Thin Silt Layers Research Articles

New and interesting records of marine fishes (Actinopterygii) from the Maltese Islands (central Mediterranean)

The occurrence of three bony ray-finned fishes, Thorogobius ephippiatus (Lowe, 1839), Chlopsis bicolor Rafinesque, 1810, and Grammonus ater (Risso, 1810) is reported for the first time in the scientific literature from Maltese coastal waters. The leopard-spotted goby, T. ephippiatus, was mostly recorded within the 8–32 m depth range on soft sediment and rocky bottoms within caves, but one individual was recorded on a rocky bottom with a thin layer of silt at a depth of 117 m where no cave was present. The bicolour eel, C. bicolor, was recorded within the 318–518 m depth range on rocky bottoms covered with a thin layer of muddy-detritic sediment; and the cusk-eel G. ater was recorded at a depth of 10 m within a cave. Notes on these three species as well as on another bony fish, the Azores rockling, Gaidropsarus granti (Regan, 1903), including new information on their bathymetric range and habitat association, are presented.

Shear Characteristics of Soil—Concrete Structure Interaction Interfaces

The shear characteristics of the interfaces between soil and concrete structures are essential for the safety of the structures. In this study, a large-scale direct shear test apparatus was developed to measure the mechanical parameters of soil–concrete interfaces under conditions with different soil types, soil moisture contents, and interfacial filling materials. The results showed that the shear stress of the soil–concrete interface increased initially and then became stable with the increase in the shear displacement. The shear displacement of the sandy soil when the shear stress became stable was smaller than that of the clayey soil. The silty sand–concrete interface had a smaller friction angle than the interface with the medium-coarse sand. Moreover, with the increase in the soil moisture content, the friction angle of the clayey soil–concrete interface decreased rapidly, whereas the cohesion first increased and then decreased, and the peak cohesion was near the plastic limit of the soil. Under the same moisture content, the friction angle and cohesion of the clay–concrete interface was reduced by filling the interface with a thin layer of sandy soil, while filling the silty sand–concrete interface with a thin layer of silt reduced the friction angle and increased the interfacial cohesion. Nonetheless, the filling had little impact on the overall shear strength of the interface.

A multidisciplinary approach to characterizing coastal alluvial aquifers to improve understanding of seawater intrusion and submarine groundwater discharge

Coastal aquifers are affected by seawater intrusion (SWI), which causes their salinization, and yield submarine groundwater discharge (SGD), which feeds marine ecosystems. Characterizing groundwater dynamics in coastal aquifers is fundamental for understanding both processes and their interaction. In order to gain insights into SWI and SGD, we developed a 100 m-scale experimental field site located in a coastal alluvial aquifer at the mouth of an ephemeral stream on the Maresme coastline (Barcelona, Spain). Given the complexity of coastal aquifers and the dynamism of the processes occurring therein, understanding of the coupled processes can be achieved by combining methods and approaches across different hydrogeological disciplines. In this study, we conduct a detailed aquifer characterization based on the four pillars of hydrogeology: geology (lithological description and core samples analyses), geophysics (downhole and cross-hole measurements), hydraulics (pumping and tidal response tests) and hydrochemistry (major and minor elements, together with stable and Ra isotopes). Each discipline contributed to the characterization of the aquifer: (1) geological characterization revealed that the aquifer consists of fluvial sediments, organized in fining upwards sequences with alternating layers of gravel, sand and silt; (2) geophysics helped in identifying silt layers and their continuity, which play a segmenting role in the aquifer hydrodynamics; (3) hydraulics tests, specifically tidal response tests, evidenced that tidal loading, rather than hydraulic connection to the sea, drives the tidal response; and (4) hydrochemistry revealed a surprising high reactivity, as most ions reflect some reaction, beyond the expected cation exchange. The summary is that the aquifer, which initially looked like a homogeneous unconfined aquifer 22 m thick, effectively behaves as a multi-aquifer and reactive system with freshwater discharging beneath saltwater at several depths. The fact that thin silt layers caused such a significant impact opens new paths beyond this study both for coastal aquifer management (the possibility of transient pumping for freshwater resources) and marine ecology (expect diffuse groundwater discharge).

