ALBERT

Description: Phosphatized bioclastic limestones of Early Tertiary age have been dredged from the crest of a seamount in the equatorial Atlantic near 9°N, 21°W. The samples were recovered from water depths of 700 to 970 m but clearly were deposited in a high-energy reefal environment, thus indicating significant Tertiary subsidence. The vertical movement is beleived to reflect reactivation of an old structural line, the Guinée 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 Palaeocene or Eocene in age and of shallow water origin. Pervasive phosphatization of the bioclastic limestones appears to have been 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 phosphoritesof high economic grade. P2O5 contents are in the range of 37 to 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 information. 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. It is argued that the cessation of phosphorite formation reflects major changes in the surface circulation of the Atlantic during the Eocene.

Description: Chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the New England Seamounts, the Sierra Leone Rise and the Mid-Atlantic Ridge near the Equator have been determined in an investigation of regional differences in Atlantic ferromanganese deposits. Most encrustations are clearly of hydrogenous origin, consisting mainly of todorokite and delta MnO2, but several recovered from the equatorial fracture zones may be hydrothermal accumulations. Differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition. Fine-scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations.