bluuebunny/crossref_metadata_2025 huggingface dataset:
Time and space distribution of megaripples and associated bedforms, Middle Atlantic Bight, North American Atlantic Shelf
Authors: SWIFT D. J. P. , FREELAND G. L. , YOUNG R. A.
Published: June 2006

Three genetically distinct size classes of lower regime transverse bedforms have long been known from laboratory studies, and from studies of the intertidal zone; ripples, megaripples, and sand waves. These features are also present on the subtidal shelf surface of the Middle Atlantic Bight, and their distribution in time and space allows us to draw inferences concerning the time and space pattern of sediment transport. Transverse bedforms in the Middle Atlantic Bight occur in response to tidal flows at estuary and inlet mouths and on tide‐dominated banks; on the shelf surface, however, they are primarily responses to wind‐driven flows. Ripples are the most widespread of the three classes. They are current‐formed during peak storm flows, but are probably remade as oscillatory wave ripples as the flow wanes. Megaripples are found primarily on the inner shelf, also as responses to peak storm flows. Sand waves of several metres amplitude occur on the inner shelf in the vicinity of topographic highs; low amplitude sand waves (< 2m), solitary or in trains, are widespread on the inner shelf. They survive through many seasons of storm flows. Megaripples are especially interesting as records of specific flow events. They are widespread on the inner shelf during the winter, occurring in fields up to several kilometres in diameter. On a portion of the Long Island inner shelf during December 1976, megaripple fields covered approximately 15% of the shelf surface. They tend to be erased during the succeeding summer months. Both megaripples (short‐term response elements) and sand waves (long‐term response elements) indicate that sand transport in the Middle Atlantic Bight is directed to the southeast, parallel with the regional trend of the isobaths.
A geochemical and mineralogical investigation of some British and other European tonsteins
Authors: SPEARS D. A. , KANARIS‐SOTIRIOU R.
Published: June 2006

The volcanic origin of a number of major tonstein horizons has been established from mineralogical compositions and from trace elements that are quantitatively retained during diagenesis. Discriminant function analysis using the variables Ti/Al, Cr/Al, Zr/Al and Ni/Al allows the tonsteins to be classified according to original composition. Two main groups are recognized amongst the British tonsteins. Those formed from acid volcanic ash are comparable in composition with French and German tonsteins and a common source is postulated, whereas tonsteins formed from basic volcanic ash contain variable amounts of detrital sediment and are thought to originate from local eruptions. Possible lateral equivalents between tonstein horizons in Britain and elsewhere in Europe are suggested.
Non‐annual cycles of varve‐like sedimentation in Walensee, Switzerland*
Authors: LAMBERT† ANDRÉ , HSÜ K. J.
Published: June 2006

Dating of recent varve‐like sediments from the perialpine Lake of Walenstadt (=Walensee) indicates that the number of laminae deposited in the 165 years between 1811 and 1976 ranges from 300 to 360 depending on sample location. Direct evidence that up to five graded laminae may be deposited during one year was found in 1912 by an engineer of the Swiss Federal Hydrological Agency who was using sediment traps to determine the annual accumulation rate. These layers are considered to be deposits of continuous‐fed turbidity currents generated by hyperpycnal inflow during river‐flood stages. Current measurements revealed that these underflows can occur sporadically throughout the year, but are especially common during the snow‐melting season and after heavy rainfalls. Currents with bottom‐velocities up to 50 cm s−1 were detected during summertime when the lake is thermally stratified, indicating that such stratification is no obstacle to the generation of turbidity underflows. The laminated sediments of the Walensee do not represent deposition of annual cycles and these non‐annual varve‐like sediments seem to be less regularly rhythmic than the annual varves of Lake Zurich.
Sedimentation in a closed trough north of the Iberia Abyssal Plain in the northeast Atlantic†
Authors: ADDY SUNIT KUMAR , KAGAMI HIDEO
Published: June 2006

