Mamu Formation Research Articles

99/02357 Studies on the progressive extraction and depolymerisation of coal macerals

The rank and composition of Upper Cretaceous-Tertiary coals from five localities (Enugu, Orukpa, Okaba, Ihioma and Azagba-Ogwashi) in Southern Nigeria are described. Ranks determined by huminite reflectance measurements indicate a range from sub-bituminous C to sub-bituminous A coalification grade for the Lower Maastrichtian coals of the Mamu Formation whereas the Oligocene-Miocene coals in the Ogwashi-Asaba Formation are predominantly of lignite rank. The highest grade of sub-bituminous A was attained in the Enugu area (Onyeama mine) in the south and this decreases towards the Okaba mine where ranks are within the sub-bituminous C coalification grade.Composition was determined by maceral analyses. Results indicate that the Lower Maastrichtian sub-bituminous coals are characterized by moderate to high concentrations of huminites (humodetrinites, humocollinite and humotellinite) and varied concentrations of inertinites and liptinites. Concentration of large amounts of liptinites (especially resinite, sporinite and cutinite) at the bottom of the Orukpa seam indicate the wet conditions of a reed swamp depositional environment, whereas the large abundance of inertinites (mainly fusinite) on the top parts of the Onyeama and Okaba seams is interpreted as a shift to drier conditions, possibly forest swamp facies. The occurrence and predominance of humodetrinite (attrinite and densinite) in the Tertiary lignites of Ihioma and Azagba-Ogwashi are interpreted as indications of reed swamp depositional environments.We conclude that peat depositional environments for the Lower Maastrichtian coals was dominated by alternating high and low groundwater tables which resulted from recurrent subsidence. Forest and reed (marsh) swamps appear to have developed in delta plain environments. Lignites in the Oligocene-Miocene stages accumulated in reed swamps of a less telmatic depositional environment.

Depositional environment and diagenesis of carbonates at the Mamu/Nkporo formation, anambra basin, Southern Nigeria

At Leru-Okigwe, the Enugu-Port Harcourt expressway cuts through the Campanian Nkporo Shales which pass upwards into the cyclic, ripple laminated sandstones and shales of the Lower Maastrichtian Mamu Formation. The Mamu Formation at this locality consists of a 60 m thick shale-sandstone sequence with the basal and middle part of the section consisting of a total of 9 carbonate units. These carbonate units vary from 10 to 70 cm in thickness, cyclically interbedded with shales and are overlain by coarsening upwards sandstone bodies. Detailed mapping and petrographic studies indicate that the carbonate units are divisible into a lower finely laminated mudstone which passes upwards into oolitic packstone/grainstone in the middle and is overlain by an upper set of laminated mudstones. The lowest mudstone unit (dark grey to greenish rock) is finely laminated, pelleted, oncolitic and sparsely fossiliferous. The oolitic packstone/grainstone consists of oolites cemented together by siderite microspar. Identifiable bioclasts include tests of small size benthic foraminifera, pelecypods and rare ostracod carapace. This unit attains a maximum thickness of about 70 cm. The upper mudstone units consist essentially of uniformly recrystallised siderite microspar. Intraclasts include micritised pelecypod fragments and small foraminifera tests. Ovoid, flat bottomed and biconvex vugs developed good geopetal structures in the mudstone. Petrographic, Xray diffraction and microprobe analyses indicate that the carbonate constituent in these units consists of solid solutions of FeCO 3MgCO 3CaCO 3 and minor MnCO 3. Sideritization, the dominant replacement process has led to the recrystallization of the micritic matrix and microcrystalline siderite is commonly associated with goethite, chamosite relics and quartz. The carbonates with associated chamosite are thought to have formed in a shallow marine subtidal to intertidal environment developed during periods of rise and fall in sea level. Formation of chamosite-bearing oolites record periods of increasing wave energy corresponding to storm conditions between quiet shallow marine sedimentation. At least five diagenetic stages involving micritization, dissolution of the primary chamosite, replacements of chamosite by siderite cement, growth of blocky calcite and a continuing replacement of the preexisting minerals by goethite were establised from textural and compositional evidence. The recognition of shallow marine subtidal to intertidal environments for carbonates at the Mamu/Nkporo Formation transition supports continuous marine influences with periodic subaerial exposure of sediments in the Mamu deltas after the deposition of the prodeltaic Nkporo Shales.

