Muddy Layers Research Articles

Sedimentary and hydraulic characterization to identify hydrofacies in Barreiras Formation, Rio de Janeiro state - Brazil

The use of analogous outcrops in characterizing hydrogeological heterogeneities is widespread worldwide. It is a powerful tool for sediment characterization and determination of hydraulic parameters in rural and urban areas lacking hydrogeological information. This study investigates and characterizes the depositional and post-depositional factors that control the quality of the porous aquifer of the Barreiras Formation, Campos Basin, Rio de Janeiro, Brazil. The integrated analysis between petrological and hydraulic parameters in two outcrops analogous to the Barreiras aquifer demonstrated the role of post-depositional processes, which altered the original deposition conditions. The two outcrops are predominantly composed of sandy and muddy facies, with hydraulic conductivity ranging between 10−4 to 10−5 cm/s and 10−5 to 10−8 cm/s, respectively. This allowed the definition of two hydrofacies, formed by sandy layers as reservoirs and by muddy layers acting as hydraulic barriers. In this context, the Barreiras Formation can be characterized as a poor aquifer, differing from the typical braided fluvial deposits. This evidence draws attention to the indiscriminate use of typical sedimentary models in environments with a high rate of chemical-biological weathering, not only in underground flow models but also in studies of underground resources protection and exploitation, such as the Barreiras aquifer, which consists of one of the main water sources for public, industrial and agricultural supply.

Mud redeposition during river incision as a factor affecting authigenic 10Be/9Be dating: Early Pleistocene large mammal fossil‐bearing site Nová Vieska, eastern Danube Basin

ABSTRACTThis study examines the suitability of the authigenic 10Be/9Be dating method to the dating of the deposits of an incising river, taking as an example the Nová Vieska river terrace, which accumulated during the neotectonic inversion of the Danube Basin (western Slovakia). The succession was formed by a wandering river with minor preservation of proximal floodplain muds. The frequent occurrence of mud intraclasts reflects significant input of eroded material from underlying, older successions. The ages of 13 authigenic 10Be/9Be dating samples formed three groups: (1) samples from below the base of the river terrace yielded dates of ~4.13–3.70 Ma (including uncertainties); (2) muddy intraclasts from the river terrace gave an age range of ~2.79–1.96 Ma; and (3) in situ muddy layers had ages in the range of ~1.91–1.39 Ma. The large mammal fossil assemblage from channel thalweg deposits yielded a biostratigraphic age of ~3.6–2.2 Ma, matching the age of intraclasts, and thus emphasising the redeposited origin of those fossils. The relatively wide range of authigenic 10Be/9Be dating ages is interpreted as a result of the redeposition of mud from older strata on three scales: decimetre‐scale intraclasts, millimetre‐scale rip‐up clasts mixed into the newly formed beds, and formation of two authigenic rims with different age and 10Be/9Be records around individual particles. Considering these observations, an age range of in situ layers of ~1.91–1.39 Ma is proposed as the depositional age of the river terrace, with the most probable age falling within the most recent part of this interval. The effect of redeposition is thus shown to be potentially limiting to the application of authigenic 10Be/9Be dating to incising rivers, and stands in marked contrast to aggrading river settings, where redeposition of older sediments is limited and the degree of 10Be/9Be variability is low.

Salvamento paleontológico, ambiente deposicional e correlação estratigráfica de uma seção da Formação Ponta Grossa (Bacia do Paraná)

The urban area of Ponta Grossa (PR) occurs on one of the most abundant fossiliferous fields in the country, which records an endemic fauna that occurred in the Devonian of the Paraná Basin. The growing urbanization of the city increasingly buries these outcrops, and new works must be accompanied by a specialized team to safeguard the paleontological heritage. This is the case of the Bosque Mistral project, which exposed many layers of the Ponta Grossa Formation, rich in fossils and trace fossils. This study reports the main fossiliferous occurrences in the section, correlates the section with classical outcrops from the literature, and interprets the main depositional environments as well stratigraphic cycles using an integrated analysis of sedimentology and ichnology. The ichnological distribution also evidences the retrogradation pattern, and, particularly, the occurrence of ichnofabrics dominated by Zoophycos at the top of the section is a signature of high-frequency cycles associated with the transgressive systems tract. Macrofossill biodiversity varies according to the facies, in the sandy and silty layers different organisms from the muddy layers occur. In the former, the epibiont fauna (mainly trilobites and brachiopods) is predominant while in the muddy layers the predominance is the endobiont and semi-endobiont fauna (bivalve mollusks and infaunal brachiopods). In other words, the distribution of taxa across the layers is not random.