Mirrol Trough of the northeast Atlantic contains five NNE‐SSW trending, en echelon, turbidite‐filled basins deeper than 5500 m, each ranging from 4 to 10 km in width and 19 to 65 km in length. Trough deposition has consisted mainly of turbidites from adjacent hills and ridges as indicated by the physiography of the region, sediment isopach map, the nature of the sediments in the trough, and benthic foraminiferal depth indicator species. The sedimentation rate on abyssal hills and ridges, as deduced from palaeomagnetic evidence, is 1.36 cm/103 years. Using this sedimentation rate, it is estimated that Mirrol Trough subsided relative to the surrounding area and began receiving sediments between 8.3 and 11.5 m.y. ago; and the deposition of the most recent turbidite has occurred sometime between 29,000 and 44,000 years b.p. Tilting of the base of the most recent turbidite with respect to the basin floor is observed, and this is attributed to relative sinking of the eastern margin of the trough after the deposition of the most recent turbidite.
Calcification in a coccoid cyanobacterium associated with the formation of desert stromatolites
Authors: KRUMBEIN W. E. , GIELE C.
Published: June 2006

In the Borrego Desert (California) and in the Sinai Desert (Israel) laminated, microbially mediated carbonate crusts have been found and analysed biologically and mineralogically, and further studied with scanning electron microscope methods combined with energy dispersive X‐ray analyses. All morphological and biological features of the extant crusts justify the term ‘desert stromatolite’, a term applied to stromatolites from desert regions which form under permanent exposure to the atmosphere. These stromatolites are never covered by standing water, and running water (heavy rainfall) covers them for only a few hours during the year. Carbonate deposition is achieved principally by the cyanobacterium Pleurocapsa sp. which exhibits characteristic yet different stages of calcification. Calcification occurs in the sheaths of single cells (including baeocytes) as well as in mature colonies. The specificity for calcification in Pleurocapsa sp. is discussed.
A simulation model of alluvial stratigraphy
Authors: BRIDGE JOHN S. , LEEDER MICHAEL R.
Published: June 2006

The quantitative model presented simulates the development of a two‐dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter‐related effects which influence the distribution of channel‐belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel‐belt deposits decrease with increasing floodplain width/channel‐belt size, mean avulsion period, and channel‐belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half‐graben). Then channel‐belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine‐grained alluvium to accumulate on the inactive side. Individual channel‐belt deposits thicken during aggradation, although a self‐regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel‐belt deposits of avulsive origin.
Mechanisms to explain the loss of heavy minerals from the Upper Palaeozoic tillites of South Africa and Australia and the late Precambrian tillites of Australia
Authors: GRAVENOR C. P. , GOSTIN† V. A.
Published: June 2006

Heavy mineral studies on Pleistocene tills from North America, Upper Palaeozoic tillites of South Africa and Australia, and late Precambrian tillites of South Australia show that the heavy mineral suites of the Pleistocene tills are dominated by amphiboles, the Upper Palaeozoic tillites by garnet, and the late Precambrian tillites by zircon and tourmaline. About half of the garnets in the Upper Palaeozoic tillites show evidence of having been rounded, and retain delicate surface chattermark trails, which indicates that these garnets have not undergone chemical attack since deposition. Although the remainder of the garnets show, by way of etching, that intrastratal solutions were active in the sediments, it is suggested that amphiboles, pyroxenes and epidote, which must have been present in the original Upper Palaeozoic heavy mineral suites, were lost primarily by the action of sorting and mechanical abrasion in beach environments prior to, and during interglacial periods. The absence of garnet and the etching of tourmaline and zircon in the late Precambrian tillites is attributed to the action of alkaline intrastratal solutions over the long time interval during which the tillites were buried in the Adelaide Geosyncline.
Changes in carbon and oxygen isotope composition during limestone diagenesis
Authors: DICKSON J. A. D. , COLEMAN M. L.
Published: June 2006

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ13C values (+ 1·8 PDB) similar to modern, shallow‐water marine skeletons, but the δ18O values (−6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re‐equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ18O of −6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3‐8 to + 1‐2‰. and oxygen from − 2‐6 to − 12‐4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault‐controlled dolomite, and is terminated by vein‐filling calcite and dolomite. The younger calcite, interpreted as a near‐surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ13C − 6‐8‰.
Carbonate deposition and facies distribution in a central Michigan marl lake
Authors: MURPHY DAVID H. , WILKINSON BRUCE H.
Published: June 2006

