Sandy Lithology Research Articles

The Enigmatic Pockmarks of the Sandy Southeastern North Sea

AbstractNatural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (<50 m). Their formation has previously been linked to methane seepage from the seafloor. Here, we characterize over 50,000 pockmarks based on their morphology, geochemical signature, and the subsurface pre‐conditions using a new integrated geoscientific data set, combining geophysical and sedimentological data with geochemical porewater and oceanographic analysis. We test whether the methane seepage is indeed responsible for pockmark formation. However, our data suggest that neither the seepage of light hydrocarbons nor groundwater is driving pockmark formation. Because of this lack of evidence for fluid seepage, we favor the previously suggested biotic formation but also discuss positive feedback mechanisms in ocean bottom currents as a formation process. Based on a comparison of pockmarks to the central and southeastern North Sea, we find that local lithology significantly affects pockmark morphology. Muddy lithologies favor the formation of larger, long‐lived structures, while sandy lithologies lead to short‐lived, small‐scale structures that are large in area but with shallow incision depth. We conclude that pockmarks in sandy environments might have been overlooked globally due to their shallow incision depth and recommend reevaluating the role of hydrocarbon ebullition in pockmark formation.

Preliminary study of liquefaction potential on the NYIA-Kulon Progo Toll Road Construction Project (Klaten Interchange STA 21+800 – 22+300)

Abstract The Solo-Jogja NYIA Kulon Progo toll road construction project section 1 package 1.1, which connects Kartasura to Klaten Regency. Based on the regional geological conditions, the Klaten interchange at STA 21+800-22+300 is situated in the Opak Merapi-Merbabu fault area predominantly composed of sandy lithology. Consequently, there is a potential for liquefaction when an earthquake occurs. This research analysis of liquefaction potential using a field test data approach, namely the SPT (Standard Penetration Test) was conducted to assess the seismic and subsurface conditions at Klaten Interchange STA 21+800 - 22+300. The seismic conditions at the Klaten interchange, located 6-7 km from the Opak fault, result in a Peak Ground Acceleration (PGA) value of 0.78 g, corresponding to class 5 on the BMKG MMI scale, potentially causing serious damage to buildings. The VS30 value in the research area ranges from 278.9 - 654.3 m/s, indicating a medium soil type. Subsurface conditions at STA 21+800-21+300 are divided into three zones based on the distance of each borehole. The SPT result shows that zone 1 is dominated by Silty Sand with Loose to Medium Dense consistency, zone 2 is dominated by Silty Sand to Sand with Medium Dense to Very Dense consistency, zone 3 is dominated by Silty Sand with a Medium Dense to Very Dense consistency. The results of the liquefaction potential calculation study indicate that several borehole points have the potential for liquefaction because the safety factor (FS) value is less than 1. Zone 1 has three points that have the potential for liquefaction: DBs-01, DBs-05, and DBs-07. Zone 2 includes two points with potential liquefaction: DBs-08 and DBs-09. Zone 3 has one point with potential for liquefaction: DBs-10.

Assessment of Potential Potable Water Reserves in Islamabad, Pakistan Using Vertical Electrical Sounding Technique

