bluuebunny/crossref_metadata_2025 huggingface dataset:
Amalgamated accumulations resulting from climatic and eustatic changes, Akchar Erg, Mauritania
Authors: KOCUREK GARY , HAVHOLM KAREN G. , DEYNOUX MAX , BLAKEY RONALD C.
Published: June 2006

The Akchar Erg of the Sahara of western Mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late Pleistocene and Holocene deposits that reflect eustatic and climatic events. Mapping, trenching, and dating by 14C methods and artefacts show that the prominent complex linear dunes (draas) of the Akchar Erg are actually composite features showing at least three constructional and two destructional phases. The constructional phases are represented by three convex‐up layers: (i) a modern veneer moulded into superimposed crescentic dunes, which partially mantle the larger linear bedforms; (ii) a middle, partly root‐turbated sand deposited sometime during the last 4000 years; and (iii) a core of linear dune sand formed during the last glacial period (13 000–20 000 yr BP), which today shows relict relief, intense root‐turbation, and pedogenesis. These constructional phases are separated by super bounding surfaces that coincide with erg destructional phases. Surface 2 bounds the middle aeolian sand, and is marked by a lag surface of small granules. Surface 1 is a very prominent surface with an abundance of Neolithic artefacts, and represents stabilization of the linear dunes during the humid, interglacial period (4000–11 000 yr BP). Interdraa deposits originated during the interglacial period, and consist of continental lacustrine limestones and sandstones, humic sands deposited in marshes, and sabkhas on the coast. The sabkhas originated during interglacial highstand of sea‐level when interdraa areas were marine embayments, and subsequently dried during regression. The draa and interdraa sequences, therefore, in spite of being adjacent facies, actually represent different events and were not formed simultaneously. The upwind sand‐sheet margin of the Akchar Erg shows exposures of the middle and core aeolian sands (which were previously protected from deflation by vegetation) being progressively cannibilized in the current phase of erg construction, and revealing a crystalline basement rock. In this proximal area, conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record.
The anatomy of an early Dinantian terraced floodplain: palaeo‐environment and early diagenesis
Authors: ANDREWS JULIAN E. , TURNER MICHAEL S. , NABI GHULAM , SPIRO BARUCH
Published: June 2006

The lowermost Carboniferous rocks in the Cockburnspath area of east Berwickshire (southern Scotland) are interpreted as coastal floodplain sediments. A lower mudstone‐dominated unit is composed of silty mudstones and shales with subordinate sandstones and argillaceous ferroan dolomites (cementstones). These are interpreted as distal floodplain sediments with periodic crevasse‐splay deposition. The dark grey colour of the mudrocks suggests deposition in reducing conditions, probably in floodplain lakes. Most of the cementstones are concretionary, some with septarian cracks, suggesting an early diagenetic origin. An immature palaeosol suggests periodic pedogenesis under improved drainage. A synsedimentary erosion surface indicates incision of a valley into the floodplain, presumably in response to base‐level lowering.

An upper sandstone‐dominated unit starts with fine‐grained rippled sandstones, cut by small channel sandstones. These are interpreted as floodplain lake deposits fed by crevasse channels. A distinctive conglomerate with cementstone clasts, wood fragments and fish remains is interpreted as a major overbank deposit, dumped into a pre‐existing floodplain lake. A bivalve fauna was established in the overlying mudstones, followed by a thin limestone with a restricted marine fossil assemblage, showing that seawater flooding of the lakes occurred at times.

Mudrocks throughout the sequence contain no pyrite, except for the marine band which has an organic‐carbon/sulphur ratio and degree of pyritization value typical of marine sediments. The concretionary cementstones have δ13C values around —4 to —6%0/00 PDB which are interpreted as indicative of anaerobic oxidation of organic matter. The combined geochemical data suggest a significant involvement of iron reduction in cementstone formation, although the δ13C values are ambiguous in assessing the relative involvement of methanogenesis and methane oxidation. Limited seawater inundation of the floodplains might have supplied magnesium and calcium ions for dolomite formation assuming that any H2S derived as a result of sulphate reduction was oxidized by iron reduction. Alternatively a weathering source for solutes might have been involved.
Sedimentology of a Proterozoic erg: the Venkatpur Sandstone, Pranhita‐Godavari Valley, south India
Authors: CHAKRABORTY TAPAN
Published: June 2006

