bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantitative relationships between numbers of fluvial cycles, bulk lithological composition and net subsidence in a Scottish Namurian basin
Authors: READ WILLIAM A. , DEAN J. MICHAEL 
Published: June 2006

ABSTRACTThis detailed quantitative basin analysis of fluvial deposits in a subsiding Namurian structural basin is aimed at discovering underlying statistical relationships between numbers of fluvial cycles, bulk lithological composition and net subsidence which could be used as a background to future sophisticated computer simulation experiments and would also facilitate comparison with other ancient basins. The succession studied lies between two widespread marine bands, one of Arnsbergian (E2) and one of Kinderscoutian (R1) age, and is dominated by upward‐fining cycles: 94% of the semi‐cycles containing sand, here termed grain‐size cycles, are upward‐fining and the ratio of fine members (mudstone+siltstone) to coarse members ranges from 0·23 to 5·0.Trend‐surface analysis reveals basement structures, which influenced sedimentation, including a Caledonoid graben. Correlation coefficient values and results of principal component analysis demonstrate that the numbers of rooty horizons and grain‐size cycles, together with the total thicknesses of sandstone and of mudstone+siltstone, all tend significantly towards a linear relationship with the total thickness of strata and hence net subsidence. The average thickness of grain‐size cycles tends towards an inverse linear relationship with net subsidence. This probably reflects the presence of stacked, relatively thin, channel‐fills within persistent channel belts. These belts tend to be localized in the areas of greatest subsidence within the basin and follow courses basically similar to channels already discovered in the underlying deltaic sediments. The highest concentrations of mudstone+siltstone tend to lie on the flanks of the basin, but high local sand concentrations found at points on the margin where channels entered the basin from the NW and NE and left to the SW, effectively disrupt any significant relationship between net subsidence and the proportion of sand.Coal has been selectively preserved on the flanks of the basin and the number of rooty horizons is greatest in an area of low subsidence which lay somewhat remote from the main channel belts.
Some effects of compaction and geological time on the pore parameters of argillaceous rocks
Authors: BORST ROGER L. 
Published: June 2006

ABSTRACTGraphical statistics have been applied to the pore‐size distribution curves of argillaceous rocks to characterize the changes in pore parameters that result from compaction and geological time. The most striking characteristic of recently deposited sediment is the high variability in mean pore size and in the sorting and skewness of the pore system. The mean pore size ranges from 15 to 980 nm, sorting ranges from very well sorted to poorly sorted, and skewness varies from systems in which small pores predominate over large ones to systems in which large pores predominate. This high variability in pore structure represents the many environmental and mineral‐related variables that affect the pore system of newly deposited sediment.The mean pore size of shales decreases with increasing compaction and approaches a limiting value of about 3·5 nm at depth. Within a geological time span of 50 m.y. and/or depth of burial of about 1200 m, most sediments have reached an irreversible, well sorted pore‐size distribution. Early diagenetic processes apparently affect the skewness of pore systems more than compaction, such that within about 50 m.y. the pore system is negatively skewed, with small pores predominating over large.Sediments buried to a depth of 500 m or less exhibit a porosity range of 40‐‐85%; below 500 m, porosity decreases linearly with burial depth. No correlation exists between the surface area of shale pore systems and depth of burial, geological age, and the pore parameters mean pore size, sorting, and skewness.
Sedimentation in the Conway Trough, a deep near‐shore marine basin at the junction of the Alpine transform and Hikurangi subduction plate boundary, New Zealand
Authors: CARTER L. , CARTER R.M. , GRIGGS G.B 
Published: June 2006

