Showing papers in "Journal of Sedimentary Research in 1999"
TL;DR: The Udden-Wentworth grain-size scale is widely used as the standard for objective description of sediment, but it inadequately covers gravel, the dominant fraction in many environments such as alluvial fans.
Abstract: The Udden-Wentworth grain-size scale is widely used as the standard for objective description of sediment, but it inadequately covers gravel, the dominant fraction in many environments such as alluvial fans. The scale is most detailed in the sand and mud fractions, where grades such as "fine sand" are defined by particle intermediate axial length (dI). We propose similar detailed grades for gravel with dI boundaries consistently determined by extending the Udden-Wentworth scheme of multiples of 2 (whole increments). The 2 to 4 mm granule class (-1 to -2 ) in this system consists of just one grade, but the pebble class comprises four: fine pebbles with dI from 4 to 8 mm (-2 to -3 ), medium pebbles from 8 to 16 mm (-3 to -4 ), coarse pebbles from 16 to 32 mm (-4 to -5 ), and very coarse pebbles from 32 to 64 mm (-5 to -6 ). Coarser grades are fine cobbles with dI from 6.4 to 12.8 cm (-6 to -7 ), coarse cobbles from 12.8 to 25.6 cm (-7 to -8 ), fine boulders from 25.6 to 51.2 cm (-8 to -9 ), medium boulders from 51.2 to 102.4 cm (-9 to -10 ), coarse boulders from 102.4 to 204.8 cm (-10 to -11 ), and very coarse boulders from 204.8 to 409.6 cm (-11 to -12 ). These terms can be used in Folk's texture classification to derive detailed descriptions such as "angular, poorly sorted, fine to coarse boulder conglomerate". This grain-size scheme is further extended to account for particles coarser than boulders (dI > 4.1 m), which we collectively call megaclasts, and the sediment they comprise megagravel or, if lithified, megaconglomerate. Megagravel is divided into four classes based on dI, including blocks from 4.1 to 65.5 m (-12 to -16 ), slabs from 65.5 to 1049 m (-16 to -20 ), monoliths from 1 to 33.6 km (-20 to -25 ), and megaliths from 33.6 to 1075 km (-25 to -30 ). The first three classes cover the coarsest sediment currently known. Their grades are fine blocks, with dI from 4.1 to 8.2 m (-12 to -13 ), medium blocks from 8.2 to 16.4 m (-13 to -14 ), coarse blocks from 16.4 to 32.8 m (-14 to -15 ), very coarse blocks from 32.8 to 65.5 m (-15 to -16 ), fine slabs from 65.5 to 131 m (-16 to -17 ), medium slabs from 131 to 262 m (-17 to -18 ), coarse slabs from 262 to 524 m (-18 to -19 ), very coarse slabs from 524 to 1049 m (-19 to -20 ), very fine monoliths from 1.0 to 2.1 km (-20 to -21 ), fine monoliths from 2.1 to 4.2 km (-21 to -22 ), medium monoliths from 4.2 to 8.4 km (-22 to -23 ), coarse monoliths from 8.4 to 16.8 km (-23 to -24 ), and very coarse monoliths from 16.8 to 33.6 km (-24 to -25 ). These grades also can be used in Folk's texture classification for objective sediment description. We reserve the megalith class and five attendant grades for even coarser megaclasts, with dI spanning from 33.6 to 1075.2 km (-25 to -30 ).
369 citations
TL;DR: In this paper, the internal Froude number (Fi) was used to predict facies variation in turbidites and pyroclastic deposits where topography has been an important control during deposition.
Abstract: The flow of turbidity currents and pyroclastic flows into regions with topography produces spatial variation in flow. This variation (the flow nonuniformity) affects not only the loci of deposition but also the depositional facies. The nonuniformity around topography can be characterized by a dimensionless parameter, the internal Froude number, Fi, that is dependent upon flow velocity, the reciprocal of flow height, and the density stratification. Low values of Fi result in decoupling of the flow into a lower, denser part that moves around the topography and an upper part that moves up or over it. We present an example from an Oligocene turbidite system interacting with a bounding slope. Massive sands on the open basin floor pass laterally into thinner sands with abundant traction structures adjacent to the paleoslope. We use outcrop constraints to develop a simple model of the basinal paleotopography, and the nonuniformity of flows resulting from their interactions with it. The observed facies variation with respect to the paleotopography corresponds closely with predictions based on this pattern of nonuniformity. Currents were deflected by the slope, but there is also evidence of reflection where the lower-density parts of the flows ran up the slope and collapsed back onto the basin floor. Application of an internal Froude number criterion indicates that the flows had a very steep density gradient, at least in their lower parts, that resulted in deflection of the basal parts of the flows. This model may be generally applicable for predicting facies variation in turbidites and pyroclastic deposits where topography has been an important control during deposition.
235 citations
TL;DR: In this article, Holocene sediment cores acquired near Ferriday, Louisiana show that Holocene deposits consist of 15-30 m (ave. 20 m) of sands, silts, and clays, which overlie Late Wisconsin sands and gravels.
Abstract: The alluvial architecture and soil characteristics of Holo cene Mississippi River floodplain deposits in the southern Lower Mississippi Valley provide evidence for significant changes in floodplain development in response to sea-level rise. Floodplain cores acquired near Ferriday, Louisiana show that Holocene deposits consist of 15-30 m (ave. 20 m) of sands, silts, and clays, which overlie Late Wisconsin sands and gravels. On the basis of differences in sediment grain size, sediment-body geometry, and the abundance of soil features, the Holocene deposits are subdivided into Lower and Upper Holocene units. Lower Holocene deposits (> 5000 yr B.P.) consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite, pyrite, and vivianite and show little evidence of soil formation. Muds encase crevasse-splay and floodplain-channel sand bodies (< 1 km wide), and collectively these deposits represent a mosaic of shallow lakes, poorly drained backswamps, and multichannel streams, similar to modern examples in the Atchafalaya Basin ( 100 km south of Ferriday). Upper Holocene deposits (< 5000 yr B.P.) are represented by large Mississippi River meander-belt sand bodies that are up to 15 km wide and 30 m thick. Natural-levee silts and sands and well drained backswamp muds are present between meander-belt sands. Upper Holocene deposits contain abundant soil features, and sandy and silty soils are Entisols, Inceptisols, and Alfisols whereas clayey soils are Vertisols. The presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that Lower Holocene sediments reflect a period of rapid floodplain aggradation during which crevassing, lacustrine sedimentation, and avulsion dominated floodplain construction. No evidence of large meandering Mississippi River channels represented by buried, thick tabular sands exists near Ferriday, and discharge in Lower Mississippi Valley flow was probably conveyed by a network of small, multichannel floodplain streams. Upper Holocene sediments record a dramatic change ca. 5000 yr B.P. from rapid to slower floodplain aggradation, which was accompanied by extensive lateral channel migration, overbank deposition, and soil formation. On the basis of differences in meander belt dimensions and numbers of abandoned channels, Upper Holocene meander belts are subdivided into simple and complex forms. Relative age relationships suggest that the smaller and older simple meander belts represent periods of divided Mississippi River flow and early attempts to establish a large, single-channel meandering regime. This type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt. Similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of Holocene sediment accumulation in the southern Lower Mississippi Valley strongly suggest that decelerating Holocene sea-level rise in the Gulf of Mexico affected floodplain development at least 300 km inland from the present-day coast. The alluvial architecture of the Lower Holocene deposits and the absence of large meandering Mississippi River channel deposits older than 5000 yr B.P. near Ferriday indicates that most of the floodplain muds were deposited by avulsion-related crevassing and lacustrine sedimentation rather than by overbank flooding of large Mississippi River channels. Similarities between the floodplain history of the Mississippi River and those of modern and ancient rivers elsewhere further suggest that avulsion, rather than simple overbank deposition, contributes to the construction of fine-grained floodplains to a greater degree than generally recognized.