Reappraisal of the Late Proterozoic Venkatpur Sandstone indicates that the bulk of the sandstone is aeolian in origin. Aeolian stratification types, namely (i) inverse graded translatent strata, (ii) adhesion laminae, (iii) grainflow strata and (iv) grainfall strata, are present throughout the outcrop belt. Nine facies have been identified that represent both aeolian and related aqueous environments within a well‐developed erg. Cosets of large cross‐beds at the Bellampalli section in the NW of the study area record dune fields in the interior of the sand sea. To the SE, at the Godavari River and Ramgundam sections, a progressive increase in the relative proportion of the flat‐bedded to cross‐bedded facies and intercalated non‐aeolian facies delineates the transition from the dune‐field to sand‐sheet environment. An alternating sequence of aeolian and marine sediments at Laknavaram, in the extreme SE, marks the termination of the sand sea. Palaeocurrent data suggest that the NW‐SE trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction.

Abundant horizontally stratified units in the Vankatpur Sandstone do not always represent the interdune sediments. On the basis of the thickness and geometry of the units, nature of bounding surfaces and associated facies sequence, the facies is variously interpreted to represent interdune, inland sabkha, sand sheet and coastal sand flat deposits.
Origin of a cool‐water, Oligo‐Miocene deep shelf limestone, Eucla Platform, southern Australia
Authors: JAMES NOEL P. , BONE YVONNE
Published: June 2006

The Abrakurrie Limestone is an areally extensive, bryozoan‐rich unit within the Eucla Platform, a Tertiary carbonate shelf which caps the central part of the southern Australian continental margin. The onshore portion, the topic of this study, has been exposed since middle Miocene time and lies beneath the Nullarbor Plain. The rocks are fine‐sand‐ to granule‐sized calcarenites, composed of bryozoans, bivalves, benthic foraminifera and echinoids with lesser numbers of brachiopods, solitary corals and serpulids. They conspicuously lack significant numbers of planktonic foraminifera and coralline algae. Most bryozoan remains are from delicate branching cyclostomes although delicate branching, robust branching, foliose, fenestrate, multilaminar encrusting and free‐living cheilostomes are variably abundant in specific units. The poorly lithified sequence is punctuated by well‐cemented layers with erosional tops, which are interpreted as hardgrounds.

The limestone is interpreted as a cool‐water, deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the Eucla Platform. A model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation. The shelf is envisaged as partitioned by the depth of the zone of wave abrasion. Sediments were produced throughout, but accumulated only below this depth. When the seafloor was above this depth it was an environment of cementation and erosion.

The vertical sequence correlates in a general way with the global sea‐level model for the mid‐Cenozoic. While accumulation rates for southern Australian carbonates are similar to rates of cool‐water carbonate deposition elsewhere (c. 2.5 cm kyr‐1), the rate of Abrakurrie accumulation is much less, suggesting that significant time periods are represented by the hardgrounds.
Late Quaternary mass movement on the lower continental rise and abyssal plain off Western Sahara
Authors: SIMM R. W. , WEAVER P. P. E. , KIDD R. B. , JONES E. J. W.
Published: June 2006

The late Quaternary development of part of the lower continental rise off Western Sahara has been determined from an investigation of short (< 2 m) gravity cores collected from a deep‐sea channel, the interchannel areas and an abyssal hill, between 30 and 33°N. Stratigraphic analysis is based on systematic variations in abundances of particular coccolith species and pelagic sediment types, referenced to the oxygen isotope time‐scale. During the last 73 000 years deposition in the channel has included volcaniclastic sand/silt turbidites and minor marl turbidites as well as pelagic sediments. The interchannel area has fewer turbidites, and the sands present were probably deposited from turbidity currents which spilt over the channel sides. The last‘event’ to give rise to sands in the channel and interchannel area occurred about 45 000 years ago. Although the channel has been inactive as an area of turbidity current deposition for the last 20 000 years, sands were deposited elsewhere on the lower rise, indicating that turbidity current transport routes have varied in time.