This study aimed to investigate the potential reserves of potable water in Islamabad, Pakistan, considering the alarming depletion of water resources. A detailed vertical electrical sounding (VES) survey was conducted in two main localities: Bara Kahu (Area 1) and Aabpara to G-13 (Area 2), based on accessibility, time, and budget constraints. A total of 23 VES measurements were performed, with 13 in Area 1 and 10 in Area 2, reaching a maximum depth of 500 m. Geologs and pseudosections were generated to assess lithological variations, aquifer conditions, and resistivity trends with depth. Statistical distribution of resistivity (SDR), hydraulic parameters, true resistivity, macroanisotropy, aquifer depth and thickness, and linear regression (R2) curves were calculated for both areas, providing insights into the aquifer conditions. The results revealed that the study areas predominantly consisted of sandy lithology as the aquifer horizon, encompassing sandstone, sandy clay, and clayey sand formations. Area 2 exhibited a higher presence of clayey horizons, and aquifers were generally deeper compared to Area 1. The aquifer thickness ranged from 10 m to 200 m, with shallow depths ranging from 10 m to 60 m and deeper aquifers exceeding 200 m. Aquifers in Area 1 were mostly semi-confined, while those in Area 2 were predominantly unconfined and susceptible to recharge and potential contamination. The northwest–southeast side of Area 1 exhibited the highest probability for ground resource estimation, while in Area 2, the northeast–southwest side displayed a dominant probability. The study identified a probable shear zone in Area 2, indicating lithological differences between the northeast and southwest sides with a reverse sequence. Based on the findings, it is recommended that the shallow aquifers in Area 1 be considered the best potential reservoir for water supply. In contrast, deeper drilling is advised in Area 2 to ensure a long-lasting, high-quality water supply. These results provide valuable information for water resource management and facilitate sustainable water supply planning and decision making in Islamabad, Pakistan.

Water Infiltration in the Soil Quaternary Volcanic Rocks in Rawadanau Serang Banten

Calculating water storage capacity can estimate how much rainwater infiltrates the ground. The soil infiltration and permeability test can estimate water storage capacity. This study is to evaluate how much water infiltrates into the soil. Research in Rawadanau, Serang Regency, has been conducted on relatively undisturbed land and mixed agriculture at the soil quaternary volcanic. The infiltration capacity (f) was measured using the double-ring infiltrometer, and the permeability (k) was measured in the laboratory using a permeameter. The test results of both methods and calculations using Horton’s equations showed that f varied from 6.6−4 to 1.89−2 cm/sec. In contrast, the k was between 3.74−7 to 6.05−3 cm/sec. The highest value of the f was found in land with sandy lithology on mixed forests, the highest of k at forests, and mixed agriculture volcanic soil.

Multifactor analysis of specific storage estimates and implications for transient groundwater modelling

Specific storage (SS) has considerable predictive importance in the modelling of groundwater systems, yet little is known about its statistical distribution and dependency on other hydrogeological characteristics. This study provides a comprehensive overview and compiles 430 values of SS from 183 individual studies, along with complementary hydrogeological information such as estimation methods, lithology, porosity, and formation compressibility. Further evaluation of different approaches to determine and utilize SS values for numerical groundwater modelling, along with the scale and source of uncertainty of different measurement methods, was carried out. Overall, SS values range across six orders of magnitude (from 3.2 × 10–9 to 6 × 10–3 m–1) with a geometric mean of 1.1 × 10–5 m–1 and the majority (> 67%) of values are in the order of 10–5 and 10–6 m–1. High SS values of ~10–4 m–1 were reported for glacial till and sandy lithologies, particularly for shallow and thin strata where leakage may obscure the estimation of SS. A parallel assessment of 45 transient regional-scale groundwater models reveals a disconnect between findings of this study and the way SS is treated in practice, and that there is a lack of foundational SS data to conduct quantitative uncertainty analysis. This study provides the first probability density functions of SS for a variety of lithology types based on the field and laboratory tests collated from the literature. Log transformed SS values follow a Gaussian/normal distribution which can be applied to evaluate uncertainties of modelling results and therefore enhance confidence in the groundwater models that support decision making.

