Oblate-prolate Index Research Articles

Pebble morphometry and sieve analysis: tools in determining the depositional environments of sediments in oron and environs

Pebble morphometry and grain size analysis were carried out in parts of Benin Formation exposed along Oron and environs to determine the depositional environment of the sediments. Quartz pebbles (900 pebbles) collected from 60 different locations around stream channels and quarries were subjected to axial measurement of long (L), short (S) and intermediate (I) axis. Fifteen (15) sediments were further subjected to sieve analysis. Results from morphometric studies shows that the pebbles are dominantly compact-bladed (CB) and elongate (E) with mean values for flatness ratio (FR), elongation ratio (ER), maximum projection sphericity index (MPSI), oblate-prolate index (OPI) and braided ratio (BR) as 0.48, 0.61, 0.67, 3.54 and 0.63 respectively with mean roundness value of 32%. Sieve analysis revealed that the sediments are dominantly medium-grained, well sorted, skewed to near symmetrical and leptokurtic to mesokurtic. Bivariate Plots of roundness versus elongation ratio, sphericity versus OP index, FI versus MPSI, and skewness versus sorting indicates deposition of the sediments predominantly in the fluvial environment. Thus, this study showed that the sediments in Benin Formation were deposited by fluvial depositional processes.

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.

Morphometric analysis of pebbles in verification of transport processes and interpretation of palaeoenvironment: A case study from the Ogwashi Formation (Oligocene), Niger Delta Basin, Nigeria

The present paper discusses a palaeoenvironmental interpretation of the Oligocene Ogwashi Formation (Niger Delta Basin, Nigeria) through morphometric analysis of pebbles, a research method essentially depending on the quantitative evaluation of pebble size and shape which change during transport processes in a range of depositional environments. The relationship of bivariate and ternary-diagram plots of independent functions was determined from freshly exposed sandstone sections in quarries at Ibusa, near Asaba, Nigeria. The grain size of pebbles ranges from fine (11.00 mm) to very coarse (41.33 mm), with a mean size of 21.05 mm (coarse pebbles). A bivariate plot of the flatness index vs maximum projection sphericity index shows 37% of the pebbles to fall within a beach (marine) environment, and 33% within a fluvial environment; the remaining 30% are uncertain. The maximum projection sphericity index vs oblate-prolate index bivariate plot indicates that 47% of pebbles fall within a marine environment and 22% within a fluvial environment, while the remaining 31% are uncertain. The sphericity-form ternary diagram plot shows that the pebbles primarily comprise bladed, platy, very bladed and very platy of near-equal proportions with minor elongate and very elongate stones, which suggests that the pebbles are diverse in origin. The sphericity-form bivariate plot indicates that the pebbles are largely of disc and blade shapes with a minor number of spherical and rod shapes. The near-equal proportion of pebbles in different segments of the bivariate and ternary diagram plots, and the wide distribution of the pebble grain sizes and shapes suggest the Ogwashi Formation is composed of redeposited sediments that likely were transported and deposited in a mix of marine and fluvial settings with possible transitional environments. In other words, the pebbles can be interpreted as marine-influenced fluvial sediments in marginal-marine settings.

DEPOSITIONAL ENVIRONMENT OF THE TERTIARY KERRI – KERRI FORMATION IN THE GONGOLA SUB-BASIN, NORTHERN BENUE TROUGH: PEBBLES MORPHOMETRIC AND GRAIN SIZE ANALYSIS APPROACH

Pebbles have been reported in the Sandstones units of the Kerri - Kerri Formation, but their application as a tool for interpreting the depositional environment is limited. In this context, 510 quartz pebbles and 16 sandstone samples were collected for pebble morphometric and grain size analyses in order to assess the depositional environment of the formation. For the pebble morphometric study, the long (L), intermediate (I), and short (C) axes of each pebble were measured. For each pebble, four geometrical parameters related to depositional environmental studies were computed, these are: Maximum Projection Sphericity, Oblate Prolate Index, Coefficient of Flatness, and Disk - Road - Index. The standard sieving method was used to sieve the sandstone samples. Mean grain size (Mz), sorting (I), skewness (SKI), and kurtosis (KG) were determined as statistical parameters of the grain size. The mean value of the computed pebble shape parameters for the pebbles of the Kerri – Kerri Formation indicates that the pebbles were deposited in a fluvial environment. Based on the grain size analysis, the sediments were deposited by fluvial action with moderate to low energy of deposition setting closer to the source of the sediments. The bivariate plots show that all the pebbles and sandstone samples lie clearlywithin the river field, which suggests fluvial origin