Turbidity current deposition on the abyssal plain and low‐lying continental rise appears to be related to distinct sliding events involving transport of material from various sources. Thin marl turbidites are interbedded with pelagic sediments in the area of sediment drape. There is a strong correlation between these and the thick marl turbidites on the abyssal plain, suggesting that the same turbidity current‘events’, occurring about once every 25 000 years, gave rise to both sets of deposits. The thinner units probably represent deposition from the outer parts or tails of the large turbidity flows. The turbidites occur at glacial/interglacial transitions, suggesting that the slides that created them were triggered by mechanisms related to climatic change. Several volcaniclastic sand/silt units within the channel and in interchannel areas occupy mid‐stage stratigraphic positions, perhaps indicating a different triggering mechanism for slides around volcanic islands. A debris flow deposit (debrite), between 30°N, 21°W and 31°N, 24°W, is related to the Saharan Sediment Slide, a major mass movement feature on the continental slope over 1000 km to the southeast. Stratigraphic correlations indicate that this slide produced a large turbidity current as well as a debris flow.
The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England
Authors: SCOTCHMAN I. C.
Published: June 2006

Concretions from the Kimmeridge Clay Formation are of three types: calcareous concretions, septarian calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe‐reduction, sulphate‐reduction and decarboxylation zones.

The septarian concretions show a long and complex history, with early initiation of growth and development spanning several phases of burial, each often resulting in the formation of septaria. Growth apparently ceased in the transitional zone between the sulphate‐reduction and the methanogenesis zones. Very early growth in the Fe‐reduction zones is also seen in one sample. The non‐septarian concretions began growth later within the sulphate‐reduction zone and have had a simpler burial history while the pyrite/calcite concretions show carbonate cementation in the sulphate‐reduction‐methanogenesis transition zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs.

Development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment, the latter determining the organic matter input to the sediments. Calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur in organic‐rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan dolomite nodule grew under very high sedimentation rates.
Morphogenetic impact of microbial mats on surface structures of Kimmeridgian micritic limestones (Cerin, France)
Authors: BERNIER P. , GAILLARD C. , GALL J. C. , BARALE G. , BOURSEAU J. P. , BUFFETAUT E. , WENZ S.
Published: June 2006

For several years, palaeoecological research has been conducted on micritic limestones of late Kimmeridgian age in the southern Jura Mountains. The sedimentary environment is that of a lagoon with an irregular bottom, which was repeatedly exposed. Between two stages of lime deposition, a microbial mat grew over the muddy surface, giving cohesion to the sediment, restraining erosion and preserving fossil remains and reptile tracks. Various structures at the microbial mat surface can be observed: crescentic wavy, radial wavy, torn, petee and mixed structures. They imply the presence of desiccation periods and a slight bottom slope leading to a downward sliding of the microbial mat. Such features may also be generalized to explain superficial microbial structures of other shallow carbonate‐mud environments.
Coral‐stromatolite reef framework, Upper Miocene, Almería, Spain
Authors: RIDING ROBERT , MARTIN JOSÉ M. , BRAGA JUAN CARLOS
Published: June 2006

Stromatolitic crusts on stick‐like and platy Porites corals forming Messinian reefs in Almería played an important role in supporting and binding the brittle corals. The crusts were previously regarded as probable marine cements. However, their clotted, peloidal, and micritic fabrics are directly comparable with those of stromatolites. They accreted allochthonous grains even on vertical faces, and include bushy fabrics closely comparable with those produced by cyanobacterial calcification. They contain numerous fenestrae, exhibit rapid fabric variation, and locally form micro‐columns and laminated domes. Their similarities to peloid micrite crusts in Recent reefs suggest that at least some of these Recent crusts are microbial in origin, even though they have widely been interpreted as marine cements.

