ABSTRACT

Cores collected by drilling in the deep ocean are providing samples of deeply buried in situ ocean sediments in the process of becoming sedimentary rocks. Carbonate sediments have been dominant in deep sea sediments since at least the Jurassic, except during the Eocene when siliceous sediments became dominant. The physical properties of the sediments follow the expected theoretical trends, except that the sonic velocity is consistently lower than the velocity determined from refraction measurements. Studies of long continuous sequences of cocolith ooze have revealed a progressive sequence of diagenetic changes from break up of individual cocoliths to solution and recementation. Dolomite rhombs are commonly reported from deep sea sediments and their occurrence appears to be rela ed to either igneous activity, to evaporitic conditions prevailing in the past, or to unknown geochemical control in a reducing environment. Chert, both nodular and bedded, has been reported from both the North Atlantic and Pacific oceans, but not the South Atlantic. Chert is found associated with all lithologies and ages of sediment but is most common in siliceous sediments of Eocene or older age. The silica in nodular cherts is biogenic in origin. The source of silica in the bedded cherts is unknown. Basal sediments in the oceans are commonly enriched in iron. The iron-rich sediments formed when the basement was near the crest of the mid-ocean ridge and have moved to their present positions by seafloor spreading.