Imo Formation Research Articles

Evaluation of Geology and Hydrochemistry of Nawfia-Agulu Axis of Anambra State, Southeastern Nigeria

The investigation of the geological, hydrochemical, and engineering characteristics of the Nawfia – Agulu area was carried out. The study aimed at mapping the geology through field data collection, outcrop logging, and laboratory analysis. Consequently, eleven soil and ten water samples were collected and analyzed through particle size distribution test; Atterberg limits tests and hydrochemical analysis to provide insights into the lithology, sediment characteristics, and water quality. The results indicated the dominance of the Ameki/Nanka Formation and Imo Formation as the underlying geology of the study area. The grain sizes of the sand samples ranged from medium to coarse grained, moderately to poorly sorted, and strongly positive to positively-skewed. The depositional environment was nearshore/beach, influenced by nearshore waves and turbulent currents. Engineering properties highlighted the prevalence of uniformly to well-graded soil and moderately to highly plastic behavior, as indicated by the mean plasticity index (PI) value of 26%. These findings suggest potential challenges for infrastructural development in some parts of the area. The water quality indicated slightly acidic water. The physical parameters and the major ions were within the permissible limits of World Health Organization (WHO) and the Nigerian Standard for Drinking Water Quality (NSDWQ). However, elevated concentrations of heavy metals, including lead, chromium, mercury, iron, and cadmium, in water sources exceeded the respective permissible limits for drinking water indicating contamination/pollution. The dominant water type was Ca2+ – Mg2+– SO42- – Cl– and the dominant hydrochemical process was simple dissolution or mixing and ion-exchange.

On Paleozoic platycerate gastropods

The platycerate gastropods Orthonychia yutaroi Ebbestad, sp. nov. (Ordovician, Boda Limestone, Sweden), O. enorme (Silurian, Sweden, Gotland), O. parva (Pennsylvanian, Finis Shale Member, USA), and Orthonychia sp. (Mississippian, Imo Formation, USA) are studied including their protoconch morphology. Orthonychia yutaroi is the oldest known species in Orthonychia. Platycerates contain species with both, openly and tightly coiled protoconchs. This is the first report that tightly coiled protoconchs occur in Orthonychia. This and previously published observations blur the diagnostic difference between orders Cyrtoneritimorpha (openly coiled protoconch) and Cycloneritimorpha (tightly coiled protoconch). We suggest to treat Cyrtoneritimorpha and Cycloneritimorpha as synonyms of Neritimorpha. The monotypic Devonian genus Pragoserpulina is morphological so close to the Orthonychia species reported herein that synonymy of both genera seems to be possible (and thus of the families Pragoserpulinidae and Orthonychiidae). Protoconch morphology and dimensions suggest that the studied platycerate species had planktotrophic larval development. By contrast, two studied Carboniferous euomphaloid species (one with an openly and the other with tightly coiled protoconch) have paucispiral, large protoconchs indicating non-planktotrophic larval development. We assume that openly and tightly coiled protoconchs were present in various Paleozoic gastropod clades and that selection acted against the openly coiled protoconch morphology. It has previously been proposed that increasing predation pressure in the plankton was the reason for the demise of openly coiled protoconchs (Paleozoic plankton revolution). The presence of larval planktotrophy in platycerates excludes the possibility that they belong to extant basal gastropod clades such as Patellogastropoda, Cocculiniformia, and Vetigastropoda. However, a previously proposed close relationship to Neritimorpha is corroborated.