Assessment of Environmental Pollution using Electrical Resistivity and Logging Techniques over A Municipal Dumpsite of Ijagun Ijebu Ode

The present study aims to assess environmental pollution using the resistivity method and geological logging technique over a municipal dumpsite of Ijagun Ijebu Ode, underlain by the Afowo Formation within the Dahomey Basin southwestern Nigeria. Forty vertical electrical resistivity soundings stations and six 2D electrical resistivity imaging profiles using Schlumberger and Wenner array respectively with maximum spread-length of 40m at each sounding and profile length of 60 m and 120 m and one borehole log were carried out. Three geoelectrical layers were obtained in the control area, their resistivity values with their corresponding depth of sediment materials is 126 Ωm – 724 Ωm at depth range 0.9 m – 1.1 m (topsoil), 608 Ωm – 2517 Ωm at depth range 3.5 m – 17.7 m (sandy layer), and 800 Ωm – 6046 Ωm (dry sandy layer). Along with the 2D resistivity imaging profiles over the control (1 and 2), four geo-electric layers characterized by Resistivity values range of 40 Ωm – 126 Ωm (loam), 135 Ωm – 418 Ωm (loamy sand), 500 Ωm – 1500 Ωm (sandy), and >3000 Ωm (dry sand) were revealed. The lithology recovery from the logging techniques shows the subsurface is underlined chiefly with sand, which shows a significant correlation with the geophysical method. The lower values of resistivity at the dumpsite are clear evidence of the subsurface contamination of the sandy layer. The 2D images revealed the contaminated zone thickness ranges from 2 m in the northwest to 25 m in the southeastern part. The protective capacity map derived from the VES showed that the dumpsite is underlain by poorly protected sandy lithology, prone to leachate infiltration. Relocation of the dumpsite to a better protected environment or underlining the present area with an impermeable geotextile layer would halt further leachate invasion into the subsurface.

The internal division of the Pliocene Lillo Formation: correlation between Cone Penetration Tests and lithostratigraphic type sections

The highly fossiliferous Lillo Formation (Pliocene, southern margin North Sea Basin) is formally subdivided in five lithostratigraphical members. While these members are generally relatively easily identifiable in outcrops, they are much more difficultly identifiable in boreholes, hampering geological studies. In the Port of Antwerp area, however, an already existing and dense network of Cone Penetration Tests (CPTs) provides an alternative and additional tool for the recognition of the different lithologies in the Lillo Formation. CPTs reveal a clear geotechnical threefold division of the Formation. The lower and upper intervals, herein named CPT units Li-A and Li-C respectively, are characterized by high qc and low Rf values and correspond to shell-bearing, sandy lithologies. CPT unit Li-A can further be divided in a shelly basal subunit Li-A1 and an overlying sandy subunit Li-A2. CPT units Li-A and Li-C are separated by CPT unit Li-B characterized by low qc and high Rf values which correspond to more clayey lithologies. Four CPTs selected in close vicinity of four well-documented outcrops, some of them being formal stratotypes for the Formation and some of its members, allowed to correlate the geotechnical units with detailed lithological descriptions as well as the formal stratigraphy. CPT unit Li-A1 can be correlated to the Luchtbal Member and the basal part of the Oorderen Member, whereas unit Li-A2 corresponds to the superjacent sandy body of the Oorderen Member. The overlying CPT units Li-B and Li-C seem to correlate differently in different outcrops to the clayey upper part of the Oorderen Member, Kruisschans and Merksem Members. This inconsistency hinders any direct correlation between the geotechnical stratigraphy and lithostratigraphy of the Members of the Lillo Formation. Further research is needed to find the reason(s) for this inconsistency.

Understanding reservoir heterogeneity using variography and data analysis: an example from coastal swamp deposits, Niger Delta Basin (Nigeria)

