Search

Fluvial strata deposited before the advent of land plants are commonly sheet‐like in geometry, lacking in channel forms. Although some pre‐vegetation strata contrast strongly with fluvial strata from later eras, the characterization of pre‐Devonian fluvial drainage systems remains incomplete. Field studies of the fluvial facies and architecture of the Series Rouge are presented to identify the products and processes of fluvial deposition occurring at high latitudes in the early Cambrian. Three different depositional styles or facies associations are recognized. Facies Association 1 is the most abundant (67%) comprising repetitively stacked sheets of trough cross‐stratified sandstone interpreted to represent the preferential preservation of the deepest parts of low sinuosity, braided river channels, 4 to 6 m deep. Facies Association 2 is also common (30%) and displays more complex facies sequences that include stacked sheets of trough cross‐strata, sets of large‐scale tabular cross‐strata, planar strata and subordinate mudstone. These facies are interpreted to reflect deposition in braided rivers with deeper channels, 6 to 8 m deep. These channels were more sinuous and subject to steady lateral migration which resulted in preservation of both deep and shallow parts of the river. Facies Association 3 is uncommon (3%) comprising interbedded sequences of cross‐stratified sandstone and mudstone interpreted to reflect deposition on a muddy coastal plain in a mixture of braided and meandering channels, 1 to 3 m deep. The fluvial strata of the Series Rouge indicate that these pre‐vegetation drainage systems were predominantly braided fluvial in origin. However, the style of braided fluvial deposition varied according to discharge and valley slope, the braided rivers becoming more migratory with decreasing slope. Meandering channel deposition was locally evident but was restricted to channels of low discharge crossing low gradient muddy coastal plains. [ABSTRACT FROM AUTHOR]

The Maltese archipelago and south-eastern Sicily include an Uppermost Oligocene to Upper Miocene hemipelagic sedimentary succession representing the Malta-Hyblean plateau, which limits the eastern Mediterranean to the west. This succession hosts a unique and well-exposed series of condensed and allochthonous phosphate-rich beds, which were formed in a sedimentary regime of erosion, sediment reworking and frequent gravity-flow deposition. The combination of nannofossil biostratigraphy and 87Sr/86Sr isotope stratigraphy allows for the precise attribution of ages to the phosphate deposits and for the distinction of three periods of major phosphogenesis. The first phase occurred between 24·5 and 21 Ma and 25 and 18·9 Ma (clustering of ages between 25 and 22·5 Ma) on Malta and Sicily, respectively. The second and third phases of phosphogenesis are documented from the Maltese Islands and are dated as 17·2 to 13·1 Ma and 10·9 to 9·8 Ma, respectively. The phosphate-rich beds are associated with hiatuses and phases of important condensation which, for the oldest phosphogenic period, envelop the time period of 23·2 to 22 Ma for the Fomm Ir Rhi Bay section (Malta) and from 19·1 to 16·3 Ma for the sections of Sampieri and Modica (Sicily). For the second phase of phosphogenesis on the Maltese Islands, a consistent hiatus was found which embraces the time period of approximately 17 to 15 Ma. Also the third phase of phosphogenesis appears to be associated with a major hiatus, which probably envelops the time period between 12·5 and 10·9 Ma, but a better age control is needed here. The correspondence in timing of the Maltese-Sicilian phases of phosphogenesis with major phases of phosphogenesis outside the Mediterranean realm, to maxima in oceanic phosphorus-burial rates and maxima in the δ13C benthic foraminiferal record suggests that the palaeoceanographic evolution of the eastern Mediterranean was well in phase with that of other ocean basins until at least the early Late Miocene, despite its increasing isolation due to the gradual closure of the Eurasian-Arabian Strait and progressive sea-level fall. [ABSTRACT FROM AUTHOR]

The article presents a study which demonstrated significant variability in timing, nature and extent of geomorphic response to European settlement at both the reach and within-reach scale along the Upper Hunter River in New South Wales. It indicates that the analyses of floodplain sedimentology, archival records, parish maps and aerial photographs documents marked spatial variability in the pattern of channel change since European settlement in the 1820s. It notes that different types, rates and extents of change are described for seven zones of adjustments along an eight kilometer (km) study range.