Depositional Facies and Palynofacies Provenance of Clastic Deposits: Insight from Paleocene Strata in Southeast Region, Nigeria

Detailed sedimentary facies and palynofacies provenance studies were carried out on clastic deposits outcropping around the Awka area within the Imo Formation (Paleocene) of the Niger Delta Basin. Seven facies were systematically recognized based on the textural parameters, sedimentary structures and palynological constituents. Interpretation of the prevailing paleoenvironments, palynofacies provenance and depositional mechanisms were derived from the hydrodynamic controls. The sedimentary facies include shales, mudstones, siltstones, heteroliths, cross-bedded sandstones and matrix-supported conglomerates. The palynofacies showed preponderance of amorphous organic matter, marine taxa, opaque particles, with few structured phytoclasts and terrestrial microflora. These palynological microflora constituents are grouped into palynofacies association A and B based on the composition and proportion of the organic particles, stratigraphic position and paleoenvironments of the strata. Facies analysis indicates that the mudrock facies (e.g. shale, mudstone and siltstone) are deposits of deep marine depositional settings with terrestrial input while the sandstones mimic deposits associated with tidal sand waves in shallow sandy seas, braided river and tidally influenced distributary channels. The facies and palynofacies provenance model suggest dominance of terrestrially influenced shallow to deep marine environment evidenced from the abundance of amorphous organic matter and presence of gonyaulacacean dinoflagellate species, Glaphyrocysta ordinata, G. exuberans, and pollen of Proxapertites operculatus, Retitricolporites irregularis with other accompanying marine forms. In addition, the presence of texturally mature coarse- to fine-grained sandstones, variable sedimentary structures, internal bed geometries and nature of bedding are suggestive of tidally dominated shallow marine settings including distributaries channel network deposits.

Stratigraphic palynology and micropaleontological study of the Cretaceous-Paleogene succession of the Nani-1 well of the Anambra and Niger Delta basins: Implications for hydrocarbon exploration

Integrated palynological and micropaleontological analyses were carried out on the Nani-1 well drilled into the Anambra and Niger Delta basins of Nigeria. The study is aimed at reconstructing biozones and paleoenvironments of deposition, as well as deducing the sequence stratigraphic framework of the Cretaceous-Paleogene successions of an area containing key elements for hydrocarbon exploration. Encountered lithofacies succession of sandstone, siltstone, dark-grey to black shale, and coal beds comprises six lithostratigraphic units, namely the Nkporo Group (Campanian-Maastrichtian), Mamu Formation (Middle Maastrichtian), Ajali Formation (Middle-Late Maastrichtian), Nsukka Formation (Late Maastrichtian), Imo Formation (Danian-Thanetian) and Ameki Group (Eocene). Fourteen dinoflagellate zones (A-N) and seven pollen and spore subzones (A1-G1) are recognized based on the first occurrence of two or more species. Two major stratigraphic events are documented: (1) dinoflagellate cyst (dinocyst) zone G at 880 m depth marked by first appearance of Danea aff. californica and Danea cf. mutabilis in association with Cordosphaeridium fibrospinosum, and the last occurrence of Danea aff. californica and Phelodinium magnificum place the Cretaceous-Paleogene (K/Pg) boundary in the lowermost part of the Imo Formation; and (2) a hiatus characterized by a starved section observed at 380 m directly overlain by the Early Eocene assemblages of the Ameki Group indicates a sequence boundary at Imo/Ameki Boundary. Thus, a chronostratigraphic chart is developed for parts of the Anambra Basin and the Niger Delta. Six depositional environments which include marginal to shallow marine, marshy lagoon to estuarine, shallow neritic and tidally influenced fluvial channel are identified, and palynostratigraphic evidences indicate that fresh water palynomorphs are present in the Lower Maastrichtian strata. Four sequence boundaries and four maximum flooding surfaces are also identified within the Maastrichtian–Early Eocene sediments. The paleoenvironmental reconstruction shows that possible hydrocarbon reservoirs in the basin are the marine sandstone facies of the Nkporo Group and Mamu Formation. Their shale members constitute potential source rocks, while the overlying shale units of the Nsukka and Imo formations may provide regional seals for hydrocarbon habitation.

