Two distinct sedimentary facies produced by sea-floor hydrothermal activity were cored during Deep Sea Drilling Project Leg 54. The first is equivalent to the typical basal iron- and manganese-rich, clayey mud recovered at many DSDP sites. It was sampled as a dispersed component throughout the cores taken in small sediment ponds in several sites within 120 km of the East Pacific Rise at 9°N. We infer that this component was originally deposited as iron hydroxides dispersed from high-temperature vents over the axial magma chamber of the East Pacific Rise. In the sediments, the iron hydroxides have reacted diagenetically with siliceous microfossil tests and detrital clays to form K- and Fe-rich clays with variable SiO(2)/Fe(2)O(3) and Fe(2)O(3)/Al(2)O(3), ratios. [NOT CONTROLLED OCR]