors: PICHA FRANTISEK
Published: June 2006

The Pleistocene and Holocene oolitic sediments of southern Kuwait appear as parallel ridges—ancient and recent barrier beaches and coastal dunes. They are underlain by Tertiary clastic deposits and interfingered with sabkhas. Considering the superposition, primary composition and diagenetic alterations five lithostratigraphic units were distinguished within the oolitic complex. Their formation and preservation were related principally to eustatic oscillations of sea level during Quaternary times, although post‐Pliocene tectonics also played an important role both in physiographic and sedimentary developments of the region.

The large accumulation of oolites in southern Kuwait might be related, at least partly, to the existence of two tidal channels, khors, discharging waters over‐saturated with respect to calcium carbonate over broad tidal flats. The formation of oolites culminated in late Pleistocene probably due to an extreme aridity of the climate.

The subaerial diagenesis of the oolitic ridges led to gradual cementation and recrystallization of oolites. The four stages of progressive diagenesis distinguished in the area are compared with schemes suggested by Land, Mackenzie & Gould (1967) for Quaternary sediments in Bermuda and by Gavish & Friedman (1969) for aeolianite ridges in Israel.

On the sea floor the cementation has produced widespread beach rocks and hard layers. Aragonite encrustations and dripstones occur within the intertidal zone of channels and lagoons. The shallow, occasionally flooded depresssions in sabkhas are occupied by algal mats.

The amount of dolomite in sabkha sediments is rather low in comparison with similar environments of the southern Persian Gulf. This might be at least partly explained by the absence of dolomitic rocks in the Tertiary substratum in Kuwait.

The complex of Quaternary sediments in southern Kuwait represents a petroleum setting in which the organic rich source sediments of lagoons and sabkhas are closely associated with highly porous reservoir rocks of the oolitic coastal ridges.
The survivability of labile light‐mineral grains in fluvial, aeolian and littoral marine environments: the Permian Cutler and Cedar Mesa Formations, Moab, Utah
Authors: MACK GREG H.
Published: June 2006

A greater than 50% decrease in the percentage of labile light minerals occurs between the non‐marine Cutler Formation (Permian) and its facies equivalent, the marine Cedar Mesa Sandstone, in the vicinity of Moab, Utah. Both the Cutler and Cedar Mesa Formations were derived from crystalline rocks of the Uncom‐pahgre Mountains under semi‐arid to arid climatic conditions. Furthermore, diagenesis had little effect on the light‐mineral fraction. Therefore, the significant change in mineralogy where the fluvial Cutler beds grade into the littoral marine Cedar Mesa deposits can be directly related to active reworking of detritus in the shallow marine environment.

Less coarse perthitic alkali feldspar, twinned plagioclase, undulose mono‐crystalline quartz, and polycrystalline quartz in the Cedar Mesa with respect to the Cutler suggest that compositional maturation was produced by breakage of mechanically weaker grains during grain‐to‐grain collisions. Furthermore, the end products of compositional modification by shallow marine processes may be sufficiently different from those produced by weathering and dilution from sedimentary source rocks to allow evaluation of the relative effects of these processes on the origin of compositionally mature ancient sandstones.
The formation of ooids
Authors: DAVIES PETER J. , BUBELA B. , FERGUSON JAMES
Published: June 2006

Field and laboratory studies suggest that different types of ooids form during quiet and agitated water conditions. Both types have been synthesized in the laboratory. Quiet water types exhibit a radial orientation of carbonate crystals, whereas in those formed in agitated conditions, a tangential orientation is prevalent.

Successful laboratory formation of quiet water ooids was accomplished in supersaturated seawater solutions containing humic acids. Negative results were obtained from strictly inorganic solutions, and from those containing simple amino acids, single proteins, mixtures of proteins or mucopolysaccharides, soil and sediment extracts. Partly successful results were obtained using an organic extract from Bahamian ooids. The organic parameters most important in quiet water ooid formation are molecular weight, the presence of carboxyl groups and an ability to participate in hydrophobic/hydrophilic interactions, all of which are critical to membrane formation. Membranes form concentric shells which act as growth surfaces for carbonate and also induce the periodicity in carbonate precipitation.