Palynostratigraphy and Palaeoenvironmental Interpretation of Outcrop Formations in Parts of Ini-Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria

The palynological study of the outcrop Formations in parts of Ini Local Government Area of Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria has contributed to unraveling the palynostratigraphy and paleoenvironments of the outcrops through the use of diagnostic palynomorphs. The section is made up of carbonaceous shales, sandstones, mudstones and claystones. The conventional maceration technique for recovering acid-insoluble organic-walled microfossils from sediments was used to prepare twenty outcrop samples for palynological studies. The samples produced poor to fair diversity assemblages of palynomorphs. A total of 176 palynomorphs species were recorded, with 48% pollen grains, 33% spores, 12% dinoflagellate cysts, 5% fungal spores, and 2% microforam test wall linings. Based on stratigraphically selected index taxa recovered,age determination and correlation wereestablished. The samples from Imo Formation were assigned Late Paleocene-Eocene based on Psilatricolporites Crassus, Retitricolporites irregularies, Psilatricolporites sp. and Leiotriletes adriennis with the presence of Paleocene dinoflagellate cysts such as Lejeunacysta beninensis and Selenopemphix nephroides. Ameki samples were dated Early-Middle Eocene based on Proxapertites operculatus, Retistephanocolpites williamsi, Mauritidites crassiexinius, Monocolpites marginatus and Longapertites marginatus. Ogwashi-Asaba shale samples were assigned Late Eocene-Early Oligocene due to the abundant presence of Verrucatosporites usmensis, Laevigatosporites discordatus and Retitricolporites irregularis while Eze-Aku samples were tentatively assigned Paleocene because of the presence of two pollen index fossils Proxapertites operculatus and Pachydermites diederixi. Inferred depositional conditions suggest that Imo Formation was deposited in a shallow marine or near shore brackish water estuarine environment. The Ameki Formation was accumulated in a marginal marine or near shore, brackish water-estuarine environment in the upper deltaic setting. The Ogwashi-Asaba Formation was deposited in a progradational shoreline moving between the brackish water and fresh water deltaic plain. The Eze-Aku Formation was deposited in an upper foreshore fresh water environment. The palynological study of the samples are Paleogene Formations while the paleoenvironment is in a costal deltaic to shallow-marine environment and are appropriate for hydrocarbon accumulations and exploration.

Groundwater exploitation in Awka (Anambra – Nigeria) and environs: Prospects, and challenges while drilling and its mitigation measures

Groundwater exploitation (borehole drilling) was carried out around Awka and environs in Anambra State, Southeastern Nigeria, to understand the underlying rock units encountered while drilling, differentiate boreholes with confined aquifers from those with unconfined aquifers, delineate the probable aquiferous zones from the borehole data, evaluate the challenges encountered while drilling (both geologic and technical), and identify mitigation measures employed to address these challenges. Detailed geologic log information of the boreholes was produced to illustrate the rock units encountered while drilling. Four rock units were identified, namely: shale, sandstone, clay, and gravel. These rock units were exposed within the Imo Formation and the Nanka Formation that underlie the study area. Results from the geologic log information of the boreholes indicate that the water table within the study area ranges from 11.2 m to 56.5 m from the soil surface, and the probable aquiferous zones vary from 6.8 m to 23.3 m in thickness. A detailed look at the lithologic logs of the boreholes show that 50% of the drilled boreholes possess confined aquifers while the remaining 50% have unconfined aquifers. A careful appraisal of the challenges encountered, which are mainly geologic, is strictly attributed to the geologic formation of the study area. Other technical challenges have been derived from mechanical faults developed during drilling.

Sequence stratigraphical interpretation of the Paleocene to Miocene (Selandian–Aquitanian) palynofacies framework of the Niger delta basin, southeastern Nigeria

