bluuebunny/crossref_metadata_2025 huggingface dataset:
Gravel‐topped offshore bar sequences in the Lower Carboniferous of southern Ireland
Authors: COTTER EDWARD
Published: June 2006

Within the Kinsale Formation (Lower Carboniferous) of southern Ireland are pebbly sandstones and conglomerates contained in what is known locally as the Garryvoe conglomerate facies. In this facies there are three main groups of lithologies: (a) heterolithic mudrocks and sandstones characterized by a wide variety of wave‐produced structures; (b) sandstones dominated by swaley cross‐stratification (SCS), parallel lamination, and rare hummocky cross‐stratification (HCS); and (c) pebbly sandstones and conglomerates occurring as discrete beds or as gravel clasts dispersed through SCS sets. Successions of the facies comprise units of heterolithic mudrock and rippled sandstone alternating repeatedly with coarsening‐upward units of SCS pebbly sandstone capped by top‐surface granule and pebble lags.

The Garryvoe conglomerate facies accumulated in a system of offshore bars on a muddy shallow‐marine shelf that was dominated by waves and currents generated by storms. Sands and gravels were bypassed from a contemporaneous northerly coastal zone to the shelf, where they were moulded by the storm‐generated flow into low, broad, sand ridges (offshore bars). The elongate bars were spaced kilometres apart, oriented obliquely to the coast, and separated by muddy interbar troughs. Their surfaces were largely covered by hummocky and swaley forms. Long‐term, gradual seaward migration of the offshore bars concentrated gravels on landward flanks from the dispersed pebbly sands that were on the crests and seaward flanks. Exceptionally intense storms could form laterally extensive winnowed gravel lags above thinned bar sequences. Such storms could also flush gravel‐bearing turbidity currents into muddy interbar trough areas.
Boulder beds in the glaciogenic Permo‐Carboniferous Dwyka Formation in South Africa
Authors: VISSER J. N. J. , HALL K. J.
Published: June 2006

Single‐layer and massive boulder beds, which include boulder pavements, are sporadically distributed in the glaciogenic Permo‐Carboniferous Dwyka Formation. These matrix‐supported beds consist of moderately to poorly sorted, rounded boulders, cobbles and pebbles with a clast composition similar to those in the underlying or overlying diamictite. Alternatively, the clasts are composed of monolithic basement rock‐types. The clasts show a long‐axis orientation which, in the case of the boulder pavements, is parallel to the striae on the pavements.

The various types of boulder beds have a similar mode of deposition and their subglacial origin is evidenced by the clast orientation, clasts with stoss and lee sides, stacking of clasts, and the development of a cleavage in the matrix due to horizontal stresses exerted by the boulders in the subglacial sediment. Subglacial streams, kame mounds, subaqeously winnowed till, or boulder beaches supplied the coarse debris which was entrained in the basal ice by plastic flow and regelation. Selective lodgement of the transported boulders occurred down‐glacier when the basal thermal conditions changed from cold‐freezing to warm‐melting. The formation of the different types of boulder beds is thought to depend primarily on the concentration of coarse debris in the basal ice.
Contained (reflected) turbidity currents from the Middle Ordovician Cloridorme Formation, Quebec, Canada: an alternative to the antidune hypothesis
Authors: PICKERING KEVIN T. , HISCOTT RICHARD N.
Published: June 2006

The parautochthonous Cloridorme Formation is a syn‐orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle‐fan, outer‐fan, and basin‐floor deposits. The basin‐floor deposits (about 1.5 km thick) are confined to members β1, β2 and γ1, and are characterized by graded, thick (1–10 m) mud‐rich calcareous greywacke beds previously interpreted as deposits of concentrated, muddy, unidirectional turbidity currents that locally generated backset (antidune) lamination in internally stratified flows.

The dominant flow directions were from east to west, but west to east transport also occurred, as seen in the orientation of ripples, climbing ripples, flutes, consistently overturned flames, and grain imbrication. We believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180° one or more times during deposition of the lower sandy part of the beds. Flow reversals are consistent with the sharp grain‐size breaks and mud partings within sandy divisions. Measurement of grain fabric relative to stratification in the most celebrated ‘antidune’ bedforms indicates flow from west to east; thus, the bedforms were produced by west‐to‐east migration of megaripples, not by the upcurrent migration of antidunes.

