bluuebunny/crossref_metadata_2025_split huggingface dataset:
Morphology and Quaternary sedimentation of the Mozambique Fan and environs, southwestern Indian Oceans*
Authors: KOLLA VENKATARATHNAM , KOSTECKI JOHN A. , HENDERSON LAWRENCE , HESS LILLIAN 
Published: June 2006

ABSTRACTMost of the Quaternary sediments of the Mozambique Fan have been derived from Africa‐Madagascar and deposited by turbidity currents in Pleistocene time. Currents caused by movement of the Antarctic Bottom Water also played a significant role in reworking and redepositing sediments along the marginal areas of the fan. The inner or upper Mozambique Fan is characterized by a single, leveed valley. Due to the effects of the Coriolis force, the natural levees to the east of the valley (left, looking downstream) are higher and contain more terrigenous sediments than those to the west of the valley. The sea floor to the west of the valley returns regular hyperbolic echoes as seen on 3·5 kHz echograms, whereas to the east of the valley, the sea floor is relatively smooth. The sediments on the valley floor are coarse‐grained (with median grain up to 2 mm) and poorly sorted, and occur often as massive turbidites, interbedded with hemipelagic sediments. Away from the valley, both to the east and the west, the terrigenous sediments are relatively fine‐grained and have been deposited as overbank turbidite sequences. We estimate the maximum velocities of the channelized turbidity currents in the upper fan to have been 8–32 ms−1. The middle fan has several distributary channels with no levees and has a relatively flat sea floor, characterized by lack of acoustic penetration. Thick, sheet‐like, turbidite sand beds, deposited primarily by unchannelized turbidity currents, characterize the middle fan. The middle fan grades, towards the margins, into the outer (lower) fan which is relatively free of channels, has good acoustic penetration and contains hemipelagic and pelagic sediments, and thin, fine‐sand turbidite and/or contourite beds. A wide zone of sediment waves, formed from the reworking of the turbidity current‐fed sediments by the Antarctic Bottom Water, forms part of the outer fan.
Terrestrial‐marine transition in the Cambrian rocks of Kangaroo Island, South Australia
Authors: DAILY B. , MOORE P. S. , RUST B. R. 
Published: June 2006

ABSTRACTThe upper part of the Lower Cambrian succession in northeast Kangaroo Island comprises three interbedded facies associations. The fine‐grained association is composed of siltstone, mudstone and minor sandstone. It contains flat lamination and abundant ripple cross‐lamination which shows bipolar palaeocurrents, and occurs in combinations of flaser bedding, lenticular bedding and wavy lamination. Although body fossils are relatively rare, trilobite traces and desiccation cracks are common, and the association is interpreted as a predominantly subtidal to intertidal deposit. The conglomerate facies association contains horizontally bedded cobble to boulder conglomerate, with subordinate trough cross‐stratified coarse sandstone to granule/pebble conglomerate. Fabrics and structures in the coarse conglomerates are consistent with alluvial transport (stream and debris flow), but not beach deposition. The conglomerate association is attributed to tectonic uplift and erosion of a Precambrian‐Lower Cambrian succession developed adjacent to the present north coast of Kangaroo Island. Southward progradation of an alluvial fan complex occurred across east‐west oriented tidal flats on which limited wave activity reworked sand and fine gravel, but not coarser material. The sandstone facies association mainly comprises trough cross‐stratified and plane‐laminated sandstone, the latter with current lineation predominantly sub‐parallel to the east‐west shoreline. Trough cross‐stratification is ascribed to onshore waves and longshore currents, and current lineation to predominantly shore‐parallel tidal currents, augmented by longshore drift and storm surge. Tectonic movements gave rise to cycles of transgression and regression as tidal and alluvial processes dominated alternately.
Hydraulic control of grain‐size distributions in a macrotidal estuary
Authors: LAMBIASE JOSEPH J. 
Published: June 2006