188 citations
TL;DR: Gravel dunes are rarely reported owing to comparative rarity of occurrence and generic confusion with antidunes and classes of gravel bars as discussed by the authors, however, there are few data on the hydrodynamic development of gravel dunes.
Abstract: Gravel dunes are rarely reported owing to comparative rarity of occurrence and generic confusion with antidunes and classes of gravel bars. A compilation of data from the literature for sediments with a median size greater than 2 mm shows that dunes have been developed in the laboratory in median grain sizes up to 28.6 mm. For field conditions, there are data for grain sizes up to 60 mm. Equilibrium and non-equilibrium gravel dunes range in length from less than 0.6 m to greater than 100 m. Heights range from less than 0.1 m to 16 m. Height and length data for the steepest three-dimensional (3-D) gravel dunes are consistent with the H:L function reported by Ashley (1990) for equilibrium dunes developed in sand. For a broad range of dune length, 3-D gravel dunes are steeper than two-dimensional (2-D) dunes, but for L > 100 m and H > 8 m there are too few data to draw any conclusion. Ashley's H:L function does not apply to a broad range of 2-D gravel dunes. A separate well defined function is proposed to describe the H:L relationship for 2-D gravel dunes of maximum steepness. There are few data on the hydrodynamic development of gravel dunes. As a guide, gravel dunes develop for a range of Froude numbers up to 0.75 given that the nondimensional mean bed shear stress () exceeds circa 0.1. Dunes reach their maximum height at = 0.25 and have reduced in height when = 0.3. Froude numbers of 0.84 and above lead to crestal flattening, dune diminution, and eventual replacement by antidunes or upper-stage plane beds. However, owing to lag effects, transitional bedforms have been recorded for Froude numbers near the critical value of 1. The development of gravel dune steepness from lower-stage plane bed through to upper-stage plane bed is well described in only one case, by Dinehart (1992a). Fredsoe's (1975) function for equilibrium dune steepness applies reasonably well to Dinehart's steepest dunes. Allen's (1984) bedform existence field based on and a characteristic grain size of the bed sediment has been extended from 5 mm to include bedstocks as coarse as 33 mm.
164 citations
TL;DR: In this paper, negative exponential functions are found to be most appropriate for describing within-link grain size changes in gravelly, alluvial systems, and a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes.
Abstract: At scales of 100 to 102 km, grain-size change in gravelly, alluvial systems is characterized by punctuated downstream fining. Significant lateral sediment inputs define a sequence of sedimentary links within which fluvial processes produce fining trends. Using a new set of surface grain-size data, eighteen such links along two Canadian gravel-bed rivers are examined. Negative exponential functions are found to be most appropriate for describing within-link grain-size changes. Development of a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes. Consideration of lithologically differentiated fining trends indicates that lithology plays an insignificant role in controlling fining rate. Examination of within-link longitudinal profiles indicates a reasonable relation between fining rate and the rate of change of channel slope. The predictive value of this relation is limited by the regional nature of the data upon which it is based and the relative difficulty of obtaining accurate gradient information. However, as previously noted for alluvial fans (although for different reasons), channel gradient is correlated with link length and a reasonable predictive tool is apparent in a relation between link length and diminution rate. The validity of the derived relation cannot be tested for want of a suitable data set. Several lines of evidence indicate that abrasion is unimportant as a fining mechanism on these rivers, and it is argued that this may be typical of sedimentary links. Fining rate must then be a function of the efficiency with which sorting processes operate, and it is suggested that this efficiency is dependent on aggradation rates associated with the lateral sediment influxes at the head and terminus of individual links.
163 citations
TL;DR: In this article, the erosional component of the basal discontinuity of the Viking Formation is defined as a boundary that was transgressively modified during subsequent relative sea-level rise, and is commonly demarcated by Glossifungites ichnofacies.
Abstract: The Viking Formation of the Joffre field comprises parts of three discrete sequences. Sequence 2 overlies an erosional discontinuity, termed BD-1, which is incised into underlying marine parasequences of the informally named "Regional Viking". The surface represents a sequence boundary that was transgressively modified during subsequent relative sea-level rise, and is commonly demarcated by the Glossifungites ichnofacies. Up to three parasequences are truncated by BD-1. Sequence 2 comprises an incised sandstone body passing basinwards into a granule- to pebble-bearing sandy mudstone. A complete facies succession consists of a thin granule to pebble lag mantling BD-1, grading upwards into thoroughly bioturbated gritty sandy mudstone, through intensely burrowed muddy sandstone, and into interbedded hummocky cross-stratified sandstone and burrowed sandstone. The facies contain diverse and uniformly distributed, open-marine trace-fossil suites displaying an upward progression from archetypal Cruziana through proximal Cruziana and into mixed Skolithos-Cruziana assemblages. The succession is interpreted to reflect a weakly storm-influenced upper offshore to proximal lower-shoreface deposit. Incised shorefaces are allocyclically generated, and may be produced by forced regressive (falling stage), lowstand, or transgressive scenarios. Sequence 2 of the Joffre area is interpreted as a transgressively incised shoreface. It is distinguished from the other two sharp-based shoreface types largely on the basis of the extent of the erosional component of its basal discontinuity. In distal positions, BD-1 remains erosional even where it is overlain by facies deposited below fair-weather wave base. This is inconsistent with forced regressive and lowstand conditions because in weakly storm-influenced shorefaces, the regressive surface of erosion and the sequence boundary, respectively, pass into correlative conformities seaward of fair-weather wave base. Facies deposited below fair-weather wave base would therefore overlie the non-erosional correlative conformity surface. In a transgressive scenario, however, ravinement during erosional shoreface retreat generates an erosional discontinuity that may lie seaward of fair-weather wave base during subsequent progradation, because the surface was cut prior to progradation and while sea level was considerably lower. As a result, facies deposited below fair-weather wave base can overlie the erosional discontinuity.
147 citations
TL;DR: The southwestern continental margin of Australia (19 degrees S-22 degrees S) in the western Indian Ocean is transitional between cool-water and warm-water carbonate realms as mentioned in this paper.