The thick muddy beds were deposited by large‐volume, muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor, much like the ‘Contessa’ and similar beds of the Italian Apennines. Large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed. Because of their mode of emplacement, we propose that these beds be called contained turbidites.
Sedimentation and palaeoenvironments of Late Cretaceous crater‐lake deposits in Bushmanland, South Africa
Authors: SMITH R. M. H.
Published: June 2006

A large diameter borehole core from an epiclastic kimberlite remnant on the farm Stompoor in the Prieska district, Cape Province, contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater‐lake during the Late Cretaceous. Three distinct facies associations reflect depositional processes that prevailed in offshore areas of the original lake. Facies Association A: matrix‐supported pebble conglomerates comprising a chaotic assemblage of pyroclastic, basement and country rocks set in a fine‐grained matrix. Flat, non‐erosional basal surfaces with ‘frozen’ rip‐up clasts, the protrusion of matrix‐supported clasts above the upper surfaces and a direct relationship between maximum clast size and bed thickness suggest deposition from debris flows that originated subaerially on pyroclastic talus cones surrounding the crater. Facies Association B: alternating thin beds of matrix‐supported granule conglomerate, structureless fine‐grained sandstone and parallel laminated mudrock. Small fining‐upward sequences within these beds are comparable to turbidite Bouma Tade, Tde. Numerous partings display petrified fish and frog skeletons, as well as bivalve, gastropod and ostracode shells, leaf impressions, insect wings and a possible bird bone. These beds were deposited by thin debris‐flows and turbidity underflows interspersed with periods of ‘pelagic’ sedimentation. Facies Association C: microlaminated mudstone beds containing scattered ‘dropstone lapilli’. The lamination is imparted by alternating Ca‐rich/Ca‐poor layers which may reflect climatic seasonality. They are interpreted as the result of seasonally influenced suspension settling through a thermally stratified water column.

Short‐term periodicities in conglomerate bed thicknesses are interpreted as the result of successive block caving of a slump scar giving rise to several debris flows from the same source area. Seismic shock from nearby volcanism may have simultaneously triggered slumps on both subaerial and subaqueous slopes. Dropstone lapilli in Type C beds and the preponderance of load casting in Type B beds support this interpretation. An estimate of the time span involved in accumulating 76 m of crater lake sediments based on the possible seasonal imprint of Type C beds gives a figure of some 220,000 yr.
Laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current
Authors: LI ZHENLIN , KOMAR PAUL D.
Published: June 2006

Important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain. A series of experiments has been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication. The experiments involved gravel‐sized spheres (ball‐bearings and marbles), natural pebbles selected for their approximately triaxial ellipsoid shapes, and angular crushed basalt pebbles. The pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined, the angle of the board being equal to the pivoting angle at the instant of grain movement.

The pivoting angles of spheres showed reasonable agreement with a theoretically derived equation, showing much better agreement than in previous studies which utilized sand‐sized spheres. A series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size. Our results are therefore consistent with the earlier studies limited to sand‐size spheres. The cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out. Similar size dependencies are also found for the ellipsoidal pebbles and angular gravel.

The experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle, being a function of the ratio of the pebble's smallest to intermediate axial diameters. This ratio controls the grain's ability to roll and pivot; with small ratios of these diameters the pebbles tended to slide out of position, whereas with ratios closer to unity (circular cross‐section) true pivoting took place and the angles were smaller. Experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position, the pivoting angle being increased approximately by the imbrication angle. The angular crushed gravel also required high pivoting angles, apparently due to interlocking of the grains resulting from their angularity.

Other factors being equal, the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres, ellipsoidal grains, angular grains, and imbricated grains. The results obtained here make possible the quantitative evaluation of these shape effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.
The behaviour of intertidal sandwaves during neap‐spring tide cycles and the relevance for palaeoflow reconstructions
Authors: TERWINDT JOOST H. J. , BROUWER MARGUERITE J. N.
Published: June 2006

Variations in migration distances and shape characteristics of sandwaves in relation to flow conditions were studied on the Ossenisse intertidal shoal in the Westerschelde estuary. The purpose was to analyse bedform behaviour, to establish the threshold and the time lags involved, to find differences in two‐ and three‐dimensional sandwaves and to determine the implications for palaeoflow reconstructions.

Sandwave migration is well correlated with the peak depth‐averaged flow velocity of the dominant tide. Thus the latter parameter may be estimated from the thickness of the tidal bundles. Other flow parameters such as shear velocity, Chezy C or roughness length do not show a correlation with the migration and cannot be used in palaeoflow analysis.