Salvamento paleontológico, ambiente deposicional e correlação estratigráfica de uma seção da Formação Ponta Grossa (Bacia do Paraná)

The urban area of Ponta Grossa (PR) occurs on one of the most abundant fossiliferous fields in the country, which records an endemic fauna that occurred in the Devonian of the Paraná Basin. The growing urbanization of the city increasingly buries these outcrops, and new works must be accompanied by a specialized team to safeguard the paleontological heritage. This is the case of the Bosque Mistral project, which exposed many layers of the Ponta Grossa Formation, rich in fossils and trace fossils. This study reports the main fossiliferous occurrences in the section, correlates the section with classical outcrops from the literature, and interprets the main depositional environments as well stratigraphic cycles using an integrated analysis of sedimentology and ichnology. The ichnological distribution also evidences the retrogradation pattern, and, particularly, the occurrence of ichnofabrics dominated by Zoophycos at the top of the section is a signature of high-frequency cycles associated with the transgressive systems tract. Macrofossill biodiversity varies according to the facies, in the sandy and silty layers different organisms from the muddy layers occur. In the former, the epibiont fauna (mainly trilobites and brachiopods) is predominant while in the muddy layers the predominance is the endobiont and semi-endobiont fauna (bivalve mollusks and infaunal brachiopods). In other words, the distribution of taxa across the layers is not random.

A geochemical approach for identifying marine incursions: Implications for tsunami geology on the Pacific coast of northeast Japan

Potential tsunami inundation areas can be predicted by the distribution of paleo tsunami deposits on land, which are mainly composed of marine-derived sands and muds. These exotic sandy and muddy layers have been identified by multiple approaches. However, there still remain uncertainties regarding the sources and weathering trends over the long term, and other useful proxies need to be investigated to identify of tsunami deposits. We examined the geochemical signatures of modern tsunami deposits from the Pacific coast of northeast Japan (2011 Tohoku tsunami) and those paleo tsunami deposits taken from the Tohoku District (Jogan, ~1080 calibrated ages before present [cal BP] and Yayoi, ca. 2000 cal BP). A geochemical ternary diagram (Seawater-Rock-(As + Metals)) showed the weathering trends of tsunami deposits over approximately 2000 years in the Sendai plain in the Tohoku. In the tsunami layers from the Tohoku, the Na/Ti atomic ratios markedly increased to 23.4 with an average value of 19.3 ± 3.0 (Jogan and Yayoi tsunami deposits), which was clearly higher than those of other layers (soil deposits, 10.3 ± 3.5 on average). These results show that Na/Ti ratios and other geochemical proxies are useful indicators of marine incursions in our case, and may contribute to the detection of muddy tsunami deposits close to the limit of inundation.

Facies associations, detrital clay grain coats and mineralogical characterization of the Gironde estuary tidal bars: A modern analogue for deeply buried estuarine sandstone reservoirs

Abstract Estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry. In order to better predict the reservoir potential of these geobodies, this study provides a modern-day reservoir analogue, describing tidal bars in the inner and outer Gironde estuary from the microscopic to the macroscopic scale. The originality of this work lies in the multi-scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high-energy environment. This work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers. Their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes. Detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars. X-ray diffraction and electron microscopes reveal that these coats are mainly composed of di-octahedral smectite, illite, chlorite and kaolinite associated with other components such as diatoms or pyrite. The best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale. An optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth. These conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones.

