Biogenic Sand Research Articles

DEM-DFM modeling of suffusion in calcareous sands considering the effect of double-porosity

Calcareous sand is a kind of marine biogenic sand formed by coral debris and marine organism skeletal bodies. One of the most important features of calcareous sand lies in remarkable void space within the particles (intra-particle voids), which, combined with the voids among particles, produces the double-porosity structure. The double-porosity structure enables both inter-particle and intra-particle flows in calcareous sand during a seepage process, which influences the migration of fine particles. This study presents a numerical investigation of internal erosion in calcareous sand with the double-porosity structure by considering both inter-particle and intra-particle flows. A gap-graded sand specimen is generated using the discrete element method (DEM), in which the coarse particles are assumed as calcareous sand and contain internal porosity. The dynamic fluid mesh (DFM) is improved with a double-porosity algorithm to consider both inter-particle and intra-particle flows. The solid–fluid interaction is simulated by coupling DEM and DFM. With this method, seepage erosion in calcareous sand is simulated, and the effect of double-porosity is revealed. Results show that the intra-particle porosity increases flow velocity and thus increases the percentage of erosion mass and the flow velocity. In addition, the intra-particle flow velocity is not affected by the fines content.

Comparison of 2D Optical Imaging and 3D Microtomography Shape Measurements of a Coastal Bioclastic Calcareous Sand.

This article compares measurements of particle shape parameters from three-dimensional (3D) X-ray micro-computed tomography (μCT) and two-dimensional (2D) dynamic image analysis (DIA) from the optical microscopy of a coastal bioclastic calcareous sand from Western Australia. This biogenic sand from a high energy environment consists largely of the shells and tests of marine organisms and their clasts. A significant difference was observed between the two imaging techniques for measurements of aspect ratio, convexity, and sphericity. Measured values of aspect ratio, sphericity, and convexity are larger in 2D than in 3D. Correlation analysis indicates that sphericity is correlated with convexity in both 2D and 3D. These results are attributed to inherent limitations of DIA when applied to platy sand grains and to the shape being, in part, dependent on the biology of the grain rather than a purely random clastic process, like typical siliceous sands. The statistical data has also been fitted to Johnson Bounded Distribution for the ease of future use. Overall, this research demonstrates the need for high-quality 3D microscopy when conducting a micromechanical analysis of biogenic calcareous sands.

Late‐Quaternary miliolite (biogenic carbonate) deposits and their implications for sea‐level fluctuations and climatic variability

AbstractThe western coast of India (Kachchh) has ubiquitous preservations of Quaternary carbonate deposits popularly known as “miliolites”. Field‐based detail documentation of the nature and distribution of miliolites supported by sedimentology and scanning electron microscopy (SEM) microtextural studies indicates that the miliolites were primarily deposited by wind as aeolinites and are termed “primary miliolites (PM)”. These were subsequently fluvially eroded and deposited and such deposits are termed “secondary miliolites (SM)”. The PM are dated between 28 ka to 16 ka largely clustering around the Last Glacial Maxima (LGM). In comparison, the fluvially reworked SM occurs as valley‐fill deposits and were deposited episodically during 23 ka to 10 ka before present. Climatically, the deposition of PMs suggests enhanced aridity whereas SMs represent episodic strengthening of the Indian Summer Monsoon (ISM). The bedding attitudes of the PMs indicate that they were deposited predominantly by the northeasterly winds whereas the presence of the deflation lag deposits suggests downwind migration of sands during the equilibrium condition (reduced sediment supply). The aeolian parameters (based on the grain size and wind velocities) for PMs indicate high shear velocity (~23 to 48 cm−1) and high transport rate (< 0.1 to 0.2 g/cm s). The short average flying distance implies that the biogenic sands were transported in multiple pulses (from coast to inland) at rates varying from ~200 m/h to ~1 km/h. Comparing our data with related occurrences along the biologically productive coasts in the mid‐latitudinal belt indicates that the regional/global aridity during the lowered sea level was responsible for inland occurrences of aeolinites, particularly during the LGM. The strengthened northeast monsoon winds during the LGM was conducive to upwelling and production of biogenic carbonates along the coasts. The subsequent reworking of the aeolinites were mainly the results of local variability in precipitation.

