Graphic Standard Deviation Research Articles

Analysis of size, shape and textural maturity of sands from the Kaligandaki River

The Gandaki River System is one of the major river basin of the Himalaya originating from China and flows through Nepal and eventually mixes with the Ganga River in India. The study aims at analyzing the textural properties (size and shape) and maturity of the Kaligandaki River sands, and their downstream variation. Sand samples were collected along the river from the Tethys Himalayan region up to the Indo-Gangetic Plain of Nepal. The sands are classified as slightly muddy sand, in which mean and median sizes and sorting increase with respect to the distance downstream from the provenance. The results indicate that coarsely skewed and platy to mesokurtic sands become nearly symmetrical to finely skewed and leptokurtic suggesting increase in fine-grained sands and enhancement of sorting downstream. The percent of fines (mud) ranges from 0 to 5% and shows no distinct downstream variation trend. Mean roundness of quartz is generally sub-angular (2 to 3 p), and downstream variation does not reflect change in roundness category. However, there occurs slight diminish of roundness with distance downstream. Inclusive graphic standard deviation (Cj) varies from 0.30 to 1.38j. After 100 km downstream, Gj decreases reflecting improvement of sorting. Sediments are immature to mature, and are sub-mature in upstream and midstream segments except for few sites where they are immature not due to flux, but probably due to local contribution of fines. Due to increased downstream distance, sediments tend to be more mature owing to enhancement of sorting

Sedimentological study of Bima sandstone in Dumne and Environs, Yola Sub-basin, Upper Benue Trough, Northeastern Nigeria.

Field mapping was carried out on Bima sandstone from Dumne and its environs with the aim to understand the geology and determine the paleoenvironment of deposition. The integrated (outcrop study with granulometric and pebble morphometric) studies were used to achieve this aim. Based on outcrop studies, nine (9) lithofacies were identified from the proximal alluvial fan facies to the mid-fan braided river: massive, matrix-supported, gravel-dominated facies (Gmm); matrix-supported gravel (Gmg); clast-supported gravel stratified (Gh); gravel stratified trough cross-bedded sandstone (Gt); trough cross-bedded sandstone (St); planar cross-bedded sandstone (Sp); ripple cross-laminated sandstone (Sr); massive sandstone (Sm) and silt and mud (Fsm). The outcrop sampled materials were subjected to granulometric and morphometric analyses. The result of the granulometric shows that the Bima sandstones of the study area have an average graphic mean (Mz) of 0.82 (coarse grains), an inclusive graphic standard deviation (δi) of 1.33 (poorly sorted), a graphic kurtosis (K) of 1.11 (leptokurtic), and an inclusive graphic skewness (Ski) of 0.17 (positive skewed). The result of the pebble morphometrics shows that the grains have an average elongation ratio (ER) of 0.80, coefficient of flatness (FI) of 60.6%, oblate–prolate index (OPI) of 0.35, maximum projection sphericity index (MPSI) of 0.77 and roundness (ρ) of 0.43. The roundness versus elongation ratio plot indicates a fluvial to transitional environment. Several bivariate and ternary plots that include the granulometric and pebble morphometric data suggest a river environment for the Bima sandstone in the studied area.

Textural Analysis of Windblown Sand as an Indicator of Susceptibility of Wind Erosion in Atbara Area, River Nile State, Sudan

Textural Analysis of Windblown Sand (TAWS) is essential for understanding the characteristics of wind sediment and behaviors of grain movement particularly in the climatically frame of wind erosion. This paper aimed to generate preliminary quantitative data on TAWS by estimating grain size distribution and some textural parameters e.g. median diameter (M50), mean grain size (Mz), graphic standard deviation (σ1) inclusive graphic skewness (Ski) and transgraphic kutosis (KG) in three selected sites namely, north, south east and south Atbara in river Nile state. Coefficient variations of the (M50) and (Mz) results exhibited a colossal similarity with minor differences between them accounted about 1.12%. In spite of the high variability of (σ1) between three sites (30%) but the data can be characterized as very well sorted (< 35), so the study area can be described as highly susceptible to wind erosion, the susceptibility decreased in the following order: north Atbara > south east Atbara> south Atbara. Regarding to (Ski) results the three sites can be described as positively skewed (north and south east Atbara) and very positively skewed (south Atbara). The (KG) data showed leptokurtic (north and south Atbara) and mesokurtic (south east Atbara). The dominant mean grain sizes for sand samples for north Atbara were fine sand 34% and medium sand 50%. Whereas the samples of south east Atbara distributed fine sand 34%, medium sand 45% and courses and 5%. South Atbara gave 25%, 38% and 12% for fine, medium and courses sand respectively.