Abstract For efficient reservoir management and long-term field development strategies, most geologists and asset managers pay special attention to reservoir chance of success. To minimise this uncertainty, a good understanding of reservoir presence and adequacy is required for better ranking of infill opportunities and optimal well placement. This can be quite challenging due to insufficient data and complexities that are typically associated with areas with compounded tectonostratigraphic framework. For the present paper, data analysis and variography were used firstly to examine possible geological factors that determine directions in which reservoirs show minimum heterogeneity for both discrete and continuous properties; secondly, to determine the maximum range and degree of variability of key reservoir petro-physical properties from the variogram, and thirdly, to highlight possible geological controls on reservoir distribution trends as well as areas with optimal reservoir quality. Discrete properties evaluated were lithology and genetic units, while continuous properties examined were porosity and net-to-gross (NtG). From the variogram analysis, the sandy lithology shows minimum heterogeneity in east-west (E–W) and north-south (N–S) directions, for Upper Shoreface Sands (USF) and Fluvial/Tidal Channel Sands (FCX/TCS), respectively. Porosity and NtG both show the least heterogeneity in the E–W axis for reservoirs belonging to both Upper Shoreface and Fluvial Channel environments with porosity showing a slightly higher range than NtG. The vertical ranges for both continuous properties did not show a clear trend. The Sequential Indicator Simulation (SIS) and Object modelling algorithm were used for modelling the discrete properties, while Sequential Gaussian Simulation (SGS) was used for modelling of the continuous properties. Results from this exercise show that depositional environment, sediment provenance, topographical slope, sub-regional structural trends, shoreline orientation and longshore currents, could have significant impacts on reservoir spatial distribution and property trends. This understanding could be applied in reservoir prediction and for generating stochastic estimates of petrophysical properties for nearby exploration assets of similar depositional environments.

Microbial community composition across a coastal hydrological system affected by submarine groundwater discharge (SGD).

Mobile Bay, the fourth largest estuary in the USA located in the northern Gulf of Mexico, is known for extreme hypoxia in the water column during dry season caused by NH4+-rich and anoxic submarine groundwater discharge (SGD). Nutrient dynamics in the coastal ecosystem point to potentially elevated microbial activities; however, little is known about microbial community composition and their functional roles in this area. In this study, we investigated microbial community composition, distribution, and metabolic prediction along the coastal hydrological compartment of Mobile Bay using 16S rRNA gene sequencing. We collected microbial samples from surface (river and bay water) and subsurface water (groundwater and coastal pore water from two SGD sites with peat and sandy lithology, respectively). Salinity was identified as the primary factor affecting the distribution of microbial communities across surface water samples, while DON and PO43- were the major predictor of community shift within subsurface water samples. Higher microbial diversity was found in coastal pore water in comparison to surface water samples. Gammaproteobacteria, Bacteroidia, and Oxyphotobacteria dominated the bacterial community. Among the archaea, methanogens were prevalent in the peat-dominated SGD site, while the sandy SGD site was characterized by a higher proportion of ammonia-oxidizing archaea. Cyanobium PCC-6307 and unclassified Thermodesulfovibrionia were identified as dominant taxa strongly associated with trends in environmental parameters in surface and subsurface samples, respectively. Microbial communities found in the groundwater and peat layer consisted of taxa known for denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This finding suggested that microbial communities might also play a significant role in mediating nitrogen transformation in the SGD flow path and in affecting the chemical composition of SGD discharging to the water column. Given the ecological importance of microorganisms, further studies at higher taxonomic and functional resolution are needed to accurately predict chemical biotransformation processes along the coastal hydrological continuum, which influence water quality and environmental condition in Mobile Bay.

Natural and anthropogenic biomarkers in recent dated sediment cores from Refome Lake, SE Nigeria: environmental implications