ABSTRACTThe Avon River estuary of Nova Scotia was studied with the intention of analysing the relations between grain‐size distributions and hydraulics. The Avon is macrotidal; tidal ranges up to 15·6 m generate tidal currents up to 1·7 m s−1. Maximum current speed increases from the mouth (seaward end) to the head (shoreward end) of the estuary. Mean grain size decreases from the estuary mouth to the head. Thus, there is an inverse relationship between mean grain size and current speed. Consequently, textural parameters do not directly reflect hydraulic conditions. Graphical dissection of cumulative frequency curves into their component grain populations reveals a large coarse population at the estuary mouth that is absent at the head. There are several relationships between hydraulics and cumulative curves. Shields’ criterion predicts that all sediment in the system can be transported so that the large coarse population at the estuary mouth is not a lag. Local maximum shear velocity nearly equals the settling velocity of the grain size at the boundary of the coarse (C) and intermediate (A) grain populations. This has been previously interpreted to signifiy a transition from traction to intermittent suspension transport, and implies that the C population is a function of traction and that the A population is related to intermittent suspension (Middleton, 1976). Each grain population is transported at a different rate; suspended grains travel almost an order of magnitude faster than grains moved by traction according to Einstein's transport formula. Sediment transport paths in the estuary were determined from bedform migration directions and the computed net sediment transport per tidal cycle using Engelund and Hansen's formula. The areal distribution of the transport paths, combined with the differential transport rates of each grain population, produces hydraulic sorting. Hydraulic sorting causes coarse sediment to be excluded from the estuary head and creates the inverse relationship between current speed and mean grain size.
Palaeosols in the Reading Beds (Paleocene) of Alum Bay, Isle of Wight, U.K.
Authors: BUURMAN P. 
Published: June 2006

ABSTRACTFossil soils occur in the Reading Beds of Alum Bay. All soils have hydromorphic characteristics, caused by either groundwater or stagnating pluvial water; some have illuviation of clay. The combination of bioturbation (striated burrows) and iron segregation may indicate that the Reading Beds in Alum Bay are of fluviomarine origin. The soils were formed in a warm climate with a marked dry season. They indicate a landscape with minor variations in surface level. The Reading Beds have clay mineral assemblages that are partly inherited and partly changed by soil formation. Some soil horizons might be used for stratigraphic correlation.
The Northwest Atlantic Mid‐Ocean Channel of the Labrador Sea: II. Deposition of parallel laminated levee‐muds from the viscous sublayer of low density turbidity currents
Authors: HESSE REINHARD , CHOUGH SUNG KWUN 
Published: June 2006

ABSTRACTVarve‐like parallel laminated and homogeneous muds on natural levees of submarine channels result from the spill‐over of the dilute upper parts of channelized turbidity currents. On the levees of the Northwest Atlantic Mid‐Ocean Channel, Labrador Sea, laminated turbidite muds show a systematic upward thickness variation of the granular silt laminae and the clay laminae. In many depositional units the former decrease logarithmically upwards in thickness while the latter increase logarithmically. The origin of parallel lamination in current‐deposited, fine‐grained sediments is interpreted in terms of the ‘burst‐and‐sweep model’ for the viscous sublayer of turbulent flows. Individual silt laminae of laminated turbidite muds form due to shear sorting during burst‐and‐sweep events which have a winnowing effect on the clay fraction. In contrast to sand‐transporting flows the viscous sublayer of silt‐ and clay‐carrying dilute turbidity currents cannot be permanently in a turbulent state. The presence of clay laminae suggests that the boundary layer relaminarizes periodically. It is speculated that this is caused by pressure waves with negative pressure gradients favouring the suppression of bursts and sweeps. The observed vertical thickness variations suggest that the periods of laminar boundary‐layer flow increase in duration as turbulence of the main current decays. The upward thickness decrease of the silt laminae is explained in terms of decreasing supply rates to the boundary layer from the main flow. A multiple bursting‐cycle mechanism is favored over a single bursting cycle mechanism for the formation of individual silt laminae by shear sorting.
Dominant particle support mechanisms in debris flows at Mt Thomas, New Zealand, and implications for flow mobility
Authors: PIERSON THOMAS C. 
Published: June 2006

