bluuebunny/crossref_metadata_2025 huggingface dataset:
Depositional subenvironments in an arid closed basin; the Wilkins Peak Member of the Green River Formation (Eocene), Wyoming, U.S.A.
Authors: SMOOT JOSEPH P.
Published: June 2006

The Wilkins Peak Member of the Green River Formation is a non‐marine dolomitic deposit that formed in an arid, hydrologically closed basin. A transect from the margin to the centre of the Wilkins Peak basin was examined in outcrop exposures in the south‐eastern corner of the basin and was supplemented with published core descriptions. The rocks were divided into seven facies based upon lithology, layering style and sedimentary fabrics: (1) AF facies—quartzose conglomerates and sandstones representing sandy alluvial fans coming from the Uinta Mountains. (2) SF facies—quartzose, peloidal, dolomitic sandstones which represent sheet flood deposits extending basinward from the alluvial fans. (3) DM facies—irregularly laminated dolomitic mudstones with abundant mudcracks which represent sheet flood deposits on a subaerially exposed mudflat. (4) EL facies—dolomitic mudstones with regular lamination and fewer but larger mudcracks representing sporadically exposed lake margins or shallow ponds. (5) PL facies—finely laminated oil shales and dolomitic mudstones with rare mudcracks which represent shallow perennial lakes. (6) SM facies—trona or halite deposits with dolomitic mudstone partings or dolomitic mudstones and sandstones disrupted by evaporites of varied morphology. These represent a central salt pan and the surrounding saline mudflat. (7) AS facies—arkosic sandstones, siltstones and mudstones which represent braided river and more basinal deltaic plain deposits.

The five dolomitic facies form poorly defined cycles representing the intersection of sheet floods with a lake which transgressed and regressed in response to climatic variations. There are at least forty 3–4 m thick cycles in the Wilkins Peak and one large‐scale cycle comprising the whole member, which mimics the facies changes of the smaller cycles. This is compatible with the transitions of the Wilkins Peak Member to the Tipton and Laney members as climatic responses and not shifts in drainage. The sheet flood sandstones, actively aggrading subaerial mudflats, and bedded evaporites of the Wilkins Peak are apparently unique within the Green River Formation. They reflect the unusual provenance and hydrology of the Wilkins Peak and suggest chemical and sedimentary models established for the other Green River members will not be applicable.
Precambrian non‐marine stromatolites in alluvial fan deposits, the Copper Harbor Conglomerate, upper Michigan
Authors: ELMORE R. DOUGLAS
Published: June 2006

Laminated cryptalgal carbonates occur in the Precambrian Copper Harbor Conglomerate of northern Michigan, which was deposited in the Keweenawan Trough, an aborted proto‐oceanic rift. This unit is composed of three major facies deposited by braided streams on a large alluvial‐fan complex. Coarse clastics were deposited in braided channels, predominantly as longitudinal bars, whereas cross‐bedded sandstones were deposited by migrating dunes or linguoid bars. Fine‐grained overbank deposits accumulated in abandoned channels. Gypsum moulds and carbonate‐filled cracks suggest an arid climate during deposition.

Stromatolites interstratified with these clastic facies occur as laterally linked drapes over cobbles, as laterally linked contorted beds in mudstone, as oncolites, and as poorly developed mats in coarse sandstones. Stromatolites also are interbedded with oolitic beds and intraclastic conglomerates. Stromatolitic microstructure consists of alternating detrital and carbonate laminae, and open‐space structures. Radial‐fibrous calcite fans are superimposed on the laminae. The laminae are interpreted as algal in origin, whereas the origin of the radial fibrous calcite is problematic.

The stromatolites are inferred to have grown in lakes which occupied abandoned channels on the fan surface. Standing water on a permeable alluvial fan in an arid climate requires a high water table maintained by high precipitation, or local elevation of the water table, possibly due to the close proximity of a lake. Occurrence of stromatolites in the upper part of the Copper Harbor Conglomerate near the base of the lacustrine Nonesuch Shale suggests that these depositional sites may have been near the Nonesuch Lake.
Polyphase erosion of subaerial omission surfaces in the Late Dinantian of Anglesey, North Wales
Authors: WALKDENs GORDON , DAVIES JERRY
Published: June 2006

The Lower Carboniferous of Anglesey consists of bioclastic carbonates and subsidiary terrigenous siliciclastics built as a cyclic succession against the northern flank of the contemporary Welsh landmass. Cycle boundaries are marked by subaerial erosive features that are unlike the normal Late Dinantian palaeokarst association, and one such surface, at Red Wharf Bay, shows remarkable evidence of polyphase erosion indicating at least three distinct periods of karstification and one of fluvial channelling. Each erosive period was succeeded by an interval of lithification during which newly arrived siliciclastic sediment was stabilized.

Diagenetic textures in the underlying limestones reveal no evidence of this complex surface history, and seen in cathodoluminescence the earliest pore filling cements evidently post‐date the entire emergent episode.

