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Pre-Salt Microbial Aptian-Albian Carbonates of the Araripe Basin, Northeastern Brazil


The Aptian-Albian succession of the interior Araripe Basin, northeastern Brazil, was deposited in the post-rift, thermal flexural subsidence stage. The laminated limestones of the Crato Member, Santana Formation are a world-class fossil lagerstätten, containing abundant plants (including angiosperms), fish, amphibians, crocodiles, lizards, pterosaurs, insects, spiders and crustaceans with extraordinary preservation. These rocks are overlain by gypsum layers, being contemporaneous to the pre-salt reservoirs under exploration in the eastern Brazil Campos and Santos Basins. A close inspection of the Crato carbonates reveals widespread microbial mat features. These include growth-related features, such as: 1) biolaminations, represented by even, millimetric laminations, as well as wavy-crinkly laminations; 2) domal buildups, made by isolated, small stromatolitic and thrombolitic domes; 3) irregular mosaic-pattern wrinkle structures of flat-topped rippled crests resembling Kinneyia. Features produced by physical mat deformation are also present, including roll-up structures (overfolded mat and “jelly rolls”), superficial, unusual crack morphologies with linear, incomplete fissures, and mat chips from desiccation of microbial films. Pyrite-rich laminae formed due to mat decay and mineralization sometimes display coccoid-shaped pyrite aggregates. Most of the carbonate rocks are mudstones, some with crenulated clay films, but carbonate allochems occur locally in the microbial mats, including peloids, oncoids and carbonate lumps with preserved coccoid and cyanobacteria (Entophysalis?) textures. Calcite pseudomorphs after halite hopper crystals precipitated within the biolaminites are conspicuously enveloped by concentric circles (“bubble wrap”), interpreted as a result of collapse or tearing of the microbial mats, caused by crystal growth. The microbial laminates are frequently interbedded with recurrent mm-cm gypsum layers and show no evidence of desiccation. Therefore the Crato microbialites were probably formed in the photic zone of a relatively shallow, yet perennial body of saline water.