Abstract ref. 1342:
Phosphatidized bioclastic limestomes of Early Tertiary age were dredged from the crest of a seamount in the equatorial Atlantic. The samples were recovered from water depths of 700-970 m but clearly were deposited in a high-energy reefal environment, thus indicating significant Tertiary subsidence. The vertical movement probably reflects reactivation of an old structural line, the Guinee Fracture Zone, which may have had important effects on the transfer of bottom waters of polar origin between the Sierra Leone and Gambia basins. Examination of foraminiferal limestone coating the shallow water material suggests that deep-water conditions were established on the seamount by Middle Miocene time. All phosphatized samples are covered with a layer of ferromanganese oxides which, from textural and faunal evidence, is Paleocene or Eocene in age and of shallow water origin. Pervasive phosphatiziation of the bioclastic limestones probably was accomplished before the end of the Eocene while the crest of the seamount was near sea level, because Upper Eocene limestone of shallow water facies is unaffected by such radical diagenesis. Phosphatization probably was associated with vigorous upwelling near the seamount and has been taken to an unusually advanced stage, resulting in a phosphorite of high economic grade. P2O5 contents are in the range of 37-41%. The purity of the phosphorites may be related to the intensity of upwelling during the Early Tertiary, but other factors such as the paucity of terrigenous detritus, frequent reworking of the sedimentary capping of the seamount, and a high primary porosity of the limestones may have been important in their formation. The Early Tertiary period of phosphorite deposition on the seamount and in northwest Africa ended before the close of Eocene time in both areas, while shallow water conditions prevailed. The cessation of phosphorite formation probably reflects major changes in the surface circulation of the Atlantic during the Eocene.