Showing papers in "Sedimentology in 1982"

Isotope signatures associated with early meteoric diagenesis

Abstract: The environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly 12C-enriched limestones. δ13C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight 18O-enrichment; (3) an abrupt shift in δ18O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine-meteoric mixing zone may be characterized by positive covariance between their δ18O and δ13C compositions; (5) the vadose-phreatic boundary may be marked by a sharp increase in δ13C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ18O and a wide range of δ13C compositions. Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian. These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations.

Saltation threshold on Earth, Mars and Venus

Abstract: New formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles. A low-air-density wind tunnel was used to extend the range of previous investigations and to separate the effects of Reynolds number and interparticle forces of cohesion. The new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the Earth, Mars, and Venus.

Bedform climbing in theory and nature

Abstract: Where bedforms migiate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of twodimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.

The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia

Abstract: 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.

Deposition of climbing‐ripple beds: a flume simulation

Abstract: Thirteen runs were made in a small recirculating flume to simulate the deposition of the climbingripple sequences commonly present in fine-grained facies of fluvial and deltaic deposits. These sequences consist of intergradational climbing-ripple cross laminae and draped laminae. The experiments were based on the assumption that stratification type depends mainly on near-bottom flow structure and uniform sediment fallout from an overloaded flow. Various combinations of curves of velocity versus time and of sediment feed versus time in runs lasting from 45 to 840 min were used in an exploratory program; conditions for each run were selected on the basis of experience in previous runs. The runs verified that Type A (erosional-stoss) climbing ripples are produced by aggradation rates that are small relative to ripple migration rate, and Type B (depositional-stoss) climbing ripples are produced by aggradation rates that are large relative to ripple migration rate. Draped lamination results from continued fallout of sediment from suspension after ripple migration ceases or almost ceases. Comparison of geometric details of the ripple stratification produced in the flume runs with that in natural sequences, supplemented by considerations on maximum and minimum migration rates of ripples, suggests times of no more than a few tens of hours for the deposition of the climbing-ripple portions of sequences 10-20 cm thick. Runs in which deposition of a 20 cm sequence took more than 10 h produced such atypical features of ripple geometry as sharp crests, planar lee-side laminae, and angular toeset-foreset contacts.

The superficial sediments of the english channel and its western approaches

Abstract: The English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal. A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous-rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply. A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Wurm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre-Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre-Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre-Holocene conditions. RESUME La Mer de la Manche et ses approches occidentales constituent une plateforme epicontinentale qui s'etend suivant un axe NE - SW d'environ 700 km, depuis le Pas-de Calais jusqu'au rebord continental. C'est un milieu sedimentaire de climat tempere, a subsidence negligeable et a faible taux de sedimentation. La couverture de depots meubles est mince (quelques metres en general) sauf au niveau des bancs sableux et des paleovallees souvent comblees s'alluvions anciennes. La repartition des sediments superficiels est essentiellement controlee par les courants de maree particulierement forts le long des cotes armoricaines et en Manche centrale. En Manche occidentale, un important gradient de vitesses decroissantes se developpe du Sud-Est au Nord-Ouest. Les houles s'exercent principalement sur la frange cotiere ou'leur action devient dominante. Dans l'ensemble, les depbts fins sont rares et concentres dans les baies et les estuaires ou les suspensions se decantent. Par contre, les sediments graveleux a caillouteux sont largement repartis. Une opposition nette se dessine entre une province orientale, surtout terrigene, et une province occidentale au materiel bioclastique abondant presentant un gradient de dispersion en direction du Sud-Ouest. Ces caracteres morphologiques et sedimentaires du milieu sont fortement marques par son histoire quaternaire, notamment par la derniere oscillation marine. Un schema de mise en place et d'evolution du modele sedimentaire au cours du Quatenaire recent est propose. Partant de la zone en permanence submergee durant le Wurm, la transgression holocene progressa sur la Manche. Le Pas-de-Calais s'ouvrit vers 9000 BP et peu a peu les conditions hydrodynamiques actuelles se mirent en place sous un climat devenu tempere. Progressivement les depots terrigenes anciens furent contamines par du materiel bioclastique holocene.