Principal component analysis of textural characteristics of fluvio-lacustrine sandstones and controlling factors of sandstone textures

AbstractTextures are important features of sandstones; however, their controlling factors are not fully understood. We present a detailed textural analysis of fluvio-lacustrine sandstones and discuss the influences of provenance and depositional environments on sandstone textures. The upper Permian – lowermost Triassic Wutonggou sandstones in the Bogda Mountains, NW China, are the focus of this study. Sandstone thin-sections were studied by point counting and their textures were analysed using statistical and principal component analysis. Fluvial lithic, fluvial feldspathic, deltaic lithic, deltaic feldspathic, littoral lithic and littoral feldspathic sandstone were classified and compared. These comparisons indicate that lithic and feldspathic sandstones from the same depositional settings have significant differences in graphic mean, graphic standard deviation and roundness; in contrast, sandstones from different depositional settings but with similar compositions have limited differences in textures. Moreover, three principal components (PCs) are recognized to explain 75% of the total variance, of which the first principal component (PC1) can explain 44%. In bivariate plots of the PCs, sandstones can be distinguished by composition where lithic and feldspathic sandstones are placed in different fields of the plots along the axis of PC1. However, sandstones from different depositional settings overlap and show no clear division. These results indicate that provenance, mainly the source lithology, is the most significant controlling factor on sandstone texture, whereas the depositional environment has limited influence. This study improves our understanding of textural characteristics of fluvio-lacustrine sandstones and their controlling factors, and shows the potentiality of principal component analysis in sandstone studies.

Determination of the Depositional Environment of Outcrop Section at Odoro Ikpe South Eastern Nigeria Using Pebble Morphometry

The depositional Environment of Odoro -Ikpe, South East Nigeria was interpreted using pebble morphometry and sieve analysis. A field observation, sieve analysis and pebble morphometric analysis was carried out on the area which comprises of conglomerate, pebbles, sand stone and intercalation of shale's and clay. The lithofacies observed are lateralitic layer, pebbly sand, alternating layers of sand and conglomerates and layers of massive sand stone. The graphic mean and skewness of the grain size analysis shows that the sediments are very coarse which indicates a high energy environment. Graphic standard deviation gives a clue that the sediment were very poorly sorted, which is indicative of a fluvial deposit with high energy , while kurtosis result revealed very coarse sediment deposits. This implies that the particles were not transported very far. Pebble analysis shows the geometric forms and this was deduced from the plot on shape measurement triangle. The shape is compact bladed which is common with river deposits. The bivarate plot of mean diameter against standard deviation also indicated a fluvial environment as well as bivarate plots of co-efficient of flatness against sphericity and maximum projection sphericity against oblate, prolate index which comprises of river and beach environment. All results showed or indicated that the environment of deposition was of river.

Environments of upper miocene sediments in the Hanoi depression interpreted from grain-size parameters

The grain-size analysis is carried out for 49 samples of Upper Miocene sediments taken from three boreholes in the Hanoi depression. The statistic parameters of the grain size distributions such as percentiles, median, graphic mean, sorting or inclusive graphic standard deviation, inclusive graphic skewness, and graphic kurtosis are used to decipher the depositional environments and transportation mechanisms of sediments. Bivariate plotting of these parameters, the discriminant functions with four basic parameters of graphics mean, sorting, inclusive graphic skewness and graphic kurtosis, and the CM pattern with the one percentile (C) and median (M) diameters are applied for this determination. The plots of graphics mean versus sorting suggest 14% of samples being the sand sheet, 46% related to rivers and 40% of the estuarine environment. The sorting - inclusive graphic skewness plot shows a river-related environment for all samples. Linear discriminant functions show the sandstone of upper Miocene in Hanoi depression are deltaic sediments formed in unstably transporting and depositional environment, impacted by turbidity current in the coastal river-mouth. The CM pattern shows sediment transportation for most of the samples in three modes: rolling, suspension and rolling, and uniform suspension. The environmental dynamics are relatively complex and change over time.