ABSTRACTThe Conway Trough is a 40 km × 10 km, 1000 m deep, rectangular, tectonically controlled sedimentary basin situated on an active plate boundary. The basin lies at the junction of the Alpine transform and Hikurangi subduction sectors of the Indo‐Australian/Pacific plate boundary. It is incised into Cainozoic sedimentary rocks and contains a thick fill of late Neogene sediment.Study of the continental shelf near the Conway Trough indicates: (1) that transport mechanisms operate in a net north‐easterly direction, and (2) that the neritic sediment drift system is confined to within a few kilometres of the shoreface, with the result that (3) large areas of the shelf south of the trough comprise bare bedrock, or relict late Pleistocene sediment. Despite reaching to within 3 km of shore, the Conway Trough receives little modern bedload sediment, as indicated by a paucity of sand fill. However, the trough has intercepted some Holocene bedload, since the main channel contains redeposited graded gravel and sand with a C‐14 age of 4670 yr. Satellite imagery and piston cores reveal that abundant mud is provided to the trough, much of it from river sources up to 200 km to the south. Some of this sediment is inferred to move by turbid layer transport through Conway Trough and out via Kaikoura Canyon into nearby Hikurangi Trough. Nonetheless, sedimentation rates in the Conway Trough may be as high as 1‐7 m (1000 yr) 1.The Conway Trough forms a modern example of a transform basin, varying Neogene examples of which are found widely in north‐eastern South Island. Sedimentation models are constructed to compare transform and transduction basins, particularly with respect to the effects of Pleistocene sea‐level changes.
Cryptalgal‐metazoan bioherms of early Ordovician age in the St George Group, western Newfoundland
Authors: PRATT BRIAN R. , JAMES NOEL P. 
Published: June 2006

ABSTRACTBioherms are common in the St George Group, a sequence of shallow‐water carbonate rocks deposited on the western continental shelf of Iapetus Ocean. They range from small heads and metre‐sized mounds to extensive banks and complexes many metres thick and hundreds of metres in lateral extent. The cores of these bioherms are principally composed of thrombolites (unlaminated, branching, columnar stromatolites), structures quite distinct from laminated stromatolites which are common in intertidal beds. Associated with thrombolites is a diverse fauna of burrowing invertebrates, trilobites, nautiloids, pelmatozoans, brachiopods, gastropods, rostroconchs and archaeoscyphiid sponges.On the basis of framework‐building components, three main bioherm types are distinguished: (1) thrombolite mounds, (2) thrombolite‐Lichenariaor ‐sponge mounds and (3) thrombolite‐Lichenaria‐Renalcisreef complexes. The framework of the last is the most complex, with abundant cavities and a demonstrably uneven growth surface of thrombolites, corals and free‐standingRenalcisheads, walls and roofs. Some cavities were active sediment conduits while others were protected, their roofs draped withRenalcisand their walls coated by cryptalgal laminites.These bioherms possess the attributes of shallow‐water ecologic reefs. They span a critical time gap in the development of reefs, the transition period from algal‐dominated bioherms of the Precambrian and Cambrian to the metazoan‐dominated bioherms of the Middle Ordovician and remaining Phanerozoic.
The use of settling velocity in defining the initiation of motion of heavy mineral grains, under unidirectional flow
Authors: COLLINS M. B. , RIGLER J. K. 
Published: June 2006

ABSTRACTThe results of critical threshold experiments on four commonly occurring heavy minerals are described. The data are presented, in conjunction with comparable quartz data, using the non‐dimensional Shields’ Curve and the Movability Number (U*/ws). The results indicate that critical shear stress for material of high density is overestimated by use of Shields’ Curve, under smooth boundary conditions. Grain settling velocity is found to be a good indicator for the critical shear stress for grains of a wide density range. A physical explanation for the results is proposed.
Distribution of chlorite and kaolinite in eastern Atlantic sediments off North Africa
Authors: LANGE HEINZ 
Published: June 2006

ABSTRACTChlorite in the Atlantic sediments is predominantly derived from the Atlas Mountains, kaolinite originates in the southern Sahara and Sahel zone. These minerals may be used to reconstruct the tracks of two superimposed wind systems (Trade Winds and Harmattan Wind). A modified method for determining relative abundances of chlorite and kaolinite is presented.
Oncolites from lacustrine sediments in the Cretaceous of north‐eastern Spain
Authors: FREEMAN TOM , ROSELL JUAN , OBRADOR ANTONIO 
Published: June 2006