Abstract: The southwestern continental margin of Australia (19 degrees S-22 degrees S) in the western Indian Ocean is transitional between cool-water and warm-water carbonate realms. It comprises the incipiently rimmed, flat-topped, steep-fronted Rottnest Shelf in the south, the uniform subtropical starved Carnarvon Ramp off Shark Bay, and the Ningaloo fringing reef in the north. The margin is strongly influenced by the poleward-flowing, warm, nutrient-poor Leeuwin Current, which promotes overall downwelling and strong summer equatorward-blowing winds, which generate local seasonal upwelling. The structurally quiescent northern part of the Rottnest Shelf, with minimum accommodation, is characterized by luxuriant stands of sea-grasses and macrophytes growing on coralline-encrusted hardgrounds and rooted in sediments rich in coralline algae and larger, symbiont-bearing foraminifers together with abundant cool-water elements such as bryozoans, molluscs, and small foraminifers. Halimeda is poorly calcified and does not contribute to the sediment. Azooxanthellate corals, although present, are not common. Such a sediment pattern has many analogs in the geologic record, especially the early Cenozoic. The incipient rim is a morphologically complex linear ridge system interpreted to be a buried mid-Pleistocene barrier reef or beach-dune complex. The northern part of the ridge complex is capped by the Houtman Abrolhos reef platforms. Subphotic sediments on the deep, outer shelf and upper slope, affected by seasonal upwelling, are typical cool-water, poorly sorted, bryozoan-dominated deposits rich in small foraminifers and sponge spicules. The inner part of the more structurally active Carnarvon Ramp ranges from steep eolianite cliffs to hypersaline environments of Shark Bay to the Ningaloo fringing reef. Mid-ramp sediment, especially off Shark Bay, is mostly relict or stranded and foraminifer-dominated sand with abundant Mg-calcite-cemented intraclasts. These sediments, accumulating on a relatively barren seafloor, likely represent attenuated carbonate production brought about by downwelling and episodic incursions of saline, Shark Bay-derived waters onto the ramp. The outer ramp is either planktic foraminiferal sand, sorted by strong bottom currents, or spiculitic mud. Such sedimentation patterns should typify the western margins of large ocean basins during those periods in geologic history when circumglobal equatorial circulation was active, and equatorial gateways narrow, in contrast to eras of supercontinents, when cool-water carbonate sedimentation and upwelling prevailed in similar settings. Furthermore, because low-latitude trade winds relax during glacial epochs and the cool-water, upwelling belt moves equatorward into otherwise tropical realms, temperate and subtropical sedimentation patterns in these regions should oscillate at the 100 ky scale in the stratigraphic record.
147 citations
TL;DR: Petrographic and small-scale sedimentary features of the mudstone-dominated Upper Devonian Sonyea Group of New York were examined in order to improve our understanding of mudstone facies as discussed by the authors.
Abstract: Petrographic and small-scale sedimentary features of the mudstone-dominated Upper Devonian Sonyea Group of New York were examined in order to (1) improve our understanding of mudstone facies, (2) examine coeval mudstone facies within a broader depositional context, and (3) promote a simple methodology for the study of mudstones. Six mudstone facies are distinguished, each characterized by several component mudstone types or subfacies. Sedimentary conditions and environments are reconstructed from primary sedimentary structures and bioturbation characteristics. Mudstone facies are characterized by soils and flood deposits in the coastal-plain region, by rapid sediment deposition and frequent reworking in nearshore areas, by storm-dominated offshore transport on a wide, basin-margin platform, by turbidite slope deposits below storm wave base, and by bottom currents and slow settling in the distal, "deep"-basin black mudstones. The latter, although commonly thought of as deposits of a stratified anoxic basin, contain indications of benthic life, such as burrows, disrupted laminae, and clay/silt fecal pellets. These observations are incompatible with a stratified-basin model, and indicate the need to search for alternative models of black-shale accumulation.
142 citations
TL;DR: The Gargano Promontory as discussed by the authors is a carbonate block that is part of the slightly deformed foreland of the southern Apennine thrust belt and the long-term stratigraphy of this margin and slope transect is punctuated by five major dynamic phenomena that subdivide the succession into six second-order sequences.
Abstract: The Upper Jurassic to Eocene Apulia Platform margin and the eastward transition to the adjacent basinal deposits are well exposed in the Gargano Promontory (southern Italy), a carbonate block that is part of the slightly deformed foreland of the southern Apennine thrust belt. The long-term stratigraphy of this margin and slope transect is punctuated by five major dynamic phenomena that subdivide the succession into six second-order sequences. These events include (1) a Valanginian drowning unconformity, (2) an early Aptian-Albian drowning and demise of the platform, (3) late Albian-Cenomanian platform-margin failures, (4) a Santonian-Campanian retreat of the platform margin, and (5) Eocene uplift and platform-margin collapse. The first event is documented worldwide and is probably eustatic in origin. The second is concomitant with some oceanic anoxic events (OAE). The last three processes are probably related to foreland reaction to subduction and collision in the Dinarides and Hellenides thrust belts. The ultimate cause of the Albian-Cenomanian failures is more problematic. A worldwide eustatic regression is documented at this time, but regional geology seems to favor tectonic uplift.
129 citations
TL;DR: The Cadotte splay complex, formed from the avulsively annexed Cadotte Channel, represents a small but typical area of the avulsion belt as discussed by the authors, and its characteristics and interrelationships examined from auger borings and vibracores illustrate patterns of deposition and floodplain development.
Abstract: The avulsion of the Saskatchewan River (Cumberland Marshes, east-central Saskatchewan, Canada) has resulted in new floodplain deposition over an area of more than 500 km2 since beginning in the 1870s. The avulsion deposits are predominantly fine grained and could be otherwise classifiable as "overbank" sediments in alluvial floodplains presumed to be supplied by intermittent flooding of dominant master channels. The North Angling and Cadotte channels are older pre-existing channels that were intercepted and appropriated by the expanding avulsion. Following annexation of the North Angling Channel, stable cohesive banks inhibited channel enlargement and crevassing, forcing the increasing discharges to be accommodated by overbank flow and in the process, enhancing new levee aggradation. These levee deposits coarsen upward and thin downstream as a result of greater competence and higher proximal deposition rates brought on by the larger flows. In contrast, the Cadotte Channel responded to increasing discharges by levee breaching, giving rise to splay progradation and floodbasin filling. The Cadotte splay complex, formed from the avulsively annexed Cadotte Channel, represents a small but typical area of the avulsion belt. Facies characteristics and interrelationships examined from auger borings and vibracores are presented to illustrate patterns of deposition and floodplain development. Avulsive flooding initially raised local water-surface elevations, deepening small existing lakes and creating new shallow (1-2 m deep) lakes on the floodplain surface. Deposition began when levees of the Cadotte Channel were breached, allowing water and coarser sediment to spread as sheet-like flows while finer sediment was carried and deposited farther into the basin. Later, as the proximal splay sheets coalesced and grew basinward, flows became channelized, and progradation of mouth bars into marshes and shallow lakes, together with deposition of interchannel fines, became dominant, forming coarsening-upward sequences. Further floodplain aggradation developed by overbank flooding when splay channels became stabilized and bordered by levees. Gradual abandonment was accompanied by deposition of organic-rich fine-grained sediment on top of the progradational sequences. Throughout the avulsion, the North Angling Channel has stood as an older alluvial ridge separating two broad areas of avulsive deposition. Its recent levee deposits now grade basinward into these new floodplain sediments composed of thicker heterogeneous, predominantly upward-coarsening, and rapidly deposited facies of the avulsion.
