bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagenesis of Ordovician carbonates from the north‐east Michigan Basin, Manitoulin Island area, Ontario: evidence from petrography, stable isotopes and fluid inclusions
Authors: CONIGLIO M. , WILLIAMS‐JONES A. E.
Published: June 2006

Middle to Late Ordovician subtidal carbonates in the Manitoulin Island area of Ontario are predominantly limestone in composition, but non‐ferroan and ferroan dolomite is a common cement as well as a selective or locally pervasive replacement phase. Integration of field, petrographic, geochemical (δ13C, δ18O) and fluid inclusion data indicates that lithification of these carbonates occurred during burial diagenesis, with much of the alteration controlled by regional fracturing and hydrothermal influences.

Aqueous (type 1) fluid inclusions in early calcite (pre‐dolomite) and dolomite are saline (> 29 wt% NaCl eq.) solutions with Ca and/or Mg in excess of Na and display homogenization temperatures with modes of 95 and 101°C, respectively. These temperatures can be explained by significantly more burial than can be accounted for either by the available stratigraphic information or by an unusually high palaeogeothermal gradient, which also is not well supported. The fluid inclusion temperatures are interpreted to have resulted from hydrothermal fluids which circulated during the burial diagenesis of these strata. Type 1 inclusions in late (post‐dolomite) calcite are less saline (<19 wt% NaCl eq.) and have a bimodal distribution of homogenization temperatures with a relatively well defined low temperature peak similar to those in early calcite and dolomite and a broad higher temperature grouping with a mode at 183°C. A small proportion of methane and light hydrocarbon‐bearing fluid inclusions (type 2) are present in all stages of carbonate.

Dolomitizing fluids were derived from burial compaction of argillaceous sediments in the more central parts of the Michigan Basin and the updip migration of these brines along fractures to the basin margin where the carbonates of the Manitoulin Island area were dolomitized. Alternatively, migration of dolomitizing brines downward from the overlying pervasively dolomitized Silurian sequence into fractures in the Ordovician carbonates may have occurred. Integration of the aqueous fluid inclusion data into the diagenetic history of these carbonates remains equivocal because most of the inclusions are secondary or indeterminate in origin. Nevertheless, high salinities resulting from interaction with evaporitic strata and hydrothermal effects are clearly implicated although the origin of the latter remains unclear. The alteration styles of the Ordovician carbonates in the Manitoulin area are similar to those of Ordovician hydrocarbon reservoirs described from other parts of the Michigan Basin. They indicate that fracture‐related diagenesis occurred on a basin‐wide scale and that hydrothermal effects were important.
Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation
Authors: SHANLEY KEITH W. , McCABE PETER J. , HETTINGER ROBERT D.
Published: June 2006

Detailed models already exist that outline physical and temporal relationships in marine and marginal marine strata. Such models are still in their infancy in alluvial deposits. Recognition of tidal and estuarine influence in fluvial strata is critical to the development of high resolution sequence stratigraphic correlations between marine and non‐marine strata. Strata that have previously been interpreted as low energy meandering river deposits contain sedimentary and biogenic structures that suggest a tidal influence. These structures include sigmoidal bedding, paired mud/silt drapes, wavy and lenticular bedding, shrinkage cracks, multiple reactivation surfaces, inclined heterolithic strata, complex compound cross‐beds, bidirectional cross‐beds, and trace fossils including Teredolites, Arenicolites and Skolithos. Although none of these structures is unique to tidal processes, the preponderance of data suggests that fluvial systems have been affected by tidal processes well inland of coeval shoreline deposits. These deposits rarely form a significant proportion of a depositional sequence; however, their occurrence allows time significant surfaces to be extended for tens or even hundreds of kilometres inland from coeval shoreline deposits.

In Turonian through Campanian strata exposed in the Kaiparowits Plateau of southern Utah, tidally influenced facies are recognized within at least two distinct stratigraphic levels that were deposited during periods of relatively rapid base level rise. These strata form part of an alluvial transgressive systems tract. Landward of each of the marine transgressive maxima, tidal facies are present in fluvial channels that are completely encased in non‐marine strata at distances up to 65 km inland from a coeval palaeoshoreline. Our work suggests that such deposits may have gone unrecognized in the past, but they form a significant component of alluvial strata in many depositional sequences. Although these tidally influenced fluvial deposits may be difficult to recognize, they are temporally equivalent to marine maximum flooding surfaces and provide a chronostratigraphic correlation between alluvial and nearshore marine deposits.
Reef Fabrics, biotic crusts and syndepositional cements of the Latemar reef margin (Middle Triassic), northern Italy
Authors: HARRIS M. T.
Published: June 2006