Grain size statistics and morphometric analysis of Kluang-Niyor, Layang-Layang, and Kampung Durian Chondong Tertiary Sediments, Onshore Peninsular Malaysia: Implications for paleoenvironment and depositional processes

Grain size statistics, morphometric and petrographic analyses of sediments and pebbles from the Tertiary sedimentary basins in Johor, onshore Peninsular Malaysia were carried out and presented in this article. The objective of the study was to interpret the depositional processes and paleoenvironment of the Tertiary sediments in the Kluang-Niyor, Layang-Layang, and Kampung Durian Chondong basins. Petrographic analysis reveals that the sandstones are mainly arenites and subfeldspathic arenites, based on the relative proportions of quartz, feldspar and lithic fragments. The textural immaturity of the sandstones suggests that the sediments were transported from a close source, and deposited along a passive continental margin. Grain size results indicate that the mean, standard deviation (sorting), skewness and kurtosis values for the sediment samples range from −0.57 Ф to 0.68 Ф (very coarse-grained to coarse-grained sandstones); 1.35 Φ to 2.27 Φ (indicating poorly sorted to very poorly sorted sandstones); 0.00 Ф to 0.36Ф (nearly symmetrical, finely skewed to strongly fine skewed); and 0.71Φ to 1.32Φ (platykurtic, mesokurtic to leptokurtic), respectively. The predominance of very coarse to coarse-grained sediments suggests low energy conditions of deposition and the poor sorting is the result of rapid deposition by strong, fluctuating currents. This, in conjunction with the various bivariate plots employed, including the Linear Discriminant Analysis (LDA) diagrams, has aided in discriminating fluvial from other depositional environments. Using Passega’s diagram, the C-M pattern of these Tertiary sediments indicated that they were transported by rolling and bottom suspension. Moreso, pebble morphometric results show the mean values of the various morphometric parameters as follows: flatness ratio (S/L = 0.49), elongation ratio (I/L = 0.73), maximum projection sphericity (ΨP = 0.69), Oblate-Prolate index (OPI = 0.45), and roundness (50.526). The pebbles of these basins were shaped in a river environment, based on measured and computed morphometric indices. Results of granulometric statistics, morphometric indices and petrography together with bivariate plots, as well as the absence of fossils/trace fossils suggest that these isolated Tertiary basins’ sediments were transported by rolling and bottom suspension (a very short distance of transport), deposited by a high-energy fluvial depositional system, close to the source.

Pebble morphogenesis of the Cretaceous Conglomerates from the Abeokuta Formation near the Oluwa River, Eastern Dahomey Basin, Southwestern Nigeria

Pebbles taken from the bed of the Cretaceous Abeokuta Formation paraconglomerate near the Oluwa River were used to infer the depositional setting and the nature of the source area, through the integration of bivariate and ternary (sphericity-form and Zingg diagrams) analyses. Deposition in a river environment is indicated by the high sphericity values, ranging from 0.59 to 0.88 (average 0.70). Also, bivariate plots of the maximum projection sphericity (ψp) vs. oblate-prolate index (OPI) and flatness index (FI) vs. maximum projection sphericity (ψp) point to the domination of fluvial processes. Dominantly elongated, compact elongated, compact and compact-bladed pebbles are typical for sedimentary regime with prevalence of fluvial over beach processes. Co-existence of various pebbles shapes (mainly disc, rod-, and sphere-shaped), despite of the similar, predominantly quartz composition, may occur due to the different clast fabrics. This heterogeneity also indicates various transport distances and water energies, pointing to the multiple source areas.

Paleo-environmental deduction from pebble morphometry and textural studies of sandstone deposits of Isuochi and Environs, Anambra Basin, South Eastern Nigeria

Textural analysis and pebble morphomentric were carried out on the sandstone deposits of Isuochi area in Anambra Basin, in an attempt to reconstruct the paleoenvironment of deposition. Field observation shows that the sandstone samples, collected from various locations were analyzed for sieve analysis, while two hundred fresh quartz pebbles that are greater than 2.00mm in diameter were collected for pebble morphomentric. Morphomentric parameters include; elongation ratio, flatness index, oblate index, and maximum projection sphericity were computed. Bivariate plots of Maximum Projection Sphericity against Oblate Prolate Index show that about 69% of the pebbles are of fluvial origin,while 31% is surf, bivariate plots of Flatness Index against Maximum Projection Sphericity show that about 57% is of fluvial origin, while 43% show surf; these imply that the associated pebbles are of fluvial origin. Bivariate plots of sandstone textural parameters such as skewness against standard deviation also suggest that the sediments are more of fluvial origin while multivariate parameter shows partly shallow marine environment.
 Keywords: Pebble Morphometry, Oblate-Prolate Index, Paleoenvironment.