A convolutional neural network applied to measured time series for source range and ocean seabed classification

Source localization and environmental inference are common problems in ocean acoustics requiring computationally intensive algorithms and knowledge of the search space. Convolutional neural networks (CNNs) learn useful features for making predictions directly from a gridded input signal circumventing the costly practice of selecting features or comparisons to a forward propagation model. To take advantage of these benefits, a CNN was trained and validated on simulated pressure signals generated using four different environments (sandy, muddy, and mixed-sediment layers) for several ranges (2 to 12 km) to make environment class and range predictions. The trained network was then tested on recorded signals from the APL-UW Intensity Vector Autonomous Receiver system during SBC2017. We found that the network could make predictions with 99% accuracy on the simulated validation dataset but struggled to make unbiased predictions on the measured data. The struggles highlight the need to incorporate all significant features in the training data, such as time alignment and addition of noise. The CNN has the power to make predictions on this type and clearly see the differences in the data, but investigations for creating the proper training dataset or building a deeper network are required. [Work supported from Office of Naval Research.]

Using foraminiferal test-size distribution and other methods to recognise Quaternary bathyal turbidites and taphonomically-modified faunas

Abstract The aim of this study was to develop a simple methodology using foraminifera to distinguish between sediment that has been displaced downslope (as in a turbidity current) and in-situ hemipelagic sediment. These same methods will also be useful to identify seafloor foraminiferal faunas that have suffered considerable taphonomic modification (e.g., through dissolution, winnowing, mixing), which render them unsuitable as modern analogues for paleoenvironmental reconstructions. To help develop this methodology we used Late Quaternary samples from cores containing identified turbiditic sand-mud couplets that had been deposited at lower bathyal to upper abyssal depths (1400–3000 m) off the earthquake-prone east coast of New Zealand. Comparison of the proportions of the total planktic and benthic foraminiferal tests in each of three size categories (63–125 μm, 125–250 μm, >250 μm) with those in unmodified, in-situ modern bathyal faunas from off New Zealand showed an increased proportion of larger tests in the lower sandy turbidite layers and a slight increase of smaller tests in the upper muddy layers presumably as a result of winnowing processes during deposition. A regression formula was used to estimate depositional depths based on the relative abundance of planktics in the Quaternary foraminiferal faunas (>63 μm). In all but one of our core samples, these depth estimates were considerably shallower than the cores' present depth and suggested that most sediment (both sand and mud) had been transported downslope. Modern Analogue Technique (MAT) was used to estimate depths of deposition of the core samples based on a modern benthic foraminiferal dataset (>63 μm) from around New Zealand. There was no great difference between estimates based on species % and on genus %. Some MAT depth estimates were inconsistent with those from the other foraminiferal and sedimentological proxy methods that suggested turbiditic displacement. Because of the inconsistent results and the much greater work required to acquire the species census data we exclude the MAT method from our suggested methodology. Genera were lumped into groups with similar bathymetric range and the relative abundances of these groups in each core sample were used to infer sediment provenance and mixing during downslope transport. We recommend a methodology using three elements: Test size distribution, planktic percentage, and relative abundance of bathymetric groups of benthic genera. The first two require the least amount of work and can reliably identify the majority of displaced and taphonomically-modified faunas. The third requires some taxonomic skills to identify key genera but gives more information on the provenance depth and amount of mixing in displaced turbiditic sediment. Although developed using New Zealand modern and Late Quaternary foraminiferal data, we contend that these methods should be applicable for use throughout most of the Neogene in other temperate and subtropical parts of the world's oceans. Before use elsewhere, studies in the local region may be needed to test, recalibrate and tweak the modern analogue data on local test-size distribution and planktic percentage. Our study also suggests that these methods can be useful to identify seafloor foraminiferal faunas that have suffered considerable taphonomic modification (e.g., through dissolution, winnowing, mixing), which render them unsuitable as modern analogues for paleoenvironmental reconstructions.