Flow depth does not correlate with sandwave height or with length. Consequently, neither sandwave height and length nor set height and length can be used for palaeoflow depth determination.

Sandwaves start moving when the peak dominant flow velocity exceeds 0.5–0.6 m s−1, and appreciable changes in shape occur at 0.75–0.8 m s −1. Complete reversal of sandwaves is accomplished if both the dominant and subordinate peak depth‐averaged current velocities exceed 0.85 m s−1.

Two‐ and 3‐D sandwaves appeared to have different stability fields in the velocity‐depth diagram and in the diagram of the Froude number versus the depth‐grain‐size ratio. In addition the distinction between 2‐D and 3‐D sandwaves appeared to be related to a variability in current direction during periods of appreciable sand transport. There are also differences in sedimentary structures between the two types of sandwaves.
Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic
Authors: JENKYNS HUGH C. , CLAYTON CHRISTOPHER J.
Published: June 2006

Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian‐Toarcian boundary with resolution to the zonal level. The oxygen‐isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ13C value of 4·52%PDB is attained. These intervals are known to be favoured periods of organic‐rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian‐Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’.

Some Italian sections spanning this Lower Jurassic interval contain organic‐rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δPDB and, in one section examined in detail, correlate with the calcium‐carbonate content of the shales; they may reflect a partial change to a non‐calcified planktonic biota during deposition of this lime‐poor interval, possibly responding to upwelling and increased fertility of near‐surface waters. The onset of upwelling may have been as early as spinatum‐tenuicostatum Zone time, that is, at the Pliensbachian‐Toarcian boundary.
A glove box for the fine‐scale subsampling of sediment box cores
Authors: EDENBORN HARRY M. , MUCCI ALFONSO , BELZILE NELSON , LEBEL JEAN , SILVERBERG NORMAN , SUNDBY BJØRN
Published: June 2006

The construction and operation of a glove box which allows the fine‐scale vertical subsampling of sediment box cores under a low oxygen atmosphere is described.
Microfacies and geochemistry of Middle Jurassic algal limestones from Scotland
Authors: ANDREWS JULIAN E.
Published: June 2006

Bathonian algal limestones from the Duntulm Formation of the Great Estuarine Group, western Scotland, are subdivided into three microfacies: cryptalgal laminites, thrombolites and porostromate‐micrites. The distribution of early diagenetic, carbonate spherulites and cement fringes is fabric selective. Spherulites are common in the thrombolitic microfacies, whereas cement fringes were best developed upon algal nodules with porostromate microstructure. Syndiagenetic micrites, spherulites and cement fringes have stable carbon isotopic ratios indicative of abundant, organic‐derived HCO3 incorporation (δ13C values of −0·05 to −15‰). Enhanced magnesium concentrations in these fabrics, relative to neomorphic and burial‐diagenetic spar cements, suggest an original Mg‐calcite mineralogy, probably with precipitation under the influence of microbial systems. Brecciated, early‐diagenetic fabrics and localized pockets of calcite pseudomorphs after gypsum were probably formed during subaerial exposure. Sulphate supply was probably from storm‐derived, saline lagoon‐water washed on to supralittoral stromatolitic flats. These inundations also allowed the temporary establishment of green algae and encrusting foraminifera. The combined geological and geochemical information suggests a palaeoenvironment somewhat similar to Recent, sub‐tropical humid zone, calcareous algal marshes.
Berm development on a monsoon‐influenced microtidal beach
Authors: THOMAS K. V. , BABA M.
Published: June 2006

The mechanisms of berm development along a microtidal‐high energy beach is examined. Such a beach with medium‐sized sand and monsoon wave‐controlled profile at Valiathura, south‐west coast of India, is selected for this study. The waves which very rarely fall below 1 m, often exceed 4 m during the monsoon period of May to October. The erosion‐accretion pattern of the beach shows a cyclicity and the berm development is mainly due to the onshore migration and welding of longshore bars on to the beach following the monsoon rough season. The stages of berm development in the present microtidal beach are more or less similar to the model presented by Hine for a mesotidal case, except for the following intermediate additional stages. The longshore bar develops due to the erosion of beach when the wave steepness was above 0·04, gets flattened when it falls below 0·04, and then reforms nearer to the shoreline as a swash bar. This reformed bar gets divided and the inner bar gets welded on to the beach, followed by the outer bar developing the berm. During the onshore migration of the longshore bar and berm development the beach face becomes partially reflective with the surf scaling parameter, εb between 2·5 and 33. The inshore is dissipative with the inshore surf scaling parameter, εs≫33. The offshore side of the longshore bar is partially reflective with its surf scaling parameter, εbar between 2·5 and 33. The breakers are spilling or plunging. Vertical growth of the berm is mainly due to the changes in swash‐limit caused by the variations in wave steepness, breaker height and type. Vertical growth stops when the beach‐face attains equilibrium with the grain size‐wave energy relationship, and a wave steepness below 0·02 helps to sustain this state.
Estimation of some aeolian saltation transport parameters: a re‐analysis of Williams' data
Authors: JENSEN JENS LEDET , SØRENSEN MICHAEL
Published: June 2006