Natural and anthropogenic biomarkers characteristics of two recent sediment cores collected toward the shoreline and center of Refome Lake (RS and RC; 30 cm long), southeastern Nigeria, were investigated in order to reconstruct past environmental conditions and asses humans-induced changes on the local ecosystem over the last century. Results from sediment bulk properties such as total organic carbon (TOC; 0.12–0.54%; mean—0.26 ± 0.9%), total nitrogen (TN; 0.01–0.05%; mean—0.03 ± 0.7%) and total inorganic carbon (TIC; 0.03–0.37%; mean—0.14 ± 1.0%) contents as well as TOC/TN (5–25; mean—11.55 ± 1.4) suggested slightly higher contribution from allochthonous over autochthonous organic matter (OM), lowered by low primary production and the characteristic Niger Delta sheltered basin morphology predominated by sandy lithology. Evaluation of molecular proxies such as carbon preference index (CPI(C24–C35); 1.23–1.74: mean—1.58 ± 0.4), carbon maxima (Cmax—17, 19, 26, 27, 29), long-chain/short-chain hydrocarbons (LHC/SH-C; 0.77–2.25, mean—1.52 ± 0.3) and Paq, (0.21–0.61, mean–0.45 ± 0.3) as well as C29/C27 (0.89–2.67; mean—1.53 ± 0.2) indicated an admixture slightly predominated by microbial OM (algae/bacteria) in the most recent top layer (RC1, 0–5 cm) of RC core and slightly more enhanced terrestrially derived OM in the middle layer (RS4, 15–20 cm) of RS core. The later sediment layer deposited at ca. 1947–1964 coincided with the period of greater wash-in of land-derived OM (associated with tree logging/forest clearing for foot path extension and farmland preparation authorized by the European settlers) occasioned by intense rainfall. The occurrence of gammacerane and 18α-oleanane in low levels in the lower and upper layers revealed trace contamination by petroleum hydrocarbons imported into the Nigerian economy prior to independence in 1960 and utilization of Niger Delta oil after 1960 following departure of the colonial masters (the British), respectively.

Risk assessment of human exposure to polycyclic aromatic hydrocarbons via shrimp (Macrobrachium felicinum) consumption along the Imo River catchments, SE Nigeria.

Shrimp species (Macrobrachium felicinum) collected from estuarine mangrove area of the Imo River is an important route of exposure to polycyclic aromatic hydrocarbons (PAHs). The estuarine associated sediment (EAS) composited sample showed higher TPAH, ΣAlkyl, ΣPAHcarc and ΣPAHEPA concentrations (550.84, 172.36, 413.17 and 482.11ng/g dry weight-dw) than their mean concentrations in shrimp samples (509.39±354.21, 31.38±18.49, 52.10±1.35 and 460.06±330.76ng/g wet weight-ww), respectively. Among the individual PAHs congeners, phenanthrene was the dominant species detected in the EAS accounting for 21.02% of the total PAH load and the decreasing order of 3->2->5->4->6-ring contamination was found. A different pattern predominated by naphthalene was observed for the shrimp species, suggesting that the organisms have different selectivity for a range of PAHs congeners. These variations may be attributed to different degree of bioavailability of these compounds, characteristic sandy lithology of the EAS and the protective capacity of soot particles associated with liquid fossil fuel combustion masking the uptake of high molecular weight PAHs by the organisms. Cancer risk associated with consumption of shrimps in the region was assessed using estimated daily intake (EDI) and compared with standards. The EDI values for naphthalene, benzo(a)pyrene and ∑PAHcarc were lower than the USEPA benchmarks and EFSA levels of concern values for adult and children population, suggesting low probability of developing cancer.

Distribution and Sources of Petroleum Hydrocarbons in Recent Sediments of the Imo River, SE Nigeria.

The distribution of aliphatic and aromatic hydrocarbons in surface sediments of the lower course of the Imo River (Nigeria) was investigated to determine the sources and fate of these compounds. The aliphatic fraction is characterized by a widespread contribution of highly weathered/biodegraded hydrocarbon residues (reflected in the absence of prominent n-alkane peaks coupled with the presence of 17α(H),21β(H)-25-norhopane, an indicator of heavy hydrocarbon biodegradation) of Nigerian crude oils (confirmed by the occurrence of 18α(H)-oleanane, a compound characteristic of oils of deltaic origin). The concentrations of polycyclic aromatic hydrocarbons (PAHs) ranging from 48 to 117ng/g dry weight (dw; ∑13PAHs) indicate a moderate pollution, possibly lowered by the sandy lithology and low organic carbon (OC) content of the sediments. Concentrations slightly decrease towards the estuary of the river, probably due to the fact that these stations are affected by tidal flushing of pollutants adsorbed on sediment particles and carried away by occasional storm to the Atlantic Ocean. A number of PAH ratios, including parent/alkylated and isomeric compounds, indicates a predominance of petrogenic sources, with a low contribution of pyrolytic inputs, particularly of fossil fuel combustion. On the basis of OC/ON (>10) and Per/ΣPAHpenta- (>10) values, a diagenetic terrigenous OC was proposed as a source of perylene to the river.