Ooids exhibiting a tangential orientation of batten‐like crystals have been synthesized under conditions of agitation, supersaturation and without the intervention of organic processes during the precipitation. Complete growth may be divided into agitation, resting and sleeping stages In the agitation stage, quartz nuclei induce an inorganic, heterogeneous nucleation from a supersaturated solution, which finally ceases as a result of Mg2+ and possibly H+ poisoning of the carbonate surfaces. No further precipitation occurs until the crystal surfaces are reactivated by removal of Mg2+ and H+ during the resting stage. Following a series of agitation and resting stages, precipitation is inhibited by a degree of poisoning which is not totally removed during the resting stage. For further growth, a new substrate is required and is provided by the development of organic membranes around the grains. This occurs when the grains are buried in the subsurface, the period of organic growth constituting the sleeping stage. Only 2% of an ooid's life is spent growing in the agitated environment, while 95% of its life is spent accreting organic membranes in the subsurface.

Our experimental work indicates that ooids of Bahamian type are inorganic precipitates. The tangential arrangement of battens is the result of suspension in an environment where the degree of turbulence is sufficient to induce grain to grain contact of sufficient strength and frequency to inhibit any crystal growth other than tangential. The role of organics is to provide a substrate for further growth after precipitation has slowed to a point when no further accretion is occurring.
Ferroan calcite replacement indicates former magnesian calcite skeletons
Authors: RICHTER DETLEV K. , FÜCHTBAUER HANS
Published: June 2006

The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high‐magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low‐magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods.

Using this criterion, an original low‐magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high‐magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high‐magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty‐seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes.

Ferroan calcites contain up to 6 mol % FeCO3 and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0‐03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite.

Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate‐reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric‐phreatic origin of many ferroan calcite occurrences.
Mesozoic carbonate rudites, megabreccias and associated deposits from central Greece
Authors: JOHNS D. RHODRI
Published: June 2006

In the Kokkinovrakhos Formation of central Greece, poorly sorted, grain supported, lithoclastic carbonate rudites, megabreccias and olistoliths are associated with accumulations of fine‐grained laminated carbonates. The laminae frequently exhibit grading and are interpreted as turbidity current deposits. The coarser grained rudites and megabreccias were also redeposited but were probably transported downslope by sliding and rolling, i.e. they are rock‐fall deposits. Olistoliths were emplaced into deeper water by sliding. The fine‐grained sediments of the laminated facies appear to have accumulated in pockets on an uneven sea floor during episodes of minimal rock fall activity. The sedimentary association is similar to that described from some modern fore‐reef environments and is possibly characteristic of accumulations of sediment on, or near to, steep slopes adjacent to carbonate platforms, when episodes of tectonic activity lead to the shearing off and transport downslope of masses of well‐cemented shallow marine carbonates.
Address of the retiring President of the International Association of Sedimentologists: Differences in size distributions of populations of particles among sands of various origins
Authors: FRIEDMAN GERALD M.
Published: June 2006

This address reminisces and reflects on a subject in which I have been involved for more than twenty years: the effect of depositional processes on various parameters of the size‐frequency distribution of sands. Movements of air and water generally separate particles by their sizes; the distribution of sizes relates to (1) the availability of different sizes of particles in a parent material, (2) processes operating where the sediments are deposited, particularly the competency of flow, and (3) concentrations of particles in suspension. Differences in size‐frequency distributions among sands correlate with various origins, that is with their terminal depositional environments. Reminiscences in this address express my personal experience, both its up and downs, in advocating the study of this relationship, particularly in applying the method of moments on the basis of the concept that one statistical function expresses the characteristics of the population of particles.

This address re‐emphasizes some points made in previous studies (Friedman, 1961, 1962b, 1967) on the interrelationship between textural parameters and depositional processes for beach, dune, and river sands and then complements and supplements data which have not been presented before. These new data are grouped on scatter plots for sands of various origins. Finally, this address displays on maps regional trends of process‐induced changes in size distributions of populations of particles in sands of four different settings: (1) a beach‐dune setting, as exemplified by Padre Island, Texas, on the shores of the Gulf of Mexico; (2) a point‐bar setting in the Arkansas River of Oklahoma; (3) a continental shelf‐slope setting in the Gulf of Mexico off Louisiana, Mississippi, and Alabama; and (4) a continental shelf setting of the western Atlantic Ocean off New Jersey.