The dynamics of a river bend: a study in flow and sedimentary processes

Abstract: Comprehensive field measurements of flow and sedimentary processes have been made with the aid of stable scaffolding bridges spaced along the length of a bend of the River South Esk, Scotland. At river stages between about two-thirds full and bankfull, channel width, mean depth and mean flow velocity at a cross-section vary little in the streamwise direction. Flow resistance reaches a maximum at these stages, and the bed topography is stable and in equilibrium with flow and bedload transport. Stable flow geometry is thus related in some way to energy conservation, and to maximization of flow resistance. Detailed observations over a large range of river stages of mean velocity distributions, secondary circulation, water surface configuration, bed shear stress and resistance to flow, bed configurations and bed load transport rates agree with much (but not all) of the comparable published experimental studies and selected theoretical work. Generalized physical models of flow and sediment transport in natural curved channels (Engelund, 1974; Bridge, 1977) are demonstrated to be sound in basis and can simulate the bend studied very well. Although there is a pressing need for further development of these models, the results lend confidence to their use in simulating ancient river sedimentation. Sediment deposited on point bars is the result mainly of bedload transport over a range of near-bankfull stages. The areal distribution of grain-size characteristics and bed configurations at these stages give rise, with lateral deposition, to vertical facies sequences that vary substantially in the streamwise direction.

Cryptalgal-metazoan bioherms of early Ordovician age in the St George Group, western Newfoundland

Abstract: Bioherms 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-Lichenaria or -sponge mounds and (3) thrombolite-Lichenaria-Renalcis reef complexes. The framework of the last is the most complex, with abundant cavities and a demonstrably uneven growth surface of thrombolites, corals and free-standing Renalcis heads, walls and roofs. Some cavities were active sediment conduits while others were protected, their roofs draped with Renalcis and 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.

Bedform and stratification characteristics of some modern small‐scale washover sand bodies

Abstract: Washover sand bodies commonly develop along microtidal coastlines in beach/barrier island or spit settings. Wave runup, usually in conjunction with an abnormally high water level, may overtop the most landward berm of the beach and the foredune crest, if one exists, to produce overwash and subsequent runoff across the more landward subaerial surface. Two main elements of the resulting deposit are the washover fan and runoff channel. Newly formed, small-scale washover deposits were examined along the Outer Banks, North Carolina, near Pt Mugu, California, and at Presque Isle (Lake Erie), Pennsylvania. The fans were formed in response to unidirectional landward transport, and the runoff channels in response to unidirectional flow usually in a landward direction, but sometimes in shore-parallel then seaward direction. Where overwash carried across the fan surface and entered a pond or lagoon, a small-scale delta (microdelta) developed. In this case, the washover fan consisted of two subfacies, the wetted, but ‘subaerial’ part of the fan and the subaqueous washover delta. Flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification. High velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low-angle (landward dipping) planar stratification which comprised the major part of the fan. Similarly, rhomboid forms were produced by high velocity sheet flow across the fan surface. Where flow carried into a standing body of water, delta-type foreset strata developed. For this case, the lateral structural sequence was subhorizontal, planar stratification merging landward into landward dipping, delta (tabular) foreset strata. In the runoff setting, where flow became channelized and continuous, both upper-flow and lower-flow regime currents were typical. Upper-flow regime bedforms included antidunes, standing waves, and plane beds. The most commonly observed lower-flow regime bedforms included microdelta-like bars, low-amplitude bars, linguoid ripples, and sinuous-crested current ripple trains. The sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment.

Sheet-flow deposits of graded beds and mudstones on an alluvial sandflat-playa system: Upper Triassic Blomidon redbeds, St Mary's Bay, Nova Scotia