120 citations
TL;DR: In this paper, the authors studied the relationship between the general morphology of worms, worms with appendages, sediment viscosity, and textural features of worm-like infills of individual burrows.
Abstract: Abundant trace fossils occur within rhythmically interbedded black and gray shales of the Chattanooga Shale (Upper Devonian) in central Tennessee. Burrows cross the boundaries between layers and tend to obscure the contacts between alternate layers. Infills of individual burrows often display an array of complex, convoluted features, apparently due to mixing of soft to soupy black and gray muds. Assuming that the burrowers were elongate worm-like organisms, experiments were made to study the relationships between the general morphology of burrowers (simple worms, worms with appendages, etc.), sediment viscosity, and textural features. Rubber bait worms were pulled through superimposed layers of plaster in a first set of experiments. Comparable experiments were then conducted with earthworms. The resulting textures were studied by sawing the hardened plaster blocks perpendicular and horizontal to bedding. The explored viscosities ranged from that of heavy motor oil to that of lithium grease (at 258 degrees C). The study shows that (1) convolute textures observed in plaster experiments closely resemble those seen in Chattanooga Shale burrows; (2) the degree of convolution increases as the viscosity of the substrate decreases; (3) the length of the worm determines the extent of mixing between layers; (4) mixing patterns produced by smooth worms and worms with appendages, although similar, contrast in detail; (5) earthworms and bait worms produce generally similar structures but show certain differences due to contrasting styles of locomotion (peristalsis vs. unidirectional pull). "Virtual" compaction and decompaction of digital images shows close resemblance between experimentally produced structures and burrow textures from the Chattanooga Shale, suggesting that the latter were indeed produced by worm-like animals that moved through semi-fluid muds.
TL;DR: A review of data from historical earthquakes indicates that the landslides they generated can be classified into 14 different types as mentioned in this paper, including highly disaggregated and fast-moving falls, disrupted slides, and avalanches; more coherent and slower-moving slumps, block slides and earth slides; and lateral spreads and flows that involve partly to completely liquefied material.
Abstract: Earthquakes as small as magnitude 40 may dislodge landslides from susceptible slopes, and larger earthquakes can generate tens of thousands of landslides throughout areas of hundreds of thousands of square kilometers, producing billions of cubic meters of loose, surficial sediment These landslides can have significant geomorphic effects that vary depending on the landslide characteristics and materials, and on the settings in which the landslides occur A review of data from historical earthquakes indicates that the landslides they generated can be classified into 14 different types These include highly disaggregated and fast-moving falls, disrupted slides, and avalanches; more coherent and slower-moving slumps, block slides, and earth slides; and lateral spreads and flows that involve partly to completely liquefied material Rock falls, disrupted rock slides, and disrupted slides of earth and debris are the most abundant types of earthquake-induced landslides, whereas earth flows, debris flows, and avalanches of rock, earth, or debris typically transport material the farthest Because of their abundance or long distances of transport, these landslides generally have the greatest effects on the landscape during and after earthquakes Landslide effects on alluvial fans include direct deposition of material on fan surfaces; fissuring and displacement of fan materials; alterations in drainage basins such as devegetation, denudation, and changes in channel networks; and generation of large amounts of sediment that may be transported to fans by post-earthquake water flows or debris flows
TL;DR: In this article, the composition of clays and heavy minerals as well as trace-element geochemistry of the lowermost Amazon sediments reflect source regions and the intense chemical weathering conditions.
Abstract: Compositions of clays and heavy minerals as well as trace-element geochemistry of the lowermost Amazon sediments reflect source regions and the intense chemical weathering conditions. Illite and montmorillonite indicate mountainous sources under physical weathering conditions, whereas kaolinite is related to the soil composition under humid tropical conditions, where chemical weathering predominates. The suite of unstable heavy minerals, on the other hand, reflects some sediment transport from the Andes, more supply from tributaries draining the basic and metamorphic rocks from the Southeastern Guiana Shield, and strongest influence of the upstream Amazon and Solimoes areas. These source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain. The Brazilian Shield contribution (via the Xingu River) seems to be of minor importance, although the Alter do Chao and the Barreiras Formations, which are products of erosion of the shields, supply suites of stable heavy minerals that reflect recycling. Rare-earth and trace-element data from the clay and heavy-mineral fractions provide additional clues to the source areas: in the downstream direction, superimposed upon provenance of sediments from an undifferentiated arc, the Andes, is provenance from strongly weathered continental crust, the Precambrian shields, as well as sedimentary recycling. Small-scale provenance can be delineated by the most immobile elements. Heavy rare earth elements in the heavy-mineral fraction increase downstream toward Estreitos (to the Para River).
TL;DR: In this article, the authors explored the depositional processes and stacking patterns of coarse-grained facies across a large fan delta and associated sub-lacustrine fan system offshore of the South Rukuru River using single-channel seismic data coupled with deep-water vibracores and gravity cores.
Abstract: Lake Malawi, Africa, serves as an important modern analog for understanding depositional processes in an active rift setting. This paper explores the depositional processes and stacking patterns of coarse-grained facies across a large fan delta and associated sub-lacustrine fan system offshore of the South Rukuru River using single-channel seismic data coupled with deep-water vibracores and gravity cores. A companion paper (Wells et al. 1999) discusses the details of the depositional processes of the system inferred from core data. Using 2900 km of single-channel seismic data, eight genetic facies were defined based on seismic reflection character, external geometry, accompanying core data, and location within the modern geomorphic system. Five sand-dominated facies include; (1) mouth-of-canyon-fan facies; (2) progradational-fan-delta facies; (3) channel-fill facies; (4) canyon-fill facies; and (5) basin-plain-fill facies. These facies occur as surficial deposits in water depths of more than 600 m, suggesting that coarse-grained sedimentation characterizes certain localities, in even the deepest parts of Lake Malawi. Climatically driven lake-level changes influenced the internal character of these facies, their thickness, and to some extent their location within the South Rukuru study area. However, climate was generally subordinate to tectonism in controlling the temporal and spatial distribution of the facies. Tectonic controls can be categorized as; (1) orientation of the first-order border fault and intrabasinal faults; (2) syndepositional footwall uplift and rotation; and (3) temporal changes in the rate and location of maximum subsidence. These controls affect the location and stacking of the mouth-of-canyon fans, and the presence and orientation of sub-lacustrine canyons and the zone of maximum sediment accumulation, and depositional gradients.
TL;DR: In this paper, the distribution of recent sedimentary facies is significantly different in four closely spaced, isolated carbonate platforms off the coasts of Belize and Yucatan (SE Mexico).
