MICROFOSSILIFEROUS CHERTY STROMATOLITES IN THE 2000 MA FRANCEVILLE GROUP, GABON

Cherty stromatolites, reported here for the first time, have been found in the Palaeoproterozoic (2000 Ma) cover of the Chaillu craton in Gabon. They occur in two different areas, probably in the same formation (FC), which comprises widespread beds of black shales and silexites. The buildups are 1-2 m thick biostromes comprising juxtaposed domes of pseudo-columnar to columnar stromatolites. The biostromes are transgressive over topographic highs marginal to the Francevillian basins. The environment was a protected shallow-marine area with occasional inputs of small quantities of peloidal sediment deposited between the columns. The high organic carbon content of Francevillian formations FB, FC and FD allows us to consider them as source rocks for the bitumen now found in the FA sandstones. The stromatolites and their preserved microfossils support the hypothesis that microbial activity was at the origin of bitumen production.