Abstract: The 400 m of Blomidon redbeds accumulated in a semi-arid rift valley in the subtropics. At St Mary's Bay, these redbeds are 64% sandy mudstone (playa mudflats), 25% graded beds (sandflats at the toes of alluvial fans), 10% fissile claystone (playa lakes), and 1% channel sandstone (stream channels). Flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north-east as sheet flows on sandflats and playa mudflats. Deceleration of the sheet flows deposited graded beds 2–83 cm in thickness on the sandflats and thin layers of mud on the playas. Nine sequences, consisting of arrangements of six lithologies, compose 90% of the graded beds. In order of decreasing abundance, these are: ripple cross-laminated siltstone horizontally laminated mudstone; fining-upward, ripple cross-laminated siltstone; ripple cross-laminated sandstone horizontally laminated mudstone; cross-bedded sandstone horizontally laminated mudstone; ripple cross-laminated sandstone ripple cross-laminated siltstone; rippledrift cross-laminated siltstone horizontally laminated mudstone; fining-upward, ripple-drift cross-laminated siltstone; cross-bedded sandstone ripple cross-laminated siltstone; and cross-bedded sandstone ripple cross-laminated siltstone horizontally laminated mudstone. The sheet flows, perhaps up to 1 m in depth, had a high concentration of suspended load. Deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration. There are 32 thinning and fining-up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone. The cycles range in thickness from 1·3 to 13·3 m, averaging 4·6 m. Each cycle is initiated by avulsion to a new active channel network on a fan. Gradual abandonment of the channel network produces the thinning and fining-up cycle. The cycles are grouped in three 60–70 m fining-up megacycles. Upwards within each megacycle, the packages of sandy mudstone compose a progressively larger proportion of the cycles. Each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor. Subsequent erosion gradually lowered the relief to yield a fining-up megacycle.

Sedimentology and evaporite genesis in a Holocene continental‐sabkha playa basin—Bristol Dry Lake, California

Abstract: Bristol Dry Lake, a 155 km2 continental-sabkha playa basin in the Mojave Desert of south eastern California, is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. Evaporite facies conform approximately to a bull's eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan to the centre of the playa, reveal: (1) crudely-bedded, alluvial fan clastics interfingering with (2) playa-margin sand flat and wadi sand and silt, followed by (3) gypsum, anhydrite, chaotic mud halite, and clay of the saline mud flat, and (4) salt-pan halite beds. Terrigenous clastics were deposited in Bristol Dry Lake by sheetflow and by suspension settling from ponded floodwater. Some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin. Thin nodular-like beds of anhydrite and several types of gypsum occur across most of the playa. Giant hopper-shaped halite cubes are suspended in saline mud flat facies, suggesting that they grew displacively in brine soaked sediment just below the surface. Thick beds (4 m) of halite, in the playa centre, may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods, intense evaporation and brine-level lowering, and capillary discharge of brines. The stratigraphy in the playa centre is cyclic. An ideal cycle consists of: (1) chaotic mud halite at the base overlain by (2) green to red clay with abundant, giant hoppers, and at the top (3) red clay, gypsum, and anhydrite with flaser- to wavy-bedded sand and silt. This type of cycle probably records a gradual progradation of mud-flat facies over salt pans. Bristol Dry Lake sediments are nearly identical to some of the Permian evaporites of the Permian Basin region, U.S.A. and they can serve as modern analogues for ancient-sabkha facies analysis.

Pyrite in ammonite‐bearing shales from the Jurassic of England and Germany

Abstract: Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias e, Germany) and in highly bituminous shales (Lias e, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected-light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro-environments, while the sediment generally was not wholly anoxic. Most pyrite pre-dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from −55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas-filled. In the bituminous Lias e shales pyrite occurs abundantly as early framboids and micro-nodules. Larger nodules show a variety of forms, some of which post-date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near −20. Pyrite formed over a long time-span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void-fillings and concretions, and which help define the diagenetic history of a shale.

A study of the dynamics of a marine sandwave

Abstract: The movement of the crest of a sandwave was studied using cross-sectional profiles obtained from lines of sea-bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest. The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea-bed by storm waves, caused major reductions in crestal heights. Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.