A new method for analysing observed aeolian sand transport rate profiles of the kind obtained by Williams is presented. The method involves a mathematical model of aeolian saltation. Detailed information about the saltation process can be calculated from the transport rate profile by means of this model. The method is used to perform a re‐analysis of Williams' trap data. Among the main findings of this analysis is that the grain borne shear stress appears to be a smaller fraction of the total shear stress than assumed by Bagnold & Owen in their theories of aeolian saltation. Other findings are that the probability distribution of the jump height of the grains does not depend much on the wind speed once the saltation is established, and that the vertical component of the mean launch velocity decreases with the grain size. It is approximately inversely proportional to the grain diameter. Our estimates of the landing angles indicate that estimates of the impact angles obtained from photographically recorded trajectories are too small due to biased sampling. The influence of grain shape on the transport characteristics is mainly due to changes in the grains' ability to jump when hitting the bed. It is found that angular grains have a lower mean jump height than spherical grains.
Pyrite replacement of mollusc shells from the Lower Oxford Clay (Jurassic) of England
Authors: FISHER IAN ST JOHN
Published: June 2006

Replacement of originally aragonite mollusc shells by pyrite commonly occurs in the Lower Oxford Clay. Petrographic studies show the shells to have constituted complex microenvironments in the sediment. A range of replacement textures is found showing a variable amount of solution of the original aragonite. Three distinct textures were found in crushed pyrite‐replaced ammonite shells from heavily pyritized concretions.

(1) A texture reflecting the original shell structure due to the replacement of the organic shell‐matrix by pyrite.

(2) An ovoid texture seen at several stages of replacement reflecting processes occurring at discrete centres of sulphate reduction.

(3) Euhedral crystals lining cracks and fractures in the shell.

Three types of replacement are found in small gastropods and bivalves from shell bed, some of which may relate to those seen in the ammonites.

(1) Replacement of organic shell‐matrix by pyrite preserving good shell‐microstructure.

(2) Replacement showing outwardly good preservation of morphological features but inwardly only the gross structure, such as growth lines, is preserved.

(3) Replacement of the shell in a matrix of euhedral pyrite leaving only lines of carbonate inclusions marking the margins of the shell.

The replacement textures and types appear to be dependent on the initial structure of the shell and the access of iron and sulphate into the shell.

Early stages of replacement appear to proceed by pyrite formation within the organic matrix of the shell, with little or no solution of the carbonate, this produces textures which faithfully mimic the original shell microstructure. It is thought that the lack of carbonate solution is due to a limited availability of iron, brought about by the less intensively reducing nature of the sediment. Later stages of replacement are promoted by the cracking and fracturing of the shell and are, generally, not as faithful to the original shell structure. This is due to the greater availability of iron as the sediment becomes more reducing with burial.
Conditions favourable for the formation of warm‐climate aeolian sand sheets
Authors: KOCUREK GARY , NIELSON JAMIE
Published: June 2006

Aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent. Sand sheets are ubiquitous to modern, warm‐climate sand seas, generally occurring marginal to dune fields, although they may exist within the interior of a sand sea or independent of a dune field. Sand‐sheet deposits are recognized in ancient aeolian sequences, where they may account for significant accumulations of low‐angle aeolian stratification. We suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets. A study of six modern sand sheets in North America (located at Great Sand Dunes, Gran Desierto, Dumont, Algodones, Padre Island, and Colorado River delta) indicates that the factors favourable for sand‐sheet development are: (1) a high water table, (2) surface cementation or binding, (3) periodic flooding, (4) a significant coarse‐grained sediment population, and (5) vegetation. These factors are reflected in the nature of stratification and the accessory features of sand‐sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the Triassic Dolores and Pennsylvanian‐Permian Rico formations.
The significance of mid‐channel bars in an active meandering river
Authors: Hooke J. M.
Published: June 2006