The Latemar reef buildup of the central Dolomites (northern Italy) provides a rare opportunity to examine an in‐place Middle Triassic (Upper Anisian to Lower Ladinian) platform margin that is not strongly deformed or dolomitized. The margin lies between the flat lying platform interior and steeply dipping foreslope clinoforms. Across this transition, the depositional profile relates directly to a consistent lateral facies pattern: (1) restricted‐biota grainstone of the platform interior, (2) ‘Tubiphytes’‐rich boundstone and (3) diverse‐biota grainstone that grades into (4) foreslope breccia beds. The boundstone and diverse‐biota grainstone facies comprise the platform margin.

The boundstone facies consists of a framework of small (< 10 cm) skeletal remains (< 10% by volume) with associated biotic crusts, internal sediments and syndepositional cements. Crusts and cements constitute most of the rock volume and created the boundstone fabric. Biotic crusts exhibit gravity‐defying geometries and range from a light grey, ‘structure grumeleuse’ rind to dark grey, micritic laminae. Both cements and biotic crusts occur as redeposited talus in the foreslope talus deposits, indicating a syndepositional origin.

The diverse‐biota grainstone facies primarily consists of skeletal‐peloidal grainstone with a diverse open marine biotic assemblage, in contrast to the restricted biota grainstones of the platform interior that have a low diversity, restricted marine biota. Metre scale hexacoral boundstone and centimetre‐scale sponge boundstone and microbial boundstone occur as isolated patches (tens to hundreds of metres apart) within the diverse‐biota grainstone facies.

The depositional profile, facies zonation and biotic constituents all indicate that the Latemar buildup had a shallow water reef margin, in contrast to previous interpretations that these were upper slope reefs. The syndepositional biotic crusts and inorganic cementation played key roles in stabilizing the boundstone fabric to form a wave‐resistant reef fabric.
Early diagenetic spherulitic siderites from Pennsylvanian palaeosols in the Boss Point Formation, Maritime Canada
Authors: BROWNE G. H. , KINGSTON D. M.
Published: June 2006

The Boss Point Formation of Maritime Canada comprises alternating successions of braidplain sandstone, lacustrine mudstone, and lacustrine fluvio‐deltaic sandstone and mudstone. These rocks were deposited within an active strike‐slip basin during the early Pennsylvanian (Westphalian A), at a palaeolatitude of 8°S. Palaesols comprise a small but distinct proportion of the formation, and typically occur at the tops of fining upward sandstone‐siltstone cycles.

Spherulitic siderites from the Boss Point Formation occur within sandy and silty palaeosols. They occur as ‘large’ (>200 μm) and ‘small’ (<200 μm) diameter spherulitic nodules within calcrete, and show either a radially arranged ferroan calcite and siderite spar morphology, or contain a core of ferroan calcite, surrounded by a later formed sheath of siderite. Analytical data indicate that with respect to Ca‐Mg‐Mn, wide compositional variation occurs within the siderites. The Boss Point Formation siderites are similar to, but contain less Ca for a given Mg/Mn ratio, than previously published siderite compositional data. Published data, together with the results from the Boss Point Formation, indicate that a continuum exists in freshwater siderites, between the relatively Mn‐depleted and the Mg‐enriched analytical fields that were previously reported.

The relatively low Ca values determined in the Boss Point Formation samples reflect the general lack of calcium in the sedimentary basin during sedimentation. Both chlorite and smectite clays may have been important sources of iron. It is concluded that the spherulitic siderites are eogenetic, and formed within small anoxic ponds rich in organic matter, under reducing and low dissolved sulphate conditions. With time these pools dried up, ferroan calcite precipitating as calcrete nodules around the earlier formed siderite spherulites.
Sediment suspension under waves and currents: time scales and vertical structure
Authors: OSBORNE PHILIP D. , GREENWOOD BRIAN
Published: June 2006

Field measurements of the vertical structure of near‐bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time‐dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near‐bed oscillatory velocity.

Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind‐wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near‐bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near‐bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.
Pleistocene glaciolacustrine sedimentation in a tectonically active zone, Kleszczów Graben, central Poland
Authors: KRZYSZKOWSKI DARIUSZ
Published: June 2006

The Kleszczów Graben in central Poland was formed by late Oligocene to Middle Pleistocene extensional tectonics. During the Pleistocene it was infilled with a 200 m thick sequence of predominantly glacial sediments. Four distinct formations of Elsterian and Saalian age are identified, each containing 15–40 m of glaciolacustrine strata. The boundaries between formations are marked by erosional surfaces and, in part, by angular discordances caused by tectonism. Glaciolacustrine sedimentation was tectonically controlled: the thickness of the sequences in the graben are three to five times greater than outside the area of fault‐controlled subsidence. Deposition in the proglacial lakes was controlled by differential subsidence rates within the basin: deep‐lake facies (varved clays) were deposited in sub‐basins with high subsidence rates and deltaic to shallow‐water facies accumulated in areas of moderate subsidence or occasional uplift. These variations led to the development of a very complex, ‘mosaic’ of lateral facies relationships, suggesting that several sub‐basins with differing subsidence rates were present. The Vertical successions show proximal‐distal sequences typical of glacier‐fed lakes that have limited contact with the ice sheet. However, gravity flow facies are very common, and occur both in the shallow‐ and deep‐water deposits. These deposits are interpreted to have been formed adjacent to active fault scarps which bordered the lake basin. Although several distinct phases of glaciolacustrine sedimentation occurred during the history of trough infilling, the location of the areas of high subsidence varied through time.
The effects of fluctuating base level on the structure of alluvial fan and associated fan delta deposits: an example from the Tertiary of the Betic Cordillera, Spain
Authors: FERNÁNDEZ JUAN , BLUCK BRIAN JOHN , VISERAS CÉSAR
Published: June 2006

The Villanueva Complex is a lacustrine alluvial fan ‐ fan delta system which accumulated on the northern margin of the Guadix Basin during the Pliocene. Five transitional zones can be distinguished from proximal to distal areas: proximal alluvial fan, mid‐fan, fan fringe (transition zone), proximal fan delta and distal fan delta (lacustrine).

This paper focuses on sedimentation in the fan fringe and the proximal fan delta areas, where the effects of fluctuations in the base level (lacustrine level) are more easily observed. The stratigraphical succession is here characterized by an alternation of fine lacustrine sediments (mudstones and siltstones) and gravels. The gravels appear as isolated channels, stacked channels, lenses and sheets.

The isolated channels, which have a V‐shaped profile and better developed wings towards the top, appear in the fan fringe zone and in the fan delta. The stacked channels originate in the fan fringe zone and evolve laterally and distally to isolated channels. The lenticular gravel bodies (lenses) are well represented throughout the fan delta and present small channels at the base, indicating a radial flow pattern. Finally, the sheets are characteristic of the fan fringe zone. The first episodes of channel incision occurred at lowstand lake level. The channels and lenses developed in rising lake level conditions, and sheet deposit took place at highstand lake level. Although the final form of the lithosome was heavily controlled by the fluctuations in lake level, it depended on the relative proportions of gravel and silt sedimentation. The gravel‐siltstone interface therefore represents an equilibrium surface between the proportions of sedimentation of these lithologies.

The bodies mentioned above are organized in coarsening and thickening upwards sequences tens of metres thick, in which a distal to proximal evolution can be observed from isolated channels to lenses or from stacked channels to sheets. The building and thickness of these sequences were controlled by fluctuations in the lake level. An initial fall in base level caused lengthening of the channels and entrenchment from distal to proximal areas, and a continuous increase in supply due to erosion in the drainage basin installed on the subaerial fan. When the lake level rose, lobes were formed at the channel endings and overbank processes were made possible, thus generating radial channels, whose levees were formed by amalgamation with the levees of the main lens channels. At the same time, sheets were formed in proximal areas, where the available amount of sediment was greater.
Development of cusp‐related, gravel size and shape facies at Malin Head, Ireland
Authors: SHERMAN DOUGLAS J. , ORFORD JULIAN D. , CARTER R. W. G.
Published: June 2006

Samples of beach gravels were obtained from foreshore cusp deposits at Portmore Beach, near Malin Head, Republic of Ireland. Axial dimensions (A, B and C) were measured, and the maximum projection sphericity was derived, for each of 4360 clasts. Each sample was also categorized according to depositional subenvironment. Principal components analysis and cluster analysis were used to identify potential facies zonation. Results indicate that reworking of the foreshore by incident and edge waves resulted in clast size and shape segregation both parallel and perpendicular to the beach. Patterns of zonation were distinctly different from those described in earlier facies descriptions, showing a complex set of foreshore facies. The distribution of these facies was controlled primarily by location within beach cusp systems. Primary and secondary facies demonstrate alongshore variability of a magnitude comparable to across shore differences. The results have important implications for palaeoenvironmental reconstruction, morphosedimentary description and research sampling design, and provide the basis for a conceptual model of gravel beach cusp facies.
Porosity evolution of the Cambrian Bonneterre Dolomite, south‐eastern Missouri, USA
Authors: GREGG JAY M. , LAUDON PETER R. , WOODY ROBERT E. , SHELTON KEVIN L.
Published: June 2006