РЕЗУЛЬТАТЫ ПЕТРОГРАФИЧЕСКОГО АНАЛИЗА КЕРАМИКИ ПОСЕЛЕНИЯ МЕШОКО

The article presents the results of petrographic analysis of the ceramics of the Chalcolithic settlement of Meshoko. A total of 42 fragments were examined (see appendix), 10 of which belong to the upper part of the monument's sediments (layers 1 and 2a; Fig. 1), 20 refer to the middle part of the sediments (layer 2b; Fig. 2), 12 – to the lower part (layer 3; Fig. 3). Based on the analysis, 5 groups of ceramics were identified (Fig. 4). Group 1 consists of fragments with an admixture of limestone, group 2 – with an admixture of diorite, group 3 – with an admixture of biogenic carbonates and sand, group 4 – with an admixture of calcite, group 5 – with an admixture of diorite and chamotte. Clays of smectite composition predominate in groups 1 and 2, while clays of smectite-carbonate composition predominate in groups 3 and 4. Comparison of these groups with stratigraphy revealed that most of the ceramics of groups 3 and 4 are confined to the lower layer, and groups 1 and 2 to the middle and upper layer (Table 1). In addition, the ceramics of these layers differ in the nature of the external surface treatment. Significant changes in the technology of making ceramics during the transition from the lower layer to the middle layer allow us to assume corresponding changes in the composition of the population.

Late Pleistocene sediment accumulation in the lower slope off the Rio Grande terrace, southern Brazilian Continental Margin

Here are described the analyses of sediments from the lower slope of the Pelotas Basin in the southern Brazilian continental margin, southwestern Atlantic. The samples were obtained from the core SIS-249, collected adjacent to the Rio Grande Terrace (RGT) at a water depth of 2091 m, an area poorly known in terms of sedimentary characteristics compared to the oil and gas-rich basins to the north. The age model indicates that the sampled sediments were accumulated during the last glacial period, between 112.5 and 34.47 ka (MIS 5 to MIS 3). The fine (<63 μm) fraction is dominated by silt with subordinate clay, whereas the coarse (sandy) fraction is chiefly composed of biogenic carbonate (tests of foraminifers) with subordinate very fine (63–125 μm) lithogenic sand. The composition of the lithogenic sand is essentially uniform downcore, dominated by quartz with glaucony as the dominant accessory mineral, and the presence of augite, derived from Mesozoic volcanic rocks, points to a source from the adjacent coastal plain. The uniform mineral composition indicates continuous input of sediments removed from the RGT by action of the Brazil Current (BC) and transferred to the lower slope by gravity flows. The higher proportion of lithogenic sand from the upper RGT along the intervals corresponding to MIS 5 and MIS 3 points to erosion of the terrace due to a more landward position of the BC, related to sea-level highstand and falling stage tracts. The higher amount of silt along the MIS 4 interval suggests increased transfer of sediments from the flanks of the RGT as the result of a more downslope position of the BC driven by the sea-level lowstand. Winnowing of the sortable silt fraction during MIS 5 and 3 points to influence of the southward-flowing North Atlantic Deep Water (NADW). The higher estimated accumulation rates and less negative peaks of δ13C in benthic foraminifers Uvigerina peregrina during MIS 4 could have been caused by oscillations of water masses with distinct isotopic compositions, but also by increased burial and decomposition of organic matter as a result of high ocean surface primary productivity, indicated by the correspondence between the δ13C oscillations and higher accumulation rates and proportion of biogenic sand in the core. Higher productivity was apparently driven by episodes of increased eolian transport of dust from Patagonia and central-western Argentina in milenial timescales, coeval with oscillations in strength and position of the southern westerly winds (SWW) system.

