banded iron
sedimentary
Upper Michigan
Across the world, sedimentary rocks that are more than 1.8 billion years old have deposits of iron oxide that alternate with bands of red chert. The banding was produced by a cycle unique to the Precambrian when oxygen levels in the atmosphere and oceans were low and varied seasonally. When sufficient oxygen was produced by sea-dwelling cyanobacteria, iron was deposited on the sea floor. When photosynthesis and oxygen levels dropped, silica (which forms chert) was deposited.