Pebble morphometric analysis as signatures of the fluvial depositional environment of the Katberg Formation near Kwerela River around East London, Eastern Cape Province, South Africa

Pebbles were collected along the Katberg Formation sandstone beds next to Kwerela River (R63) near East London. During the study of pebble morphology, calculations were made to derive values that were used for bivariate plots to confirm the depositional environment of the Katberg Formation. Bivariate plots of MPs against OP showed that 79% of pebbles fall in fluvial environment, whereas 21% of pebbles fall in the beach environment. The plot of flatness index versus sphericity index shows that 87% of pebbles have a sphericity of greater than 0.65 falling into fluvial environment and 13% falling into a beach environment. The average morphometric indices indicated the dominance of river pebbles with an average sphericity of 0.73, which is above 0.65 and considered the limit of sphericity belonging to fluvial environment. The occurrence of a small proportion of beach pebbles suggests that the river reached the marginal marine environment during its flow. The river pebbles have the lowest roundness, highest sphericity index, and neutral Oblate-Prolate Index. Based on calculated indices, it is evident that the pebbles of the Katberg formation were shaped in fluvial environment. Majority of the pebbles yielded a bladed compact shape with a dominating sphericity index symptomatic of fluvial sediments. All the bivariate plots illustrate fluvial depositional environment for the sediments of the Katberg Formation.

Pebble Morphometric Study of the Ogwashi-Asaba Formation at Ubakala and Environs in Southeastern Nigeria

Pebble morphometric study was carried out for the Ogwashi-Asaba Formation so as to understand its paleodepositional environment. The three axes; the long (l), the intermediate (i) and the short (s) were measured for the two sets of one hundred and forty (140) and one hundred and fifty (150) pebbles collected from two outcrops of the Ogwashi-Asaba Formation. Pebble form indices such as Maximum Projection Sphericity (MPSI), coefficient of flatness (FR %), elongation ratio (ER) and oblate-prolate (OP) index were computed from these measurements. Roundness was estimated using Sames image sets. The mean values of the pebble form indices ranges from 49.78 to 52.02 for coefficient of flatness (FR %). 0.71 to 0.73 for sphericity, 2.60 t0 3.53 for oblate-prolate index, and 0.66 to 0.69 for elongation ratio. The calculated indices are suggestive of fluvial sedimentation for the Ogwashi-Asaba Formation. Bivariate plots of coefficient of flatness (FR %) versus maximum projection sphericity index (MPSI) and plots of sphericity (MPSI) versus oblate-prolate index (OP) are also indicative of fluvial origin. Scatter plots of roundness versus elongation ratio shows that 100% of the pebbles are in a littoral field. Plots of bivariate parameters such as simple skewness versus simple standard deviation, and second moment skewness versus second moment standard deviation suggests the Ogwashi-Asaba Formation as a product of the fluvial depositional processes. Keywords: Niger Delta Basin, Ogwashi-Asaba Formation, Pebble morphometry, Pebble Form indices. DOI : 10.7176/JNSR/9-6-06 Publication date :March 31 st 2019

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.

Characteristics of terrestrial basaltic rock populations: Implications for Mars lander and rover science and safety

We analyzed the morphometry of basaltic rock populations that have been emplaced or affected by a variety of geologic processes, including explosive volcanic eruptions (as a proxy for impact cratering), catastrophic flooding, frost shattering, salt weathering, alluvial deposition, and chemical weathering. Morphometric indices for these rock populations were compared to an unmodified population of rocks that had broken off a solidified lava flow to understand how different geologic processes change rock shape. We found that a majority of rocks have an sphericity described as either a disc or sphere in the Zingg classification system and posit that this is a function of cooling fractures in the basalt (Zingg [1935] Schweiz. Miner. Petrogr. Mitt., 15, 39–140). Angularity (roundness) is the most diagnostic morphometric index, but the Corey Shape Factor (CSF), Oblate–Prolate Index (OPI) and deviation from compactness (D) also sometimes distinguished weathering processes. Comparison of our results to prior analyses of rock populations found at the Mars Pathfinder, Spirit, and Curiosity landing sites support previous conclusions. The observation that the size-frequency distribution of terrestrial rock populations follow exponential functions similar to lander and orbital measurements of rocks on Mars, which is expected from fracture and fragmentation theory, indicates that these distributions are being dominantly controlled by the initial fracture and fragmentation of the basalt.