The Cambrian Bonneterre Dolomite of south‐eastern Missouri, USA, hosts the large Mississippi Valley type (MVT) lead‐zinc‐copper ore deposits of the region. The Bonneterre Dolomite consists of dolomitized algal bioherms, oolitic grainstone and associated lithologies that were deposited on a carbonate platform surrounding the Precambrian age St Francois Mountains. Porosity was determined by point counting thin sections from cores and mines in the Bonneterre Dolomite and by gas porosimetry. Volumes of epigenetic cements were estimated by point counting cement filling micro‐ and mesoporosity using cathodoluminescence. Cement volumes were added to present porosity to estimate porosities during various stages of mineralization. Prior to the onset of mineralization, micro‐ and mesoporosity in the Bonneterre Dolomite averaged approximately 19%. Precipitation of early dolomite cement (roughly concurrent with the main period of sulphide deposition) reduced average porosity to approximately 7% and closed off much of the intercrystalline pore space. Later cementation by dolomite (prior to late stage sulphides) reduced porosity to approximately 5%, and late cementation by quartz further reduced porosity to the present average value of <4%. Periods of carbonate dissolution during MVT mineralization enhanced large scale megaporosity associated with fractures and breccias but did not significantly increase smaller scale porosity. Dolomite cementation associated with MVT mineralization, porosity and permeability were facies controlled. Today, as a result of mineralization, large scale fractures and breccias control porosity and permeability. This study indicates that dolomite porosity may undergo significant change during basinal brine migrations associated with MVT mineralization.
Rip currents as a geological tool
Authors: GRUSZCZYŃSKI MICHAŁ , RUDOWSKI STANISŁAW , SEMIL JULIA , SŁOMIŃSKI JAN , ZROBEK JERZY
Published: June 2006

This paper considers the nature and sedimentary significance of rip currents. Rip currents are the main factor responsible for the transport of coarse sediments from the littoral zone to greater depths. Such sediments, deposited outside the zone of wave deformation, may be identified as storm rip current increments within sediments deposited during fair weather. Composite beds deposited during a given wave cycle by storm rip currents are closely similar to ‘turbidites’ and many so called ‘fluxo‐turbidites’ described from flysch deposits.

Using data gathered in studies conducted on the modern Baltic coast, supplemented by experimental work and theoretical considerations, a sedimentary model is proposed. The model may be used to interpret possible rip current deposits among shallow water ‘turbidites’, and both modern storm sediments and ancient ‘tempestites’.
Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley
Authors: RENAUT R. W.
Published: June 2006

Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake.

The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X‐ray microanalysis) or 2–18 (d‐value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na2CO3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by‐product of the reactions. Chemical analyses suggest that pore waters supplied some of Na+, and possibly K+ and SiO2. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca2+ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.
Stable isotopic compositions of Recent freshwater cyanobacterial carbonates from the British Isles: local and regional environmental controls
Authors: ANDREWS J. E. , RIDING R. , DENNIS P. F.
Published: June 2006

Recent (<50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean δ18O value of −5–9‰ PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (−7–5‰SMOW) assuming a mean water temperature of 9°C. The mean δ18O of lake data, −4–5‰ PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to −12‰ PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light ‘soil zone’ carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO2 and aqueous HCO−3.

We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary δ18O values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.
Facies and depositional processes in an Upper Jurassic to Lower Cretaceous pelagic sedimentary sequence, Antarctica
Authors: WHITHAM A. G.
Published: June 2006

The Nordenskjöld Formation (?Oxfordian‐Berriasian age) is exposed on the east coast of the Antarctic Peninsula, where it consists of interbedded ash layers and biosiliceous mudstones which accumulated under anaerobic to dysaerobic bottom waters. The mudstones were deposited by pelagic settling and the ash layers by pelagic settling from suspension or as fallout from subaerial eruption columns. The lower part of the succession accumulated in a basinal setting under anaerobic bottom waters and is characterized by parallel bedding. Mudstones deposited in this setting preserve abundant zooplankton faecel pellets. Compaction of these pellets has given rise to a bedding parallel fissility. The upper part of the succession accumulated under dysaerobic bottom waters in a slope setting. The sequence is wavy bedded and contains abundant evidence of post‐depositional sediment instability and resedimentation, much of which was caused by tectonic activity. Discrete slide masses are absent from the slope sequence and it appears that slope processes were dominated by creep. Examination of the mudstones shows that as levels of dissolved oxygen in bottom waters increase, pelleted mudstones give way to structureless mudstones before visible bioturbation is noted.
The origin of massive diamicton facies by iceberg rafting and scouring, Scoresby Sund, East Greenland
Authors: DOWDESWELL JULIAN A. , WHITTINGTON ROBERT J. , MARIENFELD PETER
Published: June 2006