Temporal variability of flooding events of Guadiana River (Iberian Peninsula) during the middle to late Holocene: Imprints in the shallow-marine sediment record

Sedimentological, geochemical and benthic foraminiferal proxies were used to interpret changes of depositional environments in a mud entrapment inside the Guadiana River paleo-valley, northern Gulf of Cadiz, to understand the temporal variability of fluvial flooding events and to detect patterns of latitudinal climatic variability in western Iberia.The period between ca. 5800 and ca. 1250 cal yr BP was characterized by slowly accumulating coarse-grained sediments, high content of biogenic sand components, and high abundances of shallow-water benthic foraminiferal species. After ca. 1250 cal yr BP, the sedimentary environment was dominated by fine-grained sediments, with high abundances of opportunistic benthic foraminiferal species and successful colonizers, and high values of geochemical ratios indicative of enhanced terrigenous supply. Sedimentation rates increased drastically over the last ca. 500 years.The genesis of these environments was mainly driven by the variable frequency of fluvial flooding events driven by the regional climate variability. Low terrigenous sediment input under dry conditions prevailed in the older sedimentary unit. Flood frequency and terrigenous sediment supply increased during the younger unit. The variability of the North Atlantic Oscillation at centennial scales highly influenced the sediment supply during the last ca. 500 years, with high terrigenous supply during negative NAO conditions. Our results corroborate an N-S gradient along the Atlantic Iberia during middle and late Holocene, with more humid conditions in the northwestern and drier conditions towards the southeast. The gradient weakened over the past ca. 500 years, with increased rainfalls and flood events during the Little Ice Age.

On the Problem of Coastal Protection of the Varadero Resort (Cuba) with an Artificial Beach

The recent coastal dynamics of Varadero resort located on the Hicacos Peninsula (northern Cuba coast) demonstrate a trend of progressive beach erosion and degradation. This paper proposes an approach to coastal protection based on creating an artificial beach that should withstand extreme storm impact. The parameters for protecting a beach composed of biogenic sand (the most available material in the region of interest) are determined from the model [4, 8]. It is argued that material coarser than 0.4 mm can be used to build the beach in the central part of the Hicacos Peninsula. The maximum elevation of the beach should exceed 3.3 m. For an expected coastal advance 10, 20, or 30 m, sand in a volume of about 100, 180, or 250 m3 m–1 should be required. In the root part of the Hicacos Peninsula, where coastal slope is rather steep, construction of artificial beach is not economically practical, so another manner of coastal protection should be selected.

Oceanographic conditions and sediment dynamic of the Barrang Caddi Island (Spermonde Archipelago, Indonesia)

Small islands are vulnerable to long-term natural disasters like coastal erosion due to their size and topography. Barrang Caddi is one the small island in the Spermonde Archipelago (South Sulawesi) that encountered serious coastal erosion. Several attempts have been done by the relevant parties like by building a wave breaker to prevent erosion. But in fact some parts of the island are still eroded. A comprehensive oceanographic study of the wave climate and coastal processes at work to delineate the factors responsible for shoreline chance and to identify the location that need protection is needed. In this study, physical oceanographic data including waves, currents, tide, bathymetry, sediment characteristics and sediment transport were collected in the Barrang Caddi Island to analyze the factors responsible for shoreline chance (erosion) in the island. Results of the study showed that tide in the study site is mixed tide, predominantly semidiurnal with tidal range of 118 cm. Current measurements using a electromagnetic current meter revealed that current velocities at the study site were relatively low and vary spatially and temporally with magnitude of 0.02 – 0.58 m/s. Under normal conditions (no storms) the significant wave height (H 1/3) varied from 0.04 to 0.20 m. The wave height decreases from the fore reef to the reef flat due to the presence of coral reefs that reduce wave energy (wave height). Sediments were dominated by biogenic sand with grain diameter of 0.38 – 1.04 mm. Island erosion analysis showed that wave action was a main factor that responsible for shoreline chance (erosion) at the island. Current velocity alone with average of 0.19 m/s was not strong enough to move (erode) sediments at the island.