Palynofacies analysis of the outcropping Paleogene to early Neogene formations in the Niger Delta provides a snapshot of the palynofacies framework and events related to sequence stratigraphic cycles during the Cenozoic. Five palynofacies groups defined by particulate organic matter were synthesized into four main palynofacies sequence stratigraphic events denoted as A to D, recognized from base to top of the Imo, Nanka and Ogwashi formations. Palynofacies sequence stratigraphic event A in the basal dark grey shales of the Imo Formation is dominated by amorphous organic matter (AOM), opaque particles, and marine palynomorphs, and small numbers of terrestrial microflora suggestive of a transgressive systems tract (TST) for the Selandian section of the formation. High percentages of AOM and opaque particles, paucity of structured phytoclasts, and maximum numbers of dinoflagellate cysts were observed in palynofacies stratigraphic cycle B in the Thanetian section of the Imo Formation, indicating deposits of an early TST. The absence of lath-shaped opaque debris and few equidimensional forms indicated marine depositional origin for the transported opaque debris. A lowstand systems tract (LST) was interpreted for the fining upward sandstone unit separating these TST packages. Palynofacies sequence stratigraphic cycle C, which was identified in the Nanka Formation, was characterized by an increase in terrestrial microflora, structured phytoclasts, and opaque particles, with relative small numbers of dinoflagellate cysts and AOM signifying late highstand systems tract (LHST) for the Ypresian sediments. The attributes of palynofacies stratigraphic cycle D established in the Ogwashi Formation were the maximum numbers of terrestrial palynomorph taxa, structured phytoclasts, opaque debris, abundant AOM and limited amounts of marine species, suggestive of the LST during the Priabonian to Aquitanian. The Niger Delta Basin sediments are interpreted as deposits of the earliest Danian to Thanetian transgression, Ypresian highstand interval and Priabonian to Aquitanian regression interrupted by minor marine transgressions. The palynofacies sequence stratigraphic cycles model demonstrates that an overall basin-wide retrogradation was succeeded by progradation of the delta during the later Cenozoic.

Palynomorph assemblage biozonation of Paleogene strata in Bende\u2013Umuahia Area, Niger Delta Basin, southeastern Nigeria

Cenozoic sediments form extensive outcrops in the Niger Delta Basin. Detailed palynostratigraphic study was undertaken across Paleogene sequences exposed in Bende–Umuahia Area in up-dip sectors of the Niger Delta Basin, southeastern Nigeria, to establish different palynomorphs assemblage zones, with their corresponding ages. Palynological analysis was carried out on 27 selected outcrop samples, using the conventional maceration technique for recovering acid-insoluble organic-walled microfossils from sediments. Three Cenozoic lithostratigraphic units, including Imo Formation, Ameki Formation, and Ogwashi Formation, are exposed in the study area. Lithologies are sandstone, carbonaceous shale, mudstone, limestone, and the lignite. A total of 65 species of sporomorphs and 51 dinoflagellate cysts were identified. The recovered spores and pollen grains were used to establish six informal palynomorph assemblage zones, labeled as zone A – zone F, based on the first and the last occurrences of two or more species. These palynomorph assemblage zones include: (1) zone A — middle Paleocene Scabratriporites simpliformis–Bombacidites annae zone; (2) zone B — late Paleocene Foveotricolporites crassiexinus–Mauritidiites crassiexinus zone; (3) zone C — early Eocene Striatopollis catatumbus–Momipites africanus zone; (4) zone D — middle Eocene Margocolporites umuahiaensis–Gemmastephanocolporites brevicolpites zone; (5) zone E — late Eocene Cicatricosisporites dorogensis–Perfotricolpites nigerianus zone; and, (6) zone F — Oligocene–early Miocene Verrucatosporites usmensis–Magnastriatites howardii zone. The erected palynozones were correlated and compared with existing biozones in subsurface, down-dip sectors of the Niger Delta Basin, with pantropical palynological zones in tropical areas of Africa, and with palynofloral provinces of northern South America. A comparison of palynozones studied in southeastern Nigeria with other international palynozones, in this study, will assist in establishing the correlation of sediments for these areas.