Discrimination of Depositional Environment Using Statistical Parameters of Conglomeratic Outcrop Sections, Obubra Southeastern Nigeria

Statistical analyses reveal information of the hydrodynamics and environments of deposition of sedimentary rocks. The sediments of the outcrop sections from Uwakande 1, Obubra southeastern Nigeria were studied for their textural variations. Grain size analysis, and pebble morphogenesis carried out shows that the sediments are coarse to very coarse in size, poorly sorted, and very positively skewed. This in conjunction with the various bivariate plots employed, aid in discriminating between beach and river depositional environments. Pebble morphometric analysis of the conglomerates showed that the mean values of the various morphometric parameters range as follows: flatness ratio (S/L = 0.397 to 0.514), elongation ratio (I/L = 0.657 to 0.784), maximum projection sphericity (ΨP = 0.605 to 0.899), Oblate-Prolate index (ŌP = -1.738 to 1.594), coefficient of flatness (39.70 to 51.37), and roundness (-0.367 to 0.722). This study is significant in providing evidence of fluvial conditions during the deposition of the Eze-Aku Formation in the Turonian, and reveals the type of transporting medium; the mode of sediment deposition; as well as the environment into which the sediments encountered in the study area were deposited. Original Research Article Onu et al.; BJAST, 6(1): 24-33, 2015; Article no.BJAST.2015.064 25

Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria

An integrated sedimentological and geochemical evaluation has been carried out on the Cretaceous sediments of the Calabar Flank. This study is to characterize the provenance, depositional environments and hydrocarbon potentials.The techniques involved field descriptions, textural parameters, petrographic analysis and biostratigraphic studies using standard sedimentological methods. The geochemical studies involved the determination of major oxides and trace elements using Inductively Coupled Plasma–Mass Spectrometry (ICP–MS); Total Organic Carbon (TOC) and Rock Eval Pyrolysis.Results show that sandstone from Awi Formation have elongation ratio ranging from 0.4b to 0.9, oblate–prolate index and maximum sphericity index range from 9.6 to 9.7 and 0.5 to 0.9 respectively. The sandstone units are arkosic and mineralogically immature (MI=3); ZTR indexes range from 54.6% to 82.5%, with tourmaline, zircon, staurolite, garnet, apatite, augite and rutile grains being angular-sub-angular. This suggests nearness to source, and that Awi Formation was deposited in a fluvial environment. The limestone deposit of Mfamosing Formation is predominantly bioclastic consisting of algal stromatolites, oolitic and pelloidal grainstones/packstones with high carbonate content. The dark grey fissile shales of Nkporo and Ekenkpon Formations indicate deposition in quiet oxic and/or anoxic conditions. Average TOC suggests good source rocks. Predominance of Type III kerogen, Tmax and hydrocarbon source potential of Mfamosing, Ekenkpon, New Netim Marl and Nkporo Formations suggest marginal mature to mature source rocks deposited in shallow continental to open marine setting that some gas may have been generated. The sediments are derived from passive continental margin in plutonic humid palaeoclimatic setting of continental block province.

A 3-dimensional seismic method to assess the provenance of Mass-Transport Deposits (MTDs) on salt-rich continental slopes (Espírito Santo Basin, SE Brazil)