The karsts occupy positions adjacent to channels and are attributable to overbank flooding rather than atmospheric weathering. Autocyclic mechanisms may be responsible for the polyphase development but the overall control of sedimentary cyclicity is considered to have been eustatic.
The stabilizing role of bonding agents on the entrainment of sediment by wind
Authors: NICKLING W. G.
Published: June 2006

Well sorted fine sand pretreated with varying concentrations of two soluble salts (MgCl2 and CaCl2) was tested for threshold shear velocity in a recirculating wind tunnel. These tests demonstrate that even relatively low surface salt concentrations can significantly increase threshold shear velocity. However, the relationships between surface salt concentration and threshold shear velocity for the two salts were not significantly different, which is similar to the results for monovalent chlorides presented by Nickling & Ecclestone. A general relationship between threshold shear velocity and total, surface soluble salt content is presented.
Sedimentary facies of the Nova Scotian upper and middle continental slope, offshore eastern Canada
Authors: HILL PHILIP R.
Published: June 2006

A detailed survey of the upper and middle Nova Scotian continental slope at 42°50′N and 63°30′W indicates a complex morphology dominated by mass movements on various scales and an immature turbidity current channel. The range of sediment facies is diverse including hemipelagic and turbidite muds, turbidite sands and gravelly sandy muds of debris flow origin. Deformed units, interpreted as slump deposits are also observed. Several facies associations, related to discrete morphological environments, are recognized. Thick turbidite sand units with minor intervening mud beds are characteristic of the high‐relief uppermost slope and channel margin. Thinner turbidite sands, deformed slump beds and various mud facies are associated with small‐scale, hummocky mid‐slope topography. Sand beds are more abundant in the depressions than on intervening hummocks indicating the preferred transport paths of small turbidity currents. At the lower end of the main turbidity current channel, frequent turbidite sand beds with relatively minor mud beds are deposited on a depositional lobe. In areas unaffected by mass movements, alternating bioturbated mud and sandy muds make up the core sequences. A local model of sedimentation is proposed for this area and illustrates that simple models of continental slope sedimentation only apply to a limited range of settings.
Side‐scan targets in Lake Superior—evidence for bedforms and sediment transport
Authors: FLOOD ROGER D. , JOHNSON THOMAS C.
Published: June 2006

A high‐resolution side‐scan sonar survey of the lake bed off the Keweenaw Peninsula, Lake Superior, demonstrates that bottom currents are affecting lake bed morphology at depths up to 240 m. Numerous lineations which run parallel to the shore appear to be sand ribbons. A field of sedimentary furrows which occurs in one area demonstrates the long‐term directional stability of the near‐bottom flow. Large (100–300 m in diameter, 2–5 m deep), unusual ring‐like or arcuate depressions are common throughout the western half of Lake Superior. These rings themselves do not appear to have been formed by bottom currents, but may have developed as water was released by the rapid compaction of glacial sediments which underlie the lake bed. Off the Keweenaw Peninsula the forms of the rings have been modified by bottom currents. The bottom currents which have modified the lake bed are probably generated when storms cross the lake at times when it is poorly stratified.
Compositional suites of terrigenous deep‐sea sands of the present continental margins
Authors: VALLONI R. , MEZZADRI G.
Published: June 2006

The compositional variability shown by sands of present continental margins was determined through conventional microscopic techniques. This allowed the establishment of a descriptive classification of deep‐sea terrigenous sands based on a restricted number of commonly used parameters. The fundamental sand types include: (1) QF suite—sands in which quartzose and/or feldspathic grains predominate. Carbonate‐L‐bearing sands (on average richer in lithics) occur within this suite. (2) QL suite—in which quartzose and/or lithic grains predominate. Little compositional diversification is apparent within this suite. (3) FL suite—in which lithic and/or feldspathic grains predominate. Two compositional members can be identified based on the K‐feldspar/plagioclase ratio. (4) LV suite—consists of sands in which volcanic grains largely dominate the lithic types. Vitric and serpentine sands occur within this suite.

The suites are related to distinct source‐area types defined as follows: (1) cratonic block; (2) sedimentary orogenic belt; (3) volcanic orogenic belt; (4) volcanic arc.