Mid‐channel bars are common on many active meandering gravel‐bed rivers, but specific information on timescales of development and on the occurrence of bars is lacking. Data from the River Dane in Cheshire, NW England, are presented here which indicate that a sequence of formation from accumulations of coarse gravel on the centre of riffles through to attachment of bars to floodplains is common and takes a period of 5–15 years. The sedimentological and vegetation changes through the sequence are described and the ro̊1e of flow events is discussed. About 10% of bends in this meandering reach contain mid‐channel bars at any time. Bars develop in steeper sections of the channel and downstream of sharp corners and rapidly eroding apices. Many of the bars are caused by excessive erosion of the banks which overwidens the channel. In the longer‐term the bars become incorporated in the developing meanders.
Submergent shorelines in the SW Pacific: evidence for an episodic post‐glacial transgression
Authors: CARTER R. M. , CARTER L. , JOHNSON D. P.
Published: June 2006

ABSTRACT High resolution seismic profiles, supported where possible by radiocarbon dates and regional stratigraphic data, indicate that the last post‐glacial transgression in the SW Pacific was episodic, comprising major stillstands punctuated by rapid rises in sea‐level.

On the terrigenous continental shelf east of South Island, New Zealand, a succession of shorelines (S8‐S1) are recognized, as follows: S8 =c. −113 m, 18,000 yr BP; S7 =c. −88 m, 17,000 yr BP; S6 =c. −75 m, 15,000 yr BP; S5 =c. −56 m, 12,000 yr BP; S4 =c. −46 m, 11,000 yr BP; S3 =c. −28 m, 9,500 yr BP; S3a =c. −24 m, 9,000 yr BP; S2 =c. −9 m, 7,500 yr BP; S1 = 0 m, 6,500 yr BP.

With the exception of S8, and possibly S2, the shorelines are associated with wedges of sediment, the size and presence of which imply that (1) sea‐level stabilized at some shorelines for a considerable period of time (up to 1‐2,000 yr); and (2) the intervening rises of sea‐level, estimated to have been at least 10‐12 m 103 yr−1, were too rapid to allow the reworking of the wedges into a transgressive sediment sheet, as favoured in some current models.

On the Great Barrier Reef shelf, off Queensland Australia, shorelines S8‐S1 have also been recognized, with a further shoreline feature S4a occurring at c. ‐39 m. Shorelines S1a (0 m/0 yr BP), Sib (+ 2‐3 m/6,000 yr BP) and Sic (0 m/6,500yr BP) are recognized as discrete aspects of the post‐6,500 yr BP sea‐level behaviour in north‐eastern Australia. The rapid rise in sea‐level, at least between shorelines S5 (12,000 yr BP) and S3 (9,500 yr Bp), is known to have outpaced reef growth, causing in situ drowning of reefs located along the deeper shorelines. All modern reefs so far drilled and dated began their development at or above S3 (‐28 m, 9,500 yr BP).

Some of the shorelines, particularly S5, appear to correlate between the northern and southern hemispheres on the basis of age, succession and general depth of occurrence, suggesting (1) that they may be global features controlled by the post‐glacial pattern of ice‐sheet decay; and (2) that hydro‐isostatic adjustment may exert only a minor control on the depth of particular shorelines, at least during the earlier parts of the post‐glacial transgression.
Carbon‐sulphur‐iron‐variations and sulphur isotope patterns of Silurian Graptolite Shales
Authors: DILL H. , NIELSEN H.
Published: June 2006

ABSTRACT Black shale horizons (Graptolithenschiefer) in Silurian and Lower Devonian strata of the Franconian‐Thuringian Highlands (Frankenwald‐Thüringer Wald) of Germany have high metal concentrations (e.g. U,V,Mo). Organic carbon, sulphur and iron contents, degree of pyritization (DOP) and sulphur isotope ratios from the Lower Graptolite Shales, along with faunal and sedimentological evidence show that the depositional environment was anoxic. Although some data (e.g. C/S plots) do not suggest completely euxinic conditions, the DOP versus C plot indicates a Black Sea‐type environment. From the sulphur isotope data the environment fluctuated between‘open’and‘closed’with respect to sulphate supply, with higher carbon contents reflecting an increased bacterial reduction of the available sulphate. Diagenetic effects were responsible for somewhat unusual plots of C/S ratios, though iron availability may have been influential.
The Marl Slate: a model for the precipitation of calcite, dolomite and sulphides in a newly formed anoxic sea
Authors: SWEENEY M. , TURNER P. , VAUGHAN D. J.
Published: June 2006

Detailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ13C, δ18O, δ34S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite‐rich and dolomite‐rich carbonates.