ABSTRACTCompact micritic oncolites up to 8 cm in maximum diameter occur within Maestrichtian (Upper Cretaceous) Garumniense continental marls of Sierra del Montsec (Lerida Province) of north‐eastern Spain. Synsedimentary development is documented by patches of terrigenous quartz that occur among oncolitic protuberances. Soluble nuclei (limestone fragments and bivalves) further suggest an origin through accretion, rather than that of soil pisolite. Similarities between the petrography and isotopic compositions of the oncolites and those of interbedded Garumniense limestones suggest similar sedimentary origins. However, these lacustrine oncolites, like modern counterparts described by others, probably developed through in situ metabolic precipitation of calcium carbonate. Evidence of this origin is their high degree of concentricity, which is unlikely to have developed through sedimentary accretion, inasmuch as the oncolites ‘float’ in quiet‐water marls.
Sedimentation in a deep glacier‐fed lake—Lake Tekapo, New Zealand
Authors: PICKRILL R. A. , IRWIN J. 
Published: June 2006

ABSTRACT Surface sediments, cores and seismic reflection profiling delineate sedimentary environments and processes of sedimentation in Lake Tekapo. Sedimentation is dominated by the Godley River which forms an extensive delta in the northern third of the lake. Delta growth accounts for 55% of annual sediment deposition. In winter sandy muds are deposited at the top of the delta slope, where they may move under gravity as a surficial slide. Oversteepening of the upper slope also generates deep seated failures. The entire 20 km2 of delta slope is subjected to rotational slumping which episodically reworks large volumes of sediment. Down the delta slope sedimentation rates decrease, surface sediments get finer and varves become better developed.In the lake basin sediments are parallel bedded varves, which contain typical winter‐summer annual cycles as well as minor, non‐annual flood varves. Annual varve thickness and semi‐annual varve frequency are determined by variations in the discharge of the Godley River. Sedimentation in the basin accounts for 40% of the budget and sedimentation rates decrease with distance from the delta, except at the distal end of the basin, where turbid underflows are stopped by the rising lake floor. Beyond the basin, sedimentation rates decrease abruptly. Coriolis deflection of inflowing river water increases sedimentation rates down the eastern shore. The remaining 5% of the sediment is deposited on the lateral slopes and shoulders where sediments form a thin muddy veneer over basement, which occasionally slumps to the basin floor.
Interpretation of moment measures derived from water‐lain sediments
Authors: SLY P. G. , THOMAS R. L. , PELLETIER B. R. 
Published: June 2006

ABSTRACT Although mean size provides a useful value for characterizing sediments, it is an inadequate expression of the interaction between sediments and hydraulic regimes and many authors have explored the significance of other statistical moment measures in different parts of the particle size spectrum. Few publications, however, have described such relationships over a very wide range of particle sizes. Using a combined data base (of marine and lacustrine sediments) we have been able to demonstrate the following:The distributions of standard deviation (and entropy) values are repeated in similar form on either side of the sand size class, describing mixtures between sands and gravel and sands and clay.Skewness is modified by the hydraulic response of the sediment and a skewness divide is present at about 2.7 φ, where it coincides with the minimum shear velocity required to initiate particle motion. This divide may be used to distinguish between high and low energy regimes.Kurtosis, like standard deviation, is a measure of the mixing of end member populations but it is more sensitive to changes in the tails of distribution curves. The maximum positive peak of kurtosis, at about 2.5 φ, lies close to the skewness divide (in our data).By using skewness/kurtosis plots it is possible to define separate field distribution for sediments which, to a greater or lesser degree, appear to conform to hydraulic equilibrium.By using sediments which are close to equilibrium (and lie at or close to our skewness/kurtosis boundary curves, it should be possible to define hydraulic shear velocities, at the time of formation. Values may be established by empirical relationships or by defining the break‐point (traction and intermittent suspension) between linear components of individual sample φ size/probability distribution plots. Such relationships should be enhanced by using hydraulic equivalent diameters instead of sieve diameters.In fine sediments, the use of fractional (silt: clay) ratios offers sensitivity at a level comparable to that of skewness/kurtosis plots and may be more easily computed.
Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences
Authors: EYLES NICHOLAS , EYLES CAROLYN H. , MIALL ANDREW D. 
Published: June 2006

