• DK Asiedu, D Atta-Peters, E Hegner, A Hegner, T Shibata
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    ABSTRACT: The Late Ordovician to Early Cretaceous Sekondian Group of southern Ghana is made up of seven major sedimentary formations. One important feature of the Sekondian Group is the abundance of fairly fresh plagioclase in the lowest two formations, i.e., the Ajua Shale and Elmina Sandstone, and at the uppermost formation (i.e., Essikado Sandstone); but a virtual absence throughout the rest of the Group. The basal part of the Sekondian Group has been interpreted as of glacial origin whereas the upper formations are thought to have formed in a warmer stage. The abundant plagioclase in the lowest two formations could, therefore, be due to less pervasive chemical weathering rather than erosion of a distinct source. In this paper, geochemical data for fine-grained sedimentary rocks of the Ajua Shale and the Takoradi Shale (that overlie the Elmina Sandstone) formations are used to investigate the influence of palaeoweathering on the composition of the Sekondian Group.The concentrations of the transition trace metals Cr, Ni, V, and Cu, and Cr/V-, Zr/Y-, and Ba/Co-ratios are similar in the samples from both formations and suggest that they were largely derived from the granitoids that intrude the Birimian supracrastal rocks. However, the Takoradi Shale samples are enriched in Rb, Cs, Th, U, and Nb relative to the Ajua Shale samples, and this may be related to differences in the degree of weathering in the source rocks. The Takoradi Shale samples are characterized by high chemical index of alteration (CIA) values and Rb/Sr ratios (CIA > 75; Rb/Sr > 1) indicating high degrees of weathering of their source rocks. In contrast, the Ajua Shale samples have low CIA values and Rb/Sr ratios (CIA < 62; Rb/Sr < 0.5) that indicate low degrees of weathering of their source. The tectonic setting at the time of deposition of both formations was passive margin suggesting that the change in paleoclimatic conditions rather than erosion rates played a more dominant role in the observed compositional changes in these shales.
    03/2011; 12(1). DOI:10.4314/gm.v12i1.64364
  • R Anan-Yorke, D Atta-Peters
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    ABSTRACT: Subsurface data indicate over 6000 metres thick of sedimentary infill in the Voltaian Basin on the West African craton made up of three major lithofacies (groups) separated by distinct unconformities. These are: Lower Voltaian Group composed of over 2000 metres thick of induratedquartzitic sandstones with subordinate shales at the base, overlain by dense, greenish, poorly sorted and graded bedded sandstones and siltstones with subordinate shales of about 2000 metres thick; Middle Voltaian Group consisting of the triad: conglomerate – dolomitic limestone – bedded chert with intercalations of sandstone, siltstone, mudstone and shales of over 700 metres thick, and overlain by about 550 metres thick of greenish, flysch-type sediments. The Upper Voltaian Group is composed of distinctly continental facies (molasses) of reddish brown, chocolatebrown,brown coloured, poorly sorted sandstones, conglomerates, siltstone and mudstones of about 400 metres thick, and a sequence of thinly bedded, micaceous sandstones with clay galls of about 120 metres thick capped by massive cross-bedded arkosic sandstone of about 300 metresthick. Reviews of the classification schemes proposed by various geologists are basically similar. However, general discrepancies, inconsistencies and contradictions in the stratigraphic positions of some of the rock units have been observed, as well as terminologies to describe rock unitswhich are inconsistent with stratigraphic concepts and principles.
    10/2009; 11(1). DOI:10.4314/jgsa.v11i1.46998
  • D. Atta-Peters, M.B. Salami
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    ABSTRACT: Cretaceous dinoflagellate cysts and miospores have been recovered from the Tano 1-1 and 1S-3AX wells, offshore of western Ghana. Based on marker palynomorphs (Afropollis jardinus, Elaterosporites klaszii, Elaterocolpites castelainii, Sofreipites legouxae, Reyrea polymorphus, Cyclonephelium vannophorum) an Aptian-early Cenomanian age has been assigned to sediments from the well Tano 1-1. The taxa Auriculiidites reticulatus, Spinizonotriletes echinatus, Buttinia andreevi, Longapertites spp., Echitriporites trianguliformis, Dinogymnium spp., Trichodinium castanea, Andalusiella spp., which are elements typical of Campanian–Maastrichtian sediments have been recognized in the sediments from 1S-3AX.Palaeoenvironmental interpretation based on identified palynomorphs in the light of their ecological preferences and also on the relative proportions of palynomorph groups, indicate that the Aptian-lower Cenomanian sediments were deposited in a marginal marine environment with vegetation on wetlands, under a relatively dry climate. The Campanian–Maastrichtian sediments suggest a fluctuation between marginal to open marine (inner shelf) conditions. The sediments have characteristic elements indicative of the Albian–Cenomanian “Elaterate Province”, and the late Cretaceous “Senonian Palmae Province”, which supports a warm tropical climate.