Some sands cannot be easily assigned to a single compositional suite because petrologic parameters overlap with an adjacent suite. Such critical compositions are commonly arkosic. Using roundness of the framework components, it is possible to define a category identified as matured sands, which are greatly enriched in resistates.
Morphology and internal structure of sandwaves in the Bay of Fundy
Authors: DALRYMPLE ROBERT W.
Published: June 2006

Intertidal sandwaves in the Minas Basin and Cobequid Bay, Bay of Fundy, occur under a wide range of conditions (mean grain size 0.274‐1.275 mm; velocity strength index (V1)0.46‐3.34; and velocity symmetry index (V2) 0.011‐0.294), and they vary from symmetrical to strongly asymmetrical in cross‐section. Heights and wavelengths average 0.81 and 37.9 m respectively. They are straight to weakly sinuous and laterally continuous in plan, occasionally show crestal branching reminiscent of wave ripples, and are commonly skewed relative to the strongest currents because of differential migration rates along their length. The average migration rate is 0.11 m/tidal cycle. Megaripples occur on each sandwave crest, at least during spring tides, but the areal extent, sinuosity and size of the megaripples increases as the dominant current speed increases. The megaripples have heights averaging 24% of the sandwave height, are oriented perpendicular to the fastest dominant currents, and have life spans of several tidal cycles. They are believed to be in quasi‐equilibrium with the sandwaves and play a key role in sandwave dynamics and internal structure formation: periods of lee face steepening and rapid forward migration (megaripple crest at sandwave brink) alternate with times of non‐deposition or erosion and slowed or reversed migration (trough at brink).

Dominant‐current cross‐bedding predominates in the two intergradational varieties of translation structure observed: Inclined Cross‐Bedding—decimetre‐scale cross‐beds separated by gently inclined (9° average) erosional surfaces; and Large‐Scale Foresets—cross‐beds with thicknesses greater than half the sandwave height, interrupted by weakly erosional to conformable discontinuity surfaces. These are overlain by a vertical growth or repair structure, Complex Cross‐Bedded Cosets, that consists of nearly equal volumes of dominant‐ and subordinate‐current cross‐beds stacked without a preferred set‐boundary dip. The translation structures correspond well to forms predicted by Allen (1980a, fig. 8) but the inclined set boundaries and discontinuity surfaces (master bedding planes) are produced by megaripple troughs rather than by current reversals. Consequently, Allen's regime diagram is unable to predict structure occurrences. The repair structures suggest that ‘curvature‐related mass‐transport’ (Allen, 1980a, b) is important in tidal sandwave maintenance, although it is not necessarily responsible for sandwave initiation.
Seasonal and spatial variability of particulate matter of a muddy intertidal flood front
Authors: ANDERSON FRANZ E. , MAYER LAWRENCE M.
Published: June 2006

The concentration and composition of suspended particulate matter (SPM) were measured weekly for a period of one year in the flood front waters traversing a muddy tidal flat. SPM concentrations were lowest in the winter when portions of the tidal flat were covered with ice, and biological activity was minimal. In contrast, the summer months had the highest SPM concentrations which reflected increased bottom resuspension.

The two main sources of SPM were suspended matter carried in from offshore on the flooding tide and resuspended in situ bottom sediments. The offshore source was characterized by low SPM concentrations, coarse textures, and a high content of protein and chlorophyll ‘a’. Samples taken during resuspension events (storms/showers) had high SPM concentrations, finer textures, and were enriched in dead detrital organic material (phaeophytin). At any one time the SPM was primarily an admixture of these two sources.

The highest SPM measurements were taken during storm events, with rainfall seeming to play a dominant physical role in aiding resuspension. SPM concentrations, textures, and compositions collected during the storms closely approximated SPM measurements made over newly dug ‘clam flats’.
Deltaic sedimentation in the Lake Hazar pull‐apart basin, south‐eastern Turkey
Authors: DUNNE L. A. , HEMPTON M. R.
Published: June 2006

Lake Hazar lies within a small pull‐apart basin along the East Anatolian Transform Fault in south‐eastern Turkey. Deltas are formed where streams debouch into the low‐energy lacustrine environment. The facies constituting the deltas include delta plain debris flow, braided stream, and marginal lacustrine deposits; delta front foreset and mouth bar deposits; prodelta and lacustrine deposits. The facies are spatially restricted with sharp transitions. Facies sequences and relationships indicate two distinct styles of deltaic sedimentation. Fan deltas with a tripartite structure characteristic of Gilbert‐type deltas comprise the marginal drainage system and form along the basin margins. Mouth bar deltas develop where the axial drainage system of the basin debouches into the lake. The distribution of the two deltaic types is thought to be a function of gradient and controlled by position relative to faults within the basin.
Stable isotope study of carbonate sediments from the Coorong Area, South Australia
Authors: BOTZ REINER W. , VON DER BORCH CHRISTOPHER C.
Published: June 2006

ABSTRACT

The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange.