C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ18O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for 18O and 1·5% for 13C in calcite. The δ34S values of pyrite are isotopically light (mean value = ‐ 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins.

The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in the Zechstein Sea.
Submarine limestones in the nearshore environment off Kuwait, northern Arabian Gulf*
Authors: KHALAF F. , MILLIMAN J.D. , DRUFFEL E.M.
Published: June 2006

Even with careful petrographic and mineralogic characterization of marine limestones, intertidally and subtidally lithified rocks are often difficult to differentiate, thus hindering an accurate delineation of the diagenetic environment. Limestones from water depths of 6 to 8 m off Kuwait vary in petrographic character from oosparite and biosparite (in which the cement is entirely aragonite) to oomicrite and biomicrite (in which at least some of the cement is microcrystalline magnesian calcite). Carbon‐14 dates suggest that the oosparite may have lithified at depths shallower than at present (possibly intertidally) during a lower stand of sea‐level. In contrast the biosparite, oomicrite and biomicrite appear to be contemporaneous and to have lithified subtidally.
Glacial sedimentary processes and environmental reconstruction based on lithofacies
Authors: SHAW JOHN
Published: June 2006

Glacigenic sediments exposed in pits around Villeneuve, near Edmonton, Alberta, are subdivided into facies based on grain size, sedimentary structure, glacially‐induced deformation and faulting, and groove marks. Two diamicton facies are recognised, one of which is interpreted as a primary till, deposited directly from glacier ice, and the other as a product of mass‐movement. The diamicton facies are closely associated with current bedded facies interpreted as fluvioglacial deposits. The stratigraphic sedimentological and tectonic aspects of these fluvial deposits suggest subglacial deposition in channels and cavities. At any one place the glacier appears to have alternated between being attached to the bed, causing thrusting and sole marking, and being separated from the bed by a cavity in which fluvial and mass‐movement sediments accumulated. The net result is a highly complex and laterally variable stratigraphy produced by a single glacial advance. The correct interpretation of such sequences is essential if lithostratigraphy is to be used to establish glacial history. In addition, the interpretations presented here have implications regarding the formation of soft zones in ‘till’. They indicate that the soft zones are beds of sorted sediment redeposited by mass‐movement.
Magnetic fabric characteristics of bioturbated wave‐produced grain orientation in the Bridport‐Yeovil Sands (Lower Jurassic) of Southern England
Authors: HOUNSLOW M. W.
Published: June 2006

There is little visible primary hydrodynamic lamination preserved in the Bridport‐Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave‐produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non‐bioturbated fine‐grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long‐axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non‐bioturbated sandstone.
Carbonate turbidite sequences deposited in rift‐basins of the Jurassic Tethys Ocean (eastern Alps, Switzerland)
Authors: EBERLI GREGOR P.
Published: June 2006

Syn‐rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault‐bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates.

The most abundant redeposits are bio‐ and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high‐ and low‐density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow‐water carbonates and skeletal debris from nearby submarine highs.

After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge‐shaped with a horizontal upper boundary and consists of a thinning‐ and fining‐upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin‐bedded turbidites and hemipelagic limestones. The thick‐bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick‐bedded turbidites, thinning‐ and fining‐upward cycles are common. The overlying thin‐bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine‐grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin‐plain association developed comprising fine‐grained and thin‐bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.
Selective and reversible carbonate—silica replacements in Lower Cretaceous carbonate‐bearing turbidites of the Eastern Alps
Authors: HESSE REINHARD
Published: June 2006

In the diagenetic history of calcareous sandstones, silicacementation and silicification may be followed by carbonatecementation and replacement and vice versa, and the change‐over from one to the other may occur more than once. This is well illustrated by calcareous and siliciclastic turbidites of the Gault Formation (Aptian to Albian) of the Eastern Alps which have been interpreted as deep‐sea trench plain and deep‐sea fan deposits.