Facies architecture of the fluvial Missão Velha Formation (Late Jurassic–Early Cretaceous), Araripe Basin, Northeast Brazil: paleogeographic and tectonic implications

Abstract Sedimentological analysis of the Missão Velha Formation (Araripe Basin, northeast Brazil) is the aim of this paper through detailed facies analysis, architectural elements, depositional systems and paleocurrent data. The main facies recognized were: (i) coarse-grained conglomeratic sandstones, locally pebbly conglomerates, with abundant silicified fossil trunks and several large-to-medium trough cross-stratifications and predominantly lenticular geometry; (ii) lenticular coarse-to-medium sandstones with some granules, abundant silicified fossil wood, and large-to-medium trough cross-stratifications, cut-and fill features and mud drapes on the foresets of cross-strata, (iii) poorly sorted medium-grained sandstones with sparse pebbles and with horizontal stratification, (iv) fine to very fine silty sandstones, laminated, interlayered with (v) decimetric muddy layers with horizontal lamination and climbing-ripple cross-lamination. Nine architectural elements were recognized: CH: Channels, GB: Gravel bars and bed forms, SB: Sand bars and bedforms, SB (p): sand bedform with planar cross-stratification, OF: Overbank flow, DA: Downstream-accretion macroforms, LS: Laminated sandsheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. The lithofacies types and facies associations were interpreted as having been generated by alluvial systems characterized by (i) high energy perennial braided river systems and (ii) ephemeral river systems. Aeolian sand dunes and sand sheets generated by the reworking of braided alluvial deposits can also occur. The paleocurrent measurements show a main dispersion pattern to S, SE and SW, and another to NE/E. These features imply a paleodrainage flowing into the basins of the Recôncavo-Tucano-Jatobá.

Research on meandering river reservoir deposition architecture and 3D modeling of the Gudao Oil field in the Bohai Bay Basin

The distribution of remaining oil is controlled by internal sand body heterogeneity in the middle and late stages of oil field development. Reservoir architecture analysis has been conducted in the third sand group in the Guantao Formation of the Gudao Oil field. Different scales of reservoir architecture units have been identified, including complex meandering river belt, single meandering river belt, single point bar and inner point bar. Four different single channel signatures have been found in the research area to identify the single channel belt. The single point bar has been distinguished based on point bar development characteristics. The architecture of the inner point bar has been analyzed based on subsurface well data and empirical formulas. The lateral accretion angle of the muddy layer ranges from 5° to 8°, the interspace between lateral accretion muddy layers ranges from 20 to 35 m, and the horizontal width of a single lateral accretion sand body is 55–120 m. Based on the above-mentioned reservoir architecture analysis, a reservoir architecture model has been established using the hierarchical modeling method. A three-dimensional porosity and permeability model has been developed within architecture model constraints and could provide a reliable geological template for oil field development and production.

A method for the division of the conglomerate depositional cycle under Milankovitch cycles

The conglomerate layer at the upper section of the 4th member of the Shahejie formation of the Yongan district at the Donying depression is a well-developed sedimentation of several periods. It lacks stable muddy layers and sophisticated classification of the sedimentation periods and the proportion of sedimentary layering in each period has long been a difficult task for geologists. In addressing this problem, this paper attempts to introduce the theory of climatic cycles driven by astronomical periods from astronomical stratigraphy on the basis of the characteristics of the sedimentation under the turbidity current in the region of study. Through studying the conditions for the formation of the conglomerate layer and the factors of control, we pinpoint the formation of the layer in chronology and differentiate the cycle interface and correlation in the same formation period. Milankovitch analysis is conducted on the sedimentation of the conglomerate layer in the region of study to determine if the stratigraphy cycle of the region is primarily controlled by the eccentricity cycle and calculate MSC1 and MSC2 thicknesses of 189.3 m and 78.05 m, respectively. Milankovitch theory is the primary tool used in the analysis, in conjunction with petrographic analysis. The stratum at the is classified into four IV-grade sequences and 11 V-grade sequences. The information on the dominant cycle frequency is used for wave filtering of the well logs and to determine the significant Milankovitch wave log. With the data from this curve, we may compare the stratigraphy cycle with the characteristics of the standard cycle and classify and compare the sedimentation periods of the conglomerate layers in further detail.