ABSTRACT Increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills. These are based to a large degree on theoretical considerations of likely depositional processes. The classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in Quaternary and Pre‐Quaternary successions. This situation is compounded in many Quaternary terrains by the continued description of ‘tills’ in terms of laboratory‐derived analytical data only, reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment. Detailed sedimentological logging of lithofacies is rarely undertaken. There is thus considerable confusion as to what is being described or sampled when analytical data are presented for many Pleistocene ‘tills’. The same remarks apply to Pre‐Pleistocene ‘tillites’.A lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites; the term‘till’is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice. Use of a four part code, in conjunction with codes already published for fluvial sediments, allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work. The value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late Pleistocene glacigenic sequence at Scarborough Bluffs, Ontario.
Experiments on the production of transverse grain alignment in a sheared dispersion
Authors: REES A. I. 
Published: June 2006

ABSTRACT A series of experiments was designed to investigate grain orientation in flows with concentrations of granular solids in the range from 0.5 to 15%. Fourteen experiments were carried out in which sand‐sized material was dispersed in liquid plaster of paris and the dispersion allowed to flow down a slope until it came to rest. The preferred orientation of the grains was then estimated by measuring the anisotropy of magnetic susceptibility of specimens cut from the solidified flows.Five of the experiments showed signs of deformation, both in the gross characteristics of the flow and in the grain alignment. The remaining nine had grain alignment dependent on grain concentration. At concentrations of 1.2% and less and 11.1% and more, by volume, alignment of long axes was parallel to flow. These observations were consistent with the existing theories for low concentrations, in which preferred alignment is shown to result from the varying rate of rotation of grains in a shearing flow, and for high concentrations, in which preferred alignment results from the transfer of angular momentum between colliding grains.At some intermediate concentrations transverse alignment was observed. A theoretical explanation combining the theories of the extreme cases is suggested. The observation of transverse alignment is compared with a similar observation in some turbidites.
Chalcedonic quartz and occurrence of quartzine (length‐slow chalcedony) in pelagic sediments
Authors: KEENE J. B. 
Published: June 2006

ABSTRACT Chalcedony is the most abundant form of quartz in silicified pelagic sediments from the North Pacific. Varieties of chalcedonic quartz present in chert of deep‐sea origin include chalcedony (length‐fast and zebraic), quartzine (length‐slow), and lutecite.These occurrences of quartzine in known pelagic sediments emphasize the dangers of using quartzine as an indicator of former evaporitic environments. Quartzine is a diagenetic mineral and does indicate pore fluids rich in sulphate and magnesium. In pelagic sediments, it is always associated with authigenic barite and in many cases with authigenic dolomite. Quartzine should not be used, by itself, as an indicator of any particular environment of deposition.
Turbidite depositional patterns and flow characteristics, Navy Submarine Fan, California Borderland
Authors: PIPER DAVID J. W. , NORMARK WILLIAM R. 
Published: June 2006

