Discovery of fossilized hydrothermal silica deposit within the Cenomanian fluvio-deltaic detrital facies in the eastern Atlantic margin, Douala sub-basin, Cameroon

The silica deposits have never been reported before in the Atlantic margin of Cameroon, subdivided in three sub-basins, from north to south the Campo, Douala, and Rio-del-Rey sub-basins. The studied siliceous deposit is located in the Douala sub-basin with Lower Cretaceous to Quaternary infilling. Mineralogical and geochemical analyses coupled with sedimentological studies were performed to characterize its origin and environment of sedimentation. XRD (X-ray diffraction) and SEM (scanning electron microscope) studies reveal that the siliceous deposit is well crystallized with euhedral quartz and sometimes centripetal growth, probably linked to digenetic alteration of amorphous deposit. Discrimination plots such as SiO 2 vs. Al 2 O 3 , Si vs. Al, Y/P 2 O 5 vs. Zr/Cr, and Y/Ho vs. Sm/Yb suggest a hydrothermal origin. Moreover, the Y/Ho vs. Sm/Yb and Na vs. Mg binary plots are consistent with a non-marine deposit. In this case, freshwater conditions, and the Y/Ho vs. Nd plot shows that this silica deposit is purely chemical, precipitated within the detritus clayey facies. The presence of lithofacies such as convolute sands (Sc) and various trough cross-bedded sands (St) and the association of positive and negative sequences indicate the siliceous precipitation in a fluvio-deltaic and active tectonic environment related to the generation and evolution of the South Atlantic Ocean during the Cenomanian period. The Precambrian fault and fracture reactivations facilitated the meteoric waters’ flow and the upwelling of hot water springs with silica supersaturation. The studied siliceous deposit, chert probably, located in the coastal area with no link with the sea environment, as those from the northern Jiangxi region, South China, result mainly from direct seawater precipitation. This oldest siliceous deposit of Bonépoupa (Cenomanian), compared to those of Patagonia or Claudia (Jurassic, Argentina), Queensland (Paleozoic, Australia), and Rhynie (Paleozoic, Scotland), is monofacies, massive, with no microbial communities, such as cyanobacteria, that have been reported.