What is siliceous ooze mostly composed of?

Siliceous ooze is a siliceous pelagic sediment that covers large areas of the deep ocean floor ... Radiolarians are marine protists that also construct microscopic shells composed of opaline silica and are distant relatives of the foraminifera.

This sediment is the result of the silica shells of organisms settling on the sea floor. These are phytoplankton that bloomed and died off in masses following changing environmental conditions.