Sequence Stratigraphic Interpretation of Late Cretaceous-Early Paleogene Sediments in the Central Part of Southern Anambra Basin, Nigeria

ABSTRACT The identification of additional hydrocarbon reserves to buffer pressure from increasing energy demands in Nigeria necessitated the application of sequence stratigraphic framework in the analysis of sedimentary successions in the central portion of southern Anambra basin. Wireline logs and biofacies data from three exploratory wells (S-1, S-2 and S-3) were used for the study. The stratigraphy of the study area observed from the three wells showed an overall regressive succession with short-lived transgressions. Lithofacies associations deduced from well log signatures defined four lithostratigraphic units: The Mamu and Ajali formations of kate Maastrichtian age; the Nsukka Formation of kate Maastrichtian - Danian age and Imo Formation of Paleocene age. The study further used the depositional sequence model to identify sequences and accompanying systems tracts that are bounded at the top and bottom by unconformities. Dating of identified key stratigraphic surfaces was achieved by correlating chronostratigraphic biofacies data to third order cycle charts. Correlation across the three wells highlighted spatial distributions of reservoirs and some useful stratigraphic and structural discontinuities that could form hydrocarbon traps. It also showed profitable stratigraphic surfaces that would aid basin-wide correlation for improved horizon(s) mapping and hydrocarbon prospectivity in the Anambra basin. Erosional unconformities identified in the wells that correlated with major drops in global sea level, fingerprinted the influence of eustacy on sedimentation and sequence development. Other factors such as subsidence and sediment supply have direct relationships to the identified structures (faults) which also initiated the accommodation created for sedimentation in the study area.

Assessment of some pristine sand deposit from Imo Formation (southeastern Nigeria) as construction aggregate and industrial material

Ebenebe sand deposits from the designated Imo Formation (southeastern Nigeria) were evaluated using geologic, geochemical, and geotechnical techniques to ascertain the suitability for use as fine aggregate and industrial material. Results show that this formation consists of mainly quartz that show predominance of angular to subrounded sphericity, oblate to equant mineral shapes, while elongate and platy shaped grains are rare. It exhibits poorly sorted and well-graded particles that range from fine to coarse sand-sized grains. Of all the specimens taken from this geological deposit, geochemistry is dominated by a very high percentage of SiO2 (> 96%). The concentrations of Al2O3 and Fe2O3 vary from 0.58 to 2.2% and 0.09 to 0.89%, respectively. Sands show insignificant activity on soaking with water and caustic soda which demonstrate the paucity of deleterious minerals and organics. The properties of the sand deposits are evaluated based on the depositional and geomorphological environment and processes. The properties of the Ebenebe sand deposits were compared with the characteristics of other sand deposits having proven service performance and those specified by some referenced standards which implies that the studied sands possess a high potential for use as aggregate as well as industrial material, with some beneficiation.

Oil Shale Prospects of Imo Formation Niger Delta Basin, Southeastern Nigeria: Palynofacies, Organic Thermal Maturation and Source Rock Perspective

The search for oil and gas in the Nigerian inland basins and efficacy of the unconventional oil shale in hydrocarbon generation have necessitated the palynofacies characterization, thermal maturation and source rock evaluation of the Imo Formation, updip Niger Delta basin to determine its palynofacies constituents and oil shale potential. The standard acid maceration technique, palynofacies description as well as palynomorph colour index chart systematically compared to thermal alteration index and vitrinite reflectance index values were utilized for the study. The palynofacies components of the Imo Formation is dominant of yellow and brown amorphous organic matter, marine taxa, opaque particles, with few dark brown structured phytoclasts and terrestrial microflora. The source rock content is interpreted as Kerogen Type II - oil prone with abundant marine palynomorphs, amorphous organic matter and few terrestrial microfossils. The spore/pollen colour of the strata is light brown signifying mature and oil generation zone. This correlates to thermal alteration index (TAI) 2+ to 3 and vitrinite reflectance index (%Ro) values of 0.5% to 1.0%. The abundance of the amorphous organic matter of marine origin suggests good source rock which indicates good oil shale prospects. The oil shale prospects of the Imo Formation in the study area are hindered by the low abundance of the palynofacies associations. Field evidence indicate that the grey to slightly light grey shales in hand specimen, as against black carbonaceous shales rich in organic matter suggests that the environmental conditions affected the quality of the organic kerogen. The oil yields may be compensated by the great volume of marine shale deposited in the formation and the abundance of amorphous organic matter of marine origin which is known to be an excellent indicator of oil. The palynofacies model enhances the understanding of palynofacies events and oil shale quality which can be tied to the development of unconventional oil shale, oil fields and subsequent exploration drive in the area.