ABSTRACTThe late Pleistocene and Holocene stratigraphy of Navy Fan is mapped in detail from more than 100 cores. Thirteen 14C dates of plant detritus and of organic‐rich mud beds show that a marked change in sediment supply from sandy to muddy turbidites occurred between 9000 and 12,000 years ago. They also confirm the correlation of several individual depositional units. The sediment dispersal pattern is primarily controlled by basin configuration and fan morphology, particularly the geometry of distributary channels, which show abrupt 60° bends related to the Pleistocene history of lobe progradation. The Holocene turbidity currents are depositing on, and modifying only slightly, a relict Pleistocene morphology.The uppermost turbidite is a thin sand to mud bed on the upper‐fan valley levées and on parts of the mid‐fan. Most of its sediment volume is in a mud bed on the lower fan and basin plain downslope from a sharp bend in the mid‐fan distributary system. Little sediment occurs farther downstream within this distributary system. It appears that most of the turbidity current overtopped the levée at the channel bend, a process referred to as flow stripping. The muddy upper part of the flow continued straight down to the basin plain. The residual more sandy base of the flow in the distributary channel was not thick enough to maintain itself as gradient decreased and the channel opened out on to the mid‐fan lobe.Flow stripping may occur in any turbidity current that is thick relative to channel depth and that flows in a channel with sharp bends. Where thick sandy currents are stripped, levée and mid‐fan erosion may occur, but the residual current in the channel will lose much of its power and deposit rapidly. In thick muddy currents, progressive overflow of mud will cause less declaration of the residual channelised current. Thus both size and sand‐to‐mud ratio of turbidity currents feeding a fan are important factors controlling morphologic features and depositional areas on fans. The size‐frequency variation for different types of turbidity currents is estimated from the literature and related to the evolution of fan morphology.
Interaction of the Western Boundary Undercurrent with the continental margin off Newfoundland
Authors: CARTER LIONEL , SCHAFER CHARLES T. 
Published: June 2006

ABSTRACTSedimentation on the Newfoundland rise is strongly influenced by the Western Boundary Undercurrent (WBU). The upper rise (2600‐2800 m) is swept by a rapid (Ū= 8·5 cm sec−1), south‐flowing core of the WBU which has generated a sandy contourite facies characterized by coarse gravelly, sandy sediments; current‐induced bedforms such as scour moats, lineations and lee drifts; ferro‐manganese‐stained gravel clasts; a high proportion of broken foraminiferal tests and a diagnostic benthic foraminiferal assemblage. The overlying nepheloid layer, when compared to adjacent waters, is thickest (800 m), most sediment laden (80 μg 1−1), contains the highest proportion of terrigenous sediment and exhibits the best developed bottom mixed layer (∼ 15 m thick). Comparisons with earlier data from the same area imply the dimensions and sediment load of the nepheloid layer vary with time. Empirical considerations, based on near‐bottom current meter records from Labrador and Newfoundland, suggest the WBU is capable of transporting bedload with threshold friction velocities (u*) of around 0·87‐1·14 cm sec−1 for between 1 and 15% of the time. The prevailing transport direction is southwards along the rise, but this may be punctuated periodically by brief incursions to the north.The erosional regime of the upper rise is bordered by a regime of fine‐grained deposition typified by muddy contourites. Both the lower slope and lower rise are mantled by bioturbated muds, the former zone having terrigenous mud and the latter, biogenic calcareous mud. The accompanying nepheloid layer is thin, biogenic‐rich and devoid of an identifiable mixed layer.
Post‐glacial seismic stratigraphy, central Great Barrier Reef, Australia
Authors: JOHNSON D. P. , SEARLE D. E. 
Published: June 2006

ABSTRACTA regional programme of continuous seismic (boomer) profiling in the central Great Barrier Reef Province has identified a widespread shallow seismic discontinuity (reflector A) which is interpreted as the pre‐Holocene surface.Nine seismic facies units are distinguished primarily on the basis of the seismic records, but also with the aid of additional criteria such as location and surface sediment types. Two units underlie reflector A and are pre‐Holocene. These units are interpreted as: (a) Permo‐Carboniferous bedrock, and (b) Pleistocene/? Tertiary sediments, consisting of both shelf‐wide terrigenous units, and carbonate mounds and platforms under present reefs. Seven units are post‐glacial and overlie reflector A. These units are interpreted as: (c) fluvial/estuarine channel fill, (d) relict delta‐front deposits, (e) relict transgressive veneer, (f) coastal coarse and (g) fine deposits, (h) modern reef and (i) reef talus.In general post‐glacial sediment cover is very thin and in many places on the mid‐shelf the pre‐Holocene units crop out. Substantial post‐glacial accumulations are limited to protected coastal embayments and to offshore reef masses.