Abstract: Distribution of recent sedimentary facies is significantly different in four closely spaced, isolated carbonate platforms off the coasts of Belize and Yucatan (SE Mexico). These differences are largely controlled by variations in submarine topography, which are likely the result of differential subsidence and karstification from platform to platform. A variety of depositional styles and patterns is found in platform interiors that developed during the same Holocene sea-level rise. Composition and texture of surface sediment samples were used to define four major sediment types and to distinguish nine depositional environments within the platforms. Facies maps are based on analysis of 454 sediment samples and high-resolution satellite imagery. Glovers Reef is characterized by a circular facies distribution. The nearly continuous reef margin is composed of a coral-red coralline algae-Halimeda grainstone. Mixed nonskeletal (peloidal)/skeletal wackestones and packstones are present in shallow lagoon parts (< 5 m water depth) behind the margin. The deep interior, from 5 to 18 m water depth, is characterized by mollusk-foram-Halimeda wackestones with over 860 patch reefs of coral-red algae-Halimeda-mollusk packstones. Lighthouse Reef also has a coral grainstone and coral-red algae-Halimeda grainstone rim, but the facies distribution in the interior is strongly asymmetrical. The 8-m-deep eastern lagoon is floored by mollusk-foram-Halimeda wackestones and packstones; the 3-m-deep western lagoon has the mixed peloidal/skeletal wackestone to grainstone facies. A linear trend of coalescing patch reefs and skeletal grainstone facies separates the two lagoons. Banco Chinchorro is similar to Lighthouse Reef, inasmuch as facies distribution patterns of the interior are strongly asymmetrical. Mollusk-foram-Halimeda wackestones-grainstones are present in eastern lagoon areas, whereas shallow (< 5 m) western parts of the lagoon are composed of mixed peloidal/skeletal grainstones, packstones, and wackestones. Most of Turneffe Islands is protected on the east by Lighthouse Reef. Accordingly, Turneffe has a narrower reef and skeletal grainstone rim than the other two platforms. Restricted interior lagoons are up to 8 m deep, and are enclosed by wide mangrove rims and surface sediments dominated by Halimeda-rich wackestone rich in organic matter. Coral patch reefs are absent. In contrast, the unprotected northernmost part of Turneffe has a wide reef and grainstone rim and the open lagoon area consists of mollusk-foram-Halimeda wackestones-packstones with abundant patch reefs.
TL;DR: In this article, a theoretical relationship between fining rate and drainage basin area is derived from established alluvial-fan relations, which suggest that fining is achieved through the mechanism of selective transport but that the overall control on fining patterns is the volume of sediment supplied from within the drainage basin.
Abstract: A wide range of rates of downstream fining in gravel-bed rivers has been reported. As a consequence, explanations of the phenomenon range from those that consider only abrasion to others that consider only selective sediment transport. This apparent confusion results in part from inconsistent definitions of downstream fining. Analysis of results from a small gravel-bed stream is used to show how the method of sediment sampling and the percentile used to characterize fining rates can affect the results. When fining is adequately defined, a theoretical relationship between fining rate and drainage- basin area is derived from established alluvial-fan relations. This relationship is remarkably consistent with a range of studies by numerous authors using often different methods. Such results suggest that fining is achieved through the mechanism of selective transport but that the overall control on fining patterns is the volume of sediment supplied from within the drainage basin. As such, the rate of size decline in sedimentary deposits such as river terraces has the potential to be used to infer past changes in climate, tectonics, and/or relative base level. Models of this are presented, and these are shown to be broadly consistent with observations made in an ancient sandstone body. Full use of changing rates of fining as paleo-environmental indicators requires further work on the rate of adjustment of fining profiles with respect to the rates of change in the forcing environmental conditions.
TL;DR: In this paper, the authors present techniques that can be used to calculate ancient Earth-Moon distances and a modern high-tide data set, which results in calculated estimates of lunar orbital periods and an EarthMoon distance that fall well within 1 percent of the actual values.
Abstract: Tidal rhythmites are small-scale sedimentary structures that can preserve a hierarchy of astronomically induced tidal periods. They can also preserve a record of periodic nontidal sedimentation. If properly interpreted and understood, tidal rhythmites can be an important component of paleoastronomy and can be used to extract information on ancient lunar orbital dynamics including changes in Earth-Moon distance through geologic time. Herein we present techniques that can be used to calculate ancient Earth-Moon distances. Each of these techniques, when used on a modern high-tide data set, results in calculated estimates of lunar orbital periods and an Earth-Moon distance that fall well within 1 percent of the actual values. Comparisons to results from modern tidal data indicate that ancient tidal rhythmite data as short as 4 months can provide suitable estimates of lunar orbital periods if these tidal records are complete. An understanding of basic tidal theory allows for the evaluation of completeness of the ancient tidal record as derived from an analysis of tidal rhythmites. Utilizing the techniques presented herein, it appears from the rock record that lunar orbital retreat slowed sometime during the mid-Paleozoic.
TL;DR: In this article, the footwall-derived alluvial-fan strata are arranged into 3-10 m-thick cyclothems representing three events: (1) development of calcic paleosols that were contemporaneously or subsequently truncated by channels up to 4 m deep, (2) infilling of channels by turbulent-flow, horizontally bedded and cross-bedded conglomerates, and (3) deposition of hyperconcentrated-flow pebbly sandstones.
Abstract: Six steep (0.6-4.3°), short (radial length 1.5-5 km), deeply incised Pliocene to early Pleistocene paleofans and associated axial-fluvial strata are preserved adjacent to the footwall of the Palomas half graben in the southern Rio Grande rift. Two different scales of interbedding of lithofacies are evident in the exposed basin-fill sediment. The footwall-derived alluvial-fan strata are arranged into 3-10 m-thick cyclothems representing three events: (1) development of calcic paleosols that were contemporaneously or subsequently truncated by channels up to 4 m deep, (2) infilling of channels by turbulent-flow, horizontally bedded and cross-bedded conglomerates, and (3) deposition of hyperconcentrated-flow pebbly sandstones. These cyclothems probably resulted from paleoclimatic changes on the scale of 150 kyr. On a larger geographic and temporal scale, the axial river (ancestral Rio Grande), which primarily deposited cross-bedded pebbly sand, experienced kilometer-scale lateral movement toward the footwall block, resulting in one thick (80 m) axial-fluvial lithosome in the northern part of the basin and two or three thinner (8-25 m) axial-fluvial lithosomes in the south. A protracted period of fault movement and consequent basin tilting was the probable driving force behind lateral movement of the river, and toe cutting of the fans by the river was the dominant process.
TL;DR: The Gaskiers Formation is similar to the cap carbonate bed of the Newfoundland cap bed as discussed by the authors, which is the only known example from a volcanic terrane of an active orogenic belt.