Isochronous fluvial systems in Miocene deposits of Northern Pakistan

Abstract: A Palacomagnetic isochron dated at about 8.1 Myr BP and detailed lithostratigraph of a 40 m interval exposed along strike for 40 km establish the depositional patterns of two contemporaneous, interfingering fluvial systems in the upper part of the Meddle Siwalik sequence. The two systems, referred to as the buff and blue-grey, differ in unit shape, lithofacies, bedding sequence, palaeocurrent direction and sand composition. Interfingering occurs along the south-west-north-east strike of the outcrops, with the palaeodrainage directions of the two systems generally perpendicular to this line. The axis of the blue-grey system, which deposited widespread sheet sands and silts, lay toward the south west end of the study area. The more complex axis of the buff system, which deposited shoe-string sand bodies and lage volumes of silt and clay, lay toward the north-east. The source area for both systems was the rising Himalyan belt to the north and noth-east of the study area. At maximum extent the blue-grey system occupied a channel belt at least 25 km wide. Channel belt widths and depths for the buff system are 1–3 km and 3–7 m, respectively. Current directions averge 94° for blue-grey sands and 136° for buff sands. Blur-grey sands contain 20% more rock fragments and are otherwise less mature than buff sands. The buff system shows a verticla pattern of avulsion, palaeosol formation and floodplain aggradation which we attribute to autocyclic processes of parallel rivers. The blue-grey system shows phases of erosion accompaniced laterally by plaeosol formation, folowed by valley fill and overfowing of interfluve surfaces. Theis pattern may be caused by allocyclic presses affecting the source area. We interpret the blue-grey system as a major drainage from the interior Himalayas (perhaps the ancestral Indus) and the buff system as a complex of smaller drinages along the mountain front which were probably ributaries to the bluegrey syste. Vertebrate fossils including hominoid primates from the area are almost exclusively associated with lithofacies of the buff system, and this probably refects both taphonomic and palaeoecological differences between the two systems.

Depositional model for a chenier plain, Gulf of Carpentaria, Australia

Abstract: The southern coast of the Gulf of Carpentaria, Northern Australia, consists of a broad chenier plain which is comparable in size and morphology to those reported on the coasts of Surinam and south-western Louisiana. The Carpentaria chenier plain has, at some locations, prograded over 30 km since the Middle Holocene by deposition of low-tide muds over subtidal muds during periods of increased sediment input by the rivers. Chenier ridges are formed during periods of reduced sediment flux by wave induced sorting and redistribution processes. An alternation of increased and decreased sediment flux from fluvial sources has occurred several times during the upper Holocene. Such variation in sediment supply is believed to be related to temporal fluctuations in rainfall over the drainage basin which surrounds the southern and eastern Gulf.

Bed‐material size and the riffle‐pool sequence

Abstract: Relationships between the bed-material size distribution and riffle-pool geometry of a small coarse-bedload upland stream channel are described. Downstream sequences of bed-material size are highly variable, although series analysis reveals some persistence in the data over short distances. Cross-correlation of bed-material series with bed-elevation series indicates riffles tend to coincide with coarser bed material sequences. Comparison of the bed-material size distributions of individual riffles, pools and bars is, however, more revealing. Mean particle size was found to be the most useful criterion for distinguishing between bed forms, and in these terms bars are the most distinctive features. Moreover, riffles and pools can be viewed as simply downstream oscillations in elevation of a single coarser-grained sedimentary unit. Coarse sediment supply from erosion scars created where the channel meets confining bluffs in valley-fill deposits does mean that the average particle size of adjacent and downstream bed forms is increased. However, conclusions remained unchanged after reanalysis following the identification and subtraction of immobile particle diameters. Plan geometry and coarse sediment supply in the upland environment are also shown to influence bed-material size distributions. Tightly curved pools tend to have the finest pool sediments but also the least difference between bar and pool sediments. Size of material in pools is also negatively related to an index of cross-section asymmetry.

Sand waves and sediment transport around the end of a tidal sand bank

Abstract: Two detailed surveys have been made of the north-western end of the Haisborough Sand off the Norfolk coast using echosounder, 3.5 kHz reflection profiler and side-scan sonar. Asymmetrical sand waves indicate north-westerly sand movement on the southern side and south-easterly sand movement on the steeper northern side of the bank. Secondary, superimposed megaripples, which are probably better indicators of sediment movement, give evidence of a cross-bank component. Between the north-westerly and south-easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves. These are usually taken to indicate zero net transport, but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests. This suggests the route by which sand travels around the end of the bank to form a roughly closed circulation. Sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north. Analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing, though that could allow more than 0.25 km shift to the north east in 100 years to pass undetected. A box model is drawn up for the estimated sediment fluxes around the end of the bank, and implications for residence times and circulation rates are drawn from it.

