Within the Upper Albian marl/limestone pelagic to hemi-pelagic successions of Fahdene basin (northern Tunisia), small-scale thrombolitic and coral buildups were recognized for the first time. The exceptionally well-exposed and well-preserved buildups (i.e. Oued Siliana and Jebel Srassif buildups) were investigated for their biostratigraphic, petrographic, and genesis character. The vertical succession is characterized as shallowing upward and comprise four to nine conical and/or lenticular bioherms/biostromes (up to 4 m in diameter). Basal buildups consist of four to six deep water microbial thrombolites and are generally associated with the uppermost organic-rich black shale of the Mouelha member which corresponds to the Oceanic Anoxic Event OAE1d. These buildups change stratigraphic upward into three hermatypic hexacorallian thrombolites with abundant fossils and bioturbation traces. The geometry and the observed internal structures of the thrombolites suggest that microbial activity thrived carbonate precipitation and subsequent lithification, which in turn favoured preservation of the original structure of the buildups. The microbial thrombolites formed within or near the anoxic zone (related to the Oceanic Anoxic Event OAE1d) whereas the coral thrombolites grew in the photic zone. We suggest that the onset of the microbial thrombolites was induced by the presence of an eutrophic, hypersaline, and anoxic environment which is enhanced by the Late Albian sea level rise. Nevertheless, flourishing of metazoans that lead to the coral thrombolites was associated with an oxygenated and oligotrophic environment. Halokinetic dynamics and tilted-block fault systems provided conduits for nutrient-rich fluids and hydrocarbons which fueled chemosynthetic-based communities.

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