Relatively pure lacustrine carbonates referred to as marl are being deposited in Littlefield Lake, central Michigan, a hard‐water lake with little terrigenous clastic influx. Thick accumulations of marl form both progradational marl benches along lake margins, and islands or lakemounts in the lake centre. Marl benches develop flat platforms up to 20 m wide in very shallow water and steeply inclined slopes, up to 30°, extending into deep water. The flat landward platform is frequently covered by algal pisoliths while the upper portion of the lakeward‐sloping bottom is overgrown by Chara which in the summertime becomes thickly encrusted with low‐magnesian calcite. Marl islands are flat‐topped features that formed over relict highs on Pleistocene drift which underlies the lake basin. These are fringed by marl benches identical to those found along lake margins. Marl benches are composed of four units: two thin facies deposited on the shallow‐water bench platform and two thicker faces deposited on the bench slope developed in moderate water depths. These in turn overlie a fifth facies deposited in deep water. A coarsening‐upward sequence is developed in these sediments as a result of both mechanical sorting, and primary production of carbonate sand and gravel in shallow water. In addition to facies sequences and size grading, trends upsection of increasing carbonate content and decreasing insoluble content may serve to identify temperate‐region lacustrine carbonate deposits in the rock record.
Penecontemporaneous folding, sedimentation and erosion in Campanian Chalk near Portsmouth, England
Authors: GALE A. S.
Published: June 2006

Campanian (Upper Cretaceous) Chalk exposed in a quarry near Portsmouth is unusual in the occurrence of a contemporaneous anticlinal fold, probably diapiric in origin. Hardgrounds and local slumps developed over the structure, and part of the chalk succession thins considerably over its crest. Erosional channels, sometimes containing bioclastic lag deposits, formed at two levels. Movement on the anticline occurred three times during the part of the Lower Campanian represented in the section; the third phase resulted in extensive fracturing of the crestal region of the fold. Burial of the structure and the onset of normal chalk deposition concluded the Cretaceous phase of its history.
Suitability of monosaccharides as markers for particle identification in carbonate sediments*†
Authors: BÖHM LOTHAR , DAWSON RODGER , LIEBEZEIT GERD , WEFER GEROLD
Published: June 2006

The identification of the remains of organisms contributing to carbonate sediments by means of scanning electron‐microscopy is limited to particles of the 2–20 μm size class. Mineralogy and the content of Mg, Sr and trace elements alone are usually insufficient to solve the problem of identification, especially in the differentiation between algal and coral aragonite. The organic matrix of calcareous organisms consists of stable biopolymers such as polysaccharides and glycoproteins which are intimately associated with the carbonate skeleton. Analysis of these hydrolysed compounds gives rise to characteristic arrays of monosaccharides which provide independent criteria for producer identification. The calcareous green algae Halimeda, Penicillus and Udotea show high xylose and low fucose levels. Xylose and fucose levels are elevated in the red algae Amphiroa but only fucose is prominent in the brown algae Padina. The corals Oculina, Porites, Millipora and Montastrea are relatively rich in fucose and show little or no xylose. In the bivalves Arca, Codakia and in Argopecten mannose may be characteristic. Analysis of artificial and natural sediments demonstrates that coral and algal aragonite can be distinguished on the basis of the total sugar concentration and respective xylose and fucose levels. The applicability of the technique in comparison to geochemical and mineralogical methods has been demonstrated for surface sediments from varying water depths of Harrington Sound, Bermuda.
Early diagenetic, Na‐alunites in Miocene algal mat intertidal facies, Ras Sudar, Sinai
Authors: GOLDBERY RON
Published: June 2006

An early diagenetic non‐hydrothermal deposit of Na‐alunite [(Na,K)Al3(SO4)2(OH)6] is described from the Miocene Ras Maalab Group at Ras Sudar Sinai. The alunite is nodular and occurs along two horizons parallel to bedding. Displacive growth structures and disrupted bedding within the host rock, together with enterolithic growth structures within the nodules, indicate a period of growth prior to compaction. Linear arrays of fabric elements within the nodules, observed under the microscope, further support early diagenetic growth. The host sequence, believed to be the product of sedimentation within barred coastal lagoons has been subdivided into two lithofacies, one reflecting a subtidal open‐lagoonal environment, the other an intertidal restricted‐circulation environment. Anaerobic conditions of early diagenesis conducive to generation of H2S and FeS2 preceded an oxidation event produced either by contact with free‐circulating oxygenated waters or by emergence. The sulphuric acid generated lowered the pH and clay minerals were converted to alunite.
Basin‐wide turbidites in a Miocene, over‐supplied deep‐sea plain: a geometrical analysis
Authors: LUCCHI FRANCO RICCI , VALMORI EMIDIO
Published: June 2006

