Phosphorite nodules were newly found in laminated lacustrine diatomaceous sediments of the Middle Pleistocene Nogami Member in Kusu Formation northern Kyusyu (Japan) which deposited in palaeo-Kusu lake. The phosphorous content of these nodules is about 25% in P₂O₅. These nodules resemble marine phosphorite nodules in composition, consisting primarily of carbonate-fluorapatite.
Phosphorite nodules were formed from phosphate originally contained in diatom during early diagenesis. These nodules show compositional zoning and color banding due to dissolution by meteoric or hydrothermal water. The fixation of phosphorous is likely to have been controlled by concentration of Ca.