The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ13C = ‐1 to ‐2%0; δ18O=+3 to +5%0) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ13C=+3 to +4%0; δ18O=+5 to + 6%0) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO2 probably by the dolomitization of aragonite.
Closed‐system marine burial diagenesis: isotopic data from the Austin Chalk and its components
Authors: CZERNIAKOWSKI LANA ANN , LOHMANN KYGER C. , LEE WILSON JAMES
Published: June 2006

ABSTRACT

The carbon and oxygen isotopic composition of the Austin Chalk was examined in cores representing a range of depths from surface to 3000 m in order to document the effects of burial diagenesis on carbon and oxygen isotopic composition. Low magnesium calcite oysters were separated (from 500 um wide areas) and analysed to estimate the starting composition of Cretaceous marine sediment. These gave an average value of ‐2·5%δ18O; + 2·0%δ13C (PDB). The compositions of micrite, intergranular cement, and fracture cement were analysed, and their deviation from this original marine composition was evaluated to document the progression of chalk diagenesis. Interestingly, micrite exhibits only minor variation in composition from marine values despite present burial depth ranges in excess of 3000 m. The average deviation from δ18O marine is less than 1·5. Furthermore, intergranular cement and particularly fracture cements, which occur only in the deepest cores and which clearly post‐date micrite lithification, are generally indistinguishable from micrite in composition. Isotopic compositions exhibit no correlation with depth of burial despite abundant petrographic evidence of deep burial diagenesis. This uniformity in composition is interpreted as reflecting a closed, rock‐dominated diagenetic system in which the compositions of precipitated carbonate cements were controlled by the composition of dissolving carbonates during lithification. As such, the composition of burial cement is not representative of the rock‐water temperatures during precipitation.

Thus, in the context of isotopic analyses from other carbonate systems, unless the degree of openness of the diagenetic system is known, oxygen isotopic signatures of cements cannot directly be converted to the rock‐water temperatures at which they were precipitated unless the composition of the ambient porefluid is also known.
Cephalopod limestone deposition on a shallow pelagic ridge: the Tafilalt Platform (upper Devonian, eastern Anti‐Atlas, Morocco)
Authors: WENDT J. , AIGNER T. , NEUGEBAUER J.
Published: June 2006

As a result of early Variscan tectonic movements and of differential subsidence, a platform and basin topography was created along the northern margin of the Sahara Craton during the late Devonian. In the Moroccan Anti‐Atlas Mountains, the Tafilalt Platform is an approximately N‐S running ridge which developed since the late Middle Devonian. It separated a slowly subsiding shallow basin in the east (Tafilalt Basin) from a rapidly subsiding furrow in the west (Mader Basin). Platform deposits are characterized by highly reduced thicknesses, shallow subtidal to supratidal deposits in the late Frasnian and by unconformities at the Lower/Upper Frasnian and the Frasnian/Famennian boundaries. After a local transgression over emergent areas in the north, water depth probably never reached more than several tens to about 100 m in the lower Famennian. Cephalopod limestones of this age, deposited on the platform, represent a very diverse facies pattern comprising quartz‐rich brachiopod coquinas, crinoidal limestones, thick‐bedded cephalopod limestones and nodular limestones. Sedimentation rates ranged from 1 to 5 mm/ 1000 yr. In the late Famennian more uniform marl and nodular limestone facies suggest slightly deeper environments. Platform margins are characterized by higher rates of subsidence, debris flow deposits and slump structures. In the relatively shallow Tafilalt Basin, marls with intercalated nodular limestones were deposited. In the Mader Basin, sandy and calcareous turbidites suggest deeper water conditions in the late Devonian. During the Strunian/Tournaisian the whole area was overwhelmed by a thick deltaic sequence. The general facies distribution is in agreement with depositional models of other Upper Devonian and Lower Carboniferous cephalopod limestones in the European Variscan orogenic belts. In all these cases, condensed cephalopod limestones occupy a distinct palaeogeographic position in predictable facies sequences that reflect pre‐orogenic phases in the Variscan geodynamic cycle. Moreover, close parallels exist with condensed sequences in the Triassic and Jurassic that occur in a very similar position within the Alpine orogenic cycle.
Early diagenetic phosphate cements in the Albian condensed glauconitic limestone of the Tatra Mountains, Western Carpathians
Authors: KRAJEWSKI K. P.
Published: June 2006

Early diagenetic phosphate cements are described from the Albian condensed glauconitic limestone of the Tatra Mountains, Western Carpathians with regard to their macro‐ and micromorphology, distribution, classification, and genesis. The cements occur within stratigraphically condensed semi‐pelagic foramini‐feral‐glauconitic layers and are associated with mature hardgrounds within the Tatra Albian limestone. Phosphate cement fabrics consist of crypto‐ to microcrystalline carbonate‐fluorapatite, and they occur as: (i) rim envelopes, (ii) infillings of intraparticle porosity, (iii) rim cement, (iv) multiple rim cement, (v) palisade fabric and (vi) cluster cement. Micromorphological variability of the cement fabrics results from varying texture of the cemented sediment, the nature of original porosity, as well as from presence of associated microbial fabrics. The microbial fabrics are interpreted as fossilized coccoid cyanobacteria.

