Petrography, lithofacies and depositional environment of carbonate-clastic strata in part of the north-western Douala Basin, Cameroon

Abstract

The lithofacies, petrofacies and sedimentary environments of carbonate-siliciclastic strata of the Tertiary Nkapa Formation have been examined. This paper provides new data on the carbonates whose location and composite thicknesses of outcrop lithologies represent an important segment of the broader West-Central African Platform. The main carbonate outcropping belt is along the Bongoue River and its tributaries. Field and petrographic investigations reveal that carbonates in the study area consist of a number of lithologically distinct facies. These facies are non-skeletal to skeletal in composition including olithic, perlithic, bioclastic and marly limestones which occur in alternation with sandstones and shales. The studied limestones were mostly grain-supported exhibiting textures that range from grainstone, packstone to wackestone. Non-skeletal bioclasts are mainly made up of superficial and composite ooids, and peloids showing grainstone and packstone texture, respectively. The ooids have grain diameter varying from 0.1 to 1.5 mm. The pellets are of faecal origin. The bioclastic limestones are characterised with a packstone to wackestone texture (fifty–fifty), typically associated with nodules and abundant skeletal fragments suggesting deposition in shallow marine barrier reef settings. This interpretation is further supported by the presence of worm tubes in the limestones. Micritic and sparry calcite cement and opaque grains (FeS2) make up the matrix and infill within skeletal fragments. The limestones range from abundant biosparites to minor biomicrites.The marly limestones are characterised with a wacke-to-packstone texture, mainly made up of abundant lithoclasts and minor bioclasts indicating their deposition in areas closer to the continent with large influx of terrigenous clastic material. Sandstones are mostly sublitharenites (>75 %) with minor litharenites (≤25 %). The different lithofacies (carbonates, sandstones, shales) investigated demonstrate that these sediments were deposited in a wide range of environments that varies from continental shelve zones, shallow marine (reef related) to relatively deep marine environments.