The Cambro-Ordovician Cap Enrage Formation, Queebec, Canada: conglomeratic deposits of a braided submarine channel with terraces

Abstract: The Cambro-Ordovician Cap Enrage Formation is interpreted as a deep submarine channel complex of conglomerates, pebbly sandstones and massive sandstones. The formation is up to 270 m thick, and crops out in a coastal belt 50 km long. In general terms, it has previously been interpreted as a deep sea channel deposit, with the channel about 300 m deep, at least 10 km wide and trending south-westward, parallel to the coastal outcrops. Eight facies have been defined in this study and they have been grouped into three major facies associations. In the Coarse Channelled Association, conglomerates with carbonate boulders up to about 4 m are associated with graded-stratified finer grained conglomerates. Facies of this association make up about 25% of all the beds in the formation. The association is also characterized by abundant major channels 1–10 m deep and up to 250 m wide. Excellent outcrop allows the reconstruction of topographic highs (bars) within the channels and the association is interpreted as a braided channel and bar system. The second association, Multiple-Scoured Coarse Sandstones, contains some graded-stratified fine conglomerates, along with massive to structureless coarse and pebbly sandstones, and rare cross-bedded pebbly sandstones. Deep channels are absent, but multiple channelling on the scale of 0.5–1 m is characteristic. In the absence of the very coarse conglomerates and deeper channelling, this association is interpreted as being deposited on topographically higher terrace areas adjacent to the main braid plain. The third facies association, Unchannelled Sandstones, is characterized by massive sandstones with abundant fluid-escape structures, classical turbidites and thin shales. In the absence of any scouring deeper than a few tens of centimetres, this association is interpreted as being deposited on an even higher and smoother terrace, farther from the braid plain. Palaeoflow directions for conglomerate facies indicate fairly consistent south-westward transport, apparently parallel to the base of the Cambro-Ordovician continental slope. Flow directions in the finer-grained facies are rather variable, suggesting complex bar development and overbank spills. Thinning-and fining-upward sequences are present on two scales. The smaller, 1–10 m sequence, is related to channel filling and abandonment. Thicker sequences (10–100 m), with facies of the Multiple Scoured, and Unchannelled Sandstone Associations, may indicate switching of a main channel away from the area and its subsequent burial by marginal terrace and higher terrace deposits.

Geometry and origin of large‐scale deformation structures in some ancient wind‐blown sand deposits

Abstract: Large-scale deformational features that disrupt the cross-bedded strata of some ancient wind-blown dune sand deposits (Jurassic Aztec and Navajo Sandstones, U.S.A.) can betraced laterally for at least tens of metres. Information from four exposures leads to an idealized deformation style characterized by (1) a ‘head’ portion marked by collapse features, (2) a middle portion marked by a thrust ramp or a large recumbent fold, and (3) a ‘toe’ portion marked by a planar shear zone with small recumbent folds, drag folds, and possibly small horizontal shear faults. An hypothesis involving earthquake-induced liquefaction and collapse of the dunes is proposed. Sands liquefied during earthquakes cannot support an unequal surface load, resulting in the collapse of surface dunes above the liquefied substrate. Compensatory lateral squeezing of liquefied sand and buried strata produces the shear zone in the ‘toe’ portion. Engineering studies suggest that buried strata underlying interdunal lows, where over-burden stress is least, will be most susceptible to liquefaction. Other considerations suggest that it will be the steeper lee sides of advancing surface dunes that collapse, squeezing liquefied sand forward or downwind, i.e. away from the lee slope of surface dunes. If the liquefied condition persists, load structures can be formed when unliquefied layers founder into the liquefied substrate.