Sediment characteristics, floods, and heavy metal pollution recorded in an overbank core from the lower reaches of the Yangtze River

Overbank sediments contain abundant information about floods, paleoenvironmental changes, and recent pollution history of a river basin. For the purpose of understanding modern overbank sediments and their environmental significance, this multidisciplinary study sampled a core from the overbank and a short core from the overbank outcrop at Nanjing in the lower reaches of the Yangtze River. The overbank deposit comprises fine-grained discontinuous strata, including silt, sandy silt, silty sand, and sand layers. Field and laboratory observations indicated that the overbank sediment contained a series of layers of sandy silt coupled with thin silty sand layers, each a few millimeters thick, which formed muddy layers that were dozens of centimeters thick. Between the muddy layers were coarser sand layers. Grain size analyses, 137Cs dating, and heavy metal measurements contained information on flood events and pollution history over the past 60 years. The coarser sand layers corresponded with severe floods in 1954 and 1983. Between the two severe floods, the study area experienced an overbank building stage. From the 1990s to the early 2000s, the overbank sedimentation rate declined, reflecting human activities—especially the dams in the Yangtze River Basin that trapped more sediment. The average heavy metal concentrations in the overbank sediments are higher than the background levels for suspended sediment in the Yangtze River. Heavy metals in overbank sediments were assessed by enrichment factors and sediment quality assessment values for freshwater sediment. Cr and Ni values were high, especially from the early 1980s to the early 2000s. The impacts of floods, economic development, and environmental management on these heavy metal variations were discussed.

Multi-level stratigraphic heterogeneities in a Triassic shoal grainstone, Oman Mountains, Sultanate of Oman: Layer-cake or shingles?

ABSTRACT A fundamental question in the correlation of 1-D sedimentologic data is whether to use a layer-cake or shingled correlation approach. The resulting reservoir geometry has important implications for the characterization of reservoir heterogeneities and fluid flow. On the Saiq Plateau in Oman, epeiric carbonate ramp deposits of the Triassic Sudair Formation are well exposed and can be investigated in detail over several kilometers. There, reservoir heterogeneities on different scales have been documented by creating various outcrop wall panels and 2-D correlations. Multi-level architectural elements with different depositional geometries were discovered, which were linked to a sequence-stratigraphic hierarchy consisting of three levels. Level 1: A “layer-cake”-type stratigraphic architecture with minor thickness variations over several kilometers becomes apparent when correlating fourth-order cycle set boundaries. Level 2: The correlation of fifth-order cycle boundaries reflects horizontally continuous geometries, within which, however, internal grainstone layers were discovered to be arranged in a shingled fashion. Muddy layers in between these shingles illustrate sixth-order mini-cycle boundaries. Level 3: Within sixth-order mini-cycles another scale of a shingle-like architecture can be observed. Amalgamated cm-thick grainstone units form thin wedges with subtle but clearly inclined dipping geometry. Fourth-order cycle sets and fifth-order cycles can be traced over several kilometers, and therefore assumed to be related to allocyclic stratigraphic processes. The internal shingle geometries within fifth-order cycles are traceable over 100s of meters and presumably reflect an autocyclic lateral migration of a shoal complex. Cm-thick shingling grainstone wedges within sixth-order mini-cycles are interpreted as storm-related spill deposits. Their event-driven character is reflected by frequent amalgamation and reworking of the preceding deposits. The results of this study of epeiric carbonate ramp deposits suggest that a “layer-cake” correlation approach is appropriate when correlating 10s of m-thick grainstone units over a distance of several kilometers. However in the documented example, these thick grainstone units consist internally of small-scale architectural elements, which show inclined geometries and require a shingled correlation approach. These small-scale heterogeneities within an overall “layer-cake” architecture might have an impact on fluid flow in similar subsurface reservoirs and should be taken into account for detailed reservoir correlations and static reservoir models.