Sequence stratigraphic interpretation of parts of Anambra Basin, Nigeria using geophysical well logs and biostratigraphic data

The Anambra basin constitutes the southeastern lower portion of the Benue Trough, which is a large structural depression that is divided into lower, middle and upper parts; and is one of the least studied inland sedimentary basins in Nigeria. Sequence stratigraphic interpretation had been carried out in parts of the Anambra Basin using data from three wells (Alo-1 Igbariam-1 and Ajire-1). Geophysical well logs and biostratigraphic data were integrated in order to identify key bounding surfaces, subdivide the sediment packages, correlate sand continuity and interpret the environment of deposition in the fields. Biostratigraphic interpretation, using foraminifera and plankton population and diversity, reveals five maximum flooding surfaces (MFS) in the fields. Five sequence boundaries (SB) were also identified using the well log analysis. Four 3rd order genetic sequences bounded by maximum flooding surfaces (MFS-1 to MFS-6) were identified in the areas; four complete sequences and one incomplete sequence were identified in both Alo-1 and Igbariam-1 wells while Ajire-1 has an no complete sequence. The identified system tracts delineated comprises Lowstand Systems Tracts (progradational to aggradational to retrogradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational to progradational packages) in each well. The sand continuity across the fields reveal sands S1 to S5 where S1 is present in Ajire-1 well and Igbariam-1 well but not in Alo-1 well. The sands S4 to S5 run across the three fields at different depths. The formations penetrated by the wells starting from the base are; Nkporo Formation (Campanian), Mamu Formation (Late Campanian to Early Maastrichtian), Ajali Sandstone (Maastrichtian), Nsukka Formation (Late Maastrichtian to Early Palaeocene), Imo Formation (Palaeocene) and Nanka Sand (Eocene). The environments of deposition revealed are from coastal to bathyal. The sands of lowstand system tract and highstand system tract found in Ajali, Nsukka, Nkporo and Imo (Ebenebe Sandstone) Formations show good continuity and as such good reservoir qualities while the shales of the transgressive system tracts which includes the Imo Formation, Mamu, and Nkporo Formations where most of the maximum flooding surfaces were delineated, can serve as seals to the numerous reservoir units. Combinations of the reservoir sands of the lowstand system tract and highstand system tract and the shale units of the transgressive system tract can form good stratigraphic traps for hydrocarbon and hence should be hydrocarbon exploration targets.

Stratigraphic relations and habitat of the Bende fish teeth, southeastern Nigeria

The age and habitat of the Bende fish fossils have remained a matter of controversy with interpretations ranging between the Paleocene Imo Formation and the Eocene Ameki Formation. Outcrop-based sequence stratigraphic analysis presented in this work shows that the Imo and Ameki Formations together include three laterally extensive estuarine central basin/offshore mud rock units that are associated with the 59.7, 58.1, and the 56.8 ma maximum flooding surfaces, respectively. Field evidence and stratigraphic analysis show that the Bende fossil bones are hosted by the mud rock unit containing the 58.1 ma maximum flooding surface, while the Ameke bone fossils are contained within the condensed section associated with the 56.8 ma maximum flooding surface. Apart from highlighting the relevance of sequence stratigraphy in paleobiological interpretations, this study shows that the Bende fish fauna which Arua first assigned to the Eocene Ameki Formation indeed belongs to the Paleocene Imo Formation.

Tainoceratid and Liroceratid Nautilids from the Upper Mississippian Imo Formation of Arkansas, Midcontinent North America

Abstract. As the third installment of our study to describe the cephalopod fauna of the Upper Mississippian Imo Formation, four species of late Chesterian (= Serpukhovian; late early Carboniferous) nautilids are recorded from the dark gray shale of the formation in northwest Arkansas, Midcontinent North America. They include a tainoceratid, Tylonautilus gratiosus (Girty), and three liroceratids, Bistrialites bicostatus (Gordon), Condraoceras? sp. and Peripetoceras kummeli sp. nov. Diagnostic features of P. kummeli separating it from comparable species are its depressed reniform whorl sections with relatively high width/height ratios, large convergent angle of the flanks, and subcentral to subdorsal siphuncular position.