    Journal of African Earth Sciences 11/2006; DOI:10.1016/j.jafrearsci.2006.07.002 · 1.38 Impact Factor
  • D.K. Asiedu, E. Hegner, A. Rocholl, D. Atta-Peters
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    ABSTRACT: Geochemical and Nd-isotopic data are reported for 24 shale and sandstone samples comprising the Upper Ordovician to Lower Cretaceous Sekondian Group, southern Ghana. The data are interpreted in terms of the provenance of these siliclastic sediments and the Paleozoic and Mesozoic geologic development of southern Ghana. The sandstones and shales generally have trace element characteristics typical of material eroded from the upper continental crust. Cr and Ni concentrations are low suggesting their derivation from dominantly felsic sources. There appear to be chemical and Nd-isotopic differences between the lower formations (i.e., Ajua Shale and Elmina Sandstone) and the uppermost formation (i.e., Essikado Sandstone) on one hand, and the upper formations (i.e., Takoradi Sandstone, Takoradi Shale, Effia Nkwanta Beds and Sekondi Sandstone) on the other. The upper formations are characterized by evolved major element compositions, high large-ion lithophile element abundances, large negative Eu-anomalies (0.58–0.73), and low εNd values (−10.8 to −4.1), indicating stable craton settings. On the other hand, the Ajua Shale, Elmina Sandstone, and Essikado Sandstone formations have less evolved major element compositions, lower large-ion lithophile abundances, and higher Eu-anomalies (0.69–0.88), and this may reflect an active margin setting of the source (juvenile-type Birimian felsic volcanics and/or granitoids) rather than Phanerozoic active continental settings. The upper formations have TDM model ages of 1.8–2.3 Ga indicating that the Paleoproterozoic Birimian rocks are the ultimate sources, whereas the lower formations and uppermost formation have slightly younger TDM values of 1.6–1.9 Ga suggesting of a component from the Pan-African mobile belt. These differences in model ages may, however, be related to lithology (rather than stratigraphy) where the shales (1.6–2.3 Ga; mean, 1.9 Ga) typically give older ages than the sandstones (1.6–1.9 Ga; mean 1.7 Ga).
    Journal of African Earth Sciences 04/2005; 41(4):316-328. DOI:10.1016/j.jafrearsci.2005.05.003 · 1.38 Impact Factor
  • D. Atta-Peters, M. B. Salami
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    D Atta-Peters, And M B Salami
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    ABSTRACT: Miospores recovered from the interval 6,700 ft-1,260 ft (2,042 m-384 m) in well ST-8 offshore Tano Basin (Ghana) have been studied and discussed. The miospore assemblage is dominated by angiospermic pollen. Pteridophytic or trilete spores are mainly laevigate forms whilst the sculptured forms are few in the assemblage. Pollen taxa are largely monocolpates with other subordinate aper-tural types, showing appreciable variation in the exine. The miospore association, together with fun-gal and algal spores, provides evidence of freshwater swamp or marsh environment. The monocol-pates are attributed to the palmae and belong to the tropical-subtropical Senonian Palmae Province of Africa and South America (ASA) region. The assemblage is indicative of a warm and humid clima-te. Stratigraphically significant miospore taxa point out to Campanian to Eocene age for the sedi-ments. Resumen Se han estudiado y discutido mioesporas extraídas en el intervalo entre 6.700 y 1.260 pies (entre 2.042 m y 384 m) del sondeo litoral ST-8 de la Cuenca de Tano (Ghana). La asociación de mioespo-ras está dominada por polen de angiospermas. Esporas triletes o de pteridofitas se presentan como for-mas levigadas, mientras son muy pocas las formas ornamentadas. Los taxones de polen son princi-palmente polenes monocolpados con otros grupos subordinados de diferente tipo apertural, mostrando una variación importante en la exina. La asociación de mioesporas, junto a la de esporas de algas y hongos, nos facilitan evidencias de la presencia de medios dulceacuícolas pantanosos. Los pólenes de tipo monocolpados puede ser atribuidos a palmaceas, incluidas en la provincia Palmae, provincia subtropical-tropical africana de edad Senoniense situada en la región Sudamericana (ASA). La asociación es indicativa de clima húmedo y cálido. Estratigraficamente, los taxones de mioespo-ras más representativos nos indican una edad desde el Campaniense al Eoceno.
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    D Atta-Peters, And R Anan-Yorke
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    ABSTRACT: Pteridophytic spores recovered from the Takoradi Shale Formation at Essipon Beach near Sekondi in western Ghana are abundant and diverse. They are mainly trilete spores, both laevigate and sculp-tured. In all, 36 species are described and illustrated. The spores and the alga Tasmanites indicate a freshwater swamp or marsh environment of deposition. Organic-walled microplankton (acritarchs) associated with the spores imply marine or brackish incursions. Stratigraphically significant spore taxa indicate a latest Devonian (Strunian Tn 1a)–Early Carboniferous (Tournaisian Tn 1b) age for the sediments by comparison with similar taxa reported especially from western Europe, the former USSR and North America. Resumen Se describe la presencia abundante y diversa de esporas de pteridofitas en la Formación Takoradi Shale, en Essipon Beach, en las cercanías de Sekondi, oeste de Ghana. Principalmente son esporas de tipo triletes, lisas y ornamentadas. Se describen e ilustran 36 especies. Las esporas y los restos del alga Tasmanites indican la existencia de un medio de deposición de tipo pantano o marisma. La pre-sencia de microplancton de paredes orgánicas (acritarcos) asociado con las esporas indica la exis-tencia de incursiones marinas o salobres. La presencia de determinados taxones de esporas significa-tivas desde el punto de vista estratigráfico indica una edad Devónico Superior (Struniense Tn 1a)–Carbonífero Inferior (Tournaisiense Tn 1b) cuando se comparan con asociaciones similares des-critas en Europa occidental, en la antigua URSS y Norteamérica.

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