Granulometric and Sedimentologic Study of Beach Sediments, Lagos, Southwestern Nigeria

Grain size analysis and sedimentology of beach sediments in Lagos, southwestern Nigeria have been studied. Ten fresh beach sediment samples were randomly collected at a depth of 25 cm - 40 cm along Lagos and Badagry axis. The sediments were analyzed by wet sieving method. Ternary diagrams indicate that the beach sediment is silty. The graphic mean ranges from 1.02ф (medium grained) to 2.21ф (fine grained) with an average of 1.61ф (medium grained). The preponderance of medium grained sediments and lack of coarse sands suggest moderate to high energy conditions of deposition. Graphic standard deviation ranges from 0.01ф - 0.81ф (very well sorted to moderately well sorted), with an average of 0.52ф (moderately well sorted). This is probably due to rapid back and forth flow of the depositing medium. Inclusive graphic skewness ranges from -0.38 to 1.41 (strongly coarse skewed to strongly fine skewed), with an average of 0.41 (strongly fine skewed). The graphic kurtosis values are between 0.44 and 1.37, which indicates very platykurtic to leptokurtic; with an average of 0.78 (platykurtic). This variation suggests that part of the sediment achieved its sorting elsewhere in high-energy environment. Histograms of the sediments exhibit both unimodal and bimodal trends. The arrangement is dominantly asymmetrical with varying modal class, which could be attributed to variation in the energy of the transporting medium. Bivariate plots (simple skewness vs. standard deviation and mean size vs. standard deviation) show the samples plotting mainly in the river sand zone, which suggests that the sediments were initially river sediments before reworking to the present site; other parameters that discriminate between river channel deposits, overbank deposits and overbank-pool deposits show the beach sediments plotting in the river channel zone. Linear discriminant functions analysis indicates a shallow marine beach environment in agitated water, with inputs from aeolian source. The Passega C-M diagram suggests suspension and rolling as the mode of deposition.

Granulometric and pebble morphometric applications to Benin Flank sediments in western Anambra Basin, Nigeria: proxies for paleoenvironmental reconstruction.

Integrated granulometric and pebble morphometric study of Late Cretaceous rocks of the Benin Flank, Nigeria, were used to delineate depositional process and environment of the rocks in the study area. Granulometric analysis reveals that the sandstones of the Lokoja Bassange Formation have an average graphic mean (Mz) of 0.59Φ (coarse grains), inclusive graphic standard deviation (δi) of 1.36Φ (poorly sorted), graphic kurtosis (Ka) of 1.12Φ (leptokurtic), and inclusive graphic skewness (Ski) of - 0.14Φ (stongly coarse skewed). The sandstones for the Mamu Formation have an average Mz of 1.42Φ (medium grains), inclusive graphic standard deviation (δi) of 0.74Φ (moderately sorted), graphic kurtosis (Ka) of 1.43Φ (leptokurtic), and inclusive graphic skewness (Ski) of 0.02Φ (nearly symetrical). The sandstones for the Ajali Formation have an average Mz of 1.23Φ (medium grains), inclusive graphic standard deviation (δi) of 1.21Φ (poorly sorted), graphic kurtosis (Ka) of 1.46Φ (leptokurtic), and inclusive graphic skewness (Ski) of - 0.17Φ (nearly symetrical). Pebble morphometric analysis of the larger grains reveals that the mean values of elongation ratio (ER) = 0.69, coefficient of flatness (FI) = 54.90%, oblate-prolate index (OPI) = 3.34, the maximum projection sphericity index (MPSI) = 0.75, form = 0.67, and roundness (ρ) = 36.28% for the Lokoja Bassange Formation; ER = 0.76, FI = 37.07%, OPI = -3.20, the MPSI = 0.53, form = 0.38, and ρ = 52.93% for the Mamu Formation; and ER = 0.72, FI = 47.44%, OPI = 1.12, the MPSI = 0.67, form = 0.54 and ρ = 42.68% for the Ajali Formation. The integration of the various granulometric and pebble morphometric data, different bivariate and ternary plots indicate fluvial environment for the Lokoja Bassange Formation, dominantly shallow marine environment for the Mamu Formation and fluvial - shallow marine environment for the Ajali Formation. This study shows that the Benin Flank, western Anambra Basin has a depositional setting radiating between fluvial and shallow marine environments.