Geological characteristics and exploration of shale oil in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin, NW China

Abstract A set of shale-dominated source rocks series were deposited during the heyday of lake basin development in the Member 7 of Triassic Yanchang Formation, Ordos Basin, and the thickness is about 110 m. Aimed at whether this layer can form large-scale oil enrichment of industrial value, comprehensive geological research and exploration practice have been carried out for years and obtained the following important geologic findings. Firstly, widely distributed black shale and dark mudstone with an average organic matter abundance of 13.81% and 3.74%, respectively, lay solid material foundation for the formation of shale oil. Secondly, sandy rocks sandwiched in thick organic-rich shale formations constitute an oil-rich “sweet spot”, the average thickness of thin sandstone is 3.5 m. Thirdly, fine-grained sandstone and siltstone reservoirs have mainly small pores of 2–8 μm and throats of 20–150 nm in radius, but with a large number of micro-pores and nano-throats, through fracturing, the reservoirs can provide good conductivity for the fluid in it. Fourthly, continued high-intensity hydrocarbon generation led to a pressure difference between the source rock and thin-layer reservoir of up to 8–16 MPa during geological history, driven by the high pressure, the oil charged into the reservoirs in large area, with oil saturation reaching more than 70%. Under the guidance of the above theory, in 2019, the Qingcheng Oilfield with geologic oil reserves of billion ton order was proved in the class I multi-stage superimposed sandstone shale reservoir of Chang 7 Member by the Changqing Oilfield Branch through implementation of overall exploration and horizontal well volume fracturing. Two risk exploration horizontal wells were deployed for the class II thick layer mud shale interbedded with thin layers of silt- and fine-sandstones reservoir in the Chang 73 submember, and they were tested high yield oil flows of more than 100 tons per day, marking major breakthroughs in petroleum exploration in class I shale reservoirs. The new discoveries have expanded the domain of unconventional petroleum exploration.

Identification of Quaternary Sediments and the Existence of Biogenic Gas Sources in the Topang Delta, Meranti, Riau

Delta Topang, located in Topang Island, Riau Province is known to have the potential of biogenic gas. This research was conducted to identify the Quaternary sediment and the existence of biogenic gas reservoar. In this research has been conducted 75 points of surface geoelectrical measurements (38 points in Parit Jawa and 37 points in Parit Bintang) and 2 points of core drilling (BH-1 in Parit Jawa and BH-2 in Parit Bintang). Total Organic Carbon was also carried out on 10 samples (5 samples from each drilling core). Based on resistivity value of geoelectrical measurements from all points, in general are determined 2 sediment types, very fine sediment (silt and clay) and fine sand. We discussed in detail only point 11 representing Parit Jawa region and point 39 representing Parit Bintang Village. Both in point 11 and point 39, very fine sediment was identified from the surface down to 59 m and 58.5 m respectively, mostly dominated by hydrous clay. Below these, very fine sediment is identified as fine sand. Core drilling BH-1 and BH-2 (40 m length each) composed of thick layer of hydrous clay from the core surface down to bottom part, intercalated with thin layers of silt and fine sand. We considered fine sand found at depths between 24 to > 90 m (from all geoelectrical measurement points) as the closure of biogenic gas. At Parit Jawa biogenic gas closures are found at 2 locations, which are in south west and north east measurement area. At Parit Bintang biogenic gas closures are found at 3 location which are one in south and two in north measurement area. Total organic carbon analyzed from BH-1 indicate the highest percentage at 26-27 m depth with percentage 71.6%. From BH-2 the highest value is indicated at 33-34 m depth with percentage 78.0%. From all this information it is known that the formation of biogenic gas from the abundance of TOC is in the layers of hydrous clay and clay where is in an anaerobic sulfate-reduction environment.