Distinction and grain-size characteristics of intertidal heterolithic deposits in the middle Qiantang Estuary (East China Sea)

The routine sampling procedure for grain-size analysis of intertidal heterolithic deposits runs a high risk of inadvertent mixing of two or more different sedimentation units, which would consequently complicate data interpretation. Traditionally, sedimentologists pay less attention to muddy layers due to a lack of internal structures, although the grain-size populations of such layers should encode more information on fine-mud flocculation processes than sandy layers. In this paper, individual muddy and sandy layers of nine short cores from the Da-Jian-Shan tidal flats of the middle Qiantang Estuary in the East China Sea, which experiences tidal bores, were sampled separately for grain-size analysis. A core taken at Huang-Jia-Yan from the lower estuary, not affected by tidal bores, served for comparison. A curve-fitting method was employed to decompose each grain-size distribution into two Gaussian populations. Cumulative plots indicate that intertidal sediments are mostly dispersed as intermittent and uniform suspension loads, traction loads being absent or very subordinate. This is conceivably linked to flows agitated by tidal bores, and to the highly dynamic nature of fine sand and coarse silt particles. Selective transport and deposition have produced three distinct sedimentation units, namely, tidal-bore deposits, tidal sandy deposits, and tidal muddy deposits. These can also be discriminated on bivariate plots of any two textural parameters. Shoreward attenuation of tidal flows is reflected in the gradual fining and thinning of sandy layers from lower-flat massive sands, through middle-flat hybrid deposits (alternations of massive sands and tidal rhythmites), to upper-flat tidal rhythmites. This gradient is also well represented in slightly decreasing (increasing) sorting and decreasing (increasing) proportions of the coarser (finer) hydraulic populations in the muddy layers. Although no corresponding trends are discernible in the hydraulic populations of the sandy layers, these can be distinguished on the basis of characteristic sedimentary structures. The floc limit and floc volume fraction, estimated from the modes and proportions of the finer hydraulic populations, are 8~10 μm (16 μm) and on average 41.73% (26.41%) for muddy (sandy) layers, respectively. The most plausible explanation is that the floc limit sensitively responds to subtle changes in the suspended sediment composition and the ambient hydraulic and hydrochemical settings. In comparison, the Huang-Jia-Yan core features blurred bedding and higher contents of fine mud and flocs, these being consistent with the weaker energy on the upper tidal flat of the lower estuary where neither tidal bores nor bigger waves occur.

Late Holocene climatic oscillations traced by clay mineral assemblages and other palaeoceanographic proxies in Ria de Vigo (NW Spain)

This work aims to study recent climatic oscillations and their influence on sedimentation in the Ria de Vigo, a coastal embayment in Galicia, NW Spain. It is based on the study of clay mineral assemblages, in conjunction with other proxies (granulometric, geochemical, geochronological and microfaunal), in the core KSGX 24. A Benthic Foraminifera High Productivity (BFHP) proxy was used to determine changes in the flux of organic matter (OM) at the bottom of the study area. Total organic carbon (TOC) content is not a suitable proxy to estimate changes in the past supply of OM due to diagenetic processes. The sedimentation was finest in 3 sections: ~ 230-214 cm, ~ 185-73 cm and ~ 20-0 cm. These muddy sections are characterised, in general, by higher proportions of detrital minerals, concentrations of several chemical elements related to lithogenic sources and BFHP values. In addition, these sections are impoverished in carbonates, Ca, Sr and La when compared with the layers with the highest sand content. The clay mineral assemblage of the studied site, characterised by the dominance of illite, intermediate concentrations of kaolinite and minor amounts of smectite and chlorite, reveals the prevalence of a typical temperate humid climate in the last 3 ka BP, the estimated age for the core base. However, the quantities of illite and chlorite increase in the muddy layers. The characteristics of these muddy layers were interpreted as representing relatively cold climatic oscillations associated with the strengthening of northerly winds and the prevalence of an upwelling regime corresponding to well-known periods, such as the first cold period of the Upper Holocene (~ 2.9 ka cal BP), the Dark Ages (between ~ 2.2 - 1.2 ka cal BP) and the Little Ice Age (~ 0.6 ka cal BP).