In these turbidites silicification selectively affects ooids and a few other biogenic carbonate fragments rich in organic matter (algae and bryozoans) which form a small fraction of the bulk sediment. The type and sequence of diagenetic changes are largely controlled by host‐rock composition and may vary vertically within individual beds as a result of compositional grading. In the carbonate turbidites, silicification follows widespread calcite cementation. The process is slow, resulting in relatively coarsely crystalline replacement quartz. In ooids with quartz nuclei, rim‐quartz forms mostly monocrystalline ‘overgrowths’ by outward replacement of the concentrically laminated carbonate cortex. This type of silicification is often incomplete leaving parts of the ooid cortices unaffected. In quartz arenites and sublitharenites silicification precedes calcite cementation. There the process is rapid, forming microcrystalline quartz. Even if the ooid nucleus consists of quartz, a syntaxial ‘overgrowth’ does not normally form. The replacement quartz is almost always polycrystalline. Late‐stage diagenetic calcite and dolomite which develop euhedral crystal shapes and cut across grain boundaries may replace the earlier secondary rim‐quartz of the ooids as well as other minerals.

Possible sources of the silica are pressure‐solution of quartz, dissolution of opaline silica of radiolarian tests and of sponge spicules, and feldspar in the host bed. In a number of examples an increase in the degree of silicification can be observed towards the lower bedding planes of individual turbidites requiring an additional external source of silica which seems to necessitate cross‐formational flow of pore solutions. Silicification in both the carbonate and the siliciclastic turbidites probably took place at about the same time; in the carbonate turbidites it was preceded, however, by calcite cementation, which significantly reduced porosity and permeability before silicification took place.

The greater degree of alteration experienced by the Gault turbidites of the Falknis and Tasna Nappes, which are more internal structural units of the Alps (compared to the Flysch Zone of the Eastern Alps), is reflected by the growth of quartz ‘beards’ and spikes from the ooids in the direction perpendicular to maximum stress. This is the only case observed where the rim quartz of the ooids grows beyond the original grain boundaries.
Mass transport in European Cretaceous chalk; fabric criteria for its recognition
Authors: BROMLEY RICHARD G. , EKDALE A. A.
Published: June 2006

Although it is a pelagic sediment, fine‐grained calcareous ooze may be mobilized prior to general lithification and redeposited as allochthonous units. Numerous occurrences of allochthonous chalk have been reported in recent years, having been recognized by large‐scale bedding features seen in outcrop. Smaller‐scale internal features, such as contorted laminae, and larger features, such as smeared burrows and imbricated flint nodules, attest to a significant amount of soft‐sediment deformation and synsedimentary slumping in European chalk sections of Late Cretaceous age.

Truly autochthonous chalks contain complex, tiered ichnofabrics and in some cases exhibit a diagenetic nodular fabric that is undisturbed by transport. In some situations, such as stagnant water conditions, autochthonous chalks may exhibit primary lamination, although this is very uncommon in European chalk sequences.

Different types of redepositional processes produce an array of varied allochthonous fabrics. Glide and slump units, for example, contain internal deformational features produced during sliding. Ooze flow causes plastic deformation of chalk units, internally as well as externally. Resuspension and fluid flow of chalk sediment produces a deposit having a totally new fabric, such as a conglomerate composed of detrital chalk clasts.

In this paper, typical macroscopic, sedimentary fabric types are illustrated, and the means of identifying them are discussed in terms of bioturbation features, in situ diagenetic nodules versus detrital clasts, physical deformation structures and development of flints.
Channel formation and migration by mass‐flow processes in the Lower Carboniferous fluviatile Fell Sandstone Group, northeast England
Authors: TURNER BRIAN R. , MONRO MARY
Published: June 2006

Small, steep‐sided narrow channels, locally developed within the braided fluviatile Lower Carboniferous Fell Sandstone of northeast England, are attributed to mass‐flow processes initiated by channel‐bank slumping. The channels have sharp undisturbed margins and are filled with moderately well‐sorted medium to coarse sandstone identical to the sandstone which they cut. Most channel‐fills are structureless, but a few show diffuse concentric marginal laminations. Laminations also occur towards the top of channel fills, but they are less regular and show low‐amplitude trough‐like undulations. Channel margin inclinations range from 13° to 55°, and channel depths vary between 1·7 m and 2·5 m. The channels, which are only seen in profile, are orientated perpendicular to the local and regional palaeocurrent trend, and cut through structureless sandstone and planar cross‐stratified sandstone characterized by compound and compound‐complex bar bedforms. Only one channel has both margins preserved; and of the remainder (seven) only one has its northern margin preserved.