Sedimentology of Surficial Cool­water Carbonates on the Continental Shelf of Tasmania, Australia

The continental shelf of Tasmania and Bass Strait is veneered by surficial carbonate and terriginous sediment, which represents the coldest carbonate assemblage on the southern shelf of Australia. Three major facies are recognized based on dominant components, and are further separated into eight sub­facies. Quartzose sand is a nearshore facies, whereas Holocene biogenic sand and gravel is found outboard in deeper waters. Relict sand and gravel is located between these facies on the majority of the shelf, although it is rare on the narrow, eastern shelf. When Holocene biotic sediments are considered, five facies can be identified based on the abundance of bryozoans, molluscs, benthic foraminifer, sponge spicules, and coralline algae. Molluscs and bryozoans dominate the most widespread facies; molluscs are numerous nearshore, in large bays, and on granitic ridges, and bryozoans are abundant in deeper, or nutrient rich waters. Again, eastern and western shelves differ, in that molluscs, which are common in the east, only occur near rivers on the western shelf. The key factor controlling this distribution is the level of trophic resources available. Dominant bryozoan growth forms provide detailed resolution of the Holocene sediment and relative importance can be linked to water energy or substrate. In general, delicate branching forms are found in deeper water on the eastern and western shelves, as well as the eastern side of the Bass Strait, whereas articulated zooidal forms dominate higher energy waters and areas with unstable or fine substrate, such as the southern shelf and western Bass Strait.

A Brissus genus (Echinoidea) első előfordulása a magyarországi badeniből és a Brissus mihalyi n. sp. leírása

A budapesti Örs vezér téri üzletközpontok alapozása során nagy számban fordultak elő tengeri sünök is a badeni korú Lajtai Mészkő Formáció rétegeiben. Az Árkád 1 alapgödréből előkerült egy Magyarországról eddig nem ismert tengeri sün nemzetség, a Brissus. &#x0D; A genust eddig a Paratethys térségéből csak burgenlandi és nyugat-ukrajnai lelőhelyekről írták le. A Budapesten megtalált mindkét Brissus egyed jó megtartású. Az egyik példányt a Brissus abeli (Reidl, 1941) fajba lehetett besorolni, ami csak a Paratethys késő-badeni korú üledékeiből ismert. Ezen felül előkerült egy az egész európai-mediterrán térségben eddig ismeretlen Brissus forma, amely morfológiai bélyegeit tekintve egyik európai vagy egyéb mediterrán formával sem egyezik meg. Emellett hordoz sajátságos morfológiai elemeket is, úgy, mint az erősen anterior helyzetű apicalis lemez, az anterior szirompár által bezárt szög mértéke, valamint a periproct elhelyezkedése. Mindezek alapján a forma új fajként való elkülönítése indokolt, melynek neve – Mihály Sándor tiszteletére – Brissus mihalyi nov. sp.&#x0D; Maga a Brissus genus ma széles földrajzi elterjedésű, de kis egyedszámú; a ma élő együtteseket részletesen nem vizsgálták. Az európai neogénben ritka, így a magyarországi megjelenés a forma eocéntől máig zajló evolúciójának megismeréséhez is fontos információt nyújt.

A note on influence of stress anisotropy on the Poisson's ratio of dry sand

In this study, extender and bender element tests were conducted investigating the small-strain Poisson's ratio of variable sands, with a focus on the effect of stress anisotropy in order to quantify the sensitivity of Poisson's ratio to the applied deviatoric stress. Four different uniform sands were tested, including a biogenic sand, a crushed rock and two natural sands, covering a wide range of particle shapes. From these sands, eleven samples were prepared in the laboratory and were tested under variable stress paths, maintaining a constant mean effective pressure while increasing the deviatoric compressive load. Under the application of these given stress paths, the data analysis indicated that the sensitivity of Poisson's ratio to the stress ratio was more pronounced for sands with irregularly shaped particles in comparison to sands with fairly rounded and spherical grains. For sands with very irregularly shaped particles, the increase of Poisson's ratio from the isotropic to the anisotropic stress state reached 50%, while this increase for natural sands with fairly rounded particles was in the order of 20%.