Depositional environment of the Fort Member of the Jurasic Jaisalmer Formation, (western Rajasthan,.

Lithofacies and granulometric analysis were carried out to decipher the depositional environment of the Fort Member of the Jurassic Jaisalmer Formation. Based on field data nine lithofacies have been identified including trough cross-bedded sandstones, planar cross-bedded sandstones, matrix supported conglomerates, thinly bedded siltstone and sandstones, herringbone cross-bedded sandstones, wave rippled sandstones, laminated sandstones, hummocky cross-bedded sandstones, limestones and shales. Granulometric analysis of the sandstones samples has been carried out for their statistical and textural parameters. Bivariant plots of textural parameters such as graphic skewness versus graphic standard deviation and skewness versus standard deviation confirm the high energy (beach) origin of sandstones. These results suggest a wide spectrum of marine environments ranging from inner shelf to upper shoreface for the Fort Member sandstones.

Estimation of Hydraulic Conductivity from Grain Size Distribution: A Case Study of Sediments From Panzara River, Tapi Basin, Northern Maharashtra (India)

The aim of this work was to determine the grain size of the sediments and estimation of hydraulic conductivity. A total 20 sediment samples were collected from 13 different locations from both the bank of Panzara river channel, Dhule district (Maharashtra). Geologically, this region belongs to Deccan Volcanic Province of upper Cretaceous to lower Eocene age. Weight percentage frequencies and cumulative weight percentage frequencies of the sediment samples were computed. The grain size parameters like graphic mean (Mz), graphic standard deviation (ϭ 1), graphic skewness (SK) and graphic kurtosis (KG) were also computed. The coefficient of curvature (Cc), coefficient of uniformity (Cu) and hydraulic conductivity (Hc/K) were also calculated. The grain size results showed that sediments are moderate to poor sorted.

Provenance Analysis of Sediments in ILU-TITUN and Environs, Eastern Dahomey Basin, Ondo State, Nigeria

Twenty eight sediment samples were obtained from road cuts and stream cuts in Ilu Titun area of Ondo State, Nigeria and were subjected to Grain Size Distribution (GSD) analysis, from which statistical parameters were calculated and used to determine the environment of deposition of the sediments. Six representative samples were selected for petrographic studies to determine the heavy mineral content of the sediments with a view to establishing the sediments source of Ilu Titun and environs which mark the eastern margin of the Dahomey basin. The Graphic Mean (Mz) ranges from 0.700F to 1.667&Phi while the Inclusive Graphic Standard Deviation (&sigma i ) range from 0.960 to 1.319. The Inclusive Graphic Skewness ( SK I ) of the samples ranges from -0.665 to 0.803 while Kurtosis range from 1.652 to 3.309. The bivariate plot of mean size against the Inclusive graphic standard deviation show that the sediments were deposited by river and beach processes. The plot of skewness against the Inclusive graphic standard deviation confirmed that the sediments were deposited by river processes. The results of the petrographic studies show that Slides EK 01 and EK 08 have Kyanite and Staurolite. Slides EK 13 and EK 24 have Staurolite, Tourmaline and Garnet. Slides EK 26 and 28 have Tourmaline and Zircon present in them. Bivariate plots using the statistical parameters show that the sediments were deposited by fluvial processes. The results from the petrographic studies indicate that the sediments were sourced from metamorphic rocks and pegmatites.