Palaeokarst, tidal erosion surfaces and stromatolites in the Silurian Eke Formation of Gotland, Sweden

Abstract: Irregular, scalloped erosion surfaces in the shelf carbonate sequences of the lower Eke Formation (Ludlow Series, upper Silurian) in the east of Gotland, Sweden, comprise series of shallow hollows separated by sharp-crested ridges, and cavities with sculptured, undercut walls, cut into lithified sediment These represent analogues of the solution basins formed in modern coastal and subaerial karst terrains Discrete erosional cavities merge on enlargement by breaching of the intervening walls to leave remnant, tapering ridges Sets of the ridges and basins seen in surface view show an average width of basins of 1–2 m, with relief of 40–50 cm and pronounced N-S axes for the ridges; this alignment may reflect the local drainage direction The lowermost erosion surface passes laterally into a planar, mineralized horizon at the top of the underlying Hemse Group that was resistant to and forms the base level of erosion Because of restricted exposure of higher Eke Formation sediments the upper limit of erosion remains unknown There is no evidence of caliche or subaerial diagenetic textures, but solution vugs are common in the eroded limestones Marine hard-bottom biota attached to some surfaces, and transition from scalloped to planar surfaces indicate erosion in tidal zones, but subaerial karstic erosion is also inferred The resubmerged karst-eroded topography is overlain by shallow marine carbonates, including small organic buildups Finely-layered stromatolitic mats developed over the initial infill, in subtidal environments, and grew to form domed mounds within the erosional cavities They abut sharply against bounding side walls and overhangs Some emergence is evident from desiccation features in the upper parts of mounds Biostratigraphical evidence dating the events from initial uplift and karstic erosion to covering of the drowned relief topography places the whole sequence within upper Ludlow times

Origin of siltstones: glacial grinding versus weathering

Abstract: Siltstones are unusual rocks. They are mainly made up of quartz. Glacial grinding is considered by some authors to be the main, or even the only, process generating silt. This is supported by: (1) the presence of silt in basal tills from present ice-caps and, above all, (2) by the volumetric importance of Quaternary loess. However, ancient glaciogenic sediments contain only a little silt scattered in the matrix of tillites, or forming thin intercalations in pro- and periglacial deposits. These siltstones represent less than 5% of the total volume of Upper Proterozoic and Upper Ordovician glaciogenic sequences of West Africa. On the other hand, silt is abundant in weathering profiles of tropical, equatorial and mediterranean zones. This silt is generated by splitting of minerals, mainly quartz, inherited from the parent rock. Quartz grains are partly dissolved in situ and their fragments cemented by iron oxi-hydroxides in tropical and equatorial zones and by calcite in mediterranean zones. Silt is particularly abundant in tropical soils, comprising up to 50-75%. Secondary dissolution of the iron or calcite matrix induces disaggregation of these soils and the release of silt which is later reworked and concentrated by wind or running water. Weathering processes, and especially those operating in tropical zones, are the main phenomena generating silt. Glacial grinding would generate only a little silt. A large part of the material of Quaternary loess may be derived from glacially reworked weathering profiles.

Variation in particle size distribution over a small dune

Abstract: The variation in particle size distribution for a sequence of sand samples, collected from the surface of a small barchanoid dune along a sampling line parallel to the windflow, is studied by means of the hyperbolic distribution. The modal (or typical) value of the log size increases linearly with distance from the windward foot to the crest of the dune, and falls off steeply on the lee side. The spread of the log-size distribution, locally near the modal value, is roughly constant on the windward side but decreases significantly down the slip face. Two other parameters of the four-parameter hyperbolic distribution can be considered largely constant. A comparison of the log size distributions for heavy and light mineral grains, based on a selected set of the samples, showed that heavy and light minerals follow different hyperbolic patterns. However, the difference in modal size of the distributions for the two types of minerals was virtually constant among the samples.

The oxygen and carbon isotope composition of Carboniferous fossil components: sea-water effects

Abstract: The aragonitic molluscs and lime-mud of the Pennsylvanian Buckhorn asphalt (Deese Group) of southern Oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea-water. In addition, δ18O values indicate that the pelecypods precipitated their shells during the warmer months of the year. The coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year. For the orthocone nautiloids, the δ18O values suggest that they precipitated their shells in deeper/cooler water. The low-Mg calcite brachiopods of the Mississippian Lake Valley Formation of New Mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea-water. The δ18O and δ13C values of the Buckhorn and Lake Valley faunas, in conjunction with other published results, suggest that Carboniferous sea-water was, on a average, depleted in δ18O by 1·5 ± 2‰, PDB, relative to Recent sea-water. However, the δ13C value of +2.6 ± 2‰, PDB, for average Carboniferous sea-water is similar to that of Recent ocean water. Early diagenetic alteration of metastable carbonates probably occurs in a meteoric-sea-water mixing zone. In this zone the oxygen and carbon isotopic compositions of these components are increased by about 2-4‰, PDB over their marine composition.

