Abstract
The extensive, thick Paleogene carbonate platform (Amapá Formation) of the Foz do Amazonas Basin, probably the largest coralgal‐foraminiferal platform of the geological record, developed through four depositional cycles expressing global eustatic changes of sea level. It shows five environmental belts: slope, apron, coralgal platform, large foraminifer shoal, finger coral bank and restricted lagoon, all of which associate bioaccumulated and bioclastic microfacies. At all times, a terrigenous environment consisting of fan deltas and lagoonal fine‐grained clastics existed immediately behind the carbonate platform and was connected with the open ocean by transverse troughs filled with shales and carbonate olistoliths. This combination of clastic and carbonate environments is unique in the geological record, and today only the Belize Shelf displays comparable conditions.The patterns of distribution of porous carbonates are different for each depositional cycle and the percentage of porous carbonates increases from cycle II to cycle IV. This indicates distinct and successive reservoir conditions generated during episodes of subaerial exposure of an expanding platform by large‐scale circulation systems at the end of each depositional cycle when high‐stand sea level changed to low‐stand. Porosity is mainly enlarged interparticle and moldic with a minor contribution of intercrystalline type related to dolomitization by mixing freshwater‐marine waters. It is not known to what extent the terrigenous environment contributed to the meteorically supplied groundwater system and related dolomitization, but the scale of the circulation is comparable to that presently displayed by Florida and Yucatan.The possibility of hydrocarbon flushing from such carbonate reservoirs is an important factor in the evaluation of the petroleum potential of the Amapá carbonates.
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