The sedimentary effects of crust development substantially affected both the morphology and relief of the reefs and the generation of reefal clasts. Binding of the reef‐frame in the Pinnacle and Thicket zones in the lower and middle parts of the reef created a rigid margin which shed large (commonly up to 5 m) cuboidal blocks of coral‐stromatolite frame. The blocks broke along planes of weakness provided by the vertical Porites sticks and were deposited on the Fore‐Reef Slope. In the uppermost parts of the reefs crusts dominate the structure, constituting 80% or more of the rock. Veneers up to 15 cm thick encrust thin irregular Porites plates to create a solid Reef Crest Zone which has not been recognized before.

The coral‐stromatolite framework is associated with echinoids, crustose corallines and halimedacean algae which, together with the scleractinians, indicate normal marine salinity throughout reef growth.
Sedimentation from gravity currents generated by turbulent plumes
Authors: SPARKS R. S. J. , CAREY S. N. , SIGURDSSON H.
Published: June 2006

Sedimentation from radially spreading gravity currents generated at the top of ascending sediment‐laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current, resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current. For radial spreading this model predicts a Gaussian distribution of sediment accumulation away from the source with an exponential constant, B, which depends on flow rate, Q, and particle settling velocity, v (B=nv/Q). In the experiments described, sedimentation occurs from gravity currents generated by ascent of buoyant, particle‐laden plumes of fresh water in a tank of salty water. The sediment accumulation shows close agreement with the theoretical model, and the Gaussian decay constant, B, can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line In(S/S0) = ‐Br2, where r is the radial distance, S is the sediment mass flux per unit area and S0 is the value of S at r=0. Data from the dispersal of volcanic ejecta from a large (c. 24 km high) plinian eruption column in the Azores also show good agreement with the theory, confirming that it is general and independent of scale and the nature of the fluid. The experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity. Sedimentation of coarse grain sizes, between the source and the corner, occurs from the inclined plume margins and does not behave as predicted by the theoretical model.
Interpretation of Holocene sea level tendency and intertidal sedimentation in the Tees estuary using sediment luminescence techniques: a viability study
Authors: PLATER A. J. , POOLTON N. R. J.
Published: June 2006

It is demonstrated that detailed examination of the photo‐ and thermoluminescence (PL, TL) of Holocene intertidal sediments can reveal important information regarding their depositional history in the context of sea level tendency and storm surge activity. The technique is particularly applicable to thick deposits of relatively uniform composition, and is demonstrated with a 1·7‐m core of marine to brackish water silty clay taken from Cowpen Marsh in the Tees estuary, north‐east England. This sedimentary unit is intercalated with an upper and a lower terrestrial peat bed, which have been radiocarbon dated to 5250 ± 45 and 7065 ± 4514C years BP respectively. An investigation of the mineral composition and diatom assemblage of the silty clay facies reveals a marked hiatus in deposition towards the top of the minerogenic unit. Analysis of the luminescence profiles of the sediment illustrates that, below this hiatus, sedimentation was slow and continuous, with approximately 1·0 m of sediment being deposited in 1800 ± 400 years, whilst above, sedimentation was rapid, with 0·7 m of sediment being deposited in 0 ± 200 years. It is shown that variations in the luminescence of the sediments can be interpreted in terms of the onset of a positive sea level tendency period coupled with the occurrence of a low‐frequency extreme water level event.
Flow and sediment dynamics in a low sinuosity, braided river: Calamus River, Nebraska Sandhills
Authors: BRIDGE JOHN S. , GABEL SHARON L.
Published: June 2006

