Paleodepositional environments, lithostratigraphic bounding surfaces and basin-fill model of Mundeck Formation in Yabassi, NE Douala sub-basin

Abstract

Lithofacies, sedimentologic and x-ray analytical techniques were employed to reconstitute paleodepositional environments, bounding surfaces and basin-fill model of Mundeck Formation during Lower Cretaceous in Yabassi, NE Douala sub-basin. The basin-fill comprises fissile gravel, sand and mud (GSF) facies from which eighteen sub-facies were identified and grouped under five facies associations (FA1-FA5). These correspond to alluvial settings (FA1), braided channels (FA2), cross-bedded tidal channels (FA3), ephemeral fluvio-lacustrine lake with short-lived marine incursion (FA4), fluvial channels with deeply incised erosional surfaces (FA5). Four hierarchical lithostratigraphic bed bounding surfaces were defined; incipient or intraformational hardgrounds, erosional surfaces abutted by ripped-up mud clasts, an obvious transgressive surface marked by the occurrence of ammonitic in lamellibranch and gastropod-rich silty-mudstones and shales. Humid and warm climates during Lower Cretaceous could be inferred from secondary mineral suites: clays (kaolinite, chlorite, illite and smectite); oxides (hematite and hydralsite); and oxy-hydroxides (goethite). Hematite-rich incipient hardgrounds surfaces depict warm and dry climates, while paleosols suggest channel abandonment, subaerial exposure and pedogenetic processes. Sediment transport regimes have evolved from low velocity cohesive flow regime in braided channels and overbank to high velocity and cohesion-less flow regime in multi-storey cross-bedded sand bars. Granulometric results reveal angular to sub-rounded, coarse-grained to gravelly clast sizes with unimodal, bimodal- and trimodal distributions, whereby all analyzed samples are positively skewed towards fine-grained sands. Morphoscopic analysis reveals 65–90% of very angular to angular and shinning quartz (VAAS), compared to 10–35% sub-angular and sub-rounded (SASR) and 0% rounded (R) shapes which indicate short transport distance from source areas. Sorting index (So) and deviation quartile (dQ) values of analyzed sands vary from 1.52 to 2.89, and 0.38–0.88 respectively and are indicative of moderate to very poor sorting.