Distribution of chlorite and kaolinite in eastern Atlantic sediments off North Africa

Abstract: Chlorite 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.

Shape effects in aeolian grain transport

Abstract: Wind tunnel experiments are reported, the results of which show that grain transport rate is responsive to characteristic grain shape. It is relatively less sensitive to changes of sorting unless these involve the fraction smaller than 50μm. A cubic relationship between transport rate and velocity gradient appears to be valid only for well worked natural grains having high sphericity.

Size–density sorting of sand‐size spheres during deposition from bedload transport and implications concerning hydraulic equivalence

Abstract: It is well known that sediment sorting according to size, shape and density occurs, but the exact mechanisms involved are poorly understood. To assess the effects of size and density, sand-size spheres of two densities were transported and deposited under controlled flume conditions. Observations on the motion of discrete particles show that grains smaller than bed-roughness grains move continuously and have the same transport velocities regardless of density. For grains near and slightly larger than the roughness, movement is intermittent and, for a given size, heavy particles move more slowly than lights. For grains much larger than bed roughness grains, movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities. Analyses of bulk sediment deposited from plane-bed transport, show that the size and proportion of heavies decreases and that of lights increases with distance transported. For ripple bed transport, however, the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable. These results suggest that size-density sorting in plane-bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple-bed transport is related to deposition on, and recycling through, the bed forms. Application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept to be useful.

Burrows and trails produced by Quinqueloculina impressa Reuss, a benthic foraminifer, in fine-grained sediment

Abstract: The benthic foraminifer Quinqueloculina impressa Reuss, was buried in various types of sediment in order to assess its capability for producing sediment disturbances and thus, ichnofossils. Silts and silty-clays showed distinct burrows; fine sands did not. Two types of burrows were produced: fairly straight, vertical burrows from 4 cm below the water-sediment interface to within 1 cm of the sediment surface, and a horizontal and vertical maze-like burrow system in the top centimetre of the sediment. Individuals moving on the sediment surface also produced visible trails. When the sediments were dried the burrows were always destroyed; in some cases the surface trails were preserved. We propose that the vertical burrows are escape structures and that the horizontal and vertical maze-like structures are living burrows. Ichnofossils similar to the escape structures and surface structures have been described. Presence of these ichnofossils indicates a low energy sedimentary environment and a lack of macrofaunal bioturbation. The presence of escape structures may indicate a pulsatory pattern of sedimentation.

Temporal and spatial variation in intertidal sedimentation rates

Abstract: Sediment deposition and erosion rates are reported for an intertidal zone in the Burry Inlet, South Wales. Measured deposition rates over the salt marsh are compared with deposition rates calculated from observed suspended sediment concentrations. Notably, it is concluded that residual turbulence at slack water should not be discounted when calculating deposition rates. Grain-size distributions of suspended sediments over the marsh surface, during flood and ebb tides, contrast with the grain-size distribution of deposited marsh sediments, the latter being significantly coarser. These data in conjunction with mass budget calculations are used to relate total annual deposition and sediment supply by tidal action during settled meteorological periods. The analysis suggests that episodic storm-induced sediment transport is probably an important mechanism for introducing coarse sediment on to the marsh surface. Finally, it is noted that seasonal reworking of the sandy non-cohesive sediments may be related to variations in the intensity of wave-breaking throughout the year.

The use of settling velocity in defining the initiation of motion of heavy mineral grains, under unidirectional flow

Abstract: The 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

Bed‐thickness and grain size of turbidites

Abstract: In measured sequences of limestone- and greywacke-turbidites the bed-thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed-thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain-size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed-thickness on maximum grain size is well defined and resembles a fall-velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee-forming processes.