Abstract: Discontinuous outcrops of diamictite-bearing units exposed along the southern coast of Conception Bay, Newfoundland, are shown to be Neoproterozoic glacial deposits. These are equivalents of the Gaskiers Formation, a well-documented glaciogenic diamictite exposed on the southern Avalon Peninsula. A newly discovered 50-cm-thick cap limestone bed directly overlies these Varanger-age glacials and is the only bedded carbonate unit in a nearly 15-km-thick Neoproterozoic succession of volcaniclastic flysch and molasse deposits. The cap limestone contains a strongly negative carbon-isotope signature, which is believed to be primary and is similar to many cap carbonate beds deposited globally. Deposition of cap carbonate is a response to a sudden increase in shallow-seawater alkalinity. This postglacial isotopic signature has been interpreted as either (1) the result of oceanic turnover and mixing of alkaline, 13C-depleted deep water with surface water, (2) the sudden equilibration of a CO2-charged atmosphere after meltback of a snowball Earth, or (3) the near cessation of primary productivity in the oceans. The Newfoundland cap bed is a rare deep-sea example that may be the only known example from a volcanic terrane of an active orogenic belt. Carbonate clasts in the underlying diamictite also have highly negative isotopic signatures. Samples from one exposure of the cap bed show a stratigraphic shift in 13C from -6o/oo at the base to -1.5o/oo at the top. The initial shift from moderately to highly negative values recognized in Neoproterozoic cap carbonate beds from Australia and Namibia is not preserved in the Newfoundland cap bed. The minor thickness of the bed and its incomplete isotopic signature is likely due to deposition in an active tectonic setting among island arcs. This setting would have been characterized by high rates of subsidence, high accumulation rates of siliciclastic sediment, high equilibrium bottom slopes, and deep-water depositional environments. All of these factors may have contributed to a delayed initiation and short duration of carbonate deposition relative to other cap carbonate deposits worldwide. A high paleolatitude for Avalonia at this time may have also caused a shortened and delayed development of the cap carbonate bed.
TL;DR: In this paper, the authors show that the horizontal extent of individual facies units (lithotopes) are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution.
Abstract: Cambro-Ordovician carbonate lithofacies units in the Elbrook and Conococheague formations exposed at Wytheville, Virginia, as well as those in many other Phanerozoic peritidal sequences, exhibit exponential thickness frequency distributions. That is, occurrence frequency decreases exponentially with linear increase in unit thickness. Such distributions are characteristic of waiting times between independent Poisson events. This relative frequency of spaces of different size between horizons of lithologic change is what one would expect if the horizons were distributed randomly throughout carbonate successions. Abundances of different lithologies, both as net stratigraphic thickness and as number of occurrences, also decreases exponentially among successively rare sediment types, with each lithology being about 60% as plentiful as the next more abundant rock type. Relations between net thickness and number of occurrences for each facies define a linear trend coincident with a mean thickness for all Wytheville units of 0.48 m, a relation indicating that thickness distribution is independent of facies type. Similar relations are apparent for the horizontal extent of carbonate sediment bodies from the Holocene Florida-Bahama platform. Areal extents of individual facies units (lithotopes) are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution. Although regional gradients in sediment texture and composition are also apparent along most transects from platform margin to interior, such frequency distributions indicate that lateral extents of individual sediment reflect a largely stochastic distribution of facies boundaries across this Holocene surface. Lithotope abundances also yield trends of exponentially decreasing dominance among successively subordinate facies, with each being about 70% as extensive the next more abundant sediment type. Relations between areas and abundances for all lithotopes define a covariant trend corresponding to a mean area of 2.2 103 km2 for all Florida-Bahamas lithotopes. We consider several numerical models of stochastic carbonate accumulation; although not demonstrably unique, scenarios incorporating the sequential superposition of randomly placed coniform lithotopes result in thickness and area frequency distributions that are the same as those observed in ancient and modern platform deposits. Such simulations of Poisson processes of sediment accumulation are in general agreement with stochastic models of lithologic heterogeneity that have been more widely applied to petroleum reservoirs and groundwater aquifers. To the as yet unknown degree that peritidal lithofacies area and thickness are correlated, data from Paleozoic and Holocene platforms suggest that carbonate units should exhibit length/height ratios of approximately 105. Given the decimeter scale over which facies are designated in most Paleozoic peritidal successions, these relations predict mean lateral extents on the order of several tens of kilometers, a value in general agreement with the few data that exist on spatial continuities of peritidal lithotopes in Paleozoic carbonate sequences.
TL;DR: The Aptian to Albian interval of the vast Arabian Platform was studied in northern Oman (Nahr Umr Formation) for its paleoceanographic and sea-level record as discussed by the authors.
Abstract: The Aptian to Albian interval of the vast Arabian Platform was studied in northern Oman (Nahr Umr Formation) for its paleoceanographic and sea-level record. The sections indicate seven transgressive-regressive cycles overprinted by small-scale sea-level fluctuations. The peak of regression lead to emergence and exposure of the sea floor and overprinted marine hardground surfaces. The subaerial overprint of marine hardgrounds is difficult to recognize in the field. However, four independent lines of evidence document emergence. These are: (1) Pronounced negative excursions in carbon and partly in oxygen isotope composition beneath hardbottoms. They are attributed to the influx of light, soil-derived carbon and light oxygen from meteoric water. (2) Sparite-filled rudists beneath hardbottoms contain primary brackish-water fluid inclusions of very low salinity interpreted as cement precipitation in the phreatic marine/meteoric mixing zone or "pollution" of meteoric freshwater by residual salt. (3) Symbiont-bearing and thus light-dependent orbitolinids display small, conical morphotypes in the shallow, well illuminated water prior to exposure and hardbottom formation. Large, flat morphotypes are characteristic of the transgressive shales above those surfaces and document deeper, less illuminated waters. (4) The presence of mycorrhiza (Microcodium). The exposure surfaces overlie facies-incomplete shallowing-upward successions or subtidal sediments, but unequivocal evidence for erosion before or during emergence is lacking. Correlation of transgressive-regressive events in Oman with other sea-level curves suggests the record of spasmodic regional tectonism combined with a strong eustatic component.
TL;DR: The Ironshore Formation (up to 19 m thick) in the Rogers Wreck Point area of Grand Cayman is divided into the unconformity-bounded units A, B, C, and D as mentioned in this paper.
Abstract: The Ironshore Formation (up to 19 m thick) in the Rogers Wreck Point area of Grand Cayman is divided into the unconformity-bounded units A, B, C, and D. The unconformities are highlighted by caliches and/or terra rossa. Th/U dates from aragonitic corals indicate that unit A formed at > 400 ka, unit B 346 ka, unit C 229 ka, and unit D 131 ka. These ages correlate with the highstands of the last four interglacial periods and probably represent marine isotope stages 5 (unit D), 7 (unit C), 9 (unit B), and 11 (unit A). The limestones in units A-D, which have similar biotic and lithologic characteristics, represent deposition in shallow water on a narrow coastal shelf. Unit A has a high faunal diversity and was deposited in open-marine conditions, whereas units B, C, and D were deposited in quieter-water lagoons that probably had fringing reefs along their sea-ward edges. Although Grand Cayman is located on the Cayman Ridge close to an active spreading center and transform fault, the island has undergone little or no vertical movement over the last 500 ky. Available evidence indicates that sea level at the time of deposition of each unit relative to present sea level was: unit A, -9.0 to -5.5 m; unit B, -3.0 to +0.5 m; unit C, -2.5 to +1.1 m; and unit D, +2.5 to +6.0 m. These data are good estimates of eustatic sea-level highstands over the last 500 ky.
