Mechanisms of sedimentation in lacustrine environment of Babouri-Figuil and Mayo Oulo-Lere basins deposits (North Cameroon) during the Southern Atlantic opening: sedimentary structures and geochemistry constraints

Abstract

Sedimentary structures are very useful to characterize sedimentation environments. In this study, sedimentary structures are documented in the Lower Cretaceous Babouri-Figuil and Mayo Oulo-Lere intracontinental Basins whose genesis is linked to the opening of the Southern Atlantic Ocean. Geochemical data is also presented with the aim to discriminate the paleoenvironment. Field investigations and facies analyses revealed synsedimentary and postsedimentary structures related or not to tectonic. Synsedimentary structures not affected by tectonic are lamination or bedding (rhythmic), graded bedding, cross-stratification, loads casts, channels, and ripple marks. Those affected by tectonic are convolute bedding, slump (Slump sensus stricto and olistolith). Desiccation cracks and print marks are postsedimentary structures not related to tectonic; others are stratigraphic unconformity, oblique bedding, clastic injection, and volcanic veins. The bottom of the sequence in the two basins is fluvio-lacustrine deposits with oblique stratification and global positive graded bedding; sometimes there is alternation of positive and negative graded beddings related to progressive variation of energy of sedimentation from high to low and low to high. The intermediate level is lacustrine dominated by lamination or bedding that characterizes a calm sedimentation environment marked by regular seasonal fluctuation. The ripple marks, desiccation cracks, and dinosaur footprints indicate the emersion period, and their fossilization indicates slow phases of sedimentation. Channels and load casts correspond to a rapid sedimentation phase. Seismic shocks at time break the fragile stability of deposited materials causing gravitational flows of the sublacustrine avalanche type and give olistolith and convolute bedding to slumps. The clastic or chemical injections and volcanic veins are related to brittle tectonic that affects consolidated deposits. From the geochemical data, the high accumulation of LREE (average: 142.93 ppm) over HREE (average: 13.10 ppm); La/La* generally < 1.3, the negative and positive Gd/Gd* (0.97–1.30), Th/U ratios (> 2) and the slight to very slight negative Ce/Ce* (0.87–0.99) suggest the nonmarine deposition, and the Y/Ho ratios (< 45) and ratios indicate river condition or transportation. The Sr/Ba ratios, ranging from 0.55 to 6.41, suggest a freshwater environment with fluctuant salinity and sometimes hypersaline (Hypersaline Lake). Thus the synsedimentary structures were essentially formed in lacustrine milieu, and some structures such as graded bedding and cross-stratification affecting the basal deposit indicate a fluviatile to fluvio-lacustrine environment.