bluuebunny/crossref_metadata_2025 huggingface dataset:
Evinosponges in the Triassic Esino Limestone (Southern Alps): documentation of early lithification and late diagenetic overprint
Authors: FRISIA‐BRUNI SILVIA , JADOUL FLAVIO , WEISSERT HELMUT
Published: June 2006

A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement‐filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low‐Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non‐luminescent under cathodoluminescence, but both cements and host rock are cross‐cut by micro‐fractures filled with bright‐luminescent calcite, related to late void‐filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ18O(PDB)=−5‰ to −12‰ and from δ13O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non‐luminescent calcite of early diagenetic origin.

The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine‐meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny.
Spheroidal dolomites – origin by hydrocarbon seepage?
Authors: GUNATILAKA ANA
Published: June 2006

Spheroidal dolomite cements are a pervasive and distinctive feature in five exposed lithologies of different geological age in Kuwait. The spheroids range in diameter from ∼5–350 μm, have concentric zones and nuclei of fluid inclusions with or without a radial fabric, and compositions varying from near stoichiometric to Ca56 Mg44. All spheroids appear to be primary in origin with no evidence of replacement. Some concentric zones show selective calcitization and/or leaching. Most of these features are confined to within 30 m of the surface. Consequently, spheroidal dolomites recognizable at depth in ancient sequences may be indicative of a local unconformity.

The dolomite distributions are areally correlative with known areas of hydrocarbon seepage, the latter being confined to prominent oil field anticlinal trends. It is proposed that the dolomitization was caused by groundwaters carrying hydrocarbons to the surface, which were then oxidized to carbon dioxide. The gas bubbles may have acted as nuclei for spheroid growth with a possible inducement by bacteria which have an affinity for hydrocarbons. The implication is that the regional distribution of spheroidal dolomites may be an indicator of a potential reservoir facies at depth.
Crystallographic and chemical variations during pyritization in the upper Barremian and lower Aptian dark claystones from the Lower Saxonian Basin (NW Germany)
Authors: DILL H. , KEMPER E.
Published: June 2006

In the Lower Saxonian Basin, cores from three drill holes provide a cross‐section from the basin centre towards the margin through the upper Barremian/lowermost Aptian pyritic claystones and marls. Microscopic analyses distinguish six types of pyrite (I: ‘cones and tubes’, II: ‘tubes’, III: ‘isometric crystal aggregates’, IV: ‘pseudomorphs of fossils’, V: ‘star‐like concretions’, and VI: ‘filiform pyrite‐marcasite intergrowths’) which are associated with subordinate amounts of Fe carbonate, chalcopyrite, sphalerite and wurtzite. The crystal morphology of these sulphides shows a clear‐cut diagenetic sequence from pyrite crystals dominated by the octahedron to those dominated by the cube.

Among these early to late diagenetic pyrites, the conversion of crystal habits is accompanied by a striking variation in trace element contents, some of which have extremely low values (e.g. Au, Se, Te, Tl, Co). In this context, classification of crystal habits and analyses of As and Ni establishes a sequence of pyritization which may help define a basin zonation for these argillaceous sedimentary rocks, categorized as ‘normal facies’—oxygenated bottom waters—interrupted by short episodes of bottom water oxygen depletion (‘bituminous facies’). Substitution of the cube for the octahedron in the Fe bisulphide aggregates led to a continuous removal of As and Ni from the pyrite, attesting to a steady decrease of those elements in the pore fluids with time. By contrast, within each type of pyrite, As and Ni contents increase from the margin towards the centre of the basin, owing to pre‐concentration of both elements in the basinal sediments, which are more abundant in organic matter. Some other elements, however, such as Mn, Cu, Sb and Ag, do not follow this trend and are unrelated to the lattice transformation of the Fe bisulphides. Their sometimes anomalously high quantities in Fe bisulphides are controlled by the host rock chemistry (presence of volcaniclastic material) and accessory minerals intergrown with pyrite (e.g. chalcopyrite).

