The oxygen then combined with dissolved iron in Earth 's oceans to form insoluble iron oxides , which precipitated out , forming a thin layer on the ocean floor , which may have been anoxic mud ( forming shale and chert ) .
This combined with dissolved iron in Earth 's oceans to form insoluble iron oxides . These precipitated out , forming a thin layer on the substrate , which may have been anoxic mud ( forming shale and chert ) .