Geodynamic processes on sandy slope gullies in central Spain field observations, methods and measurements in a singular system

Gullies developed on sandy lithologies are scarce and few studies have been reported on these landscapes. This paper presents an approach to study such singular landforms. The studied gullies appear on the slopes of a group of mesas and cuestas of Upper Cretaceous sediments located in the Northern piedmont of the Guadarrama Mountains, Spain. Landforms of these gullied areas were catalogued, characterized and quantified with reconnaissance methods, providing information about the most active geomorphic processes. These are being monitored in a 1.32 ha representative gullied catchment, the Barranca de los Pinos. In its high gradient slopes, where mass movements occur, high resolution topographical surveys are being carried out by Terrestrial Laser Scanning (TLS). On low gradient slopes, runoff and rain splash are being monitored in micro plots; and in the main channel, sediment transport and water discharge are being measured. This ensemble of methods, some of them novel, is providing patterns of sediment movement within the gully system, and a hypothesis of high activity rates has been confirmed. High gradient sand slopes without carbonate caprock erode fourfold compared to the capped slopes. In the low gradient slopes, those ungullied produce more runoff while exposed sands yield more sediment; sands covered by litter produce the least runoff and sediment. Notably, this catchment yields mainly bedload. © 2011 Lavoisier SAS. All rights reserved

Monitoring of test gullies development from the Moldavian Tableland in 1986 - 2008 period

In a study area in Romania covering about 25,000 km 2 over 9000 gullies were mapped. The areal distribution of the gullies indicated some areas where a higher density occurs. These areas are dominated by hillslope orientation towards the northwest and, respectively, the southeast, slope inclination between 20 to 30 m/100 m, slope length between 300 and 500 m and by a dominantly sandy lithology. Detailed field surveys were made of 17 gullies. Each gully was marked with a network of stakes to show the location of cross sections. The distances between cross sections were around 3-4 times the gully width. Topographical maps at scales of 1:200 to 1:500 were made for gully geomorphological surveys. In 2007-2008 we were interested in the evolution of the test gullies and if the evolution pattern we had proposed in 1999 was verified by the new measurements. In this work we will focus on the monitoring methods of the test gullies from our work database. Finally, we intend to present the calibrated pattern of the gully evolution. The distribution of gullies indicates that the highest gully density reflects hillslope orientation, inclination and length, and a sandy lithology. In Moldavia the average number of gullies per kms is between 2 to 4, with a maximum of 20 gullies per kms; average density is between 0.1 to 1 km per kms, with a maximum length value of over 3 km per km. Gully number and density per surface unit are considered as classification criteria of gully erosion. The areas with the highest susceptibility of gully initiation in Moldavia are at the intersection of following factors: hillslope orientation towards NW and SE, respectively (hillslopes of consequent valleys), slope inclination between 20 to 30%, slope length between 300 and 500 m and a dominantly sandy lithology. These areas have a high density of points. Detailed investigations on 17 gullies sampled from the different areas of Moldavia between the Siret and Prut rivers in the period 1986-2008 show that some of the properties of gully geometry favour using discriminating factors to produce a diagnosis and a possible assessment of geomorphic development. Factor identification can be used to identify the most important combinations of geomorphic processes in gully development. The surface deposits of gullies studied vary from silty-clays in the northern half of the territory, to silts in the middle to southern area and sands in the southern end of the region. For the cases studied, the sidewall processes contribute 1-5 times more to the gully evolution than the incision processes. The nature of the processes in a gully may be inferred by the grain size analysis of the surface material. The rate of gully head cutting is over 1.5 m/year for gullies cut in sandy deposits and under 1 m/year for the gullies cut in marls and clays. A model of gully development is proposed which shows an accelerated rate of gully development immediately downstream after their initiation and a reduced and even cessation of advance on attaining an equilibrium length.