Pyrite I through III formed in tubular hollows of burrowing organisms, and type IV in tests of fossils. Types V and VI did not result from replacement or void filling but were generated by diffusion of sulphate along inhomogeneties, such as bedding planes, shrinkage cracks or water escape structures.
Brecciated diamictons from Mohawk Bay, S. Ontario, Canada
Authors: MENZIES JOHN
Published: June 2006

Examination of sediments along the north shore of Lake Erie at Mohawk Bay reveals a relationship between the formation of intensely brecciated diamictons and the presence of sand‐block intraclasts. It is postulated that the sand blocks were subglacially deposited within a meltwater environment, and later frozen prior to being eroded and transported within a mobile subglacial debris layer. On immobilization the frozen sand blocks, encased within the diamicton, acted as a heat sink creating cryostatic stresses within the surrounding diamicton as a result of the advance of a frost front and related frozen ‘fringe’. The effect of these anisotropic stresses resulted in porewater migration to the frost front. Subsequent development of intense brecciation occurred as aureoles around the sand intraclasts due to localized high tensile stresses causing fracturing within the fine‐grained matrix of the diamicton.
Horizontal lamination and the sequence of bed phases and stratification under upper‐flow‐regime conditions
Authors: CHEEL R. J.
Published: June 2006

Two forms of horizontal laminae have been produced under upper‐flow‐regime conditions in a flume by deposition on a plane bed and on low in‐phase waves. Upper plane bed laminae consist of approximately equal numbers of fining‐upward (FU) and coarsening‐upward (CU) textural laminae; heavy minerals, when present, are typically within or form CU laminae. In horizontal laminae formed under low in‐phase waves, FU laminae may predominate and are significantly thicker than CU laminae; heavy minerals are associated with the tops of some FU laminae and are absent from CU laminae. Deposits aggraded on plane beds and low in‐phase waves both display low‐angle (13–14°) upstream grain imbrication. Upper plane‐bed horizontal laminae are attributed to the effects of the burst/sweep cycle on local bedload transport (bursts form FU laminae and sweeps form CU laminae). Some FU laminae in in‐phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest. Heavy minerals are concentrated in the tops of these FU laminae due to their enrichment on the crests of the migrating bed waves. Other FU and CU laminae in in‐phase wave deposits probably formed by the action of bursts and sweeps, as on Upper plane beds.

A sequence of bed phases and stratification formed under upper‐flow‐regime conditions is described based on the observations here combined with descriptions from the literature. With increasing flow strength the gradual sequence of bed phases is: (1) plane bed, (2) low relief, downstream‐migrating inphase waves increasing in amplitude and wavelength with increasing flow strength, (3) stationary in‐phase waves, (4) upstream‐migrating (including breaking wave) antidunes. The sequence of stratification formed under these bed phases is: (1) upper plane bed horizontal lamination, (2) in‐phase wave horizontal lamination to in‐phase wave foreset cross‐laminae, (3) in‐phase wave drape laminae, (4) antidune backset cross‐laminae.
Giant flute‐like scour and other erosional features formed by the 1929 Grand Banks turbidity current
Authors: SHOR ALEXANDER N. , PIPER DAVID J. W. , CLARKE JOHN E. HUGHES , MAYER LARRY A.
Published: June 2006

Erosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side‐scan sonar images and Seabeam multi‐beam echo‐soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100‐m‐deep flute‐shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic‐reflection profiles show that the flute‐shaped scour cuts through this conglomerate and into Plio‐Pleistocene valley‐floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel‐floor erosion was inhibited by the conglomerate veneer.

Valley‐floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel‐floor erosion, increasing the volume of sediment transported by large turbidity currents.
Synsedimentary dolospar cementation: a possible Devonian example in the Camsell Formation, Northwest Territories, Canada1
Authors: MORROW DAVID W.
Published: June 2006

A unique association of beds containing fenestral pore‐filling medium crystalline dolomite with beds of dolospar and quartz sandstone occurs in the Lower Devonian Camsell Formation. Many of these sandstones consist almost entirely of dolospar sand. Sedimentological, petrographic, cathodoluminescent and chemical data indicate that the dolospar sands consist of grains that have been eroded and reworked during Camsell deposition from fenestral dolostone beds that directly underlie these sandstones. Erosional truncation of individual dolomite crystals and of intracrystalline cathodoluminescent zonations along the edges of sand grains indicates that either dolomitization, or dolomite cementation of the pore‐filling carbonate cements within fenestrae of the fenestral fabrics, occurred before erosion and deposition of dolospar sand. The fine details of the cathodoluminescent zonations suggest that the pore‐filling fenestral dolomite originated as primary cements that underwent some annealing recrystallization under the influence of high subsurface temperatures. This unique occurrence may document synsedimentary Devonian cementation by medium to coarsely crystalline dolomite.
Formation of siderite‐Mg‐calcite‐iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England
Authors: PYE K. , DICKSON J. A. D. , SCHIAVON N. , COLEMAN M. L. , COX M.
Published: June 2006