Detailed seismic lithofluid distribution using Bayesian stochastic inversion for a thinly bedded reservoir: A case study over Huntington UK Central North Sea

A geostatistical inversion scheme was designed and performed for reservoir characterization of the UK Central North Sea (CNS) Huntington Oil field, with the goal of increasing the resolution of the seismic inversion and derived attributes, and to provide statistical measures of the uncertainties involved in the seismic facies prediction. The aim of the study was to fully understand lithology and pore-fluid rock properties in the Forties sandstone reservoir, Late Paleocene Sele Formation fan system of the UK CNS. In the Forties, multiple sand beds are separated by thin layers of silt and shale. Reprocessed prestack seismic volumes and considerable well-log data were input to an advanced stochastic simultaneous inversion method. This method combines the strengths of geostatistical inversion with sequential Gaussian simulation and Bayesian inversion. The approach allowed the inversion to be built on a fine-scale stratigraphic grid at a reservoir scale rather than at a seismic scale, and it enabled a detailed description of facies distribution and associated uncertainty. A sufficiently large number of stochastic solutions (100 realizations) enabled probabilistic models of lithofluid distribution to be generated. Facies geobody connectivity can also be derived, but this was not part of this work. As a consequence of our study, a sidetrack water injector well location was evaluated, and a potential undrilled hydrocarbon accumulation was identified, which could extend the field life of the Huntington asset.

On the Stabilizing Influence of Silt On Sand Beds

Abstract In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50 = 55 µm) and sand (D50 = 300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed.

Precipitation as the main driver of Neoglacial fluctuations of Gualas glacier, Northern Patagonian Icefield

Abstract. Glaciers are frequently used as indicators of climate change. However, the link between past glacier fluctuations and climate variability is still highly debated. Here, we investigate the mid- to late-Holocene fluctuations of Gualas Glacier, one of the northernmost outlet glaciers of the Northern Patagonian Icefield, using a multi-proxy sedimentological and geochemical analysis of a 15 m long fjord sediment core from Golfo Elefantes, Chile, and historical documents from early Spanish explorers. Our results show that the core can be sub-divided into three main lithological units that were deposited under very different hydrodynamic conditions. Between 5400 and 4180 cal yr BP and after 750 cal yr BP, sedimentation in Golfo Elefantes was characterized by the rapid deposition of fine silt, most likely transported by fluvio-glacial processes. By contrast, the sediment deposited between 4130 and 850 cal yr BP is composed of poorly sorted sand that is free of shells. This interval is particularly marked by high magnetic susceptibility values and Zr concentrations, and likely reflects a major advance of Gualas glacier towards Golfo Elefantes during the Neoglaciation. Several thin silt layers observed in the upper part of the core are interpreted as secondary fluctuations of Gualas glacier during the Little Ice Age, in agreement with historical and dendrochronological data. Our interpretation of the Golfo Elefantes glaciomarine sediment record in terms of fluctuations of Gualas glacier is in excellent agreement with the glacier chronology proposed for the Southern Patagonian Icefield, which is based on terrestrial (moraine) deposits. By comparing our results with independent proxy records of precipitation and sea surface temperature, we suggest that the fluctuations of Gualas glacier during the last 5400 yr were mainly driven by changes in precipitation in the North Patagonian Andes.

Inference of the two dimensional GPR velocity field using collocated cokriging of Direct Push permittivity and conductivity logs and GPR profiles

One of the main difficulties in processing GPR surface data is to infer the ground velocity models in order to convert the radargram time scale into depth. Common conversion techniques usually fail if the ground lithology is complex. Consequently, the ground velocity has to be guessed and generally assumed laterally invariant. In this paper, we present a new geostatistical approach to the velocity analysis of sandy aquifer based on the interpolation of the Direct Push relative permittivity and conductivity logs. The first step of the method consists in scaling the radargram to maximize the correlation with the logs. Then, collocated cokriging of electrical relative permittivity and conductivity and the scaled radargram information is computed. The cokriged fields are converted in velocity field following EM hypothesis and the time to depth conversion of the original radargram is then applied using the inferred velocity field. Results on real data analysis show that the method works well in fairly homogeneous sandy aquifer with intercalated dipping thin silt layers.

