Iron silicate microgranules as precursor sediments to 2.5-billion year-old banded iron formations
Birger Rasmussen et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Posted online 18 March 2013 as part of the April 2013 issue; http://dx.doi.org/10.1130/G33828.1.
Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the approx. 2.5-billion-year-old Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF, where they define sedimentary laminations, implying a depositional origin. Study authors Birger Rasmussen and colleagues suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor and that the microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. They propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation.
Origin and impact of the oldest metazoan bioclastic sediments
L.V. Warren et al., Instituto de Geocincias, Universidade de So Paulo (USP), Rua
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