ABSTRACTWithin zones of little or no deformation by internal shearing in debris flows at Mt Thomas, about two‐thirds of the weight of large particles is supported by buoyancy and about one‐third by static grain to‐grain contact. In boundary shear zones of low velocity flows and in high velocity, turbulent debris flow, grain‐to grain contact is replaced by turbulence and dispersive pressure. Cohesive strength of the clay + silt + water interstitial fluid provides less than 2 % of the force keeping particles larger than 1 cm gravel in suspension.Excess pore pressure is generated in the interstitial fluid by the weight of coarse particles suspended in the slurry. According to Coulomb strength theory, pore pressures measured in these debris‐flow slurries reduce the shear strength of the material to less than 10 % of what it is in the unsaturated state. The excess pore pressures are slow to dissipate because of the small connections between pore spaces that result from the extremely poor sorting of the debris and the presence of silt and clay in the pore fluid. Maintenance of sufficient pore space to trap fluid and facilitate flow on low‐gradient slopes may be accomplished by dilatancy and subsequent partial liquefaction of the debris during shear.
Detrital modes of recent deep‐sea sands and their relation to tectonic setting: a first approximation
Authors: VALLONI RENZO , MAYNARD J. BARRY 
Published: June 2006

ABSTRACTComplete petrographic analyses of modern deep‐sea sands are surprisingly rare, but are needed if problems such as the influence of tectonics on petrology, or changes in petrology due to diagenesis, are to be understood in ancient sandstones. This paper is a first attempt, using data from our own study of 80 sands plus data from the literature, to establish a baseline for modern deep‐sea sands with which ancient sandstones can be compared. We assigned sands to five plate‐tectonic categories, each of which has a distinct average petrologic composition: (1) basins associated with trailing‐edge continental margins, Q62F26L12; (2) with leading‐edge continental margins, either subduction, Q16F53L31, or (3) strike‐slip, Q34F39L27; (4) back‐arc basins, Q20F29L51; (5) fore‐arc basins, Q8F17L75. The amount and type of lithic fragments and the type of feldspar are particularly important discriminators, the amount of quartz less so. Trailing‐edge and fore‐arc basins have quite distinctive compositions, but back‐arc basins and basins near the two types of leading‐edge margins show some overlap, and thus may be hard to differentiate in ancient sandstones.
Sedimentation rates in Fayetteville Green Lake, New York, U.S.A.
Authors: LUDLAM STUART D. 
Published: June 2006

ABSTRACTIn Fayetteville Green Lake, past sedimentation rates can be accurately and precisely estimated by separating annual couplets or varves in dried sediment samples. Two measures were used, which serve as upper and lower limits on estimated sedimentation rate. They agree within 5 % with average annual sedimentation rate in couplets for recent years. Between 3 and 5 replicate samples are needed to reduce the half‐width of 95 % confidence intervals on individual couplet sedimentation rates to 30 g m−2 yr−1 about 5 % of average recent rates. In the late 1800s sedimentation rate averaged 392 g m−2 yr−1 and ranged between 324 and 466 g m−2 yr−1, while in the 1970s the rate averaged 581 g m−2 yr−1 by the same measure, and ranged between 384 and 646 g m−2 yr−1. Sedimentation rate averaged for 13 years does not vary over short distances in the profundal zone, but lateral variation in sedimentation rate can be detected for individual years over the same distance. Not all this variation was associated with the non‐uniform distribution of dark sublaminae and thin turbidites which cannot be separated from the annual layers. This indicates that although precise estimates of sedimentation rates can be made at different points in the lake, estimates will have to be made at numerous points before annual sedimentation rates for the lake as a whole can be accurately assessed.
Neap–spring tide sequences of intertidal shoal deposits in a mesotidal estuary
Authors: BOERSMA J. R. , TERWINDT J. H. J. 
Published: June 2006