Vermiculite, with hydroxy-aluminium interlayer, and kaolinite formation in a subtropical sandy soil from south Brazil

AbstractThe purpose of this study was to investigate the clay mineral phases in a Rhodic Acrisol soil and to discuss their evolution in subtropical conditions. Prairie and forest soil profiles were sampled and clay fractions of the parent material and soil horizons analysed by X-ray diffraction (XRD) at the Federal University of Santa Maria, Rio Grande do Sul-Brazil. The XRD results show the presence of interstratified kaolinite-smectite and illite-smectite as well as illite in the parent material. These minerals were also found in the soil samples but with two new phases: hydroxy-aluminium interlayered vermiculite (HIV), which showed incomplete collapse with treatment at 550ºC, and a newly formed kaolinite (d= 7.17 Å). Under a subtropical climate and a sandy lithology, HIV and kaolinite appear to be a result of a specific pedogenic clay formation, related to the natural vegetation. Originally, under the prairie area, the intensity of the weathering processes was weak (within 2:1 clay minerals), as only small quantities of kaolinite and Fe oxides, and no evidence of gibbsite, were found.

Desenvolvimento e testes de mini-sonda Wenner para avaliação de contaminações rasas

The evaluation of soil and groundwater contamination has been an important task in the geosciences field and pushing on the methodological improvement. The contamination extension is normally investigated using geophysical methods or boreholes and chemical analysis. Otherwise, several contamination problems are at low depths (less than 0.5m) allowing the easy access to the contaminated region. Therefore, a low-cost and simple system (mini-probe Wenner) was developed and tested and it is aimed at low depth resistivity measurements. The system is formed by a current source (AC) and a PVC probe which contains a Wenner array with a electrode spacing of 1 cm. Laboratory tests show that the current source works adequately in the resistance range normally associated with contaminated materials while the system tests (including the current source and probe) indicated apparent resistivity values very similar to the resistivity values calculated using the equation proposed by Archie, in the 1.5 to 57 Wm interval. Preliminary field tests showed that the mini-probe is easily applicable in sandy and sand-silty soils while in clayey or hard soils is difficult its introduction. Therefore, interesting applications to the proposed system are the study of agricultural areas or the study of contamination in predominantly sandy lithologies.

PCB concentrations in sediments from the Gulf of Nicoya estuary, Pacific coast of Costa Rica.

Thirty-one sediment samples collected from 1996-2003 from the Gulf of Nicoya estuary on the northwestern coast of Costa Rica, have been obtained for PCB analyses. This is part of the first study to evaluate the PCB contamination in coastal Costa Rica. Overall, the concentrations are low, especially when compared to sediments from more temperate climates and/or sediments from more heavily industrialized areas. Values average less than 3 ng/g dw sediment, however, a few samples contained up to 7 ng/g dw sediment. Sediments with the highest concentrations were located in the Punta Morales area, where muds were sampled from among mangrove roots. The Puntarenas samples had surprisingly low PCB concentrations, likely due to their sandy lithology. The congener distribution within the majority of the samples showed signs of either recent sources or lack of degradation. However, a few sites, specifically some of the inter-gulf islands and more remote samples had congener distributions indicative of airborne contaminants and/or degradation. Considering the presence of airborne PCBs in the Gulf of Papagayo to the north, the lack of airborne PCBs and more varied congener distribution in the Gulf of Nicoya estuary was surprising.