日本海東縁海底下に分布する砂層の孔隙特性と年代 : MD179航海

In 2010 the MD179 project was undertaken aiming at recovery of deep seated gas hydrate, methane induced carbonate, and deep sediments older than 300 ka. Sediment samples were obtained in the Umitaka Spur, Joetsu Channel, Toyama Trough, Japan Basin, Nishi Tsugaru and Okushiri Ridge areas. Small amounts of sandy sediment have been retrieved as thin intercalations in Pleistocene and Holocene muddy layers, where trace fossils and strong bioturbations are commonly observed.Those sandy sediments consist of very fine- to fine-grained sand grains, and are sometimes tuffaceous. Pore-size distribution measurements and thin-section observations of these sands were undertaken, which indicated that porosities of muddy sediments are around 50 % but those of arenites range from 42 to 52%. Mean pore sizes and permeabilities of those arenites are larger than those of mudstones. While the presence of gas hydrate in intergranular pores of sands has not been confirmed, the soupy occurrence in recovered sediments may strongly indicate the presence of hydrate filling the intergranular pore system of sands. Such arenites have been recognized till now in the Mallik, NW Territory Canada, as well as in the Nankai Trough areas, which are expected common even in the subsurface sandy sediments at the eastern margin of Japan Sea. Concentration of gas hydrate may need primary intergranular pores large enough for gas hydrate to occur within host sediments likely deposited in the sedimentary environment such as deep sea channels. Small amounts of sandy sediment were retrieved as thin intercalations in Pleistocene silty layers, because supplying sediments may not be abundant due to sea level fluctuation during Pleistocene ice age. As time of deposition of coarse-grained sediments can be recognized by the thermoluminescence (TL) dating method, sandy sediments are usually tuffaceous and contain a small amount of quartz grains, and TL dating has been completed only for seven samples, which indicate 48 to 980 ka in age. This study was performed as a part of the MH21 Research Consortium.

IMPROVING THE NAUTICAL ACCESS TO ZEEBRUGGE HARBOR: A MULTIDISCIPLINARY STUDY

A multidisciplinary study was set up by the Maritime Access Division of the Flemish Community to tackle the nautical problems faced by Zeebrugge Harbor. The problem is twofold: strong cross-currents at the harbor mouth hinder the entrance of the ships around high water, and the occurrence of thick muddy layers hinders the navigation in the harbor and causes very high dredging costs. . The article focuses on the global project set up, and explains more in detail the numerical and physical modeling research.

Earthquake-related Tectonic Deformation of Soft-sediments and Its Constraints on Basin Tectonic Evolution

: The authors introduced two kinds of newly found soft-sediment deformation-syn-sedimentary extension structure and syn-sedimentary compression structure, and discuss their origins and constraints on basin tectonic evolution. One representative of the syn-sedimentary extension structure is syn-sedimentary boudinage structure, while the typical example of the syn-sedimentary compression structure is compression sand pillows or compression wrinkles. The former shows NW-SE-trending contemporaneous extension events related to earthquakes in the rift basin near a famous Fe-Nb-REE deposit in northern China during the Early Paleozoic (or Mesoproterozoic as proposed by some researches), while the latter indicates NE-SW-trending contemporaneous compression activities related to earthquakes in the Middle Triassic in the Nanpanjiang remnant basin covering south Guizhou, northwestern Guangxi and eastern Yunnan in southwestern China. The syn-sedimentary boudinage structure was found in an earthquake slump block in the lower part of the Early Paleozoic Sailinhudong Group, 20 km to the southeast of Bayan Obo, Inner Mongolia, north of China. The slump block is composed of two kinds of very thin layers—pale-gray micrite (microcrystalline limestone) of 1–2 cm thick interbedded with gray muddy micrite layers with the similar thickness. Almost every thin muddy micrite layer was cut into imbricate blocks or boudins by abundant tiny contemporaneous faults, while the interbedded micrite remain in continuity. Boudins form as a response to layer-parallel extension (and/or layer-perpendicular flattening) of stiff layers enveloped top and bottom by mechanically soft layers. In this case, the imbricate blocks cut by the tiny contemporaneous faults are the result of abrupt horizontal extension of the crust in the SE-NW direction accompanied with earthquakes. Thus, the rock block is, in fact, a kind of seismites. The syn-sedimentary boudins indicate that there was at least a strong earthquake belt on the southeast side of the basin during the early stage of the Sailinhudong Group. This may be a good constraint on the tectonic evolution of the Bayan Obo area during the Early Paleozoic time. The syn-sedimentary compression structure was found in the Middle Triassic flysch in the Nanpanjiang Basin. The typical structures are compression sand pillows and compression wrinkles. Both of them were found on the bottoms of sand units and the top surface of the underlying mud units. In other words, the structures were found only in the interfaces between the graded sand layer and the underlying mud layer of the flysch. A deformation experiment with dough was conducted, showing that the tectonic deformation must have been instantaneous one accompanied by earthquakes. The compression sand pillows or wrinkles showed uniform directions along the bottoms of the sand layer in the flysch, revealing contemporaneous horizontal compression during the time between deposition and diagenesis of the related beds. The Nanpanjiang Basin was affected, in general, with SSW-NNE compression during the Middle Triassic, according to the syn-sedimentary compression structure. The two kinds of syn-sedimentary tectonic deformation also indicate that the related basins belong to a rift basin and a remnant basin, respectively, in the model of Wilson Cycle.