Sedimentary petrology and provenance interpretation of the sandstone lithofacies of the Paleogene strata, south-eastern Nigeria

The Paleogene strata in the south-eastern Nigeria include the Imo Formation, the Ameki Group and the Ogwashi Formation. Petrographic studies suggest quartz arenite to arkosic arenite and sub-arkose for sandstones of the Imo Formation. Sandstones of the tide-dominated estuarine system of Ameki Group are classified as quartz arenites, sub-arkoses, sub-litharenites and litharenites. Sandstones of the Ogwashi Formation are classified as quartz arenites and sub-litharenites. These Paleogene sandstones are derived from mixed provenances based on the petrographic interpretation. The provenances include cratonic interiors, transitional continental blocks as well as recycled orogenic and transitional continental, transitional recycled and quartzose recycled. Paleoclimatic condition for the Paleogene region is semi-humid to humid climate. The sandstone composition suggests detritus from igneous rocks, metamorphic rocks and recycled sedimentary rocks. Heavy mineral analysis confirms the possibility of mixed provenance for the Paleogene sediments. The source areas include the Oban Massif, the West African Massif and the recycled sedimentary rocks of the older sedimentary rocks of probably the Anambra Basin and Afikpo Syncline.

Paleocene ichthyofauna and paleoenvironmental setting, Imo Formation, southeastern Nigeria

The Paleogene vertebrate fossil record of Nigeria has until recently been limited to marine deposits in the Eocene-age Ameki Formation. New work in the Imo Formation of southeastern Nigeria has revealed a diverse ichthyofauna of Paleocene age. The Imo Formation overlies the Nsukka Formation, and underlies the Ameki Formation. The uppermost bone-bearing interval of the Imo Formation is separated from the overlying Ameki Formation by a NW-SE trending, shoreface- tidally-influenced sandstone ridge that dominates the Ozuitem-Umuahia landscape. The Ameki beds outcrop to the west of the sandstone ridge, whereas the Imo outcrops to the east. Fossils described herein were recovered from an interval of interstratified crudely laminated clay/shale and calcareous lenses of well-sorted sandstone/siltstone which outcrops to the east of the sandstone ridge in the Bende District. The fauna is represented by at least seven taxa, including an actinopterygian, five chondrichthyans (represented by one ray and four shark species), and Cylindracanthus, an enigmatic incertae sedis taxon. Five of the species in the Imo fauna appear to have survived into the Eocene, whereas the remaining two are hitherto undocumented in younger faunas of Nigeria.

Arcuatoceras, a New Genus of Nautiloid Cephalopods from the Early Carboniferous in the Midcontinent of North America

A cyrtoconic nautiloid, Arcuatoceras saundersi gen. et sp. nov., from the upper shale member of the Imo Formation (late Chesterian, Early Carboniferous) in north central Arkansas, U.S.A., is described and tentatively assigned to the family Scyphoceratidae. One specimen of the new taxon retains the embryonic conch and the earliest growth stages of the post-hatching shell; this specimen is the first record of the embryonic shell of nautiloid cephalopods having cyrtoconic brevicones in the Carboniferous. The morphology of the shell ornament and the shape of the early shell suggest that the newly hatched animal was an active swimmer after hatching, whereas in later growth stages, the animal probably lived and crawled on the bottom as part of the nektonicbenthonic community. Additionally, some conchs of this new genus show the remains of color patterns preserved as dark gray markings on a lighter gray background. There is considerable variation in the color patterns. One small specimen has conspicuous longitudinal bands and spots on the venter and ventrolateral surface together with a faint chevron on the lateral and dorsolateral parts of the conch. Several larger specimens have only well developed chevrons on the venter, ventrolateral, lateral, and dorsolateral parts of their conchs. This color-pattern variation is interpreted as a change during ontogenetic development rather than as a systematic difference.