Phosphate cementation developed under peculiar early diagenetic conditions within semi‐closed microenvironments rich in organic matter in the marine phreatic environment. The cementation contributed to the formation of phosphatic fossils and hardgrounds. The accretion of the cements was due to concentration of biologically uptaken phosphorus near the sediment/water interface, enrichment of pore fluids with respect to phosphate, and its precipitation within restricted microenvironments. Phosphate cementation post‐dated seafloor formation of pelletal glauconite but predated partial decomposition of organic matter as well as dissolution or neomorphism of aragonite and high‐Mg calcite. Phosphate cementation occurred on a carbonate platform following the submersion of Urgonian reefal build‐ups. Episodes of phosphate cementation were repeated during the sedimentation of the Tatra Albian limestone as a response to rapid relative sea‐level rises and increased influence of nutrient‐rich Tethyan waters.
Upper delta plain lacustrine and fluviolacustrine facies from the Westphalian of the Durham coalfield, NE England
Authors: FIELDING C. R.
Published: June 2006

Facies analysis suggests that the productive Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England were deposited on an upper delta plain. Distributary channels crossed the plain and were separated by shallow lakes.

Detailed examination of largely three‐dimensional exposures has revealed the existence of nine laterally and vertically interrelated fluviolacustrine and lacustrine lithofacies. Facies 1 and 2 are interpreted as overbank deposits of distributary channels, Facies 3–5 are regarded as deposits of crevasse splay/minor delta systems, and Facies 6–9 are considered to have formed in areas of diminished clastic sediment supply. Facies 4 and 5 are volumetrically the most important. Facies 3–9 are interpreted as representing progressively less energetic conditions of sedimentation across a lake, from the point of entry of a crevasse splay/minor delta system.

The distribution and characteristics of the lithofacies indicate that the upper delta plain lakes were completely or effectively enclosed, up to about 8m deep and had wave fetches of the order of 20 km. These lakes were intermittently infilled by the crevasse‐initiated, minor delta systems and, to a relatively minor extent, by overbank flood sediment from channels. Infilled lake surfaces became platforms for plant colonization and peat accumulation.

The three dimensional relationships of the various lithofacies provide a model of Coal Measure lacustrine sedimentation, which may have wider implications in extending the model of interdistributary genesis and infilling proposed by Elliott, particularly with respect to detailed facies relationships and to the balance between overbank and crevasse‐derived sediment.
Syntaxial overgrowths in muddy crinoidal limestones: cathodoluminescence sheds new light on an old problem
Authors: WALKDEN G. M. , BERRY J. R.
Published: June 2006

Lower Carboniferous shallow water limestones of Asbian and Brigantian age in Britain commonly contain abundant interparticle micrite and characteristically display large syntaxial calcite overgrowths on crinoid and echinoid grains. These overgrowths appear to have developed at the expense of the micrite and are widely regarded as neomorphic replacements. However, cathodoluminescence of these has revealed growth features which indicate that they are not neomorphic but originated as passive cement fills of solution voids surrounding echinoderm grains.

We introduce the term solution corona for these grain selective voids and consider that three processes may have contributed to their development, namely: high‐Mg calcite stabilization of host grains, crystal ripening, and meteoric dissolution. Cyclic subaerial emergence was a critical factor in these processes, and we contrast the morphology of the overgrowths in question with forms produced in basinal limestones which never experienced comparable early meteoric conditions.

These early‐formed solution coronas around echinoderm grains are therefore a useful indicator of meteoric diagenesis and have important implications for porosity evolution. The syntaxial cements which fill the solution coronas show distinct phases of growth in cathodoluminescence which reflect a progression from near‐surface meteoric conditions to deep burial with pressure solution.
An electromagnetic device for automatic detection of bedload motion and its field applications
Authors: REID IAN , BRAYSHAW ANDREW C. , FROSTICK LYNNE E.
Published: June 2006

A new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse‐grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainment.
A large‐scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia
Authors: VON DER BORCH C.C. , GRADY A. E. , ALDAM R. , MILLER D. , NEUMANN R. , ROVIRA A. , EICKHOFF K.
Published: June 2006

The late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn‐rift or early post‐rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation.

The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low‐stand of sea‐level. They were subsequently filled by a fining‐upwards suite of sediments which reflects subsequent relative rise of sea‐level and carbonate platform development. Ultimately the canyon complex was buried by north‐westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle.

It is considered possible that the series of isolated outcrops of canyon cross‐sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge‐like Patsy Springs Canyon lies in more proximal regions of the basin‐slope, whereas 40 km to the north‐east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel‐fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg.

The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel‐fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.
Submarine‐fan deposited sandstone and rudite in a mid‐Cenozoic interarc basin in Maewo, Vanuatu (New Hebrides)
Authors: NEEF G. , PLIMER I. R. , BOTTRILL R. S.
Published: June 2006

The volcaniclastic, Lower Sarava Formation of north‐eastern Maewo was deposited in a Late Oligocene‐Early Miocene deep sea fan environment. Eight lithofacies can be recognized from a westward‐trending, deep‐sea fan which prograded at a depth of >4.25 km within an extinct interarc basin. These lithofacies include massive spilite‐rich rudite, graded coarse rudite‐arenite, graded fine rudite‐arenite, intraformational rudite, brown and grey siltstone, minor calcarenite and a lithofacies composed of turbidites with Bouma sequences. The sediment was essentially derived from the Vitiaz palaeoarc to the east. Non‐calcareous red and green siltstone and minor airfall tuff was deposited distal to the arenite and rudite, in areas, or during periods, of slower sedimentation.