Almost 90% of 39 m of core material recovered from Scoresby Sund and the adjacent East Greenland shelf is massive diamicton, interpreted to be formed predominantly by the release of iceberg rafted debris and reworking by iceberg scouring. There is also likely to be a contribution from suspension settling of fines derived from glaciofluvial sources. Model calculations suggest that the 14C derived Holocene sedimentation rate of 0.1‐0.3 m 1000 yr−1 in Scoresby Sund can be accounted for mainly by iceberg rafting of debris. A further 4% of core material is of gravel or coarse sand lenses, interpreted to reflect iceberg dumping of debris. Intensive iceberg scouring, which reworks sea floor sediments, is observed on acoustic records from over 30 000 km2 of the Scoresby Sund fiord system and the adjacent East Greenland shelf (69‐72°N and 75°N). The rate of iceberg production from Greenland Ice Sheet outlet glaciers, and iceberg drift tracks on the shelf, suggests that iceberg rafting and scouring may be important over a significant proportion of the 500 000 km2 area above the shelf break. The relatively extensive modern occurrence of massive diamicton, formed by iceberg rafting and scouring, together with suspension settling of fines, suggests that it may also be a significant facies in the glacier‐influenced geological record. The recognition in the geological record of the massive diamicton facies described above may also indicate the former presence of fast flowing ice sheet outlet glaciers.
Transgression of an estuarine channel and tidal flat complex: the Lower Triassic of Barles, Alpes de Haute Provence, France
Authors: RICHARDS MARCUS T.
Published: June 2006

The Lower Triassic succession of Barles, Alpes de Haute Provence, France, comprises an unconformable quartz arenite sand body of 90m thickness. The succession may be informally divided into (i) lower channellized cross‐bedded member overlain by (ii) an upper fining upward member. The lower member comprises vertically stacked, subtidal channel units separated into five major sand bodies by thin developments of fine grained channel margin and shoal deposits. Subtidal channel fill deposits are dominated by varying scales of cross bedding. These scales vary systematically from the base to the top of the member, with large scale planar sets dominating the lowest channel sand body (sand body 1), medium scale planar and trough cross bedding characterizing sand bodies 2‐4, the largest scale planar sets in the highest sand body (sand body 5). This upward change in cross bedding scale is concomitant with a decrease in both the relief of major channel sand body erosion surfaces, and the proportion of preserved interchannel shoal deposits. The succeeding fining upward member comprises small scale tidal channel units overlain by channel shoal and tidal flat deposits. Tidal flat sequences are characterized by parallel laminated, wave and current rippled sandstones separated by bioturbated, fine grained siltstones and mudstones.

The vertical variation in facies of the Lower Triassic succession suggests two main periods of deposition. The lower member is considered to preserve successively more seaward components of a transgressive estuarine complex. The overlying upper member records the seaward progradation of tidal channel, shoal and tidal flat environments.

The unconformity bounded nature of the lower member, combined with its systematic variation in facies, suggests it may represent an incised valley‐estuarine fill developed in response to an early Triassic relative sea level fall and subsequent rise. Succeeding tidal channel and tidal flat deposits forming the upper fining upward member reflect a change in sediment supply and/or rate of relative sea level rise comparable with a progradational shoreline. It is unclear whether this final depositional episode represents a period of highstand progradation or a later lowstand shoreline system developed following a further period of relative sea level fall and rise.
The anatomy of a sulphate platform and adjacent basin system in the Leba sub‐basin of the Lower Werra Anhydrite (Zechstein, Upper Permian), northern Poland
Authors: PERYT TADEUSZ MAREK
Published: June 2006

The Lower Werra Anhydrite (Zechstein, Upper Permian) deposits of the teba area originated in a deep basin setting, in shallow to deep water conditions. Facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms. This pattern of local platforms and adjacent basins developed during deposition. In basinal areas, the sequence is clearly transgressive, whereas on platforms accumulation kept pace with subsidence after an initial transgression.

Nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition. Massive anhydrite with pseudomorphs after upright‐growth gypsum crystals suggest rapid precipitation in a subaqueous environment and/or fluctuating, but generally high, salinity conditions. Massive clastic sulphate originated due to periodic high energy events and resedimentation, or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite. Massive, textureless anhydrite is locally porous and passes upward into breccia, indicating a strongly saline environment. Bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water. Bedded anhydrites contain portions which are graded. Intercalations of sulphate turbidites and upright‐growth gypsum suggest fluctuating water depths, with comparatively deep water during turbidite deposition, but shallower conditions during upright‐growth gypsum deposition.

The sequence observed in slope zones at platform‐basin margins, detrital (parautochthonous) sulphate sand to graded beds to basinal laminites, indicates that redistribution processes were important. At the onset of the Lower Werra Anhydrite deposition bathymetric relief existed between the central part of the basin and its margins, where carbonate platforms remained subaerially exposed. Formation of local platforms and adjacent basins required a relatively high subsidence rate, as pre‐existing relief cannot account for the total accumulated thickness of the Lower Werra Anhydrite deposits. One implication of this is that the main argument against ‘the shallow water ‐ shallow basin’ evaporite basin model, i.e.,a very fast rate of subsidence, may not be valid for the Łeba Lower Werra Anhydrite basin.
Fluted moraine formation and till genesis below a temperate valley glacier: Slettmarkbreen, Jotunheimen, southern Norway
Authors: BENN DOUGLAS I.
Published: June 2006

Clast fabric and morphological data have been used to determine the origin of fluted subglacial tills exposed by recent retreat of the Slettmarkbreen glacier, Norway. A new method for the interpretation of clast fabric data allows aspects of the strain and depositional history of the till to be reconstructed. The till formed by a combination of lodgement and subsole deformation by slip along discrete shear planes. Lodgement was dominant for the larger size fractions (>125 mm), while the smaller material was more susceptible to deformation. The fluted till surface reflects the tendency for the till matrix to deform into regions of low confining pressure in the lee of lodged boulders. Downglacier components of till flow are thought to have resulted in significant sediment transfer towards the margin.
Sea level control of stacked late Quaternary coastal sequences, central Great Barrier Reef
Authors: GAGAN MICHAEL K. , JOHNSON DAVID P. , CROWLEY GABRIEL M.
Published: June 2006

Lithofacies analysis, pollen assemblages and radiocarbon age dates of 20 stratigraphic drill holes are used to develop an evolutionary history for late Quaternary sedimentation in two coastal embayments landward of the central Great Barrier Reef. Different physiographic settings of the embayments result in two contrasting styles of sedimentary sequence: (a) an exposed, moderate energy, beach barrier‐lagoon system (Wyvuri Embayment) and (b) a protected, low energy, muddy inlet fill sequence (Mutchero Inlet). Despite sharp contrast in sequence style, similar depositional cycles occur in both embayments in response to late Quaternary sea level fluctuations including: (1) a last interglacial highstand (+2 m; c. 125 000 yr bp) beach barrier (Wyvuri); (2) an early to mid‐Holocene (8000–6100 yr bp) transgressive beach barrier‐lagoon (Wyvuri) and estuarine infill (Mutchero); and (3) mid‐Holocene to present highstand beach barrier (Wyvuri) and estuarine (Mutchero) progradation. Preservation of such cycles in the stratigraphic record would produce a series of vertically stacked and offset linear barrier sands surrounded by lagoonal mud and fine grained shoreface sediment juxtaposed to muddy, estuarine infills.

Sea level elevations are well recorded by the upward transition from Rhizophora‐dominated intertidal mangrove mud to freshwater swamp (clearly identified by pollen analysis) and by the basal contacts of beach barrier sediments which sharply overlie the upper shoreface. Transgressive sedimentation is interrupted in both embayments by a constructional beach barrier (Wyvuri) and abbreviated progradation (Mutchero) corresponding to a — 5 m pause in relative sea level rise at c. 6800 yr bp. Sea level control of fine scale coastal sedimentation patterns is beginning to be widely recognized and provides an accurate analogue for stacked ancient sequences.
Evaluation of a wide range laser diffraction grain size analyser for use with sediments
Authors: LOIZEAU J.‐L. , ARBOUILLE D. , SANTIAGO S. , VERNET J.‐P.
Published: June 2006

The qualities of a laser diffraction grain size analyser, the Coulter LS‐100 (range claimed by the manufacturer: 0·4–900 μm in a single measurement), are evaluated on sediments of fluvial and lacustrine origin.