TL;DR: Gomso Bay, located on the western coast of Korea, has a mean tidal range of 4.3 m and is directly open to the Yellow Sea as discussed by the authors, where the Holocene tidal-flat deposits consist of three depositional units: (1) a basal Unit I (intensely bioturbated silt with occasional remnant subparallel lamination), (2) an intermediate Unit II (highly biotiropated, faintly laminated sandy silt or silty sand), and (3) an upper Unit III (slightly to moderately biot
Abstract: Gomso Bay, located on the western coast of Korea, has a mean tidal range of 4.3 m and is directly open to the Yellow Sea. Analysis of eighteen vibracores (in two transects) from extensive tidal flats on the south side of the bay shows that the Holocene tidal-flat deposits consist of three depositional units: (1) a basal Unit I (intensely bioturbated silt with occasional remnant subparallel lamination), (2) an intermediate Unit II (highly bioturbated, faintly laminated sandy silt or silty sand), and (3) an upper Unit III (slightly to moderately bioturbated very fine to fine sand with abundant parallel lamination and cross-lamination). Units I, II, and III can be interpreted as a result of subtidal to intertidal sedimentation on the mud flats, mixed flats, and sand flats, respectively. Tidal-channel deposits are partly preserved, occurring in Units II and III. Storm deposits, preserved predominantly in Units II and III, generally have sharp erosional bases and consist of thick ( 10 cm) massive shelly sand (often graded) that is overlain by thin, waning-stage, parallel-laminated and cross-laminated sand and mud. These intertidal deposits show a coarsening-upward succession underlain unconformably by pre-Holocene semiconsolidated, oxidized mud (Unit 0). The coarsening-upward Holocene Gomso deposits represent a transgressive, retrograding phase within a parasequence, associated with low accumulation rates. The unconformable boundary between pre-Holocene Unit 0 and Unit 1 is a transgressive surface of erosion (TSE) formed during the Holocene transgression.
TL;DR: The Llandeilo-Caradoc black shales of southwest Wales, U.K. record a paragenetic sequence that requires redistribution of rare earth elements (REE) on a mineralogic scale during diagenesis.
Abstract: The Llandeilo-Caradoc black shales of southwest Wales, U.K. record a paragenetic sequence that requires redistribution of rare earth elements (REE) on a mineralogic scale during diagenesis. The main evidence for this redistribution is formation and replacement of REE-rich apatite and monazite and variations in whole-rock REE patterns. The processes that produce these mineralogic changes create a diagenetic overprint on the Llandeilo-Caradoc black shales that obscures geochemical information related to both provenance and paleoenvironmental conditions of deposition. Periods of open-system conditions existed during the diagenetic alteration of the Llandeilo-Caradoc black shales, which allow an evaluation of the extent and timing of the REE redistribution inferred from petrography. The observed variations in La/Sm ratios in the whole-rock REE patterns are strong evidence that REE were redistributed on a scale greater than an individual hand sample. This change in the whole-rock REE pattern altered the Cerium anomaly by as much as 20% and produced a range in the Sm/Nd ratio from 0.14 to 0.20. This change in the whole-rock Sm/Nd ratio indicates a disturbance in the Sm-Nd isotopic system at about 460 Ma that can be directly linked to the formation and dissolution of diagenetic apatite and monazite during diagenesis. One effect of this disturbance is to alter the range in Nd model age for the Llandeilo-Caradoc black shales from 1.6 to 1.8 Ga to about 1.4 to 2.25 Ga, thus complicating any interpretation of provenance. The ability to link petrographic observations from a black shale to measurable whole-rock geochemical changes provides us with one tool to evaluate the presence of a diagenetic overprint on trace elements commonly used in paleoenvironmental reconstruction.
TL;DR: In this paper, a series of short cores taken from a nearshore terrace were used to identify three periods of diagenesis: early shallow marine, meteoric, and post-meteoric shallow marine.
Abstract: The nearshore slope of Oahu consists of a shallowly dipping shelf extending from the shoreline out to the -20 m contour, where there is a sharp break in slope down to -30 m. Limestones recovered in a series of short cores taken from this nearshore terrace are typical of shallow-marine reef environments and comprise either a branching-coral or massive-coral facies. The composition as well as shoreward zonation of facies suggests that the terrace represents an in situ fossil reef complex. Th-U ages of in situ corals are all Pleistocene and suggest that the bulk of the feature formed during marine oxygen isotope stage 7. Later accretion along the seaward front of the terrace occurred during marine oxygen isotope substages 5a and/or 5c. Deposition during these interglacial highstands has not previously been documented in the sea-level record on Oahu. Although the diagenetic record in the cored samples is incomplete, three periods of diagenesis are identified: early shallow marine, meteoric, and post-meteoric shallow marine. Early shallow-marine diagenesis includes cementation by aragonite and Mg calcite in an active marine phreatic zone and predominantly micritization in a stagnant marine phreatic zone. Meteoric processes occurred in the vadose zone and include precipitation of calcite (needle fibers, meniscus cements, micritic networks), neomorphism, and dissolution. All limestones are now in an active marine phreatic zone. Evidence of post-meteoric shallow-marine diagenesis is found in last-generation Mg calcite cements and internal sediments occurring directly on limestone substrates that have otherwise been stabilized to calcite. The present seafloor is undergoing extensive biological and physical erosion. No Holocene limestones were recovered. Petrographic and geochemical signatures of subaerial exposure and meteoric diagenesis are recognized within the upper several centimeters of all cores. Thus, the present seafloor in the study area is a flooded Pleistocene subaerial exposure surface.
TL;DR: The authors compared quantitative X-ray diffraction, normative calculation, and whole-rock chemical analysis for Permo-Triassic quartz-lithic to litho-feldspathic sandstones from eastern Australia.
Abstract: Quantitative X-ray diffraction, normative calculation, and whole-rock chemical analysis have been compared for Permo-Triassic quartz-lithic to litho-feldspathic sandstones from eastern Australia. The quantitative XRD technique provides mineralogical percentages that correlate well with the chemical composition of the sandstones, and also with most of the mineral percentages derived from normative calculation. The need for assumptions on potassium distribution between minerals provides a limitation on estimates of K-feldspar and illite using the normative technique, although these can be improved on a sample-specific basis if necessary using complementary thin-section observations or qualitative X-ray diffractograms. Quantitative XRD analysis possibly indicates a small proportion of amorphous material in some of the sandstones, especially materials with abundant lithic fragments or clay minerals, but the proportion of any amorphous material overall is probably negligible. Both quantitative XRD and normative calculation avoid problems associated with sample heterogeneity or operator inconsistency that may affect point counting of thin sections. They can also be used to provide quantitative information on sediments with abundant rock fragments or matrix components, the actual mineralogy of which is not resolvable in conventional microscope investigations.