Eighteen stratigraphic sections, 200 m thick on average, were logged in basin plain deposits of the Marnoso‐arenacea Formation (Miocene, northern Apennines) over an area of 123 × 27 km. Turbidites form 80–90% of the facies association, hemipelagites the remainder. Thin and thick‐bedded turbidites are separated by an approximate statistical boundary at 40 cm; most prominent beds (> 1 m thick) are qualified as megaturbidites. With reference to the main supply‐dispersal system (NW to SE), the basin plain can be axially subdivided into proximal, intermediate and distal segments by means of the following parameters: bulk sand content, sand/shale ratio in turbidites, mean thickness of individual layers and component beds, and frequency of thick layers. Almost 40% of thick‐bedded turbidites can be traced over the whole study area. These basin‐wide deposits form the bulk of the basin fill. Geometrical reconstruction shows that some sandstone beds taper downcurrent from the proximal plain or the adjacent fan area while others thin upcurrent suggesting sand by pass of the fan. Mudstone beds in general thicken towards the end and the margins of the plain indicating that turbidite mud, besides bypassing the fan as a rule, was affected by ponding in the plain. Thin‐bedded turbidites have a low sand/shale ratio or are completely muddy representing either tails of sandier turbidites of the outer fan (lobe and fringe deposits) or sheets extending to a great part of or to the whole plain. Sandstone lobes advanced from fans into the plain for 40–50 km gradually thinning and shaling out over a transitional zone of 10–20 km. Their internal geometry shows simple and complex growth patterns: end members are defined as progradational and aggradational. Estimates of original length, width and volume of individual turbidites strongly suggest that flows were usually confined and deflected by basin slopes regardless of source location. Basinal deposits are thus characterized by great thickness and volume, abundance of mud and fine sand, extremely low lateral gradients of thickness and grain size (but rapid wedging near the sides). The basin plain developed as a part of an elongated, oversupplied basin with a ‘highly efficient’, probably delta‐fed, dispersal system.
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

Most 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

The 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

The 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

Fossil 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

Varve‐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

Within 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

Complete 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

In 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

In 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

Marine 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

A 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

Sandstones 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

The 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

During 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

Longitudinal 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

A 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.
The influence of peripheral sabkhas on the geochemistry and sedimentology of a Tunisian lagoon: Bahiret el Biban
Authors: MEDHIOUB KHALED , PERTHUISOT JEAN‐PIERRE
Published: June 2006

Bahiret el Biban is a restricted lagoon in an arid climate. The general geological setting is briefly analysed, especially with respect to the relations between the water body and its evaporitic bordering areas, i.e. supratidal sabkhas.

The geochemical behaviour of the lagoon waters has been studied over a 2‐year period following floodings. In spite of some exchange with the open sea, there is a general tendency towards increasing salinity, calcium sulphate impoverishment and a consequent increase of the Mg/Ca ratio. Thus, unless large floodings occur, the basin waters undergo an evaporitic evolution.

Sedimentation and early diagenesis are controlled largely by this geochemical pattern, but also by exchange with the open sea and by the water depth. Organic matter derived from the sea is deposited on entering this highly saline basin and locally produces sediments which could eventually become source rocks.