Accuracy and resolution of measurement on standard latex spheres are excellent. Reproducibility of the results on natural sediments appears to be satisfactory, but the method underestimates the fraction of clay particles with an efficiency of detection (36–70%) proportional to the clay content determined from pipette analysis. This efficiency is somewhat higher than those reported from other instruments of the same generation.

Comparison of the Coulter LS‐100 with other sizing techniques shows good agreement with the sieving method but some differences appear with the electroresistance particle technique: median and mean size values measured by the Coulter Counter TA1 are systematically lower than those obtained by the Coulter LS‐100. Analyses show good correlation with those of a Malvern Laser particle analyser but a discrepancy appears with very fine silt and clay sized sediments. The Coulter LS‐100 detects a higher clay content than that measured with the Malvern Laser Sizer 2600. Except when precise measurements of clay content are needed, the Coulter LS‐100 produces precise and accurate results in size ranges required for geological and environmental studies.
Origin of quartz cements in some sandstones from the Jurassic of the Inner Moray Firth (UK)
Authors: VAGLE GRETE BLOCK , HURST ANDREW , DYPVIK HENNING
Published: June 2006

The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix‐supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM‐CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM‐CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a‐axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ18O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ18O composition of +20·0‰. Estimates of the δ18O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ18O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.
Orbitally forced cyclical changes in the quantity of calcareous and siliceous microfossils in an Upper Jurassic to Lower Cretaceous pelagic basin succession, Bakony Mountains, Hungary
Authors: HAAS J. , KOVÁCS L. Ó. , TARDI‐FILÁCZ E.
Published: June 2006

Rhythmic changes were observed in the quantity of radiolarians and calpionellids preserved in an Upper Jurassic to Lower Cretaceous deep water succession in the Bakony Mountains of Hungary. Statistical analysis revealed that for short periods (order of 100 kyr) the correlation of the two planktonic groups is negative. Over longer time intervals definite trends are recognized, which can be explained by the combined effect of facies transition, evolution of the calpionellids and rhythmic environmental (climatic) changes. Spectral analysis and time span calculations indicate that orbital forcing in the Milankovitch frequency band influenced the observed changes.
Palaeovalleys and protozoan assemblages in a Late Carboniferous cyclothem, Sydney Basin, Nova Scotia
Authors: GIBLING MARTIN R. , WIGHTMAN WINTON G.
Published: June 2006

The early Stephanian Bonar Cyclothem of the Sydney Basin, Nova Scotia, contains an erosional surface cut through coastal plain strata with economic coals and distributary channel bodies. The erosion surface is interpreted as a palaeovalley 20 m deep and at least 7 km wide that marks a sequence boundary formed during relative fall in sea level. The palaeovalley is filled with stacked alluvial channel bodies which become more isolated as the valley fill passes upward into red, alluvial plain deposits, probably laid down in an anastomosed river system. In an adjacent, interfluve area, calcretes and red, vertic palaeosols cap coastal strata. Assemblage analysis of agglutinated foraminifera and thecamoebians indicates that the palaeovalley was filled with freshwater sediments before an initial marine transgression flooded the alluvial surface and adjacent interfluve.

Valley incision probably reflects glacioeustatic sea level fall. However, the alluvial nature of the valley deposits suggests that valley filling reflects an abundant sediment supply during lowstand and/or transgressive stages and was not a direct consequence of sea level rise. During the subsequent transgression phase, aggradation was rapid as sediment supply apparently kept pace with rising sea level. Features of both channel and extra‐channel facies suggest that seasonality intensified during the transition from coastal plain to palaeovalley and alluvial plain deposition.
The genesis of the silica sands of Black River, St Elizabeth, Jamaica
Authors: JACKSON TREVOR A. , WEST‐THOMAS JENNIFER
Published: June 2006

The only reported occurrence of quartz‐rich sands in Jamaica is on the plains of Black River, St Elizabeth, where they outcrop in isolated pockets. The sands overlie limestones of the White Limestone Supergroup and the Coastal Group, and partly underlie Holocene peat deposits.

Monocrystalline quartz, containing fluid and mineral inclusions, is the dominant mineral of the sands, with the heavy minerals. magnetite, haematite, epidote, rutile, anatase, zircon and tourmaline, occurring in minor amounts. Grain surface texture studies of the quartz grains have identified a series of mechanical and chemical features. Microtextures depicting marine, aeolian and pedological environments have been retained on the grains and represent the last three stages in the history of the grains.