Slightly more than half of the lower part of the Sarava Formation is represented by massive rudite, graded coarse rudite‐arenite, and graded fine rudite‐arenite. These deposits, and some of the calcarenites and massive arenites. were deposited from decelerating high density turbidity currents. Some of the thicker rudite‐arenites may have been induced by tsunamis. Classical turbidites deposited, perhaps, from low density turbidity currents are rare.

The petrography of igneous clasts indicates that Late Oligocene‐Early Miocene volcanism on the Vitiaz palaeoarc was largely basaltic‐andesitic but there were some dacitic eruptions.
Element mobility during alteration of silicic ash to kaolinite—a study of tonstein
Authors: ZIELINSKI ROBERT A.
Published: June 2006

A laterally persistent kaolinitic mudstone parting (tonstein) occurring in a Wyoming coal bed of Eocene age was sampled to estimate the compositional contrast with a probable silicic ash precursor, and to determine the compositional influence of leached ash on immediately adjacent coal. With the exception of Al, and possibly Ti, the tonstein is highly to moderately leached of major elements, relative to a range of compositions estimated for silicic ash of the region. In agreement with the behaviour of geochemically similar major elements, alkali trace elements (Rb, Cs) are highly leached, transition‐series metals moderately leached, and Ga is residual. Additional immobile trace elements are Zr and Hf but some other trace elements that are considered relatively immobile during low‐temperature alteration (Th, Ta, Nb, REE, Y) were apparently leached by the low‐Eh, low‐pH, organic‐rich pore fluids of the coal‐forming swamp.

The migrational range of many of the leached elements is highly limited by the intimate association of tonstein with enclosing organic matter. Mixtures of coal + minor tonstein that occur within 20 mm of contacts are consistently enriched in some elements relative to either tonstein or nearby coal (U, Cu), or relative to calculated mixtures of tonstein and coal in their measured proportions (Th, Y, REE, Pb, Ba, V, Ti). Direct observations by fission‐track radiography and electron microprobe indicate a preference of U and Fe for the organic component of mixtures. Metal fixation is probably by adsorption on insoluble organic matter (humic acids), or by precipitation as minor sulphides in these low‐sulphur coals.
Estimation of dust fallout rates in the northern Arabian Gulf
Authors: FODA M. A. , KHALAF F. I. , AL‐KADI A. S.
Published: June 2006

The penetration of dust clouds over a sea surface is modelled as a sediment transport‐diffusion problem. The model is used to determine the steady state concentration field of two‐dimensional dust clouds during offishore dust storms. The rates of dust deposition into the sea can then be calculated from the obtained concentration fields. The model was applied to offshore dust storms blowing over the Arabian Gulf, yielding estimated dust sedimentation rates of 0–8 mm yr−1. This is substantially higher, by several orders of magnitude, than dust fallout estimates in most seas of the world. The calculated fallout rates are in line with the results of existing field measurements.
A laboratory flume investigation of the formation of fossil stem fills
Authors: REX GILLIAN
Published: June 2006

The mechanism of formation of fossil plant ‘pith casts’ has been investigated experimentally using a small laboratory flume tank. The extent to which filling occurs is dependent on stem length and diameter, and on the current velocity. Stem fills produced by sediment carried in suspension show a distinct structure of two wedges of sediment deposited at either end of the stem cavity. These wedges are deposited from small flow vortices which form when the flow around the stem separates, as the flow through the stem becomes restricted. Stems filled by bedload currents do not show such a uniform structure. The experimentally produced stem fills are comparable with fossil pith casts, e.g. Catamites. The formation of cortical casts of Stigmaria is not simply explained in terms of the processes operating in open‐ended stem segments. This investigation of the mechanics of filling of hollow plant organs, and the structure of the fill produced, offers a basis for interpreting the sedimentary environment in which the fossil fills have formed, and leads to a fuller understanding of the mode of formation of such fossils.
Major bounding surfaces in aeolian sandstones—a climatic model
Authors: TALBOT M. R.
Published: June 2006

Large areas of fixed, vegetated dunes occur in the Sahel and southern margin of the Sahara. These dunes were active during Late Pleistocene times and have been stabilized as a result of a change to more humid, less windy conditions in the Holocene. The stabilized surfaces, which are in part of erosional origin, are of regional extent. Within aeolian sand sequences, such surfaces form the highest order bounding surfaces. Since regional bounding surfaces of this type occur in all the world's major deserts, it is suggested that they should also be present in some ancient aeolian sandstone sequences.
The hydraulic interpretation of turbidites from their grain sizes and sedimentary structures
Authors: KOMAR PAUL D.
Published: June 2006

Previous attempts to deduce the flow parameters of turbidity currents from their deposited sediments have focused on applications of criteria for the suspension or autosuspension of the grains and on hydrodynamic interpretations of the characteristic Bouma sequence of sedimentary structures. There has been a considerable diversity of opinion, however, as to how the transport criteria relate to the observed deposited grain‐size distribution, and no attempt has been made to determine whether the separate analyses of deposited sediments and sedimentary structures agree as to the magnitudes of the evaluated flow parameters.