Concretions cemented mainly by siderite, Mg‐calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate‐reducing bacteria are active. δ13C values ranging from −3 to −11·8% (mean −5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ18O values ranged from −6·4% to + 0·8% (mean −1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to‐sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg‐calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron.
Particle over‐passing on depth‐limited gravel bars
Authors: CARLING P. A.
Published: June 2006

An experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of ‘hump‐back’ bars and along the top of planar gravel sheets. Hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like‐sized and finer particles. For prescribed conditions, flat upper‐stage plane sand‐beds develop over the crestal location with pebbles rolling easily over the sandy bed. At the brinkpoint, flow separation ensures effective segregation of the gravel from the sand. Over the slip‐face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed‐form. Consequently, distinctive vertical assemblages of open‐work and closed contact framework gravels could be generated as another bar migrates over, and preserves, the initial structure.

In respect to the observed mechanisms of sorting over the bars, a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper‐stage plane‐bed conditions on the crestal platform. The model then is used to ascertain whether the depositional environment ascribed to certain facies in the Bunter Pebble Beds, described in a recent publication, is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle‐segregation process.
Thermoluminescence properties of fiord sediments from Engelskbukta, western Spitsbergen, Svalbard: a new tool for deciphering depositional environment?
Authors: FORMAN STEVEN L.
Published: June 2006

This study explored the relationship between high‐Arctic fiord depositional environments and the natural thermoluminescence (TL) signal of sediments. The energy and duration of light exposure during transportation and deposition controls the TL level of silicate mineral grains in the sediment. The TL signal of sediments rapidly decreases within c. 0·5 km of a glacier sediment source. The highest TL levels are from tills and ice‐proximal glacial‐marine sediments, which receive little or no light exposure during transportation and deposition. Intermediate and consistent TL levels are recorded for ice‐distal glacialmarine muds, c. 0·5–5·0 km from the glacier front, reflecting slower sedimentation rates. The lowest TL levels are for littoral and sublittoral sediments which receive extended light exposure with shoaling. The granulometry of the sediments is fairly homogeneous and is not diagnostic of a sedimentary environment with most samples dominated by silt and clay; littoral and ice‐proximal samples exhibit peak abundances in sand. These results suggest that the relative TL signal of sediments is sensitive to a depositional environment, particularly for environments proximal (within 0·5 km) to a glacier terminus and in shallow water, less than 15 m deep.
Regressive and transgressive sequences in a raised Holocene gravelly beach, southwestern Crete
Authors: POSTMA GEORGE , NEMEC WOJCIECH
Published: June 2006

Both a modern and a raised, late Holocene gravelly beach have been studied along the cliffed coast near the village of Chora Sfakion in southwestern Crete, Greece. The Holocene beach shows the record of regional, well‐documented relative sea‐level changes, in the form of regressive and transgressive sequences. The texture, fabric and depositional architecture of these sequences are described and compared with those of the modern, microtidal beach there. The depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea‐level changes.

The regressive sequences of both the modern and the Holocene beach resemble those of the mesotidal beaches in the Sker district of southwest Wales, described by Bluck (1967). The transgressive sequence of the Holocene beach resembles the mesotidal beach sequence near Newton (southwest Wales), recognized by Bluck as another type of progradational (regressive) beach. Therefore, transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive. The stratigraphic depositional patterns of gravelly beach systems are, apparently, still poorly understood.
Reflected sediment gravity flows and their deposits in flysch of Middle Dalmatia, Yugoslavia
Authors: MARJANAC TIHOMIR
Published: June 2006

The Eocene flysch of Middle Dalmatia comprises several beds that are interpreted to have been deposited from reflected sediment gravity flows. Their compositions are similar and two bed types are differentiated: complex beds that are debrite‐plus‐turbidite couplets, and turbidites.