ABSTRACTIn the mesotidal (tidal range 3·5–4·9 m) Westerschelde estuary (The Netherlands) the intertidal part of a sandbank was the subject of systematic observations of: (1) hydrographic properties, (2) the distribution and response pattern of various types of bedforms, and (3) the sedimentary structures produced. Ebb and flood usually differ considerably in strength, giving rise to a clear dominance of one over the other, which may change over the neap‐spring tide period. Parallel, long‐crested sand‐waves and irregular, short‐crested dunes have a different response to the neap‐spring variation in current velocity.Because one tide (usually the flood in our area) predominated over the other, the internal structure largely consists of unidirectional cross‐bedding, separated into a succession of tidal bundles, each formed during one tide. The tidal bundles are arranged in a lateral sequence reflecting neap‐spring tide periods and differing in character with location.Within the tidal bundle, reactivation, full vortex and slackening structures reflect acceleration, full stage and deceleration of flow respectively in the dominant tide. The full vortex structures tend to be well developed around spring tide but disappear towards neap tide. The subordinate tide carves ‘pause planes’ which enclose the tidal bundles. These pause planes are either erosional or depositional (mud).
Silica diagenesis of Neogene diatomaceous and volcaniclastic sediments in northern Japan*
Authors: IIJIMA AZUMA , TADA RYUJI 
Published: June 2006

ABSTRACTMarine diatomaceous siliceous sediments in Neogene sections of northern Japan contrast with the Monterey Shale of California in containing many intercalations of acidic volcaniclastic sediments. Diagenesis of these sediments from deep boreholes and surface sections was investigated. Three diagenetic zones—biogenic opal, opal‐CT and quartz zones—are recognized in siliceous sediments, corresponding roughly to amorphous silica, low cristobalite and quartz zones in acidic vitric volcaniclastic sediments. Opal‐CT consists almost exclusively of silica and water, while low cristobalite contains appreciable amounts of A1, Ca, Na and K.In subsurface sections, values of d(101) spacing of opal‐CT decrease progressively with increasing burial depth. The progressive ordering is not associated with additional silica cementation. In surface sections, the behaviour of d(101) spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift. The cementing opal‐CT is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal‐CT porcellanite. Opaline cherts that form during burial diagenesis are designated as early opaline chert, while those which form during uplift are later opaline chert. The later opaline chert contains two groups of opal‐CT; one is progressively ordered opal‐CT and the other is additionally cemented opal‐CT with higher d(101) spacing than that in the host porcellanite. In diatomaceous siliceous sediments, early opaline chert is scarce. Most, if not all, opaline cherts in surface sections are of later origin.
Foreslope stromatoporoid‐renalcid bioherm with evidence of early cementation, Devonian Ancient Wall reef complex, Rocky Mountains
Authors: MOUNTJOY ERIC W. , RIDING ROBERT 
Published: June 2006

ABSTRACTA small mound, 4 × 23 m in size, on the fore‐reef slope of the Ancient Wall reef complex at Mount Haultain, Alberta, consists of fenestral, renalcid micrite and wackestone. Stromatoporoids are the most abundant skeletal component but do not form an organic framework. Renalcids, together with early micrite cementation, performed a supporting and stabilizing role but only rarely directly encrusted skeletal material. Many of the primary shelter voids, interparticle voids and fenestrae were filled with internal sediments and submarine micrite cements. A considerable proportion of the micrite matrix appears to be cement.The location and development of the bioherm was controlled by a local hard substrate and suitable environmental conditions. The mound developed on and around a large block within an underlying megabreccia bed soon after it was deposited in a debris flow on the fore‐reef slope. It formed near wave base in about 30–50 m water depth and was subject to gentle current action. Laminar stromatoporoids grew on the flanks and top of the mound, and fenestral, renalcid micrite and wackestone accumulated in the central part. Deepening water and increased rates of mud accumulation terminated further build‐up of the bioherm and it was buried by basin mud.Fenestral, renalcid micrite occurs in a number of Devonian and Lower Palaeozoic organic mounds. This study suggests that early, submarine cementation was a significant factor in the development and preservation of this distinctive lithotype.
Soft‐sediment deformation of sandstone related to the Dwyka glaciation in South Africa
Authors: BELL C. M. 
Published: June 2006

