15. ORIGIN OF LOWER EOCENE PALYGORSKITE CLAYS ON THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN, EASTERN EQUATORIAL ATLANTIC1Thomas Pletsch2 |
Palygorskite was identified in lower Eocene clays and porcellanites in Holes 959D, 960A, 960C, and 961A; in upper Albian to Cenomanian clays with chert in Hole 962B; and in undated clays in Holes 962B and 962C of Ocean Drilling Program Leg 159. Palygorskite is particularly abundant in lower Eocene bluish claystones of Sites 960 and 961, which reach 20 to 30 m in thickness. Based on the microscopic texture, the mineralogical purity of the palygorskite clays, and on growth relationships, palygorskite is interpreted to be of authigenic or very early diagenetic marine origin. The lack of typical detrital lithologies or of sedimentary structures related to resedimentation and the scarcity of clearly terrigenous minerals also argues for in situ formation. Micropaleontologic data indicate that palygorskite formation took place in deep-sea environments with no indication of significant shallowing. The transformation of pre-existing clay minerals at or closely below the sediment/seawater interface seems to be responsible for the formation of palygorskite clay on the Côte d’Ivoire-Ghana Transform Margin. Palygorskite frequently occurs in lower Eocene sediments of equatorial to subtropical paleolatitudes. The formation of authigenic palygorskite clays at the Leg 159 drill sites is interpreted to be the result of elevated deep-sea temperatures and Mg- and Si-rich deeper waters. The occurrence of authigenic, deep-marine palygorskite in space and time may correspond to the areas and to the time periods, respectively, of warm, saline deep-water production. |
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