The sequence alternations in the turbidite part of the bed, opposing ripples within the same bed, and opposite flow directions indicated by flutes and ripples are indicative of flow reflections. The influence of seiches is suggested by the occurrence of symmetrical (oscillation) ripples.

The palaeotransport directions of reflected flows show wide dispersal. A geometry of small, fault‐controlled sub‐basins with centripetal palaeotransport patterns is proposed.
Facies‐controlled shrinkage‐crack assemblages in Middle Proterozoic mudstones from Montana, USA
Authors: KIDDER DAVID L.
Published: June 2006

Shrinkage‐crack morphotypes in the Libby Formation (upper Belt Supergroup) are confined to distinct environmental facies. The lower facies is characterized by flat rip‐up clasts, stromatolites, oolites, small‐scale symmetrical ripples, and fenestral fabric. These rocks were deposited above fair‐weather wave base on a periodically exposed mudflat. Shrinkage cracks in this facies are predominantly branching, incompletely connected features in plan view, except for local examples of completely connected polygonal cracks on purple argillite bed tops and rare, long spindle‐shaped cracks on bed tops of dark grey argillite. The upper facies was deposited below fair‐weather wave base and contains mainly unconnected, short spindle‐shaped shrinkage cracks, and rare slightly branching cracks.

Restriction of some crack types to certain facies better constrains interpretation of the origin of these shrinkage cracks. The cracks in the upper facies were strongly influenced by sediment loading, and may have formed by compaction‐induced expulsion of water from pore space, resulting in synaeresis cracks. In the underlying shallower facies, polygonal cracks formed by desiccation. Elsewhere in this facies, incomplete, partially connected cracks and long spindle‐shaped cracks on the same bedding plane are interpreted as having formed by desiccation.

Shrinkage cracks are an under‐used source of environmental information, but confusion as to their origin sometimes restricts their potential. More intensive analysis of properties of host sediment and crack fills may further our understanding of depositional and diagenetic influence on crack morphology. Crack cross‐sections, which are often more commonly exposed than bedding plane cracks, may provide critical additional information on crack genesis. Better understanding of crack genesis will strengthen our ability to interpret unfossiliferous muddy sequences common in Precambrian and lacustrine settings.
Sandstone‐hosted thorium‐bitumen mineralization in the Northwest Irish Basin
Authors: PARNELL JOHN , MONSON BRYAN
Published: June 2006

Low‐grade sandstone‐hosted thorium mineralization is recorded in the Carboniferous Northwest Irish Basin in the Republic of Ireland and Northern Ireland. The thorium is concentrated within bitumen nodules which are replacive within the sandstones. A source of thorium was available in a Caledonian granite in the basin watershed, and the bitumen was a product of hydrocarbons generated within the basin. The thorium occurs as thorite inclusions in the bitumen, and the development of the nodules was enhanced by the polymerization of fluid hydrocarbons by irradiation from the thorium. Thorium mobility may have been enhanced by the formation of organometallic complexes. Mineralization probably therefore occurred during hydrocarbon migration, dated as Late Carboniferous to Permian.
Rates of aerodynamic entrainment in a developing boundary layer
Authors: WILLIAMS JON J. , BUTTERFIELD GRAEME R. , CLARK DAVID G.
Published: June 2006

Despite its significance for inception of grain transport by wind, the initial dislodgement of grains from a static surface by aerodynamic forces of drag and lift in the absence of grain collision has received little attention. This paper describes a series of wind‐tunnel experiments in which the erosion of narrow strips of loose grains from the roughened surface of a flat plate exposed to a range of wind speeds was examined.

The progressive downwind development of the boundary layer over the plate provided a range of airflow conditions which permitted systematic evaluation of grain entrainment rates arising from purely aerodynamic forces. Use of closely graded size fractions in flat, single grain layers resting on identical, fixed grain support eliminated the effects of surface irregularities and impacts from saltation. Results show that erosion of strips of loose grains develops with time according to an inverse exponential function in which the entrainment rate time constant relates to Shields dimensionless shear stress function. An empirical expression defining aerodynamic entrainment rate in terms of rate of strip erosion is derived and comparisons are made between present and published data. The need for additional data to resolve several questions raised by the present investigation is stressed.

In addition, a simple, objective technique for accurate determination of the aerodynamic entrainment threshold of any loose, granular sediment is proposed.
A wind tunnel simulation and field verification of desert dust deposition (Avdat Experimental Station, Negev Desert)
Authors: GOOSSENS D. , OFFER Z. I.
Published: June 2006

The deposition of natural dust in an area of 53 ha, situated in the northern Negev desert, is investigated in detail both in the wind tunnel (dust storm simulations over a topographic scale model) and in the field. The wind tunnel results and the field results show a high degree of agreement, indicating that scale‐model simulation may be considered an important technique for future loess and desert research. More dust settles on windward slopes than on leeward slopes, which is in contradistinction with the widespread wind shadow concept. Air‐flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too.

In the case of dust deposition on topographic scale models, a restricted height distortion of the model will not necessarily lead to serious problems. In addition, wind tunnel blockage percentages up to 13% may be allowed in order to obtain acceptable dust deposition patterns for the scale model.

A mean gross dust deposition of about 200–250 g m−2 year−1 is calculated for the northern Negev desert for 1987. Thus, if the settled dust can be protected against erosion in the cultivated areas in the Negev, the dust content of the top soil will markedly increase with time. However, it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and, hence, to a higher dust content in the atmosphere. At any rate, a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields.

From the air dust concentration data and the dust deposition data, a deposition velocity of 4.7cms−1 can be calculated for Avdat dust.
A tidal‐inlet complex in the Cretaceous epeiric sea of North America: Virgelle Member, Milk River Formation, southern Alberta, Canada
Authors: CHEEL R. J. , LECKIE D. A.
Published: June 2006

The upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing‐on‐Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave‐dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non‐marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal‐inlet complex. Most inlet sections consist of an erosional base overlain by a shale‐pebble conglomerate, followed by cross‐bedded sandstones which become finer‐grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re‐activation surfaces and herringbone cross‐beds. The sequence through the tidal‐inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb‐dominated, flood‐dominated, or mixed‐tidal influence. Ebb‐dominated sections commonly contain lateral accretion surfaces whereas flood‐dominated sections contain tidal‐ramp deposits. Back‐barrier lagoon deposits are dominated by sandstones of an extensive flood‐tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb‐tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave‐induced longshore currents acting on the regionally wave‐dominated shoreline.
Sedimentology and tectonic implications of Cretaceous fan‐delta conglomerates, Queen Charlotte Islands, Canada
Authors: HIGGS ROGER
Published: June 2006

The Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast‐supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse‐to‐normal graded beds; and parallel‐stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high‐density turbidity currents.

The depositional environment was a deep‐water, gravelly fan that draped a fault‐controlled, basin‐margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the name fan delta can be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as a deep‐water fan delta. The lack of a shelf is in marked contrast to other types of fan delta.

Three facies associations are recognized in the Honna Formation: subaqueous proximal‐fan conglomerates, distal‐fan turbiditic sandstones, and pro‐fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt.

It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia‐Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west‐verging) thrust sheet whose sole‐thrust was the Sandspit Fault immediately to the east.

Deep‐water fan deltas appear to develop preferentially when eustatic sea‐level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep‐water fan deltas from base‐of‐slope (canyon‐fed) submarine fans, because the two systems differ significantly in terms of coarse‐sediment distribution.
Origin of dolomitization on the Barbwire Terrace, Canning Basin, Western Australia
Authors: WALLACE MALCOLM W.
Published: June 2006

A thick, areally extensive subsurface sequence of Upper Devonian carbonates occurs on the Barbwire Terrace in the Canning Basin of Western Australia. It is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized. Slightly deeper water marls have remained as limestones. The major, regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite (up to 600 m) with no primary calcite. A small volume of evaporitic, supratidal dolomite is present at one location. This dolomite is derived from highly saline fluids developed in an arid supratidal environment.

Replacement dolomite of the regional dolomite type has a xenotopic form, with undulose extinction, and irregular crystal boundaries. In addition, saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite. This suggests the regional dolomite type was precipitated at slightly elevated temperatures. Dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution.

Geochemically, the synsedimentary supratidal and regional dolomite types are quite distinctive. Supratidal dolomites have δ18O values which are significantly higher (δ18O=−2 to +1‰ (PDB)) than the regional dolomite type (δ18O=−9 to −2‰ (PDB)). Assuming the lowest δ18O values for the sabkha dolomite represent replacement in marine waters, the oxygen isotopic composition for Upper Devonian Canning Basin marine dolomite would be around δ18O=−2‰ (PDB).

The petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin. However, sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the Barbwire Terrace. Therefore, it is suggested that dolomitization may have taken place in a near‐surface environment with a major recrystallization event superimposed during burial diagenesis.
Anatomy and origin of a Cretaceous phosphorite‐greensand giant, Egypt
Authors: GLENN CRAIG R. , ARTHUR MICHAEL A.
Published: June 2006

Late Cretaceous epicontinental phosphorites, porcelanites/cherts, dark‐coloured shales, glauconitic sandstones and bioclastic and fine‐grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon‐rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea‐level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution‐collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron‐oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters.

This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters.
Diagenetic alunite in clastic sequences, Kuwait, Arabian Gulf
Authors: KHALAF FIKRY I.
Published: June 2006

Diagenetic alunite occurs with calcrete, gypcrete and silcrete deposits in a Mio‐Pleistocene clastic sequence at several locations in southern Kuwait, Arabian Gulf. Based on their physical properties and textural characteristics the alunite deposits were divided into (1) chalky quartzitic alunite, (2) chalky quartz‐free alunite, (3) hard pink alunitic sandstone. The chalky alunite is composed mainly of hydronium‐alunite (H3O)Al3(SO4)2(OH)6, while the hard pink alunite is composed solely of well developed potassium alunite KAl3(SO4)2(OH)6. These minerals resulted from the action of sulphuric acid on clays and K‐feldspars in the muddy sandstone and mudstone host sediments. The sulphuric acid is most probably produced by the oxidation of hydrogen sulphide that might have seeped in from the oil fields of this area. Four diagenetic stages are suggested for the genesis of the studied alunites: gypsification, sulphuratization, silicification and alunitization. In oil field areas, the occurrences of alunite would serve as an indicator for the presence at depth of hydrocarbons.
Neptunian dykes and associated breccias (Southern Alps, Italy and Switzerland): role of gravity sliding in open and closed systems
Authors: WINTERER E. L. , METZLER C. V. , SART M.
Published: June 2006

Neptunian dykes and sills of Middle Jurassic pelagic limestone within Lower Jurassic shallow‐water carbonate host rocks occur at many localities in the Southern Alps of Italy and Switzerland, especially on what were the upper slopes of tilted half‐grabens created during the Early Jurassic rifting stage of a passive margin that faced the Middle and Late Jurassic Tethyan Ocean. The host rocks were dilated by cracking, folding, and brecciation during movements of shallow‐based gravity‐driven slides and slumps of semibrittle platform strata, commonly along décollement contacts between layers of different competence. In most places, the network of cavities in the dilated strata connected to the sea floor, and pelagic sediments trickled from above into the open spaces. In other places, the brittle strata were overlain by somewhat impermeable sediments that formed a partial seal. Sudden dilation of the brittle beds resulted in forceful injection of the overlying weakly consolidated or plastic sediments into open spaces. The filling in both open and closed systems was commonly episodic, resulting in complex internal‐sediment stratigraphy and cross‐cutting dykes. Stable isotopic data on internal sediments and early‐formed cement lie within the field of normal sea water, and none of the sedimentological or stable isotopic data supports a subaerial, dissolution (karst) origin for the Jurassic neptunian dykes of this region.
The shelf sand‐plume model: a critique
Authors: SCHEIHING MARK H. , GAYNOR GERARD C.
Published: June 2006

In recent years, a new model for deposition of sand bodies in a shelf environment has appeared. This model, known as the shelf sand‐plume model, is hypothesized to result from storm‐driven currents that are deflected around a deltaic headland, stripping sand from the headland and redepositing it in a downcurrent ‘plume’ on the inner shelf. The modern analogue for this model is considered to be an arcuate shelf sand body located off the Damietta branch of the Nile Delta. However, the distribution of older deltaic and shoreline sands probably controls the arcuate outline of the sand body. The present current system has certainly reworked these sands into ridges and large‐scale bedforms but is not responsible for the overall outline of the sand body. Grain‐size range and distribution of sand on the shelf demonstrate that the source of sand in the Nile shelf sand body is not the modern Damietta headland as postulated by the shelf sandplume model.

In our view, the shelf sand‐plume model is presently unsubstantiated and has orginated as a misapplication of the original Nile example. As a geological model, the shelf sand‐plume model lacks a set of observable, consistently applicable criteria. The only common denominator to the model is the ‘plume’ geometry of a sand body located off a deltaic promontory. However, workers postulating the existence of shelf sand‐plumes have neither clearly established a ‘plume’ geometry nor shown the juxtaposition of these bodies with respect to coeval deltaic headlands in their outcrop or subsurface examples. The model does not provide criteria to distinguish a ‘shelf sand‐plume’ from other classes of shelf sand bodies, notably sand ridges and storm‐generated sheet‐like sands. Its application to the rock record should be re‐evaluated.
Eustatically‐controlled sedimentation in the Hettangian‐Sinemurian (Early Jurassic) of Poland and Sweden
Authors: PIEŃKOWSKI GRZEGORZ
Published: June 2006

In earliest Jurassic times, terrigenous, continental and marginal marine deposition occurred in a large epeiric basin along the Tornquist Line in Europe. Detailed sedimentological studies allow recognition of palaeoenvironmental fluctuations in space and time. The main earliest Jurassic transgressions occurred in the early Hettangian, early Sinemurian, mid‐Sinemurian and latest Sinemurian and formed bounding discontinuities (transgressive surfaces) of considerable correlative significance. There is a step‐wise trend of increasing marine extension and influence during the early Hettangian, early Sinemurian, mid‐Sinemurian and latest Sinemurian‐earliest Pliensbachian transgressions. Four sequences, four transgressive systems tracts, three highstand systems tracts and three levels regarded as equivalents of maximum flooding surfaces are distinguished. In the case of type 2 sequences, when incised valley‐fill deposits are not developed and regional erosion is less common, it may be rather difficult to define the sequence boundaries, which are often concealed within the amalgamated fluvial deposits occurring in the neighbouring parts of two adjacent sequences (fluvial/deltaic sediments terminate the highstand systems tracts and in this setting the transgressive systems tracts start with continental deposits prior to the transgressive surfaces). Generally, an exact correlation can be achieved between the sequence stratigraphy of the northeast and northwest European Lower Jurassic and the eustatic curve proposed by EPR (assuming some changes proposed by A. Hallam). The establishment of this correlation hopefully will stimulate future studies of the sequence stratigraphy of poorly dated siliciclastic deposits of marginal basins. In this setting even minor changes in sea‐level may cause major changes in facies development over large areas.
Transverse and linear dunes in an Upper Pleistocene marine sequence, Corinth Basin, Greece
Authors: COLLIER RICHARD E. LL. , THOMPSON JILL
Published: June 2006

A range of large‐scale dunes of oolitic calcarenite composition are exposed in the Corinth Basin of central Greece. These transverse dunes and a very large linear dune (> 15 m high) lie within an Upper Pleistocene, transgressive marine sequence. Tidal flow, accelerated by constriction through a narrow, fault‐bounded seaway, is interpreted to have generated the current velocities necessary to produce the dunes.

Marine facies in the Upper Pleistocene sequence include beach to offshore conglomerates and sandstones with wave‐modified sedimentary structures and herringbone cross‐stratification. An offshore facies association comprises variably bioturbated siltstones and sandstones with a varied marine fauna that includes thermophile species such as scleractinian corals and Strombus bubonius. Oolitic sandstone facies also occur.

Oolitic sands were apparently produced in shoal environments subject to tidal (and wave) action, and transported by dominant southerly currents over the southern part of the basin. Oolites accumulated in a linear dune 2.7 km long and 15–20 m high and in three‐dimensional transverse dunes up to 10 m high having a variety of compound and simple internal geometries. The isolated, WSW‐ENE‐trending linear form exhibits angle of repose sedimentary dips (up to 35°) of avalanche sets on its SE flank and sets typically with dips of 15–20° to the NW. Internal high‐angle discontinuities are developed in the SE‐dipping lee face. It is proposed that a dominant north‐to‐south flow crossed over the crest obliquely, resulting in both net erosional and depositional processes on the lee flank. A subordinate (?tidal) current may have locally and or periodically crossed the dune crest in a westwards direction. A string of transverse dunes, which were located adjacent to a fault/marine terrace scarp, is interpreted to have originally coalesced to form the linear dune.