Geochemical processes underlying a sharp contrast in groundwater arsenic concentrations in a village on the Red River delta, Vietnam

Abstract The spatial variability of As concentrations in aquifers of the Red River Delta, Vietnam, was studied in the vicinity of Hanoi. Two sites, only 700 m apart but with very different As concentrations in groundwater (site L: NH 4 + (∼10 mg/L), HCO 3 - (500 mg/L) and dissolved P (600 mg/L), in addition to elevated As at site H are consistent with a release coupled to microbially induced reductive dissolution of Fe oxyhydroxides. Other processes such as precipitation of siderite and vivianite, which are strongly supersaturated at site H, or the formation of amorphous Fe(II)/As(III) phases and Fe sulfides, may also influence the partitioning of As between groundwater and aquifer sands. The origin of the redox contrast between the two sites is presently unclear. Peat was observed at site L, but it was embedded within a thick clayey silt layer. At site H, instead, organic rich layers were only separated from the underlying aquifer by thin silt layers. Leaching of organic matter from this source could cause reducing conditions and therefore potentially be related to particularly high concentrations of dissolved NH 4 + , HCO 3 - , P and DOC in the portion of the aquifer where groundwater As concentrations are also elevated.

Tide‐induced groundwater head fluctuation in a coastal aquifer system with a submarine outcrop covered by a thin silt layer

AbstractWe present an analytical solution of groundwater head response to tidal fluctuation in a coastal multilayered aquifer system consisting of an unconfined aquifer, a leaky confined aquifer and a semi‐permeable layer between them. The submarine outcrop of the confined aquifer is covered by a thin silt layer. A mathematical model and the analytical solution of this model are given. The silt layer reduces the amplitude of the hydraulic head fluctuation by a constant factor, and shifts the phase by a positive constant (time lag), both of which depend on the leakances of the silt layer and the semi‐permeable layer. The time lag is less than 1·5 h and 3·0 h for semi‐diurnal and diurnal sea tides respectively. When the leakance of the semi‐permeable layer or the silt layer assumes certain special values, the solution becomes the existing solutions derived by previous researchers. The amplitude of the hydraulic head fluctuation in the confined aquifer increases with the leakance of the silt layer and decreases with the leakance of the semi‐permeable layer, whereas the phase shift of the fluctuation decreases with both of them. A hypothetical example shows that neglecting the silt layer may result in significant parameter estimation discrepancy between the amplitude attenuation and the time‐lag fittings. Copyright © 2007 John Wiley & Sons, Ltd.

Comparative performance of 5 cm2 and 10 cm2 piezocones in a lacustrine clay

A piezocone investigation has been carried out in a deep deposit of glacial lacustrine clay using piezocones with cross-sectional areas of 5 cm2 and 10 cm2. The piezocone tests formed part of a larger soil characterisation study, but this paper focuses on the relative performance of the piezocones in profiling the clay. It is shown that using a 5 cm2 piezocone rather than a 10 cm2 one significantly improved the detection of relatively thin silt layers within the lacustrine, and occasionally varved, clay. The feasibility of detecting silt layers as thin as 2–4 mm and of matching the piezocone responses associated with silt layers at different test locations, thereby allowing an assessment of their continuity, is demonstrated. There were no significant differences between the magnitudes of the cone resistance and excess pore pressure recorded in the clay with 5 cm2 and 10 cm2 piezocones. Pore pressure dissipation test results were variable but, in a region where no silt layers were detected, similar results were obtained with piezocones of each size.

Imaging of groundwater resources in glacial deposits using high-resolution reflection seismics, Sweden

Two high-resolution reflection seismic profiles were acquired in the Heby area of eastern Sweden over glacial deposits for the purpose of mapping groundwater resources. The majority of shot points were located in clay resulting in good quality data along most of the profiles. On stacked and migrated sections, the uppermost clay is about 20 m thick and is characterized by its subhorizontal reflectivity. Sand/gravel deposits below it contain more dipping interfaces and have a chaotic reflectivity pattern. Depth to bedrock is interpreted to be 90 and 65 m on the respective profiles and occurs in about a 100-m-wide trough on both profiles. Reflections from the tops of sandy gravel zones generally have higher amplitudes. Clear reflections from a thin silt layer (20 cm thick) at about 10-m depth are observed on one of the profiles. Elastic finite difference modeling and the observation of this reflection in shot gathers show that the reflection is not an artifact of the acquisition nor the processing. The modeling also shows that there is no marked low-velocity waveguide in the near surface, but that an effective low Q zone may be present. Comparison with refraction profiling on the other profile shows that there is better agreement between the reflection seismic results and penetration tests than the refraction results with these tests. Both profiles allow the thickness of the overlying clay layers to be determined, as well as the thickness of the underlying sand/gravel deposits. This is important for estimating the amount of groundwater resources in an area.

