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 FeCO₃MgCO₃CaCO₃ and minor MnCO₃. 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.