ABSTRACTSandstones of the uppermost Witteberg Group in the Cape fold belt of South Africa exhibit unusual and distinctive soft‐sediment deformation structures. These structures include folds, axial planar cleavage and micro‐fold lineations. Interfering fold patterns and intersecting sets of lineations are indicative of repeated deformation. The sandstones are immediately overlain by glacial and proglacial sediments of the Carboniferous Dwyka Group, indicating that the deformation was related to glaciation. Possible environments of deformation include: (a) subglacial dragging of unconsolidated material, (b) subaqueous slumping beyond the limit of floating ice, and (c) englacial deformation of material incorporated by freezing into the base of the glacier.
Wave‐generated structures in the Devonian lacustrine sediments of south‐east Shetland and ancient wave conditions
Authors: ALLEN PHILIP A. 
Published: June 2006

ABSTRACTThe most common wave‐generated structures in the nearshore lacustrine sediments of the south‐east Shetland basin are cosets of undulatory and unidirectional ripple cross‐lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post‐vortex) ripples, and the unidirectional cross‐sets were formed by the migration of vortex (orbital) ripples at lower strengths.Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand‐starved substrate. Some lenses were reworked during periods of more prolonged wave activity.The Inman‐Komar plot of near‐bottom orbital diameter versus ripple spacing (λ= 0.80d0 for small d0, or λ= 0.65d0 as modified by Miller & Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimum d0 conditions) is utilized in this study since wave periods in lakes are small.The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the order of 20 km wide.
A scale of dissolution for quartz and its implications for diagenetic processes in sandstones
Authors: HURST ANDREW R. 
Published: June 2006

ABSTRACTDuring diagenesis quartz grains undergo selective dissolution, controlled in location by the surface energy characteristics of the individual grains. Experimental etching in HF of isolated quartz grains reproduces comparable textures to those of natural occurrences. Some experimental results illustrate the specific effects of surface textures on their initial dissolution rates, so demonstrating the control surface energy variation has over dissolution. A hierarchy of grain surface characteristics, according to surface energies, provides a useful guide to the relative rates of dissolution during decomposition.
A pre‐split soft sediment core tube
Authors: REDDERING J.S.V. 
Published: June 2006

ABSTRACTLongitudinal splitting and reassembly of core tubes, before core sampling, solves many of the problems of hand‐operated corers. Low‐cost, re‐usable PVC core tubes have successfully been used for obtaining virtually undisturbed cores from unconsolidated sandy sediments in the intertidal and shallow subtidal zones. The entire operation can be carried out by one person.
Anoxic events during sedimentation of a Palaeogene diatomite in Denmark
Authors: PEDERSEN GUNVER KRARUP 
Published: June 2006

ABSTRACTA diatomite, about 60 m thick, of late Palaeocene‐early Eocene age crops out in northern Jutland, Denmark. The diatomite is locally termed ‘Moler’. Frustules of marine diatoms constitutec. 65% (by weight) of the diatomite, and clay minerals, chiefly montmorillonite, make up the remainder. Slight variations in the relative supply of diatom frustules and clay minerals are preserved undisturbed in laminated diatomite, while lamination is partly destroyed by burrowing organisms in weakly laminated diatomite and obliterated by total bioturbation in structureless diatomite; these three facies alternate throughout the sequence.The presence or absence of infaunal burrowing organisms is interpreted as a record of the content of dissolved oxygen in the water above the sediment‐water interface and hence of the position of the redox potential discontinuity.Interspaced in the diatomite are 179 identifiable layers of volcanic ash. These ash layers provide a means of precise lateral correlation. They show that levels of laminated diatomite may be followed throughout the basin and therefore that changes between anoxic and oxic conditions occurred simultaneously across the area. The laminated diatomites may consequently be interpreted as representing short‐term anoxic events, of which twelve have been recognized.