The distribution of transverse and linear dunes together with the palaeogeographical reconstruction suggest that a marine connection periodically existed across the Corinth Isthmus during the Late Pleistocene due to a combination of active faulting and glacio‐eustatic highstands of sea level.
Formation of 18O‐depleted dolomite within a marine evaporitic sequence, Triassic Reichenhall Formation, Austria
Authors: SPÖTL CHRISTOPH , BURNS STEPHEN J.
Published: June 2006

Dark grey, bituminous dolostones interbedded with marine‐derived anhydrite horizons occur in the Triassic Reichenhall Formation of western Austria. Fossils are rare and indicate a hostile, hypersaline depositional environment. The dolomites are finely crystalline, fairly stoichiometric, well ordered and non‐ferroan. Closely spaced samples (94 in total) of individual dolomite units have been analysed for their carbon and oxygen isotopic composition. The data indicate surprisingly low δ18O values (‐5.7 to ‐2.1%0 PDB), whereas the δ13C values are comparable to the contemporary Triassic seawater (+0.2 to +2.6%0 PDB). Sedimentological evidence, including (i) lack of any evidence for extensive dissolution, (ii) distinct oxygen and carbon isotope ratios of individual dolomite units, (iii) covariance of carbon and oxygen isotopes within some dolomite layers and (iv) inclusions of celestite in dolomite, indicates a nearly closed system after early diagenesis. Combining this information with water‐rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation. A secondary factor may be dolomite recrystallization at elevated temperatures during burial.
Rock varnish in the Sonoran Desert: microbiologically mediated accumulation of manganiferous sediments
Authors: NAGY BARTHOLOMEW , NAGY LOIS ANNE , RIGALI MARK J. , JONES WILLIAM D. , KRINSLEY DAVID H. , SINCLAIR NORVAL A.
Published: June 2006

Rock varnish occurs in virtually all environments, most commonly in arid and semi‐arid climates, including Antarctica. Rock varnish consists of thin layers of intimately mixed aeolian and chemical sediments often showing botryoidal and more rarely stromatolite‐like morphologies. Typical rock varnish samples collected at Twin Peak Mountain Park, near Phoenix, Arizona, consist of abundant quartz, with plagioclase, illite and a mixed layer, Fe‐clay mineral, probably corrensite. EDS, SEM (BSE) and TEM analyses revealed that the typical Mn, Fe minerals occur as minute particles; some of these particles and other mineral grains are attached to filaments. XRD and electron diffraction showed that the Mn.Fe‐bearing particles are poorly crystalline. The filaments, based on morphological criteria, are virtually indistinguishable from fungal filaments. Most filaments are fragments, probably broken by scraping during sample collection. Coccoid and rod‐shaped forms, resembling cyanobacteria and other bacteria, respectively, are also present. Unlike definitive minerals, these filaments disintegrated in the concentrated energy of the SEM electron beam at the instrumental and experimental conditions used. In addition, no filamentous, rod‐shaped or coccoid forms were observed in samples hydrolysed with 6 N HCl for 24 h at 100°C. Bacteria and fungi in powdered rock varnish were cultured on four media, incubated aerobically in the dark at 25°C. The culture media yielded dense growths of spore‐forming bacteria and filamentous fungi. One fungus and two Bacillus isolates oxidized and concentrated manganese. Control experiments revealed that fungi and bacteria are present on and below the surfaces of rock varnish. Free and hydrolysed, peptide/protein‐bound amino acids were identified in the rock varnish. Amino acids showed virtually no racemization with the exception of D/L asp = 0.1. Relatively high molecular weight humic matter was also separated from the rock varnish. High‐resolution mass spectrometry revealed non‐hydrocarbon moieties, similar to a Suwannee River (FL) humic acid standard. Micro‐organisms and their original biochemical compounds do not seem to be preserved for long in the accreting varnish layer. The studies showed that the filaments helped to trap mineral particles of rock varnish, and that bacteria and fungi abetted Mn concentration. Some structures in the layers of rock varnish resemble stromatolites and present definitions would allow them to be termed as such.