Structural architecture and active deformation of the Nankai Accretionary Prism, Japan: Submersible survey results from the Tenryu Submarine Canyon

Two research cruises that deployed submersible surveys were undertaken along the Tenryu Submarine Canyon to directly observe the structural architecture of the eastern Nankai Accretionary Prism off the coast of southern Japan. The surveys have demonstrated that the accreted sediments are strongly deformed turbidite sequences that occur in repeated thrust-anticline structures. From the leading edge of the prism near the trench toward the arc, the following deformation zones have been identified within the prism: Frontal Thrust zone, Prism Toe zone, Imbricate Thrust zone, and Tokai Thrust zone (or out-of-sequence thrust or OOST zone). The Frontal Thrust zone is characterized by debris deposits within the hanging wall that have an age of 0–0.43 Ma, as determined from radiolarian biostratigraphy. The Prism Toe zone is characterized by unconsolidated turbidite sequences that are 1.98–3.4 Ma; these sequences are cut by normal and thrust faults. The Imbricate Thrust zone includes consolidated muddy layers and unconsolidated sandy layers that contain numerous fracture cleavages. The OOST zone consists of highly deformed consolidated sediments, ranging in age from 0.18 to 1.03 Ma. From the Prism Toe zone to the Imbricate Thrust zone, the uniaxial compressive strength increases gradually from 0.5–3.0 to 1.0–6.0 MPa, while the anisotropy of magnetic susceptibility changes from oblate to prolate shapes, and porosity decreases from 40%–50% to 30%–50%. These data indicate that the eastern Nankai Accretionary Prism appears to have been deformed toward the Imbricate Thrust zone just south of the OOST. Stable isotope analyses of calcite veins and calcite cement of the sandstone samples from the Tokai Thrust zone have shown that fluid temperatures for calcite precipitation were 24–63 °C in the OOST zone. The occurrence of highly deformed and consolidated rocks within the Nankai Accretionary Prism likely resulted from tectonic transportation of deeply buried rocks along major out-of-sequence thrust faults, such as the Tokai OOST. We infer therefore that out-of-sequence thrust faults play a major role in transporting deeply buried, deformed rocks in accretionary prisms to the shallower depths and even to the seafloor during ongoing subduction.

Slope failures and stability analysis of shallow water prodeltas in the active margins of Western Greece, northeastern Mediterranean Sea

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10–15 m below seafloor. Shear planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the steeper (2°–6°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone. The foresets of the HST prodelta deposits display high water content (30–80%), low bulk density (1.4–1.9 g cm−3) and relatively low values of undrained shear strength (3–20 kPa). The water content of the HST distal muddy bottomsets is relatively higher (50–110%) and bulk density relatively lower (1.3–1.7 g cm−3) with low values of shear strength (2–10 kPa). The shear strength of the gas releasing sediment layer displays lower values (2–9 kPa) relative to the overlying, post failure, muddy sediments of the late 100–300 years. Slope stability was calculated using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments. This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6–29.6% g for the HST foresets and 12.4–14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs) for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments of the lower part of the HST foresets to the upper part of early HST bottomsets.