Such analyses are performed on a turbidite from the Capistrano Formation (Miocene‐Pliocene) of California. This turbidite is normally graded from medium sand at its base to very fine sand and silt at the top, and has the complete sequence of Bouma structures. Only a small degree of cementation has occurred so that samples from the layer could be disaggregated and grain sizes determined both by sieving and sedimentation balance analyses.

It was decided to employ the grain‐suspension criterion for the calculation of the flow conditions at the time of deposition, published experiments on the pipe‐flow of suspensions having demonstrated that this criterion is one of deposition versus non‐deposition of grains according to their settling velocities. The published work relating types of sedimentary structures to the sediment grain size and either the flow power or Shield's dimensionless stress is used to evaluate the flow parameters from the observed Bouma sequence. All methods employed yield estimates of the mean flow velocity and bed stress (force per unit bottom area).

The evaluated flow parameters for the transition from a flat bed to ripples (Bouma B to C divisions) are nearly an order of magnitude greater than obtained from the grain‐suspension criterion where the calculations are based on the median grain size of the deposited sediments. Agreement results only if the calculations utilizing the suspension criterion are based on nearly the coarsest grains deposited at any instant, a procedure that is difficult to justify with the expected sediment deposition from a waning current. Inclusion of other factors, such as possible lags in sediment deposition or ripple formation beneath the decelerating flow, provide no firm explanation as to the cause of the discrepancy between the results based on the two approaches, and in nearly all cases their inclusion would increase the difference. Something is amiss with our procedures for the hydraulic interpretation of sediments which can be resolved only by further study.
The sandstone memory of the evolution of a Triassic volcanic arc in the Southern Alps, Italy
Authors: GARZANTI EDUARDO
Published: June 2006

The erosion of a southern volcanic belt provided the bulk of the fine‐grained and moderately sorted siliciclastic detritus in the whole deltaic to lagoonal Carnian (Upper Triassic) sequence of the Bergamasc Alps. Volcanic input began in the Early Carnian (Calcare Metallifero Bergamasco) and became prominent in Early‐Middle (Val Sabbia Sandstone) and Late Carnian times (S. Giovanni Bianco Fm.), after an intervening period of diminished supply (Gorno Fm.).

The mineralogical and chemical composition of the sandstones compares closely with that of siliciclastics derived from modern, Pacific‐type magmatic arcs and it testifies to an evolution of the source rocks from andesites and dacites during the deposition of the Val Sabbia Sandstone, towards rhyodacitic ignimbrites for the S. Giovanni Bianco Fm. The marked reduction in feldspars at the top of the sequence contrasts with the classical trend from Undissected to Dissected Magmatic Arc provenance and is ascribed to the progressive deepening of erosion into Middle Triassic felsic volcanics. An increase in undulatory and polycrystalline quartz grains in the Late Carnian shows that the erosion did reach deeper into the crystalline roots of the arc, although non‐volcanic detritus never exceeded 10%.

After a terminal pyroclastic event in Middle‐Late Carnian times, suggested by the sudden appearance of pumiceous vitric clasts at the top of the Val Sabbia Sandstone, the mid‐Triassic orogenic magmatism ended and was followed by the tensional stage which led to the birth of Neotethys.
Sedimentation on an early Proterozoic continental margin under glacial influence: the Gowganda Formation (Huronian), Elliot Lake area, Ontario, Canada
Authors: MIALL ANDREW D.
Published: June 2006

Eight continuous cores up to 150 m long and spaced an average of 200 m apart yield a detailed local insight into the composition and architecture of an ancient continental margin sequence, the Gowganda Formation (early Proterozoic: Huronian) near Elliot Lake, Ontario. Nearby outcrops of similar facies provide important supplementary data on sedimentary structures.

Continental glaciers provided an abundant supply of coarse debris but, apart from rafting of debris by floating ice, played little or no part in Gowganda sedimentation. The basal 50 m of the Gowganda Formation in the drill‐hole area represents a continental slope depositional system. It consists mainly of gravelly and sandy sediment gravity flow deposits, interbedded with minor rain‐out units of diamictite, and argillite containing dropstones. Ten types of sediment gravity flow deposit are distinguished. An overlying submarine‐channel depositional system, 10–50m thick, consists of hemipelagic argillites containing dropstones and showing deformation structures. These are interbedded with well‐sorted channel‐fill sandstones. Submarine point bars 4·5 m thick (identified in nearby outcrops) demonstrate a meandering channel geometry. This channel‐fill sequence probably formed during a period of high sea‐level and reduced sediment supply, but the relationship to ice advance‐retreat cycles is unclear. The subsurface sequence is completed by a blanket of massive rain‐out diamictites up to 55 m thick, and a younger slope sequence of sediment gravity flow diamictites and sandstones.