Dynamic characterization of a biogenic sand with a resonant column of fixed-partly fixed boundary conditions

The dynamic properties of sands, including the small-strain shear modulus (Gmax) and the small-to-medium strain shear modulus (G) and shear damping (Ds) have been examined extensively in the literature. However, most works published over the past decades have focused on the behavior of soils subjected to isotropic stress paths. In the present study, the dynamic properties of a biogenic sand with origin from Western Australia were examined in the laboratory using a resonant column of fixed-partly fixed boundary conditions. This configuration allows the study of the small-strain shear modulus, the strain-dependent modulus and shear damping of samples subjected to stress anisotropy, which stress conditions may represent more effectively the in-situ state of a soil. For this purpose, proper calibrations of the resonant column were carried out in order to capture the rotational inertia of the apparatus in a wide range of cyclic frequencies covering, in this way, typical resonant frequencies of real soil samples. Basic geotechnical characterization of the biogenic sand was carried out quantifying the shape of the grains and conducting a series of monotonic triaxial tests capturing the peak and critical state friction angles. The dynamic test results indicated a more pronounced effect of the stress anisotropy on Gmax of the biogenic sand in comparison to the corresponding effect on quartz sands reported in the literature. For the limited set of experiments conducted in the study, the effect of stress anisotropy was found less pronounced for the small-to-medium strain shear modulus and shear damping, even though the results indicated a marked drop of shear damping at small strains when the samples were subjected to a deviatoric compressive load.

Post-Cyclic Behavior of Carbonate Sand of the Northern Coast of the Persian Gulf

The post-cyclic behavior of biogenic carbonate sand was evaluated using cyclic triaxial testing through a stress control method under different confining pressures between 50 to 600 kPa. The testing program included a series of isotropically and anisotropically consolidated, undrained triaxial compression and extension tests on samples of remolded calcareous Bushehr sand. Grading analyses (before and after each test) were used to examine the influence of particle breakage on post-cyclic behavior of Bushehr sand. The particle breakage commonly occurred in these soils even in lower values of confining pressure, yet there was not a clear correlation between the post-cyclic responses and particle breakage. Based on the present study, a concept is suggested for post-cyclic behavior of carbonate sand. It was observed that post-cyclic strength has a good correlation with cyclic stress ratio, type of consolidation, and value of residual cyclic strain. For all specimens, it is clear that the post-cyclic strength is greater than monotonic strength, irrespective of confining pressure and relative density.

Geochemical maturity of pocket beach sands from the San’in region of southwest Japan

Major and trace elements of ninety-two coastal beach sands samples, collected along the San’in region of southwest Japan, were determined by X-ray fluorescence to define their geochemical characteristics. These included Yamaguchi (n=27), Shimane (n=50), and Tottori (n=15) beach sands samples. Data of beach sands from Shimane and Tottori normalised averages show approaching patterns relative to the upper crust of the Japan arc UCJAN, and upper continental crust UCCN, reflecting the composition of the source rocks that dominate their watersheds. In contrast the normalised patterns for most elements of Yamaguchi beach sands samples are less than 1 except for CaO, Sr, and Th. The primary component of beach sands from Shimane is quartz, or silica (SiO2), Sands from Tottori are composed essentially of weathered particles of feldspar, and in contrast components of biogenic and quartz-rich sands from Yamaguchi are primarily shell fragments, quartz, and igneous rock.

Infaunal zoogeography and intergeneric character blending: The case of Metaniphargus shiroi sp. nov. (Crustacea: Amphipoda: Hadziidae), from interstitial beach water on Akajima Island, the Kerama Islands, Southwestern Japan.