The interaction between channel geometry, flow, sediment transport and deposition associated with a midstream island was studied in a braided to meandering reach of the Calamus River, Nebraska Sandhills. Hydraulic and sediment transport measurements were made over a large discharge range using equipment operated from catwalk bridges. The relatively low sinuosity channel on the right‐hand side of the island carries over 70% of the water discharge at high flow stages and 50–60% at low flow stages. As a result, mean velocity, depth, bed shear stress and sediment transport rate tend to be greater here than in the more strongly curved left‐hand channel. The loci of maximum flow velocity, depth and bed shear stress are near the centre of the channel upstream of the island, but then split and move towards the outer banks of both channels downstream. Variations in these loci depend on the flow stage. Topographically induced across‐stream flows are generally stronger than the weak, curvature‐induced secondary circulations. Water surface topography is controlled mainly by centrifugal accelerations and local changes in downstream flow velocity. The averaged water surface slope of the study reach varies very little with discharge, having values between 0·00075 and 0·00090. As bed shear stress generally varies in a similar way to mean velocity, friction coefficients vary little, normally being in the range 0·07–0·13. These values are similar to those in straight channels with sandy dune‐covered beds. Bedload is moved mainly as dunes at all flow stages. Grain size is mainly medium sand with coarse sand moved in thalwegs adjacent to the cut banks, and with fine sand at the downstream end of the island. These patterns of flow velocity, depth, water surface topography, bed shear stress, bedload transport rate and mean grain size can be accurately predicted using theoretical models of flow, bed topography and sediment transport rate in single river bends, applied separately to the left and right channels.

During high flow stages deposition occurs persistently near the downstream end of the island, and cut banks are eroded. Otherwise, erosion and deposition occurs only locally within the channel as discharge varies. Abandonment and filling of a strongly curved channel segment may occur by migration of an upstream bar into the channel entrance at a high flow stage.
Early diagenetic recrystallization of Holocene (<3000 years old) peritidal dolomites, Ambergris Cay, Belize
Authors: GREGG JAY M. , HOWARD SCOTT A. , MAZZULLO S. J.
Published: June 2006

Three peritidal carbonate crusts and associated intercrust sediments (total thickness of ∼30cm; aged <3000 years BP) on Ambergris Cay, Belize, contain 32–100% calcian dolomite (δx=72·5% dolomite) ranging in composition from 40 to 46 mol% MgCO3 (δx=43·3). Dolomite replaced high Mg calcite foraminiferal muds penecontemporaneously with sedimentation, forming partially dolomitized sediments and lithified crusts. Dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present.

Detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth; 0·4 μm near the top of the section to 1·0 μm near the base of the dolomitized section. This size increase is not accompanied by any significant decrease in porosity. Crystal size distributions appear to be log‐normal and become increasingly broad and flat with depth. Rietveld X‐ray pattern‐fitting structure refinements indicate increasing Ca and Mg concentrations on their respective sites (cation ordering) as a function of increasing depth. Most of the ordering occurs within the first 15 cm of the surface. Stoichiometry does not increase with depth indicating no relationship between the Ca/Mg ratio and cation ordering. Strong geochemical trends were observed down‐section in the dolomite, including: (1) increasing Mn content (44 to 274 ppm), and (2) decreasing δ13C values (−0·9 to −5·5‰ PDB). Oxygen isotope values range from δ18O = 1·3‰ PDB in the upper part of the section to 2·6‰ PDB in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend.

Down‐section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy‐driven dissolution‐reprecipitation process (Ostwald ripening). The observed trends in carbon isotopes and Mn content probably result from geochemical re‐equilibration during recrystallization and reflect reducing conditions and an isotopically light, organically derived, carbon source. Oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level.
Genesis of diamicton in the Oak Creek Formation of south‐east Wisconsin, USA
Authors: RONNERT LARS
Published: June 2006

The clayey diamicton of the Oak Creek Formation was deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet during the late Wisconsin deglaciation. Rapid changes in the position of the ice margin may have been facilitated by a subglacial deforming bed. However, although it is difficult to find proof either for or against a subglacial deforming bed, most observations from this investigation are more easily explained by transportation within and deposition from the ice itself. Good exposures of the diamicton units, especially of the lower contacts, along the shore of Lake Michigan, are the basis for the interpretation of basal till genesis. Basal deposition occurred mainly by stagnation and stacking block by block followed by melting out, as opposed to lodgement grain by grain. This interpretation of basal deposition is applicable in other areas and in most topographic environments.
Shingled Quaternary debris flow lenses on the north‐east Newfoundland Slope
Authors: AKSU A. E. , HISCOTT R. N.
Published: June 2006