Bahiret el Biban is a possible model which may be of considerable interest in the reconstruction of the early stages of the evolution of evaporitic basins.
Debris flow (olistostromes) and slumping on a distal passive continental margin: the Palombini limestone–shale sequence of the northern Apennines
Authors: NAYLOR MICHAEL A.
Published: June 2006

An olistostrome accumulation up to 530 m thick occurs in the Casanova area of the northern Apennines. It lies within or above the calciturbiditic Palombini limestone‐shale sequence, and is part of the allochthonous Vara Complex—sediments originally deposited on oceanic crust. The olistostromes are poorly sorted, monomict, matrix‐supported, submarine debris flow deposits with rigid plugs. They have a compactional foliation and a compaction‐modified, planar clast fabric created during flow. Although diachronous in the main part of the area, the olistostromes have a vertical gradational contact with the overlying slumped Palombini, in which recumbent asymmetric fold hinges and trains and slump boudins are present. Many criteria indicate soft‐sediment deformation. Fold asymmetry indicates a uniformly SW‐dipping palaeoslope. The textural gradation from slumps to olistostrome beds, plus slump folds and boudins as olistostrome clasts show that the olistostromes are dismembered slumps. In vertical sections, variations in limestone petrography, volume percentage and size of clasts confirm that the olistostromes were derived from the Palombini as a series of bed‐by‐bed slumps keeping pace with sedimentation of the Palombini. From olistostrome clast sizes and bed thicknesses, a depositional slope of ∼ 4° is estimated. The olistostromes are not precursor sediments shed from advancing nappes, as in the Bracco ridge model of some authors; rather, they were formed at the foot of a distal, block‐faulted passive continental margin, long before nappe emplacement.
The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia
Authors: WARREN JOHN K.
Published: June 2006

Well exposed, often laminated gypsum sequences occur in many Quarternary salt lakes in southern South Australia. The gypsum in the salt lakes is classified by increasing grain size into gypsite, gypsarenite and selenite. The salt lakes are classified by age and hydrological setting into coastal salinas which are Holocene sea‐water fed groundwater lakes, and continental playas which are late Pleistocene endorheic basins. A study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes. As the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited. Application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non‐salina gypsum deposits.
Lacustrine deltas in a Mesozoic alluvial sequence from Camp Hill, Antarctica
Authors: FARQUHARSON G. W.
Published: June 2006

Sedimentary rocks of late Mesozoic age exposed at Camp Hill, northern Antarctic Peninsula, are associated with calc‐alkaline volcanic rocks. They represent deposition on a fault‐controlled floodplain, with marginal alluvial fans, on a volcanic arc. Finely laminated mudstone and occasional graded laminae were deposited from suspension and by density underflow currents, respectively, in small shallow lakes. Thickening‐ and coarsening‐upward sandstone bodies overlying the lake deposits are interpreted as lacustrine deltas of which two types are preserved: (1) Gilbert‐type with steep foresets and (2) mouth‐bar type which lack steep foresets. Sections through the latter type reveal the presence of sub‐environments characteristic of fluvial‐dominated marine deltas, i.e. prodelta, distributary mouth‐bar and distributary channel. Abandoned mouth‐bars resulting from avulsion are recognized. It is suggested that the processes which operated during formation of the mouth‐bar deltas resulted from hyperpycnal flow. By contrast, the Gilbert‐type delta is thought to be the consequence of a reduced inflow of suspended sediment causing homopycnal flow, and thorough mixing of the river and lake waters.
Rare earth elements in the sedimentary cycle: a pilot study of the first leg
Authors: BASU ABHIJIT , BLANCHARD DOUGLAS P. , BRANNON JOYCE C.
Published: June 2006

A pilot study with Holocene fluvial sands was undertaken in order to evaluate the effects of source rock composition and climate on natural abundances of rare elements (REE) in the first leg of the sedimentary cycle. We have analysed the medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in Montana (semi‐arid), Georgia (humid), and in South Carolina (humid). Despite compositional differences between parent plutons the REE distribution patterns (but not the total absolute abundances) of the daughter sands are very similar. Averages of the three areas have a La/Lu ratio of ∼ 103 showing a depletion of heavy REE with respect to an ‘average granite’ (La/Lu = 79) or the composite of North American Shales (NAS; La/Lu = 55). Also, the Eu/Sm ratio in sands from these areas is ∼ 0·22 which is very close to the NAS ratio of 0·21. However, the overall REE distribution of these sands is not similar to that of NAS in any way. We conclude that the major rock type, but neither its minor subdivisions nor the climate (in the source area), controls the REE distribution patterns in first cycle daughter sands. But, the total and the parent rock‐normalized abundances of REE in sands from the humid areas are much lower than those in sands from the arid areas.