Designation Of The Standard Stratotype And Age Of Nsukka Formation Of Anambra Basin, Southeastern Nigeria

The type locality of Nsukka Formation was designated north of Nsukka in an area along Nadu River, without any formal description of the type section. Field mapping of the type locality was undertaken with the intention to formally define the type section. The thickest and best exposed is a 40 meter thick, stream section at Ugwu Nkalagu-Obukpa, designated the unit stratotype, and complemented by the road section at Umulolo near Okigwe as the hypostratotype. The unit stratotype contains coal towards the base and falls within the lower shaly facies of the formation. The hypostratotype belongs to the top facies without coal, having at the base ca. 0.8 m thick coquina sandstone. The palynomorphs of the type section indicate paralic sedimentation while at the complementary section marine conditions prevailed and continued across the K/T boundary into the Paleocene. The formation dates Late Maastrichtian at the type section and Paleocene (Danian) at the hypostratotype where a sharp boundary with overlying Imo Formation. Journal of Mining and Geology Vol. 43 (2) 2007: pp. 147-166

Trigonoceratid nautilids from the Early Carboniferous Imo Formation of Arkansas, Midcontinent North America

Four species of trigonoceratid nautilids from the late Chesterian (Early Carboniferous) strata in the Imo Formation of Arkansas, Midcontinent North America are described. A specimen of Aphelaeceras arkansanum Gordon, 1964 reveals the external juvenile whorl morphology. Based on the discontinuous points of its surface lirae, we suggest the possibility that the hatching shell of A. arkansanum is cyrtoconic with two septa and a length of approximately 4.9 mm. Species previously described or assigned to Aphelaeceras are reexamined. Epistroboceras lesliense sp. nov. differs from E. caneyense Niko and Mapes, 2004 and E. pitkinense Niko and Mapes, 2005 in its lower ratios of umbilical zone height per whorl height and lower siphuncular position ratios. Epistroboceras? sp. indet. is also identified in this fauna. A new genus, Imonautilus, is erected on the basis of I. meeki sp. nov. The most characteristic features of this genus are the Stroboceras-like juvenile whorl, the obsolescence of the adoral ventrolateral grooves at or near maturity, and the roundly inflated adoral venter. Based on early ontogenetic similarities, a close evolutionary relationship between Aphelaeceras and Imonautilus is suggested.

Lithofacies, palynofacies, and sequence stratigraphy of Palaeogene strata in Southeastern Nigeria

Integrated sedimentologic, macrofossil, trace fossil, and palynofacies data from Paleocene-Middle Eocene outcrops document a comprehensive sequence stratigraphy in the Anambra Basin/Afikpo Syncline complex of southeastern Nigeria. Four lithofacies associations occur: (1) lithofacies association I is characterized by fluvial channel and/or tidally influenced fluvial channel sediments; (2) lithofacies association II ( Glossifungites and Skolithos ichnofacies) is estuarine and/or proximal lagoonal in origin; (3) lithofacies association III ( Skolithos and Cruziana ichnofacies) is from the distal lagoon to shallow shelf; and (4) shoreface and foreshore sediments ( Skolithos ichnofacies) comprise lithofacies association IV. Five depositional sequences, one in the Upper Nsukka Formation (Paleocene), two in the Imo Formation (Paleocene), and one each in the Ameki Group and Ogwashi-Asaba Formation (Eocene), are identified. Each sequence is bounded by a type-1 sequence boundary, and contains a basal fluvio-marine portion representing the transgressive systems tract, which is succeeded by shoreface and foreshore deposits of the highstand systems tract. In the study area, the outcropping Ogwashi-Asaba Formation is composed of non-marine/coastal aggradational deposits representing the early transgressive systems tract. The occurrence of the estuarine cycles in the Palaeogene succession is interpreted as evidence of significant relative sea level fluctuations, and the presence of type-1 sequence boundaries may well be the stratigraphic signature of major drops in relative sea level during the Paleocene and Eocene. Sequence architecture appears to have been tectono-eustatically controlled.