This address concludes with the observation that the distribution of particles in sands will be influenced by the conditions that determine what happens in the depositional environment, hence statistical parameters that define the size‐frequency distribution may be related to formative process.
Initiation of ripple marks under oscillating water
Authors: KANEKO A. , HONJI H.
Published: June 2006

It was observed that a monolayer of glass beads which were scattered sparsely on a rigid plane floor grew into regular waves of particles under oscillatory water flow. The relative displacement of two nearby particles due to viscous fluid forces seems to be responsible for the initiation of these particle waves. It was also observed that the similar particle waves were formed on the initially flat surface of a thick sand bed and subsequently developed into oscillatory sand ripples of a common type. On the basis of these observations, it is suggested that the particle waves may be the basic cause of the initiation of general ripple marks under oscillatory flow.
Microbial alteration of Bahamian deep‐sea carbonates
Authors: ZEFF MARJORIE L. , PERKINS RONALD D.
Published: June 2006

An analysis of microborings within sediment and hardground samples collected from the Northwest Providence Channel and the western margin of the Little Bahama Bank was conducted to characterize the endolithic assemblage present, to examine the role of microboring organisms in the alteration of deep‐sea carbonates, and to evaluate the palaeoecologic potential of the aphotic microboring assemblage found. Samples examined in this study were collected at depths ranging from 210 to 1450 m. The microboring assemblage was found to contain: (a) filamentous fungal borings of five distinct types, (b) a 1.0 × 2.5 μm vermicular form of fungal or bacterial origin, (c) an 8‐12 μm tubular, branching form of probably fungal origin, (d) a subapically branched form considered to be fungal, (e) a spinate form of uncertain affinity and (f) sponge borings. Three of these forms are known only from the deep‐marine environment; the remainder are also known from shallow‐marine sediments found well within the photic zone.

Both carbonate sediments and lithified hardgrounds are highly altered through the activity of endolithic organisms. Infestation of individual skeletal fragments by microborers may be so extensive as to produce heavily bored envelopes resembling those previously reported to occur only under shallow‐marine conditions. Although the geological ranges of these microborings remain to be established, the presence of such an aphotic assemblage, coupled with the absence of photosynthetic algal borings, could provide a valuable tool in palaeoecological studies. Other potential applications include the determination of turbidite sediment sources and the establishment of relative water depths for the formation of hardground surfaces.
Holocene pisoliths and encrustations associated with spring‐fed surface pools, Pastos Grandes, Bolivia
Authors: RISACHER FRANÇOIS , EUGSTER HANS P.
Published: June 2006

Calcite pisoliths, with diameters ranging from 1 to 200 mm, are forming now on the surface of a playa (salar) in the Andean Altiplano (4500 m above MSL) of Bolivia. They are associated with active or recently active hot springs (20‐75°C) which flow onto the playa surface. Encrustations of pieces of an older caliche‐type crust, of pisoliths, of indurated mud and of older concretions are also found as well as series of small (1‐3 cm high) sinter terraces (rimstone dams). Arborescent concretions and overgrowths are common and they are reminiscent of drip‐stone textures. Water analyses demonstrate that calcite supersaturation (about twenty times) occurs mainly through CO2 loss, with photosynthesis by algae and degassing the main removal mechanisms. The two available analyses indicate slight evaporation and a calcium loss between spring and pool of 2.3 mmol per litre of water. It is thought that the hot springs pick up much of their solute load from the playa sediments. The closest analogues to these deposits have been reported from caves (cave pearls and concretions). Although the depositional processes may be similar, the environment on an evaporitic playa surface is quite different. The geological implications for this newly observed pisolith environment may be considerable.
The isotopic composition of strontium in non‐marine carbonate rocks: the Flagstaff Formation of Utah*
Authors: NEAT PATRICK L. , FAURE GUNTER , PEGRAM WILLIAM J.
Published: June 2006