A survey of biogenic coralline sands in the littoral fringe of a tropical island in Japan brought a new amphipod species to light. This species represents the first record of the subterranean genus Metaniphargus from the West Pacific. The majority of the species in this genus occur in the Caribbean, but a report from Hawaii and now from Japan defies the endemic Caribbean status it kept for so long. Metaniphargus shiroi sp. nov. is described, and morphological comparisons are made with closely resembling species from Hawaii and the Cayman Islands (genus Metaniphargus), and the Great Barrier Reef and California (genus Dulzura). Involvement of non-congeners in the comparisons is necessary as character overlap is abundant. These comparisons suggest that the presence of form-related body types in the shallow marine interstitial realm is circumtropical and follows habitat suitability rather than sudden dispersal or vicariance events.

La secuencia de Venado, un estuario lagunar tropical del Miocemo Medio, San Carlos, Costa Rica

The Middle Miocene sedimentary sequence of Venado represents a semienclosed estuary with an external bioclastic sand bar, open to the Caribbean Sea. The sequence is constituted by approximately 100m of sediments which correspond to two depositional systems: a) lagoon or back barrier and b) carbonate sand bar. The first system is composed of tuffites, coquinas, fossiliferous sandstones and lenticular bioclastic carbonate. The second system is constituted of bioclastic sand bar and tidal channel deposits. The biogenic sands are composed of molusc and balanid debris. Miorofacies criteria indicate interlittoral to sublittoral environments. Diagenetic criteria point to a temporal emersion. The paleocurrents directions of the tidal channels give transport directions mainly towards the southeast. Thus, bioclastic sand barrier had a NE-SW orientation. This Atlantic-type epicontinental tropical estuary was dominated by heavy fluvial volcaniclastic input. Freshwater influence is reflected by the paleoecology of the micro-fauna.

Sediment characteristics and molluscan fossils of the Farasan Islands shorelines, southern Red Sea, Saudi Arabia

The Farasan Islands are located about 40 km offshore of Jizan area, southern Red Sea, Saudi Arabia. This is a preliminary study aiming to collect shoreline sediments from 17 stations to describe their main lithologic characters and identify the dominant molluscs that could help to evaluate the environmental conditions of these islands. Also, a sediment core FARA2 was collected from the intertidal area of Khor As Sailah Lagoon to see the development of shoreline sediments in this area with time. The results indicated that the shoreline sediments of the Farasan Islands comprised mainly of white, biogenic sand enriched in remains of corals, molluscs, calcareous algae, and benthic foraminifera. Thirty-four mollusc species were recorded from these sediments. Among them, Strombus fasciatus, Strombus tricornis, Chicoreus ramosus, Chicoreus virgineus, Tridacna maxima, and Tridacna squamosa were the most common molluscan in/near the shoreline sediments of Farasan Islands. Frequency abundances of sediment grain-size fractions in the core FARA2 indicated that the shoreline of the Khor As Sailah Lagoon is developing towards shallow intertidal setting due to high production of biogenic sediments by reefal communities. Metal concentrations (Fe, Mn, Cu, and Zn) in the sediment of the core FARA2 were stable throughout, showing no remarkable changes. This finding could indicate that flux of detrital components by fresh water runoff or wind-blown dust over the Farasan Islands was un-detectable during the deposition of these sediments.

Provenance-related characteristics of beach sediments around the island of Menorca, Balearic Islands (western Mediterranean)