Effects of lithology on geothermal gradient on the southeast Nigeria Delta, Nigeria

A study of the effects of lithologic formations on geothermal gradients is carried out in the south-east Niger Delta, Nigeria, using continuous temperature and lithologic log data from closely-spaced petroleum wells. The gradient profiles obtained for the deep wells, logged to depths between 6500 ft (1981m) and 8500ft (2591m), were observed to have markedly different geothermal gradients with corresponding vertical lithologic units. For the upper sandy lithology, the Benin Formation, the temperature gradient ranges between 0.80 oF/100 ft (1.456 oC/100m) and 1.22 oF/100 ft (2.220 oC/100m) with an average of 1.03 oF/100 ft (1.875 oC/100m). For the paralic shaly Agbada Formation segment, the temperature gradient ranges between 0.92 oF/100 ft (1.674 oC/100m) and 1.87 oF/100ft (3.403 oC/100m) with an average of 1.40 oF/100ft (2.548 oC/100m). The depths of Benin Formation were observed to vary between 4200ft (1280 m) and 5400ft (1879 m). Geothermal gradients are thus observed to increase with depth and shale units. Key Words: Subsurface temperature, lithology, temperature gradient, thermal. (Global Journal of Pure and Applied Sciences: 2002 8(3): 325-338)

Early jurassic spiriferid brachiopodsfrom Alaska and their paleogeographic significance

Spiriferid brachiopods are described in detail for the first time from the Jurassic of North America. The specimens are referred to the genera Liospiriferina and Callospiriferina, both originally described from the Early Jurassic of Morocco and Spain by Rousselle (1977). The species identified herein, Liospiriferina rostrata and Callospiriferina tumida, are known from Europe, South America, and northern Africa, with L. rostrata also recorded from Indonesia. The new occurrences in Alaska indicate a lower-latitude placement during the Early Jurassic, at least temperate latitudes, for the Farewell (probably during the Sinemurian) and Peninsular (Sinemurian-basal Toarcian) terranes. Liospiriferina and Callospiriferina range through much of the Early Jurassic. These new occurrences are significant because they indicate further parallels between the composition of Mesozoic brachiopod faunas of Europe, where faunas were initially monographed in the nineteenth century, and those of North America and South America. The spiriferids therefore show evidence of survival in North America beyond end-Triassic extinctions, underscoring the global nature of the continuity of the Spiriferida into the Jurassic before their demise by the end of the Early Jurassic. In terms of the composition of Mesozoic brachiopod faunas spiriferids have been identified as preferring deeper-water shelf environments. It has not been determined if this was the case for these Jurassic spiriferids: the collections studied herein show Liospiriferina to be associated with sandy lithologies while Callospiriferina was collected from a black shale. Based on external morphology Liospiriferina is closely homeomorphic with the cosmopolitan Triassic athyrid brachiopod Oxycolpella. However, on closer examination and preparation of serial sections of internal structures the Alaskan specimens showed the presence of dental lamellae, a large, persistent median septum, a spiralium (spiral brachidium), and punctate shell, indicating affinities to the Spiriferida. The material had originally been tentatively identified as terebratulids.

Gully distribution and development in Moldavia, Romania

In a study area in Romania covering about 25,000 km 2 over 9000 gullies were mapped and 12 were surveyed in detail. The distribution of gullies indicates that the highest gully density reflects hillslope orientation, inclination and length, and a sandy lithology. The surface deposits of gullies studied vary from silty-clays in the northern half of the territory, to silts in the middle to southern area and sands in the southern end of the region. Factorial analysis of geometrical and granulometrical variables showed the role of the M parameter as a discriminating value of gully cross section shape. The rate of gully head cutting is over 1.5 m/year for gullies cut in sandy deposits and under 1 m/year for the gullies cut in marls and clays. A model of gully development is proposed which shows an accelerated rate of gully development immediately downstream after their initiation and a reduced and even cessation of advance on attaining an equilibrium length.