TL;DR: In this article, a large area ranging from the Pechora Sea and Novaya Zemlya in the southeast to the western Barents Sea and Svalbard has been analyzed by petrographic, mineralogical, mineral-chemical, and Sm-Nd bulk-rock isotopic methods.
Abstract: Triassic sandstones from a large area ranging from the Pechora Sea and Novaya Zemlya in the southeast to the western Barents Sea and Svalbard have been analyzed by petrographic, mineralogical, mineral-chemical, and Sm-Nd bulk-rock isotopic methods. The largest compositional variation is seen in the Lower Triassic deposits. Epidote-bearing, lithic-rich sandstones are characteristic of the eastern Barents Sea, Novaya Zemlya, and the Timan-Pechora Basin, and indicate a provenance from the Polar Urals. Quartzo-feldspathic sandstones in the Hammerfest Basin may have been derived from deeper crustal plutonic and metamorphic rocks of the Caledonides and Baltic margin, whereas the source of quartz-dominated sandstones on Spitsbergen was a paleoland in the northwest, in the vicinity of paleo-Greenland. Calculated Sm-Nd provenance ages for Lower Triassic deposits are in the range of 1500-1880 Ma for sandstones from Spitsbergen, the Hammerfest Basin, and the Finnmark Platform. Sandstone provenance ages for samples analyzed from the eastern Barents Sea are significantly younger, with a range of 600-1280 Ma and an average of 780 Ma. Sandstones in central shelf areas, i.e., the Nordkapp Basin-Bjarmeland Platform, are rich in both lithics and feldspar and have intermediate provenance ages (960-1630 Ma). The largest influence of older components occurs in samples from the Nordkapp Basin. This may be related to sediment supply both from the Caledonized Baltic margin and Uralides, and from local uplifted areas composed of Paleozoic sedimentary rocks. Middle and Upper Triassic sandstones display compositional variations at different times in different areas due to local changes in provenance. However, in the Upper Triassic there appears to be a convergence in sandstone compositions reflecting both more extensive mixing of components from the eastern and western provenance areas as well as changes to more evolved compositions of the erosional products derived from the paleo-Urals. There is also a question of increasing influence from possible paleolands in the northeast. The most important compositional change was the distinct increase in sandstone maturity that took place on a regional scale in the Arctic in the latest Triassic-Early Jurassic. This may in part have been caused by extensive reworking connected with a regional transgression and sea-level raise.
TL;DR: Grain-size parameters (mean, standard deviation, skewness, and kurtosis) were determined for 83 sediment samples from six dune cross-profiles and one 28 km long profile on dunes in the southwest Kalahari Desert in southern Africa as discussed by the authors.
Abstract: Grain-size parameters (mean, standard deviation, skewness, and kurtosis) were determined for 83 sediment samples from six dune cross-profiles and one 28 km long-profile on dunes in the southwest Kalahari Desert in southern Africa. Although some studies elsewhere and previous studies in the Kalahari have reported significant grain-size variation over dune cross-profiles, data presented here show no systematic variation of grain-size parameters except that sediments from the crest of a dune are almost invariably better sorted (indicated by the standard deviation of the sample) than samples from the base, or plinth, of the dune. The proportion of fines (< 63 µm) in plinth sediments is also greater than in crest samples. There is no significant trend in grain-size parameters along the length of a linear dune. This lack of significant pattern corresponds with similar findings for central Australian dunes.
TL;DR: In this paper, the importance of different flood stages on floodplain sedimentation was assessed from analysis of spatial variations in grain-size distributions, which are indicative of sediment transport directions.
Abstract: The importance of floods of different magnitude on floodplain sedimentation can be estimated from sedimentation measurements carried out during a series of floods. However, because these measurements represent sediment accumulation during the entire flood, they cannot be used to estimate the importance of individual flood stages on sediment deposition. In this study, the importance of different flood stages on floodplain sedimentation was assessed from analysis of spatial variations in grain-size distributions, which are indicative of sediment transport directions. As a secondary objective, the applicability of grain-size trend analysis to fine-grained floodplain sediments was tested. The grain-size trend analyses were carried out in a Geographical Information System, and applied to sediment deposited on an embanked floodplain section along the river Waal in the Netherlands, during a large flood in December 1993. Because flow patterns at the studied floodplain section vary with flood stage, comparison of the identified sediment transport pathways with computed flow patterns indicates at which discharge most sediment is deposited. The results indicate that large discharges with a recurrence time of more than two years are most effective in depositing suspended sediment on floodplain areas enclosed by a minor river dike. On floodplain sections that directly border the main channel, moderate discharges with a recurrence time of about one year seem to be more efficient. Contrary to the results of floodplain sedimentation models, low but increasing discharges with a recurrence time of less than one year and during which only low-lying areas near residual channels are inundated appear important for deposition of suspended sediment in these low-lying areas. This observation is confirmed by measurements of spatial concentration gradients and by floodplain sedimentation measurements. Comparison of the trends with additional information on sediment transport directions indicates that trends in which sediment becomes finer, better sorted, and more negatively skewed (when measured in units) are most suitable for defining transport pathways on river floodplains. Trends may, however, deviate up to 90° from true transport directions. Flocculation does not seem to have any negative effects on the applicability of grain-size trend analysis.
TL;DR: The Adams Argillite and the Nation River Formation are two sandstone-bearing units within a remarkably complete Paleozoic stratigraphic section in east-central Alaska as mentioned in this paper.
Abstract: The Cambrian Adams Argillite and the Devonian Nation River Formation are two sandstone-bearing units within a remarkably complete Paleozoic stratigraphic section in east-central Alaska. These strata, now foreshortened and fault-bounded, were originally contiguous with miogeoclinal strata to the east that formed as a passive-margin sequence along the northwestern margin of the North American continent. Seventy-five detrital zircon grains from the Adams Argillite and the Nation River Formation were analyzed in an effort to provide constraints on the original sources of the grains, and to generate a detrital zircon reference for miogeoclinal strata in the northern Cordillera. Thirty-five single zircon grains from a quartzite in the Adams Argillite yield dominant age clusters of 1047-1094 (n = 6), 1801-1868 (n = 10), and 2564-2687 (n = 5) Ma. Forty zircons extracted from a sandstone in the Nation River Formation yield clusters primarily of 424-434 (n = 6), 1815-1838 (n = 6), 1874-1921 (n = 7), and 2653-2771 (n = 4) Ma. The Early Proterozoic and Archean grains in both units probably originated in basement rocks in a broad region of the Canadian Shield. In contrast, the original igneous sources for mid-Proterozoic grains in the Adams Argillite and 430 Ma grains in the Nation River Formation are more difficult to identify. Possible original sources for the mid-Proterozoic grains include: (1) the Grenville Province of eastern Laurentia, (2) the Pearya terrane along the Arctic margin, and (3) mid-Proterozoic igneous rocks that may have been widespread along or outboard of the Cordilleran margin. The 430 Ma grains may have originated in: (1) arc-type sources along the Cordilleran margin, (2) the Caledonian orogen, or (3) a landmass, such as Pearya, Siberia, or crustal fragments now in northern Asia, that resided outboard of the Innuitian orogen during mid-Paleozoic time.