The stratigraphy is quite different in outcrop section 10 km to the west of the drill‐holes, suggesting the presence of major lateral facies changes and/or internal erosion surfaces within the Gowganda Formation. This complexity of stratigraphy and depositional processes is probably a feature of many ancient glacial units, and points to the advisability of not making climatic or tectonic interpretations from a few generalized or composite sections.
Erosion and deposition of fine‐grained sediments from the Bay of Fundy
Authors: AMOS CARL L. , MOSHER DAVID C.
Published: June 2006

Simulations of the erosion, transport and deposition of fine‐grained sediment, such as that of Greenberg & Amos and the Hydraulics Research Station, have illustrated a general lack of reliable field data. Consequently, some standard equations and constants used in modelling the sedimentation character of fine‐grained cohesive sediment were evaluated based on data from two field studies and a flume experiment with undisturbed sediment from the Bay of Fundy.

Initial results showed that the resistance to erosion of intertidal fine‐grained sediment is controlled largely by the degree of subaerial exposure and the consequent dehydration and compaction. The sediment shear strength was high (4 kPa), but generally decreased seawards across the intertidal zone. The resistance of intertidal mud to erosion can be 80 times greater than sub‐tidal counterparts.

The rate of sediment erosion varied as a complex function of the applied bottom shear stress. At stresses immediately above the critical, the erosion rate decreased asymptotically with time. At higher excess stresses, the erosion rate was linear with respect to time. Thus sediment erosion cannot be represented by a single coefficient.

The Krone method of computing sedimentation rates of suspended material was shown, by comparisons with direct measurement, to overpredict by 29%. All variables used in his method were measured in the evaluation with the exception of the critical deposition stress (τd). The closest comparisons were obtained when τd was assigned a value of 0.1 N m−2 following Creutzberg & Postma.

The in situ still‐water particle settling rate (Vo) was constant with respect to time (2.1 × 10−3 m s−1). However, the settling tube measures of settling rate, compared to in situ results, underpredicted particle settling by an order of magnitude (2.7 × 10−4 m s−1). The reason for this discrepancy is not apparent from our results.
The petrology and diagenesis of Middle Jurassic clastic sediments, Ravenscar Group, Yorkshire
Authors: KANTOROWICZ J. D.
Published: June 2006

The petrology of non‐marine clastic sediments from the Middle Jurassic Ravenscar Group was investigated to identify and to assess the significance of possible controls on diagenesis. Diagenetic modifications took place in three broad regimes: within the depositional groundwater (eogenesis), during burial (mesogenesis), and during uplift and erosion (telogenesis).

Eogenesis involved the initial interaction of the original sedimentary assemblage with its depositional pore waters. Eogenetic processes were influenced strongly by bacterial degradation of organic matter present in the finer grained sediments. This bacterial activity caused a lowering of pore water pH, and subsequently reduced Eh. Feldspar dissolution and alteration of muscovite to kaolinite occurred. Quartz overgrowths and vermiform kaolinite precipitated in sands with oxygenated and mildly acidic pore waters, whereas chlorite and quartz overgrowths formed in sands with anoxic and neutral pore waters. This caused complete reduction of porosity in both finer‐grained and texturally less‐mature floodplain‐facies sandstones. Channel‐facies sandstones were cemented only with a thin veneer of quartz overgrowths, creating a rigid but still porous quartzose framework. Bacterial ferric‐iron reduction throughout the water table subsequently established more ubiquitous conditions, raised pH, and caused rhombic siderite cementation within the remaining porosity.

Mesogenetic modifications, of the mineralogy formed at the surface, occurred when pore water composition changed during burial. Bicarbonate saturated solutions migrated through the surviving porosity, precipitating replacive, poikilotopic ferroan calcite. Carbonate dissolution occurred subsequently. Carbonate dissolution was possibly caused by the same solutions which then precipitated dickite and further authigenic quartz.

Telogenetic modifications took place during uplift and exposure. Groundwater reactions and subaerial exposure probably caused further carbonate and feldspar dissolution, as well as alteration of chlorite to interstratified chlorite vermiculite.

It is concluded that the diagenesis of these non‐marine clastic sediments reflects various factors, the complex interrelationship of which precludes the identification of any single factor as wholly controlling their diagenesis. Eogenetic modifications involved the interaction of the sediments and their pore waters. As a result of the metastability of the mineral assemblage established at this time, these modifications can still be recognized. despite the sediments' subsequent history.