The isotopic composition of strontium in surface water in continental basins is determined primarily by the geology of the basin and to a lesser extent by climatic conditions. Consequently, the 87Sr/86Sr ratios of brines in such basins can change only as a result of changes in the geology or climate. This principle of isotope geology was studied by analysis of a suite of non‐marine carbonate rocks from the Flagstaff Formation (Palaeocene‐Eocene) of Utah. The samples were collected from a section in Fairview Canyon of Sanpete County. They include both limestone and dolomite and were selected to have low non‐carbonate residues. The concentrations of strontium in calcites averages 383 ± 128 p.p.m., while those of dolomites increase from 354 ± 74 p.p.m. in the lower 43 m of section to a maximum of 2259 p.p.m. higher up. The increase in the strontium content of dolomite is interpreted as evidence for a change from steady‐state to progressively more evaporitic conditions. Two dolomites have isotopic compositions of oxygen expressed as δ18O = ‐2.75‰ (relative to the PDB standard) and are enriched in 18O relative to two calcites whose average δ18O value is ‐9.9‰. The 87Sr/86Sr ratios of the carbonate minerals range from 0.70890 to 0.71260. These values are clearly greater than the 87Sr/86Sr ratio of marine carbonates of Early Eocene age which is 0.70744. The variation of the 87Sr/86Sr ratio in this section of the Flagstaff Formation is real and reflects the occurrence of geological events which changed the isotopic composition of Sr entering Lake Flagstaff. The non‐carbonate fractions of six carbonate rocks and one sandstone fit a straight line on the strontium mixing diagram in co‐ordinates of initial 87Sr/86Sr and 1/Sr concentration. These results suggest that the isotopic composition of strontium in Lake Flagstaff may have been modulated by periodic input of volcanogenic detritus of felsic composition.
Convolute lamination in modern sands of the estuary of the Oosterschelde, the Netherlands, formed as the result of entrapped air*
Authors: DE BOER P. L.
Published: June 2006

Soft‐sediment deformation structures similar to convolute lamination were found at the sandy rim of an intertidal shoal in the Oosterschelde estuary, the Netherlands. Abundant air filled cavities within upward penetrated domes suggest that reversed density stratification resulting from trapped air in some sand layers plays a role in the deformational process.

Field observations show that air‐filled cavities are formed within fine‐grained sand; at rising tide the ground water level does not rise quickly enough to replace the interstitial air before flood water covers the sediment surface. Thus, air is trapped in the sand between the ground water level and the sediment‐water interface. Because of the weight of the overlying water column and the slow downward movement of water due to capillary action, the entrapped air is compressed and eventually may attain a pressure which enables it to lift the overlying sediment. At this time bubbles are formed, often to a depth of 20 cm. Observation and experiments show that, in layers of fine sand, bubbles develop preferentially in better sorted and coarser zones. This is probably because capillary forces are greater in finer‐grained and less sorted sand. Thus, water will penetrate by preference into the latter, pressing the interstitial air into the better sorted and coarser sand. The high content of air cavities in certain layers then provides the density instability responsible for the deformational process leading to the formation of convolute lamination. In the intertidal zone this appears to be a slow process that covers a number of ebb and flood cycles.

Convolute lamination has been described from a number of ancient sandstones that are thought to have been deposited in shallow water, near‐shore environments. In many of these cases sand size compares with that found in the Oosterschelde. Air entrapment might have played a role in the formation of certain of these occurrences of convolute lamination.
Distinguishing between fine‐grained turbidites and contourites on the Nova Scotian deep water margin
Authors: STOW DORRIK A. V.
Published: June 2006

Review of the criteria which have been proposed for distinguishing between the deposits of turbidity currents and bottom currents in deep water sedimentation shows no general agreement on their validity. It is important to compare finegrained turbidites and contourites, to recognize that different turbidity current and bottom current mechanisms exist, and that their deposits may be closely inter‐bedded in a continental rise environment.

Interbedded turbidites and contourites have been recognized in cores from the deep‐water margin off Nova Scotia. The most useful criteria for distinguishing between the two deposits were found to be: (1) fining and sorting trends: perpendicular or parallel to the contours; (2) marked textural differences between interbedded turbidites and contourites indicating differences in source and transport distance; (3) mineralogy and textural composition: regional patterns indicating transport perpendicular or parallel to the contours; (4) grain fabric: indication of downslope or along‐slope transport at the time of final deposition; (5) sedimentary structures: turbidites show a structural sequence and evidence of rapid burial; contourites are bioturbated and contain irregular lag concentrations of biogenic sand. Other criteria include grain‐size parameters, and the regional setting, distribution and depositional rate of the various facies. With due care these criteria can be applied to other regions.

Previously used characteristics of silt‐laminae abundance, layer thickness, heavy mineral cross lamination, sorting, and the nature of bed contacts are not applicable to fine‐grained turbidites and contourites. Compositional criteria depend on regional features.