Environmental pollution from coal mining activities in the Enugu area Anambka state Nigeria

Hydrogeological studies of the Enugu coal mining area were carried out which included hydrogeochemical analyses of water samples. These analyses revealed high sulphate and iron content in the acid mine drainage water as well as high total dissolved solids' (TDS) and low pH (acidity) values. The water issues from the Ajali Sandstone formation and the underlying carbonaceous Mamu Formation and is classified as hard water. As a consequence of under-mining this aquifer, huge volumes of (polluted) water has flooded the mines are channelled into some streams or rivers which in turn get chemically polluted. Remedial measures have been indicated which include the following:

Rank and petrographic composition of selected upper cretaceous and tertiary coals of Southern Nigeria

The rank and composition of Upper Cretaceous-Tertiary coals from five localities (Enugu, Orukpa, Okaba, Ihioma and Azagba-Ogwashi) in Southern Nigeria are described. Ranks determined by huminite reflectance measurements indicate a range from sub-bituminous C to sub-bituminous A coalification grade for the Lower Maastrichtian coals of the Mamu Formation whereas the Oligocene-Miocene coals in the Ogwashi-Asaba Formation are predominantly of lignite rank. The highest grade of sub-bituminous A was attained in the Enugu area (Onyeama mine) in the south and this decreases towards the Okaba mine where ranks are within the sub-bituminous C coalification grade. Composition was determined by maceral analyses. Results indicate that the Lower Maastrichtian sub-bituminous coals are characterized by moderate to high concentrations of huminites (humodetrinites, humocollinite and humotellinite) and varied concentrations of inertinites and liptinites. Concentration of large amounts of liptinites (especially resinite, sporinite and cutinite) at the bottom of the Orukpa seam indicate the wet conditions of a reed swamp depositional environment, whereas the large abundance of inertinites (mainly fusinite) on the top parts of the Onyeama and Okaba seams is interpreted as a shift to drier conditions, possibly forest swamp facies. The occurrence and predominance of humodetrinite (attrinite and densinite) in the Tertiary lignites of Ihioma and Azagba-Ogwashi are interpreted as indications of reed swamp depositional environments. We conclude that peat depositional environments for the Lower Maastrichtian coals was dominated by alternating high and low groundwater tables which resulted from recurrent subsidence. Forest and reed (marsh) swamps appear to have developed in delta plain environments. Lignites in the Oligocene-Miocene stages accumulated in reed swamps of a less telmatic depositional environment.

Coal occurrence related to sedimentary dynamics: three African examples

Three African examples of coal occurrence vary greatly in geodynamic conditions for peat (coal) development. In the Anambra basin, near Enugu (Nigeria), Maastrichtian coal bearing strata developed after settling of the prodeltaic type Enugu Shales. In the Mamu formation, coal occurred landwards of the shoreline turnaround of an ocean margin and is related to relative high stands of sea. Peat formation occurred during an early transgressive phase, as seen by palynological and ichnological associations or sand wave development. In the Westphalian Jerada basin (Morocco), from sequence analysis, there is evidence of repeated filling up to flooding succession. Coal occurred after filling up of the basin, at the beginning of a transgressive phase which probably curtailed detrital influx in the peat forming area. In some Stephano-autunian intramontane basins near Marrakech (Morocco), coal occurs in proximally braided systems (Aït Ziffa) or in flood-plain deposits associated with red beds (Haouz). Local environmental conditions allow the development of coal in restricted areas sheltered from detritic supply.

Clavate borings in a Maastrichtian woodground in southeastern Nigeria

An occurence of the clavate borings Teredolites clavatus protruding from the base of a silty sandstone into the top surface of a coaly layer is described from the Maastrichtian Mamu Formation of southeastern Nigeria. The coaly layer represents a woodground, which extends over 500 m in length. The borings have been deformed by compaction prior to lithification suggesting that they were formed while the woodground was still soft. The boring activities probably occured at an early stage in the history of the sand-filled channel within a deltaic environment.

Paleogeography, sedimentation and tectonics of the upper cretaceous Anambra basin, southeastern Nigeria

A reinterpretation of the depositional environments of the Anambra Basin succession in the light of modern sedimentological models has necessitated a revision of the late Cretaceous paleogeography of southeastern Nigeria. The phases of transgressions and regressions in the Campanian in the Campanian to Paleocene within the Anambra Basin are reviewed, based on these new interpretation. Thus, it has been shown, for example, that the Mamu Formation and the overlying Ajali Sandstone are both part of a continued controlled Maastrichtian transgression, which started in the late Campanian, superimposed on a regional geoidal eustatic sea level rise, which lasted until the Paleocene. This contrasts with the interpretation favouring a regressive phase of a short-lived Maastrichtian transregression, as has been previously thought by some workers. Tectonic movements within the basin have been shown to have been active during the late Cretaceous sedimentary cycles, as demonstrated by variations in the thickness patterns across the basin. Lateral facies variations both within and across formation boundaries, from north to south, as well as the occurrences of a large-scale slumps and other types of syn-sedimentary deformation structures attributed to tectonic activity further attest to the active nature of the basin during the period of sedimentation. The pattern of subsidence is also thought to have been influenced by earth tremors and periods of “quiescene”, particularly in the northern Ankpa sub-basin.