MINERALOGY AND RADIOACTIVITY STUDIES OF WADI YOIDER STREAM SEDIMENTS, SOUTH EASTERN DESERT, EGYPT

Wadi Yoider area is located in the South Eastern Desert of Egypt, covering an area of about 150km2. It is covered by basement rocks dissected by Wadi Yoider as a main wadi with several tributaries. The rock types cropping out at Wadi Yoider area comprise, from the oldest to youngest, metavolcanics, younger gabbros, alkali feldspar granites, muscovite granites, biotite granites, monzogranites, and wadi deposits (quaternary sediments) filling the streams of the studied area. The results of the sedimentological statistical parameters are graphic mean (Mz) indicate that, they range from 0.04 to 1.8 with average 0.85, inclusive graphic standard deviation (σI) ranging from 0.88 to 1.8 with an average 1.2, inclusive graphic skewness (SkI) ranging from 0.03 to 0.33 with an average 0.2, and inclusive graphic kurtosis (Kg) ranging from 0.68 to 2.05 with an average 1.4. These samples are characterized by fine skewed, neer skewed, coarse skewed, platykurtic and mesokurtic. All these features indicate of a turrstile. Gravity and magnetic separation techniques in addition to microscopic examination for the studied stream sediment samples were applied to identify the heavy minerals in these sediments. These minerals are represented by magnetite, ilmenite, hematite, rutile, leucoxene, garnet, titanite, zircon, and samarskite. The average concentrations of U and Th in the stream sediments of Wadi Yoider are 20.2ppm and 42.2ppm, which are higher than the clarke values for earth crust

CONTRIBUTION TO THE MIERALOGY AND RADIOACTIVITY OF STREAM SEDIMENTS OF WADI RAS BAROUD AREA, NORTH EASTERN DESERT, EGYPT

Wadi Ras Baroud area is located in the North Eastern Desert of Egypt, covering an area of about 170km2. It is covered bybasement rocks and dissected by several wadies. The rock assemblages cropping out at Wadi Ras Baroud area comprise, quartzdiorites, granodiorites, syenogranites, dykes and Quaternary sediments filling the streams of the studied area.The grain size analysis and their distribution is a fundamental descriptive measure for clastic sediments. The results of thestatistical parameters are a graphic mean (Mz) ranging from 0.4 to2, inclusive graphic standard deviation (I) ranging from1.2 to 1.9, inclusive graphic skewness (SkI) ranging from -0.3 to 0.22, and inclusive graphic kurtosis (Kg) ranging from 0.7 to1.1. These parameters clear that they are coarse to medium sand, poorly sorted, near to coarse skewed samples, and platykurticand mesokurtic, so these sediments are represented by the turbidity currents environments.The identified minerals in the heavy fractures are represented by magnetite, ilmenite, hematite, garnet, rutile, titanite, zircon,thorite, ferogusnite, and columbite. The radiometric study of the stream sediments of Wadi Ras Baroud indicates that theaverage concentrations of U and Th are 10.38ppm and 25.5ppm respectively

Size Analysis of the Late Pliocene-Early Pleistocene Upper Siwalik Sediments, Northwestern Himalaya, India

Size analysis of the Late Pliocene-Early Pleistocene Upper Siwalik sediments comprising the Pinjor Formation in the type area and adjoining regions reveals that the sediments are bimodal to polymodal in nature, medium to fine grained and are moderately sorted. The inclusive graphic standard deviation and moment standard deviation values suggest the deposition of sediments in shallow to moderately deep fluvial agitated water. The log probability plots reveal that saltation mode is the dominant mode of transportation of detritus. The sediments are continental in character and are derived from crystalline, metamorphic and sedimentary rocks of the Himalaya exposed to the North of the type area Pinjor.

Pedogenic hematitic concretions from the Triassic New Haven Arkose, Connecticut: Implications for understanding Martian diagenetic processes

