The Palaeontology and Palaeoecology of Precambrian Iron-Formations

This chapter discusses the palaeontology and palaeoecology of Precambrian iron-formations. The iron-formations of all but the latest Proterozoic were deposited before the origin of animals and of macroscopic plant life. The palaeontology of these iron-formations involves study of traces of microbial life. There is no direct evidence of the involvement of microorganisms in the deposition of Archaean iron-formations. The absence of stromatolites is consistent with the interpretation that the iron-formations were deposited in water deeper than about 100m. The palaeontological and geochemical records as a whole seem to indicate that oxygen-producing photosynthesis evolved during the late Archaean, and that the iron in older iron-formations must have been oxidized by some other mechanism. Early Proterozoic iron-formations can be considered to be of two distinct types, peloidal and banded, of which the Gunflint and Brockman iron-formations are the best-known examples. The two types are palaeontologically very different. The banded iron-formations of the Early Proterozoic are like those of the Archaean and were probably also deposited in more than 100 m of water; no stromatolites or convincing microfossils are known from these rocks. In contrast, the peloidal iron-formations are richly fossiliferous, with stromatolites and microfossils all consistent with deposition in very shallow, turbulent water.