Provenance studies of Mass-Transport Deposits (MTDs) frequently rely on the analysis of kinematic indicators. In this work, we use seismic facies analyses, measurements of preserved blocks, and correlations between MTDs thickness and salt diapir geometry to introduce a new method to estimate the provenance of MTDs on 3D seismic data. The method, applied to a high-quality 3D seismic volume from the Espírito Santo Basin (SE Brazil), provides information on the location of source areas of MTDs, their bulk composition, and on their directions of transport. Whenever blocks of strata are found, their orthogonal axes are measured to estimate parameters such as Maximum Projection Sphericity Index (MPSI), Oblate Prolate Index (OPI), and ratio of short and intermediate axes (ds/di). These parameters are used to quantify the relative degrees of remobilization and transport of MTDs. Statistical data of importance include the variation of MTDs thickness with the diameter and distance from the centre of growing salt diapirs. In addition, the presence of cubic or equant blocks with c/a > 0.4 and c/b > 0.65 ratios are typical of proximal areas in MTDs, reflecting small transporting distances. Our work is important because it shows that distinct correlation coefficients exist between the thickness of MTDs and diameter of salt diapirs. Positive coefficients of correlation between diapir diameter and MTD thickness characterise active diapirism during the deposition of MTDs. This quantification allows the interpreters to distinguish MTDs sourced from local diapirs from MTDs sourced from more distal regions on the continental slope. Thus, the new method used in this research provides an alternative technique to kinematic indicators for determining the provenance of mass transport deposits on continental margins.

Use of Statistical Parameters in the Sedimentological Study of Conglomerate Deposits in Northeastern Part of Akwa Ibom State, Niger Delta Basin, Nigeria

Conglomerates and pebbly sandstone abound along a belt of about 50km long and 11km wide in northeastern part of Akwa Ibom State. Granulometric analysis of 29 samples show the deposits to be poorly to moderately sorted, having sorting coefficient values ranging between 0.63ф to 1.41ф. Pebble morphometric analysis of the conglomerates showed that the mean values of the various morphometric parameters range as follows: flatness ratio (S/L = 0.39-0.58), elongation ratio (I/L =0.64-0.78), maximum projection sphericity (Ψ P = 0.60-0.77), Oblate Prolate index (Ō P = -1.98 to 4.13), coefficient of flatness (39.25-57.86). Roundness index determined for the pebbles through simple comparism with Power’s (1953) roundness chart averaged 0.232. Interpretation of pebble morphometric and granulometric results indicate a fluvial environment of deposition for the deposits.

Gastroliths associated with a juvenile elasmosaur (Plesiosauria, Elasmosauridae) from the Snow Hill Island Formation (upper Campanian–lower Maastrichtian), Vega Island, Antarctica

José P. O'Gorman, Eduardo Olivero & Daniel A. Cabrera. December 2012. Gastroliths associated with a juvenile elasmosaur (Plesiosauria, Elasmosauridae) from the Snow Hill Island Formation (upper Campanian–lower Maastrichtian), Vega Island, Antarctica. Alcheringa 36, 531–541. ISSN 0311-5518. One of the unresolved problems concerning the palaeobiology of plesiosaurs is the function of gastroliths. A new juvenile specimen referred to Elasmosauridae indet., collected from the Cape Lamb Member of the Snow Hill Island Formation (upper Campanian–lower Maastrichtian) from Cape Lamb, Vega Island, Antarctic Peninsula with gastroliths, provides the opportunity to add information about this issue. The specimen consists of approximately 20% of a partially articulated skeleton including 333 gastroliths. Taphonomic evidence indicates rapid burial and possibly different taphonomic pathways for the dorsal (articulated) and caudal (disarticulated) regions. Analysis of the 333 gastroliths determined the mean major axis to be 14.32 mm, the mean maximum projection sphericity to be 0.7 and the standard deviation to be 1.1. Following Krumbein classification, most of the gastroliths are disk-shaped (35.4%), spheroid (34%) and cylindrical (21.3%). According to Powers's categories, most of the gastroliths are rounded (71.2%). Petrographically, the gastroliths that are larger than 15 mm (major axis) are mostly volcanic rhyolites (78.3%) and fine-grained quartz aggregates (11.6%); together with minor granitic rocks (3.1%), mudstones (3.1%) and aphanitic volcanics (3.9%); similar percentages were present among the smaller clasts. The acidic volcanic clasts likely derive from the volcanic Antarctic Peninsula Group (GPVA; Middle–Upper Jurassic). The mean values for sphericity and oblate-prolate index (OP index) indicate a fluvial origin for the gastroliths. The method of estimation of the maximum prey-size using gastroliths is discussed. The available data provides evidence against the hydrostatic function of the gastroliths because the total weight of the gastroliths is insufficient to modify the hydrostatic balance of the juvenile plesiosaur.