The island of Menorca, one of the Balearic Islands (Spain) located in the western Mediterranean, is characterised by a contrasting geology and landscape with two major geographic domains: (1) a southern region called Migjorn, comprised of Late Miocene calcarenites and limestones, and (2) a northern region known as Tramuntana, which is composed of folded and faulted Palaeozoic, Mesozoic and Tertiary (Oligocene) siliceous and calcareous rocks. Both domains are lined by numerous pocket beaches exhibiting a high variety of surficial sediment assemblages. Grain-size and compositional analyses revealed that cliff erosion and nearshore Posidonia oceanica meadows are the main sources of sediments consisting mostly of medium- to coarse-grained carbonate sands of marine biogenic origin, with variable amounts of terrigenous rock fragments and quartz. Based on distinctly different contributions of bioclastic material, biogenic carbonates and quartz, 320 sediment samples from 64 beaches were grouped into different facies associations dominated by either (1) biogenic sands, (2) biogenic sands with terrigenous contributions or (3) terrigenous sands with quartz. Nevertheless, there is a marked regional variability in sediment texture and composition. Thus, variable mixtures of carbonate and siliciclastic sediments characterise the beaches of the northern region, whereas the beaches of the southern region are composed mostly of carbonate sands of marine biogenic origin. An exception is the central sector of the south coast, which is enriched in quartz sand (~10 %); this can be related to outcrops of quartz-rich basement rock and also to rocks exposed in some northern drainage basins captured by southern streams since the Plio-Quaternary.

Lithology Dependence of Porosity In Slope and Deep Marine Sediments

Abstract Compilation of porosity data from 11 Ocean Drilling Program legs and 53 sites show that compaction in deep marine to upper-slope sediments is, at least in part, a function of both burial depth and lithology. Overall porosities range from 4 to 94%. Despite the broad range of porosities, there is an exponential trend toward decreasing porosity with depth. Comparison among samples dominated (90 to 100%) by each of six lithologies, based on grain size and composition, reveals the lithology dependence of compaction, except for biogenic silt-size sediments and clastic sands. In the former case, this may, in part, be due to mixing of calcareous and siliceous nannofossils. Clay-size clastic sediments show a clear change from rapid porosity reduction in the top 172 meters (porosity = 82.7 e−depth/430) to less pronounced loss of porosity below (porosity = 61.4 e−depth/1671). The remainder of the lithologies, silt (porosity = 75.5 e−depth/1091), biogenic sand (porosity = 88.5 e−depth/1338) and micrite (porosity = 69.5 e−depth/1235) show a good to strong exponential reduction of porosity with depth. These relations compare well with literature studies, taking into account the less pure nature of the lithologies used in this work. The silt compaction curve is unique; no previous studies of porosity separated silt from other grain sizes. Most sediment samples are of mixed grain sizes and types. The lithology-based porosity-depth relations were used to test four approaches to decompaction in mixed-lithology sediment used in backstripping. All methods generated porosity estimates that correlated strongly to the observed values, with correlation coefficients ranging from 0.629 to 0.686. These methods that have been used for years are, for the first time, shown to be valid.

Preliminary study on the aspects of the biology of snakehead fish parachanna obscura (günther) in a Nigerian wetland

Between October 2000 and March 2001, a preliminary study on the aspects of the biology of Parachanna obscura, was carried out. Sex ratio revealed female preponderance (÷2 = 4.733, df = 1, P range 9.0-18.0 cm comprises ten food items. The items were re-classified into seven major groups consisting of detritus [coarse and fine detritus], fish [juvenile and remains of fish], insects (un-identified adult and remains of insects, macrophyte materials, mud, nematode, and biogenic sand. Fish, insects and nematodes were eaten as primary food items. There was a complete ingestion of the array of the dietaries by both sexes of Parachanna obscura. Exception of one item (coarse detritus), during the wet season, Parachanna obscura consumed all the dietaries during the seasons. Nevertheless, the later season increased in the ingestion of nine out of 10 items in conformity with optimal foraging theory. Parachanna obscura is considered a piscivore-insectivore-invertivore in feeding habits. In the wake of doubt about the viability and prospects of aquaculture, farming of P. obscura could be desirable. This species possesses a number of positive attributes when it comes to aquaculture. It is likely that efforts to culture this hardy carnivorous and fast-growing fish will spread in the coming years in Nigeria, if breeders and fish farmers will give it a chance. Further studies are, however, required to delineate the full biology of P. obscura in order to highlight its viability for domestication and cultivability. P. obscura is widely enjoyed by natives of its natural habitats all over the world as a source of protein.