Debris flow deposits are the principal component of Quaternary continental slope sediments between the north‐east Newfoundland Shelf and central Orphan Basin. In seismic profiles, these deposits occur as shingled, elongate, acoustically transparent lenses with their long axes orientated downslope. Deposits of individual flows form positive mounds on the sea floor; subsequent flows were diverted by the pre‐existing topography into bathymetric lows between older debris flow deposits. These deposits show a large variation in the area of sea floor covered by individual flows (about 60–1000 km2), average thickness of deposits (9–37 m) and volume of sediment displaced (1–27 km3). The ratio of average thickness to a measure of deposit diameter, termed the aspect ratio, has a threefold variation from 0·0006 to 0·0021. Very low depositional slopes and low aspect ratios suggest relatively low viscosities, probably due to inmixing of water during downslope transport. Stratified sediments form three distinct horizons and are locally interbedded with the debris flow deposits. These are mainly hemipelagic deposits.

The slope and rise to the west of the Orphan Basin are constructional in character. The apparent absence of upper slope erosional features and the abundance of debris flow deposits on the slope suggest that the supply of sediment to the continental slope occurred predominantly during times of maximum extent of Quaternary glacial ice. The ice sheet grounding line during several glacial maxima must have been situated at or near the present shelf break, supplying vast amounts of sediment directly to the upper slope. Oversteepening and subsequent slope failures fed material into deeper water.
Palaeo‐yardangs: wind‐scoured desert landforms at the Permo‐Triassic unconformity
Authors: TEWES DAVID W. , LOOPE DAVID B.
Published: June 2006

In Canyonlands National Park, south‐east Utah, at least 29 partly exhumed, aligned sandstone ridges trending generally N20°W occur at the upper unconformable surface of the Lower Permian (Leonardian) White Rim Sandstone. The ridges are at least 1·5 km long, 250 m wide and have up to 14 m of vertical relief (mean of 9 m). A thin lag of coarse sandstone that contains wind‐ripple laminae and granule ripples directly overlies the ridges. Angular blocks of sandstone within the lag and sand‐filled fissures immediately below the lag, within the ridges, attest to early cementation of the ridge‐forming material. SE‐dipping aeolian cross‐strata within the White Rim Sandstone and within the lag closely parallel the ridge trend. The ridges are interpreted as wind‐sculpted desert landforms (yardangs) that developed on the lithified upper surface of the White Rim Sandstone during an extended period of hyperaridity towards the end of the Permian.
The grain/bed collision in sand transport by wind
Authors: McEWAN I. K. , WILLETTS B. B. , RICE M. A.
Published: June 2006

In the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain/bed collision has been identified as having a significant role. The grain/bed collision has been studied by both physical experiments and numerical simulation. A principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system. Thus the study of the exact physical mechanism of the collision has to some extent been neglected.

The transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension. Thus the precise physics of the grain/bed collision become highly significant with regard to the physics of dust release from a surface. This paper re‐examines previous collision data and compares them with a simple collision model. This model proposed that the impinging grain strikes a single surface grain and rebounds: an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding. The collision data are shown to support the model: first, because the duration of the collisions observed on the high speed film supports a short contact time with the surface and, second, because the data fit well with the model.
Geometry and facies of Tertiary clinothems, Spitsbergen
Authors: HELLAND‐HANSEN WILLIAM
Published: June 2006

Sandstone clinothems from the Battfjellet Formation (Palaeogene) on Spitsbergen are locally exceptionally well preserved along depositional dip‐parallel mountainsides. The clinothems are more than 1 km wide and more than 100 m thick. Superposition of several sandstone clinothems separated by mudstones reflects repeated shoreline progradation and transgression. Deposition took place partly on‘post‐transgressional’ depositional shelves, and partly by contributing seaward‐sloping wedges, or clinothems, to a ramp progradation. Shorelines dominated both by mouth bar and shoreface environments have been identified. The clinothems are organized into an overall progradational architecture with a geometry having features in common with progradational seismic facies.
Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones
Authors: HESSELBO S. P. , PALMER T. J.
Published: June 2006

The Coinstone is a well known hiatus‐concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay‐hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea‐floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack‐lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV‐fluorescent and inclusion‐rich, and is similar to the first calcite generations seen in many other clay‐hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post‐exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today.