The article discusses the development of step-pool bedforms through high flow rates and high sediment influx and transport rates in the U.S. It notes that step setting events are responsible in mobilizing and rearranging the large step-forming grain sizes. Laboratory experiments are conducted and result shows that step bedforms are created during the flow movement of large, step-forming clasts. During the experiment, several step formations are proposed including particle cluster, migrating hydraulic jump, cascade, antidune and scouring formation models.

Channel-scale sedimentary units associated with bedrock-controlled riffle-pool morphology are examined in detail along Sandy Creek gorge, an ephemeral stream in arid south-eastern central Australia. Pool-fills comprise cut-and-fill assemblages of poorly sorted sediments ranging in texture from muds to boulders. Five unit types are defined based on particle size, sedimentary structures, geometry and bounding surface character: (1) coarse-grained bar platform; (2) fine-grained bar supraplatform; (3) fine-grained pool-fill; (4) fine-grained bench; and (5) modern pool-fill. The last coarse-grained unit currently lining the pools suggests an altered sedimentation style over the post-settlement period (post-ad 1860s). Situated at bedrock valley constrictions, pool-fills are compared with other sedimentary units associated with recirculating currents: eddy bars and slackwater deposits. But only the fine-grained bench units reflect eddy recirculation; the pool-fills are principally forced-bars associated with bedrock-controlled or‘forced’ riffle-pool morphology. A late Holocene palaeoflood history is proposed based on radiocarbon ages from the pool-fills: multiple phases of cut-and-fill activity were preceded by a superflood 3400–1900 years ago that eroded the pool-fills to bedrock. The resilience of the pool-fills was illustrated by the passage of a 1-in-100-year flood in 1992, which caused only minor erosion. The presence of pool-fills may provide a window to past phases of river activity that cannot be extracted from either historical records/observations or palaeoflood slackwater sediment analyses. The formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences, but the occurrence of high-magnitude floods exerts the dominant control. [ABSTRACT FROM AUTHOR]

Fine sediment deposition in the ocean is complicated by the cohesive nature of muds and their tendency to flocculate. The result is disaggregated inorganic grain size (DIGS) distributions of bottom sediment that are influenced by single-grain and floc deposition. This study outlines a parametric model that characterizes bottom sediment DIGS distributions. Modelled parameters are then used to infer depositional conditions that account for the regional variation in the grain sizes deposited by turbidity currents on the Laurentian Fan–Sohm Abyssal Plain, offshore south-eastern Canada. Results indicate that, on the channellized Laurentian Fan, the mass fraction of floc-deposited mud increases only slightly downslope. The small evolution in this fraction arises because sediment concentration and turbulent energy are associated in turbidity currents. On the Sohm Abyssal Plain, however, the mass fraction of floc-deposited mud decreases, probably as a result of lower sediment concentration at this source-distal site. Estimates of the mass fraction of mud deposited as flocs suggest that floc deposition is the dominant mode by which sediment is lost from suspension, although single-grain deposition contributes more to the depositional flux in proximal areas where high energy breaks flocs and in distal areas where low sediment concentration limits floc formation. It is concluded that, throughout the dispersal system, changes in the fraction of flocculated mud deposited from turbidity currents reflect changes in sediment concentration and energy downslope. [ABSTRACT FROM AUTHOR]

The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I—unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III—interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales: and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor. [ABSTRACT FROM AUTHOR]

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

Cumulative frequency curves and bivariate textural plots of sands collected and sieved from the medium-grained (1.94), well-sorted (0.46) Amazon-Solimões Rivers are not environmentally sensitive. Only 13 % of the samples plot as fluvial using cumulative frequency curves, and only one of the nine bivariate plots successfully identified the fluvial environment. Thus, identification of depositional environments of sands by size distribution based only on sieving is not a reliable technique—even when the results of diagenesis are not considered. [ABSTRACT FROM AUTHOR]

Attention is called to the ability of bottom-dwelling micro-organisms such as algae to stabilize sandy sediment surfaces in non-carbonate environments and to influence sand transport. [ABSTRACT FROM AUTHOR]