The Eastern Chenier Plain: An Update on Downdrift Coastal Progradation Associated with the Building of a New Holocene Delta Lobe in the Mississippi Delta Complex: ABSTRACT

The Eastern Chenier Plain: An Update on Downdrift Coastal Progradation Associated with the Building of a New Holocene Delta Lobe in the Mississippi Delta Complex: ABSTRACT

Flow Of Groundwater From Soil To Crystalline Rock

Knowledge of groundwater flow from soil or surface water to crystalline bedrock has usually been derived from indirect studies of drawdown in soil due to underground constructions, as well as from analysis of water chemistry and from tracer experiments. Infiltration into the bedrock occurs at specific sites where suitable combinations of geological and hydrological variables exist. Flow from soil to rock in the saturated zone occurs where conductors in the bedrock, such as fractures and fracture zones, are hydraulically connected to a groundwater reservoir in permeable soil or to horizons of permeable and constructive material in heterogeneous soil. Of particular importance for infiltration are the hydraulic conditions of the contact zone between soil and rock. A thin layer of silt on the bedrock surface often blocks the water flow. The micro-topography of the bedrock surface is important since fracture zones usually give depressions in the surface, in which accumulations of sorted and conductive material often can be found. A strong heterogeneity in the infiltration from soil to rock is evidenced by statistical analyses of the flow related to various geological and hydrogeological variables, as well as from analyses of groundwater chemistry and tracer experiments. In order to estimate the infiltration from soil to rock and to carry out mathematical modelling of the groundwater flow, it is necessary to have a good knowledge of the hydraulic conditions of the superficial rock and soil as well as of the conditions at the soil/bedrock contact zone. Information on the saturated flow from soil to rock is essential for calculation of water budgets, for assessments of spread of pollutants and for estimations of leakage into underground constructions.

An alluvial record of El Niño events from northern coastal Peru

Overbank flood deposits of northern coastal Peru provide the potential for the development of a late Quaternary chronology of El Niño events. Alluvial deposits from the 1982–1983 El Niño event are the basis for establishing a type El Niño deposit. Sedimentary structures suggesting depositional processes range from sheet flows to debris flows, with sheet flood deposits being the most common. The 1982–1983 deposits are characterized by a 50‐ to 100‐cm‐ thick basal gravel, overlain by a 10‐ to 100‐cm‐thick sand bed, grading into a 1‐ to 10‐cm‐thick silty sand bed and capped by a very thin layer of silt or clay. The surface of the deposit commonly displays the original shear flow lines crosscut by postdepositional mud cracks and footprints (human and animal). Stacked sequences of flood deposits are present in Pleistocene and Holocene alluvial fill, suggesting that El Niño type events likely occurred throughout the late Quaternary. A relative chronology of the deposits is developed based on terrace and soil stratigraphy and on the degree of preservation of surficial features. A minimum of 15 El Niño events occurred during the Holocene; a minimum of 21 events occurred during the late Pleistocene. Timing of the Holocene events is bracketed by isochrons derived from the archaeologic stratigraphy. Corrected radiocarbon ages from included detrital wood provide the following absolute dates for El Niño events: 1720 ± 60 A.D., 1460 ± 20 A.D., 1380 ± 140 A.D. (error overlaps with the A.D. 1460 event; these may represent a single event), and 1230 ± 60 B.C.

Pleistocene glacio-lacustrine deltaic deposits of the Scarborough Formation, Ontario, Canada

The late Pleistocene Scarborough Formation of the Lake Ontario Basin represents a lacustrine-deltaic sequence strongly affected by a nearby glacier. It shows gradual upward coarsening and thickening of layers from varve-like, clay-rich rhythmites at the base to channelized cross-bedded sands at the top. Thick, lensing, “massive” layers (Cm) of silty sand are found interstratified with thin layers of silt and clay in the lower and middle parts of the sequence. These “massive” layers were probably formed by subaqueous gravity flows of material in part derived directly from a glacier. Absence of pebbles and of other features such as glacial silt-pellets suggests dispersal away from the foot of the glacier rather than deposition under floating ice. Varve-like rhythmites, pollen and insect fauna indicate subarctic to arctic climatic conditions at the time of sediment deposition. The large amount of sand in the upper part of the formation is best explained as derived from active outwash fans. Similar fans were formed during the last deglaciation in southern Ontario and can be observed on the landscape.