First record of gastroliths associated with elasmosaur remains from La Colonia Formation (Campanian–Maastrichtian), Chubut, Patagonia Argentina, with comments on the probable depositional palaeoenvironment of the source of the gastroliths

A gastrolith set, comprising 197 elements, associated with an indeterminate elasmosaurid plesiosaur collected from the Campanian–Maastrichtian La Colonia Formation, Chubut, Argentina, is described. Most of the gastroliths are discoidal (41.9%) or spheroidal (34.8%), with a mean Maximum Projection Sphericity Index value of 0.69 and a mean OP Index value of −0.74. The values of these indices are compared with those recorded for gastroliths associated with other Upper Cretaceous elasmosaurids to see if patterns with palaeobiological relevance are evident. The mean values of the Maximum Projection Sphericity Index and the Oblate-Prolate Index allow us to infer a fluvial or estuarine origin for all the elasmosaurid gastroliths for which these indices have been calculated. This inference is palaeobiologically informative because it indicates that at least some Upper Cretaceous elasmosaurs entered into estuarine (or fluvial) environments. It also helps explain the interesting vertebrate assemblage of the Allen and La Colonia formations where a rich continental fauna is recorded but elasmosaurids and polycotylids are the only well represented vertebrates with marine affinities.

Shape development of trondhjemite pebbles and cobbles on shores in the southwestern Finnish Archipelago

The morphometry of trondhjemite pebbles and cobbles from till and esker material and from moraine-shore and esker-shore material was studied in the Airisto area, SW Finnish Archipelago. The maximum projection roundness, sphericity, and oblate-prolateness were measured on 2000 pebbles and cobbles in the 16-256 mm size range. Wave energy was low at the study sites, which owing to land uplift rise beyond the reach of littoral forces in a relatively short time (the upper beach over a period of about 800 years). Of the three indices, roundness shows the greatest variation, from 0.30 to 0.72. The oblate-prolate index also showed significant variations (-0.66 - + 1.43). The poorest indicator, but not entirely useless as a discriminatory tool, is the sphericity index, which showed least change (0.69 - 0.78). The increase in roundness and decrease in oblate-prolate index are predominantly the results of abrasion caused by the sliding and rolling of pebbles and cobbles to and fro over sand and gravel in the breaker and swash zones. On mixed gravel-cobble shores from till pebbles (16-64 mm) were most disc-like while cobbles (64-128 mm) were most disc-like on esker shores.

Paleodepositional environmental diagnosis of the Nsugbe area and environs, Anambra state, southeastern Nigeria: an application of pebble Morphometric studies.

The application of pebble morphometric studies in diagnosing the paleodepositional environment of Nsugbe area and environs was carried out using one hundred and seventy (170) vein quartz recovered from seventeen (17) outcrops widely distributed in the studied area. Results show that the pebbles are rounded to subrounded and dominantly compact to compact-bladed. Computed mean values for the flatness index (FI), elongation, maximum projection sphericity index (MPSI) and oblate-prolate index (OPI) are 0.47, 0.74, 0.67 and 0.40 respectively. When these values were integrated with bivariate plots of MPSI versus OPI and mean roundness versus elongation ratio, a dominantly fluviatile depositional environment with subordinate beach/littoral setting are indicated for the Nsugbe area and environs. KEY WORDS: Pebble morphometry, Paleodepositional environment, Fluviatile, Nsugbe, Anambra basin.

PALEOEVIRONMENT OF NIGERIA'S AJALI SANDSTONES: A PEBBLE MORPHOMETIC APPROACH.

The Ajali Sandstone is a major clastic formation of Campanian-Maastrichtian age occuring within the Anambra Basin, Southeastern Nigeria. The Sandstones have a high incidence of quartz and feldspar pebbles. Clasts of vein quartz pebbles were selected for morphometric study of decipher the depositional environment of the sandstone. The three mutually perpendicular axes of each pebble were measured and the roundness estimate with the aid of a roundness image set. Morphometric parameters such as size, flatness ratio, elongation ratio, elongation ratio, maximum projection sphericity, form geometry and oblate index were computed. Bivarite scattergrams of roundness versus oblate-prolate index were plotted. Except for the flatness ratio which is non-diagnostic, all the other results of the morphometric studies indicated a fluvial – dominated depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment based on analysis of faceis, sedimentary rock which are suitable for morphometrical research: those with isotropic constitution and high resistivity to wear such as cherts, quartz sandstones and some related types. KEY WORDS: Clastic formation, Fluviatile, Fluviodeltaic, Elongation ratio, Oblate index. Global Jnl Geological Sciences . Vol.2(1) 2004: 37-43