The petrography of the sands indicates an igneous provenance, with the major source of the detrital minerals being the Cretaceous granitoids that outcrop in the eastern half of the island. These rocks were unroofed during the uplift of Jamaica in the Late Tertiary and were subjected to extensive chemical weathering, which accounted for the breakdown of the majority of their primary silicate minerals. Fluvial transportation and deposition of detritus onto the shelf of the south coast commenced during this period and continued into the Pleistocene, providing terrigenous sand that was modified along the coast to quartz arenite by moderate to high energy marine conditions. Tidal currents, east‐west longshore currents and weathering contributed to the maturity of the sand by the time it had been transported along the shelf to the vicinity of Black River. During low sea level stands in the Pleistocene, some of this sediment that became trapped on the shelf was blown inland by south‐east winds.
Carbonate depositional sequence development on active fault blocks: the Albian in the Castro Urdiales area, northern Spain
Authors: ROSALES IDOIA , FERNÁNDEZ‐MENDIOLA PEDRO ANGEL , GARCÍA‐MONDÉJAR JOAQUÍN
Published: June 2006

The Aptian to lowermost Albian carbonate platform of Castro Urdiales (Cantabria, northern Spain) was broken up by extensional tectonic movements shortly after the beginning of the Albian. Block faulting characterized this rifting episode, the effects of which waned during the Albian. In crestal locations tilting of the fault blocks caused the subaerial exposure of parts of the older platform, resulting in intense karst diagenesis. Differential subsidence of the blocks controlled the development of a crestal residual carbonate platform (Arenillas), which was surrounded by deeper water on both sides. Seven unconformities related to platform exposure and karstification are identified on the Arenillas platform, and form the basal boundaries of seven depositional sequences (S1‐S7). On the platform, lowstand systems tract deposits consist of breccias filling caves, and grainstones and debris flow deposits filling incised canyons. Transgressive plus highstand systems tracts consist of rare marls plus shallow water rudistid, coral and chondrodontid limestones. In the basin, the unconformities are erosional surfaces at the base of resedimented limestones, marls or sandy or silty siliciclastics (lowstand systems tracts). Transgressive plus highstand systems tracts in this setting consist of marls and hemipelagic marly limestones. Tectonism is believed to be the main control on sequence formation, and only a few sequence boundaries (e.g. the base of S6 in the Upper Albian inflatum Zone) can be correlated with eustatic events.
Reefal coralline algal build‐ups within the Arctic Circle: morphology and sedimentary dynamics under extreme environmental seasonality
Authors: FREIWALD A. , HENRICH R.
Published: June 2006

Carbonate frameworks secreted by phototrophic organisms within the Arctic Circle are not well documented. Underwater surveys of the inner‐shelf off Troms, northern Norway (70°N), reveal extended fixed algal build‐ups which are fringed by rhodolith belts affected by storms. Reefal growth by coralline algae under temperature and light regimes of extreme seasonality is made possible because of a decoupling of carbon fixation during summer and utilization of stored carbon during the period of winter darkness. Although the annual growth of the framework constructing algae is comparatively low, the annual carbonate production rate is similar to subtropical‐tropical counterparts because of a remarkably high standing stock. Early diagenetic alteration is restricted to intraparticle cementation processes which start in vivo. Bioerosional destruction is the dominant control on the preservation of high latitude build‐ups. Preservation of Holocene autochthonous coralline algal biostromes is enhanced by rapid burial during storm events. Redeposition during storms is the most important process in forming a distinct sedimentary facies zonation.
Thin section grain size analysis revisited
Authors: JOHNSON M. R.
Published: June 2006

Existing proposals for converting thin section data to their sieve equivalents are all flawed in various ways, while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using φ units, and of non‐spherical (ellipsoidal) grains using both millimetres and φ units, have not been ‐satisfactorily resolved in the literature. It can be shown mathematically that the mean thin section diameters of spherical grains, or axis lengths of a series of parallel sections through ellipsoidal grains, will underestimate the dimensions of the corresponding central section (i.e. one passing through the grain centre) by 0.2023 φ when measured on a volume frequency basis. In order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains, the dimensions of a series of sections cut in 49 unique directions, symmetrically arranged and evenly spread with respect to the ellipsoidal axes, were calculated. This calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations. The data indicated that the mean true nominal diameters (D̄) of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters (d̄′) and major axes (ā′) of the central sections (derived from the observed values by multiplying the millimetre means by 1·1318 and subtracting 0·2023 from the φ means) using the following equations:

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A rough estimate (to within c. 5%) of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millimetres or φ units.