Ecological interactions between the brown seaweed Sargassum muticum and its associated fauna

The fauna inhabiting the brown seaweed Sargassum muticum (Yendo) Fensholt was studied at Friday Harbor, Washington, USA over a 2 mo period (July/August, 1978). The fauna lacked diversity, but a few species were found frequently and in great abundance, namely the amphipods Ampithoe mea Gurjanova, Aoroides columbiae Walker, Caprella laeviuscula Mayer and Ischyrocerus anguipes Kryer, and the gastropod Lacuna variegata Carpenter. All of these vagile species were more abundant on the better illuminated distal regions of the plant, where there was an abundant coating of the diatoms Biddulphia aurita (Lyngb.) de Breb and Melosira dubia (C. A. Ag.) Kutz. A comparison between plants bearing different periphyton loads revealed that Aoroides columbiae and C. laeviuscula were far more abundant on heavily infested plants. A similar trend was evident for Ampthoe mea and L. variegata, but was not sufficiently marked to be statistically significant. Plants bearing silt rather than diatoms supported reduced numbers of all the common species of associated animals and especially C. laeviuscula. The results of substratum selection experiments in the laboratory corresponded to the expectations raised by the field observations. Both A. mea and C. laeviuscula were strongly attracted to S. muticum, especially if it was coated with diatoms. Only live diatoms were attractive. C. laeviuscula shunned dead diatoms and also a thin layer of silt. L. variegata proved indifferent to S. muticum, diatoms or silt. The preferences exhibited are discussed in relation to the diet of each species as well as its need for a secure perch in a turbulent environment, and concealment from predators. The grazing rates of each of the three commonest herbivores found on S. muticum were also assessed. Ampithoe mea consumed up to 4.4 mg of S. muticum tissue per individual per day, although nest-building activities greatly added to its depredations. C. laeviuscula fed largely on diatoms and did not harm S. muticum. L. variegata was the most significant of the grazers tested. Full grown specimens consumed 8.1 mg tissue individual-1 d-1, but small specimens consumed more in relation to their body weight, 1.55 mg tissue per mg animal soft tissue-1 d-1. Estimates based on the abundance of grazers on S. muticum, their feeding rates and the growth rates of the plants revealed that in late summer, epiphytic grazers could remove more tissue than is being formed. This was, however, the slow growth period for S. muticum, and earlier in the year its very rapid growth should more than compensate for the effects of epiphytic grazers. In autumn, much of the fauna of S. muticum was lost with the natural defoliation of the plants, which therefore, must be restocked annually, probably from animals inhabiting nearby Zostera marina beds.