The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.
Aragonite laminae in hot water travertine crusts, Rapolano Terme, Italy
Authors: GUO LI , RIDING ROBERT
Published: June 2006

Small (5–30 μm) aggregates of aragonite needles occur in calcite crystal crusts of present day hot water slope travertines at Rapolano Terme in Tuscany, Italy. The aggregates are mainly concentrated in irregular, wispy and dark laminae which cross‐cut calcite crystal feathers to create a pervasive millimetre scale banded appearance in the deposit; they also occur less commonly scattered irregularly through the calcite layers. The aragonite needle aggregates are in the form of crosses, fascicles (sheaf shaped bundles, or dumbbell shaped), rosettes and spherulites. Locally, irregular masses of needles also occur. The fascicles, rosettes and spherulites have hollow centres which resemble microbial components (?fungal spores, bacterial colonies and pollen), suggesting that the aragonite crystals are biotically nucleated. The lamination is interpreted to reflect diurnal control. Stimulation of microbial activity during daylight concentrates cells in laminae and promotes aragonite calcification. Calcite feather crystals, although traversed by the aragonite aggregate laminae, have a clear appearance under the light microscope. They form more or less continuously through the diurnal cycle by abiotic precipitation. The constant association of aragonite with organic nuclei, irrespective of whether the latter are in laminae or scattered through the calcite layers, supports a biotic control on aragonite formation. Lamination in Pleistocene travertines is superficially similar to that in the present day deposits, but is diagenetically altered. In the Pleistocene deposits, the calcite feathers appear dark under the light microscope and the aragonite aggregates, where they are not altered to dark calcite, are dissolved, together with parts of the adjacent spar calcite, and therefore appear light coloured.
Sedimentology and depositional model for glaciolacustrine deposits in an ice‐dammed tributary valley, western Tasmania, Australia
Authors: FITZSIMONS SEAN J.
Published: June 2006

Quaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake‐bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium‐ to high‐relief topography.
An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates
Authors: HOWARD JEFFREY L.
Published: June 2006

The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south‐west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate‐prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δp and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade‐shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and Rw is observed, hence the abundance of disc‐shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.
Current crescents formed by marine algae (Valonia sp.): a new record of obstacle marks lacking preservation of obstacles
Authors: BHATTACHARYA ASOKKUMAR
Published: June 2006

Obstacle marks, formed around non‐permanent tools, commonly occur on the beach faces of Lower Long Sand Island and Sagar Island, two tidal islands in the Ganga River estuary, India. The marine globose algae, Valonia sp., which bloom profusely in the waters of the Bay of Bengal during winter, are deposited on beach faces after being carried by wave swash, and then act as impermeable obstacles in the path of wave backwash. This creates around the globose algal bodies U‐shaped or crescentic marks 2–3 cm in width with arms 1–3 cm in length. The globose vesicles are soon broken up by external forces or by forces of surface tension, leaving crescentic marks around a circular filamentous impression. The crescents always open toward the direction of wave backwash. These structures may prove useful not only for interpreting the palaeoslope and palaeocurrent directions but also for recognizing the palaeoecology.
Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications
Authors: CORNÉE ANNIE , DICKMAN MIKE , BUSSON GEORGES
Published: June 2006

Formation of microlaminated sediments in solar salt works along the Mediterranean coast in southern France only occurs within a restricted salinity range of 60–150 gl−1. These salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria Microcoleus chthonoplastes interbedded with detrital laminae. Transplants of the cyanobacterial mat to a less saline zone (36–60 gl−1) indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of Cladophora and Enteromorpha. Neither the snails nor the Cladophora and Enteromorpha masses are tolerant of salinities above 60 gl−1, and therefore the Microcoleus mats are restricted to those areas of the solar salt works with these higher salinities.

Analyses of salinity, conductivity, dissolved oxygen and pH in shallow salt pans (with salinities of 60–150 gl−1) established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis.

Sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats. These mats are frequently drained of their overlying water, and thus desiccation cracks divide them into polygonal plates. The development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats. No correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed. This was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons. This process results in a ‘patchwork quilt’of young and old cyanobacterial mat polygons with an irregular microlamination pattern. The presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified (meromictic) water bodies. The sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis.
The hyperbolic shape triangle as a tool for discriminating populations of sediment samples of closely connected origin
Authors: HARTMANN DANIEL , CHRISTIANSEN CHRISTIAN
Published: June 2006

This paper describes a new method for discriminating populations of sediment samples of closely connected origin. It provides a dynamical interpretation to grain size data. The descriptive modelling power of the hyperbolic distribution, the hyperbolic shape triangle and the population concept have been applied to several hundred beach sand samples from the Mediterranean coast of southern Israel. The investigated samples are from a 60 km stretch of beach, from Rafah to Ashqelon, and represent an autumn storm profile. The domain of variation of the hyperbolic shape parameters χ and ξ is a triangle referred to as the hyperbolic shape triangle. The distribution of the estimated (χ, ξ) shape positions in the hyperbolic shape triangle was studied graphically and statistically using the newly developed tools described here to present, evaluate and compare hyperbolic shape position data for analysed grain size populations. The graphical methods of contouring the scatter diagrams of the samples in the domain of the hyperbolic shape triangle and chi‐squared tests are demonstrated. They were performed on two sedimentary environments normal to the beach, namely inshore and mid‐swash zone. The qualitative and quantitative estimated changes in the populations (χ, ξ) shape positions in the shape triangle suggest that they are subjected mainly to ε erosion‐deposition processes. Their modes are located along one of the upper ‘hammock’curves in the shape triangle. The location and scale invariant hyperbolic shape parameter ρ indicates a change of the grain size distribution across the left part of the hyperbolic shape triangle, which is the domain of predominantly depositional processes. However, the mid‐swash zone sediments reflect that they are subjected more to depositional processes and the inshore sediments more to erosional ones.
Deposition and diagenesis of debris flows in Upper Ordovician limestones, Hadeland, Norway
Authors: BRAITHWAITE C. J. R. , HEATH RACHEL A.
Published: June 2006

The Upper Ordovician rocks of Hadeland, Norway, form a sequence of thin bedded nodular limestones (wackestones) and shales, hosting five distinctive sedimentary breccia complexes. These breccias contain blocks of varying sizes and shapes in a wackestone and grainstone matrix. Blocks differ in lithology, and in their included biotas and cement sequences.

The thin bedded limestones are interpreted as turbidites, deposited against a background of hemipelagic calcareous shales. The breccias occupy channels cut into this sequence. The lithologies and biotas of blocks in the breccias record deposition in differing sedimentary environments, whereas their cements are the results of contrasting diagenetic histories. Blocks were eroded from a diverse and mature carbonate platform, close to sea level, which probably lay 5–10 km east of Hadeland. The breccias are interpreted as debris flow deposits, transported as channellized flows. Following channel cutting events, perhaps triggered by sea level change, channels were characterized by deposition rather than erosion. Wackestones and grainstones associated with the breccias also reflect resedimentation, their less diverse biota suggesting local derivation on the slope. The reworking of calcarenaceous muds locally produced clean washed calcarenites (now grainstones).

A fall in sea level resulted in emergence of the upper slope and erosion of the debris flow complex to form caverns and fissures. As sea level rose again crinoidal calcarenites, now grainstones, were deposited within these cavities.

Cement sequences in blocks record early marine and burial conditions on the shelf, and also precipitation of new marine cements following downslope transport. Those cements in lithologies formed in situ document later shallowing, culminating in emergence. The localized dissolution of cements in both blocks and associated grainstones reflects the infiltration of ‘aggressive’meteoric waters through permeable channel deposits. A subsequent rise in sea level is recorded in the generation of an additional marine cement with final burial reflected in the deposition of blocky calcite. The debris flow deposits therefore maintained their distinctive character from deposition through diagenesis.