Undercutting and retrogressive slumping of the sandy channel bank is thought to have initiated a subaqueous sediment gravity flow, which moved across the main channel along a transverse scour at the base of the slip face of a large composite bar bedform. Such scours have strong asymmetric profiles with the steeper slope along the base of the active slip face (northern side) and the lower slope on the downstream (southern) side of the scour. Thus, the northern channel margin was steeper, less stable and more prone to collapse than the southern margin, and is almost never preserved. Flow reconstruction indicates that on entering the channel a hydraulic jump developed at the change in slope, and the flow became more turbulent and erosive. Scoured sand incorporated into the flow increased concentration and shearing along the boundaries. Reverse shear was also exerted by the overlying current, on top of the flow, but the central part remained relatively unaffected. The presence or absence of marginal laminations is probably related to marginal shearing and flow dilution. Deposition occurred in response to deceleration and frictional freezing of the sand flow.
Longitudinal petrographic variations in a Middle Ordovician trench deposit, Central Appalachian orogen
Authors: LASH GARY G.
Published: June 2006

Petrographic analysis of Middle Ordovician turbidite sandstones of the Greenwich slice of the Hamburg klippe (eastern Pennsylvania), inferred to be part of a fossil subduction complex, define three coeval petrofacies. The Jonestown petrofacies was derived from felsic plutonic and less abundant metasedimentary rocks, whereas the Windsor Township, the most extensive petrofacies, and Werleys Corner petrofacies were derived from sources characterized by various proportions of sedimentary/metasedimentary, plutonic, and volcanic rocks. The presence of minor but conspicuous extrabasinal carbonate and microlitic volcanic lithic fragments together with higher percentages of polycrystalline quartz, serve to distinguish the Werleys Corner from the Windsor Township petrofacies. It is conceivable that sandstones of the Greenwich slice were derived from microplates inferred to have existed to the southeast of the proto‐North American plate in Early Palaeozoic time.

The variations in sandstone composition along the length of the Greenwich slice may be explained by post‐accretion tectonic juxtaposition of petrofacies derived from various sources. An equally plausible explanation involves transverse infilling of a channelized longitudinal transport system (Windsor Township petrofacies) by sediment derived from compositionally diverse source terranes orthogonal to the trench (Jonestown and Werleys Corner petrofacies).
Evolution and stratigraphy of a regressive barrier/backbarrier complex: Kiawah Island, South Carolina
Authors: DUC AILEEN WOJTAL , TYE ROBERT S.
Published: June 2006

Analysis of 75 vibracores from the backbarrier region of Kiawah Island, South Carolina reveals a complex association of three distinct stratigraphic sequences. Beach ridge progradation and orientation‐controlled backbarrier development during the evolution of Kiawah Island, and resulted in deposition of: (1) a mud‐rich central backbarrier sequence consisting of low marsh overlying fine‐grained, tidal flat/lagoonal mud; (2) a sandy beach‐ridge swale sequence consisting of high and low marsh overlying tidal creek channel and point bar sand, and foreshore/shoreface; and (3) a regressive sequence of sandy, mixed, and muddy tidal flats capped by salt marsh that occurs on the updrift end of the island.

Central backbarrier deposits formed as a result of the development of the initial beach ridge on Kiawah Island. Formation of this beach ridge created a backbarrier lagoon in which fine‐grained estuarine and tidal flat mud accumulated. Washovers, oyster mounds, and tidal creek deposits form isolated sand and/or shell‐rich lenses in the lagoon. Spartina alterniflora low marsh prograded into the lagoon as the tidal flats aggraded.

Barrier progradation and sediment bar‐bypassing at Stono Inlet created digitate beach ridges on the northeast end of Kiawah Island. Within the beach‐ridge swales, tidal flats were disconformably deposited on shoreface and foreshore sand of the older beach ridges. Tidal creek drainage systems evolved to drain the swales. These rapidly migrating creeks reworked the tidal flat, foreshore, and shoreface sediments while redepositing a fining‐upward sequence of channel lag and point bar deposits, which served as a substrate for salt marsh colonization.

This resultant regressive sedimentary package marks the culmination of barrier island development and estuary infilling. Given enough time and sedimentation, the backbarrier sequence will ultimately prograde over the barrier island, reworking dune, beach, and foreshore sediments to form the upper sand‐rich bounding surface of the barrier lithosome. Preservation of the regressive sequence is dependent upon sediment supply and the relative rate of sea‐level rise, but the reworking of barrier islands by tidal inlets and migrating tidal creeks greatly alter and complicate the stratigraphic sequence.