The Triassic New Haven Arkose locally contains mm-scaled pedogenic hematitic concretions, within red arkosic sandy mudstones, that provide insights into concretion-forming processes on Earth and Mars. Concretions represent ~8% of the sediment by mass (~7% by volume), are irregularly distributed and have a near-normal size distribution with a mean diameter of 1.53mm and graphic standard deviation of 0.64mm. X-ray diffraction (XRD) and attenuated total reflectance (ATR) spectroscopy indicate that the concretions are composed of ambient sediments: quartz, montmorillonite, and feldspar, and a minor phosphatic phase that is also implied by geochemistry; however, the concretions preferentially exclude coarser grain sizes, and are cemented by hematite and goethite. Optical observations and synchrotron X-ray fluorescence (XRF) chemical mapping indicate that concretions are massive to weakly zoned with respect to hematitic cements. Compared to the surrounding sediments, concretion rare earth elements (REE) are elevated in total abundances, exhibit light rare earth element (LREE)-enrichment, and possess negative Ce-anomalies; Th/U ratios are lower due to elevated U. Mass balance calculations indicate that ~20% of the concretions are composed of iron oxides and that ~30mm3 of ambient sediment is required to provide the hematitic cement in a single 1.5mm diameter concretion. Element mobility during concretion formation was tested assuming different immobile elements (Al, Ti, Zr). Assuming Zr immobility provides intermediate results, and indicates the following gains in concretions: Pb (572%), Fe (322%), Mn (142%), REE (438–116%), V (138%), U (124%), Ni (84%), and Nb (58%); other elements show either gains or losses of less than ±50%. Ce-anomalies, low Th/U, and elevated V and U abundances point to a significant redox influence on element distributions in the concretions. High iron content, crude internal concentric banding, and redox controls suggest that the concretions are formed in seasonally variable, but generally moist, soil conditions with annual precipitation >130cm. Trace element enrichment patterns are broadly consistent with derivation from downward percolating weathering fluids. Although the New Haven Arkose pedogenic concretions have significant differences and were clearly formed by different mechanisms than hematitic spherules discovered on Mars, they nevertheless exhibit a number of textural (e.g., sphericity, size distributions) and geochemical (e.g., Ni enrichments) similarities that support models suggesting that the Martian spherules are formed as sedimentary concretions.

Geographic mosaic of mutually exclusive dominance of obligate commensals in symbiotic communities associated with a burrowing echiuran worm

Species richness and community composition of symbionts associated with the burrowing echiuran worm, Ochetostoma erythrogrammon Leuckart & Ruppell, 1828, were quantitatively surveyed on subtropical intertidal flats in the Ryukyu Archipelago, Japan. Overall, we recorded seven species of burrow associates, including at least six obligate commensals. According to symbiont community composition, the study sites were largely divided into two groups; one was characterized by the dominance of a snapping shrimp, Alpheus barbatus Coutiere, 1897, and the other by the dominance of a scale worm, Lepidonotus sp. Furthermore, a granulometric analysis showed that the sediment characteristics differed significantly between shrimp- and scale worm-dominant sites in terms of median diameter and inclusive graphic standard deviation. These results suggest that these symbionts have different habitat requirements, resulting in the mutually exclusive dominance pattern. Our findings indicate that habitat heterogeneity is important to the evaluation and conservation of the symbiotic diversity in intertidal flats.

Nearshore Macrobenthic Communities off Two Nuclear Power Plants in Northern Taiwan

The nearshore macrobenthic communities outside the Nuclear Power Plants Ⅰ and Ⅱ were monitored on a seasonal basis from winter, 2000 to summer, 2004. The animal and sediment were sampled by scuba diving. The macrobenthic assemblages, sediment characteristics and total organic content were analyzed. The macrobenthos assemblages in terms of species richness, density and biomass was determined. The mean grain size and sorting (i.e. inclusive graphic standard deviation) of sediment were also determined. The small-sized annelids and crustaceans were the dominant macrobenthos. A total of 96 species of macrobenthos was found in the sediment near the Nuclear Power Plant Ⅰ. The sediment was composed mainly of moderately sorted coarse sand, with silt-clay content less than 2%. The total organic Content ranged from less than 1% to 7%. A total of 86 species of macrobenthos was found in the sediment near the Nuclear Power Plant Ⅱ. The sediment composed mainly of moderately well sorted medium sand, with silt-clay content equal or less than 4%. The total organic content ranged from 1% to 4%. The short distance between two power plants resulted in about 50% of species shared by both plants. Statistical analyses showed that the macrobenthic communities in the relative open system outside the Power Plant Ⅰ were more affected by season than by the sediment characteristics. The relative protected environment outside the Power Plant Ⅱ resulted in higher habitat diversity. The sediment characters strongly influenced to the macrobenthic communities. The synoptic comparisons between two power plants showed that in spite the differences in the microhabitat, the nearshore macrobenthic communities in the Northeast Taiwan were mainly affected by the sediment characters. Seasonal variation was the secondary factor. Results of this study found that the influence of thermal effluent from the power plant to the nearshore macrobenthic community structures was rather localized. Nearshore macrobenthic communities appeared to have reached a stable condition with the constant environmental disturbances from the power plants.