A DEPOSITIONAL PROCESS OF REVERSE GRADED BEDDING IN FLOOD DEPOSITS OF THE SAKURA RIVER, IBARAKI PREFECTURE, JAPAN

The Sakura River flows in a narrow alluvial channel with gentle slope, draining the southeast area of Ibaraki Prefecture (Figs. 1 and 2). The bed consists of arcosic sand and gravel which reflect the geological condition of the upper drainage basin of the river. The investigation of flood deposits in the upper reaches of the river shows that the flood deposits have reverse grading structure (becoming coarser grain from bottom to top of a bed) and that a sedimentary unit, the lower mud layer and the upper sand layer, is formed during a single flood. It is the purpose of this paper to explain the depositional process of the reverse graded bed from the characteristics of sediment transport in the Sakura. Main results are as follows. The flood deposits are thick only on the cut-sides of the river channel, which are attacked by the main flow of the flood (Fig. 6). Levees are better developed in these areas. Medium and fine sand make up the thick flood deposits on the natural levee (Fig. 7). It begins with the deposition of a thin silt layer and is followed by a thick sand layer which displays no internal structures. Sand sizes, φ1 as well as φ50, gradually increase toward upward. Fig. 8 shows the repeated cycles of coarsening upward sequence of flood deposits on the natural levee. The maximum diameter of each flood deposit has the upper limit of about 10(0. 5mm) . The floods which left the thick flood deposits are shown in the time-stage curves of Fig. 9. Reverse grading is also developed among the thin flood deposits on the lower part of the flood plain (Fig. 7). The lowest clayey silt layer with a thickness of a few milimeters suddenly changes the upper sand layer which usually has fine lamination. Fig. 9 shows the observational results on the hydraulic regimes and the suspended sediment transports in some floods in the Sakura River. It is the common fact to all observations that the maximum concentration of suspended load precedes the peak discharge. Furthermore, it is clear from Fig. 10 that the amount of the finer sediment load of silt-clay and very fine sand is extremely high at the leginning of each flood and rapidly decreases with the rising of waterlevel. The depositional process of reverse graded bedding in flood deposits can be explained as follows and Fig. 12 schematized them. 1. Deposition of the mud layer (Fig. 12-I) It is very likely that the lower muddy part of reverse graded bed is deposited during the beginning of the flood when the concentration of finer grained suspended sediments, the socalled “wash load”, is extremely high. Flood water with large quantities of wash load is diverted into the flood plain and the stagnation of flows on the flood plain causes the deposition of this finer sediments. It can be considered that the reason why mud layers on the natural levee are very thin and rather indistinct (Figs. 7 and 8) is that the natural levee is not covered by the inundated water at this stage. Boils along the bank begin to occur in the mean while. 2. Deposition of the sand layer (Fig, 12-II) The upper sandy part is presumed to be produced by the deposition of suspended sand after the rapid decrease of wash load concentration. The upward coarsening structure in this sand layer may be produced under the condition of gradual increase of flow intensity. The sand make up the natural levee is coarser than that of suspended up to the bankfull level at the center of the channel (Figs. 7, 8 and 11). This fact is explained as follows. Boils over the bank become more and more violent. This suggests that the upward flows (kolks) near the bank become violent with increases of discharge. It is supposed that these kolks cause the coarser sediment grains to suspend near the bank. 3. Preservation of the reverse graded bed (Fig. 12-III) With the beginning of water level falling, the inundated flow turns back from the flood plain to the channel.

Pelagic redbeds in the Devonian of Germany — deposition and diagenesis

Red pelagic mudstones are a conspicuous facies type of the Variscan basins in Europe, where they occur chiefly in the Famennian. They are restricted to basinal sites without major clastic influx: in areas marginal to turbidite fans, in starved intra-geosynclinal basins, or on the slopes of rises. Thin silt layers, intercalated among the red shales, are interpreted as distal or lateral turbidites. Pelagic body fossils are extremely rare. Endobentonic trace fossils, indicative of outer fan and trough environments, are proof of aerated bottom water. Ferric iron, from which hematite is formed during diagenesis, is provided by continental weathering, and is mainly bound to the clay fraction. Red colouration in mudstones requires a minimum of about 1.5% hematite, as calculated from chemical analyses. The content of pigment may be taken below the critical level by dilution with non-red silicate or carbonate. Conversely, enrichment of the clay fraction, at places far from the source area for example, enhanced a red colouration. Drab colouration is mainly due to reduction of iron, caused by oxidation or organic matter during early diagenesis. Reduction continued during later diagenesis and tectonic deformation, as demonstrated by colour-boundaries which cut through the bedding and partly follow structured patterns. Where the sediment was permeable (e.g., prior to compaction or along structural surfaces), the reduced iron was partly removed. The red pigment could only survive where organic supply to the sediment was too low to consume the available oxygen. The deficit of organic matter in the Famennian sediments is attributed to oligotrophic conditions. These could have been brought about by decrease in continental afflux from the Caledonian source areas, which, by the late Devonian time, were much reduced by erosion and also progressively more limited in areal extent as marine transgression advanced. Oligotrophy is seen as one of the main factors governing the origin of all kinds of red-beds: terrestrial, fluviatile, estuarine, lagoonal-hyperhaline, nearshore-marine, as well as (in the Famennian) pelagic-bathyal.