Small-scale non-linear geostatistical analysis of Liocarcinus depurator (Crustacea: Brachyura) abundance and size structure in a western Mediterranean population

The small-scale spatial distribution of density and biomass of the crab Liocarcinus depu- rator off the Ebro delta was analysed using universal kriging (non-linear geostatistics), considering depth as an external trend. In order to understand the spatial distribution of crab size, a third variable was created from the residuals of the non-linear regression between biomass and density. Variogram analysis of crab density and residuals of the biomass/density relationship presented a quadratic rela- tionship with depth, while biomass showed a linear relationship with depth. All experimental vario- grams were fitted to spherical models. The patch size (range) found was 10.64 km for density, 14.53 km for biomass and 20.49 km for the residual variables. The spatial distribution of 8 sediment variables (temperature, redox potential at 1 and 6 cm depth (R1 and R6), carbonate content (CO3), median grain size (ϕ), sediment sorting (IGSD: inclusive graphic standard deviation) and organic matter content (OM)) was studied. The maps obtained were compared with those calculated for the biological variables of L. depurator using Mantel and partial Mantel tests. Distribution characteristics of this species were significantly affected by most sediment variables considered, particularly by tem- perature, organic matter content and redox potential, although overall the correlation level was low, coinciding with the generalist ecological role and opportunistic behaviour of the species. Density and biomass, as well as residuals, presented a negative correlation with the sorting coefficient (IGSD) and with carbonates. In relation to IGSD, high densities of L. depurator were found in homogeneous sed- iments. Residuals (size structure of the population) were significantly correlated with grain size (ϕ), with larger individuals occurring in finer sediments, while juveniles were preferably found in areas with very fine sand sediments. Both density and biomass presented significant positive correlations with the redox potential, indicating that the species favoured well-oxygenated sediments with a low organic content.

Análise dimensional de sedimentos fluviais: formação de Alvarães e depósitos de terraços da bacia do rio Lima (NW de Portugal)

Trata-se dum estudo das distribuições dimensionais de sedimentos fluviais Plio-Quaternários do NW de Portugal pertencendo: à Formação de Alvarâes (enchimento da «bacia de Alvarães»), aos depósitos dos terraços do rio Lima e depósitos dos terraços dos afluentes deste rio. As distribuições dimensionais foramobtidas por: crivação dos sedimentos>62um num agitador mecânico de tipo RO-TAP, segundo uma escala dimensional com intervalos iguais a V2, e pipetagem das suspensões<62um com a pipeta de Andreasen. A interpretação é baseada nos parâmetros estatísticos média, desvio padrão, assimetria e curtose, calculados segundo as fórmulas de FOLK-WARD (1957), e diagramas CM (PASSEGA 1957).

Distribuição de freqüência de tamanho da fração areia e compacidade relativa de solos desenvolvidos de sedimentos do grupo geológico barreiras

A condição de adensamento do solo tem importância para orientar o manejo de uma área. Em solos derivados de sedimentos recentes, a propensão ao adensamento pode estar relacionada com as características herdadas das condições de sedimentação. Em uma toposseqüência de solos desenvolvidos de sedimentos arenosos do grupo geológico Barreiras, no município de Caravelas (BA), selecionaram-se oito perfis de solos, sendo cinco deles sob Eucalyptus, abrangendo Gley Pouco Húmico, Podzol Hidromórfico, Podzólico Amarelo e Plintossolo. Foi determinada a densidade aparente, a qual foi posicionada entre a densidade aparente mínima e a máxima, definindo-se, assim, a compacidade relativa para cada um dos 41 horizontes. A areia foi fracionada em 21 classes de tamanho, mediante peneiramento, determinando-se o diâmetro médio, o desvio-padrão gráfico inclusivo, a assimetria gráfica inclusiva e a curtose gráfica. Observou-se que o maior teor de areia fina e mal selecionada facilitou o arranjo mais compacto das partículas, provocando aumento na compacidade relativa nos horizontes de maior resistência à penetração.