This sample consists of black wrinkly microbial mats, and green grey claystone from the 1.4 billion year old Velkerri formation, from northern Australia. The mats were dominated by cyanobacteria living on the bed of a shallow sea that covered northern Australia at the time. The organic matter contained lots of sulphur which was converted into pyrite during burial. You can sea the pyrite as the sparkly gold flecks in the close up of the black mats. This area of the sea was far from land so instead of depositing grains eroded from existing rocks, like sand, sediment chemically precipitated from the sea water. The sea was rich in iron and silica and so iron silicate clays formed on the sea bed. The iron is green because they're was little oxygen around for it to react with. This process of chemical sedimentation is very slow and so there was plenty of time for the mats to grow across the sea bed. This also means that other material can accumulate on the sediment that would be lost in areas of rapid sedimentation. So I've also found lots of volcanic ash and spherules in these layers. I've also found mysterious grains in these samples which may be impact spherules from a meteor impact, or perhaps even micrometeors too. We don't know what the banding represents. It could be seasonal growth of the mats, it could reflect how nutrient abundance changes, or could effect changes in the rate if sedimentation. It could even reflect all our none of the above. I'm going through these samples, documenting the sedimentary textures and deciding which samples I want to turn into thin sections to examine under the petro microscope, and then to digest in acid to extract the fossils from. #geologyjohnson #geologist #geology #geologyrocks #sedimentary #microbial #microbialmat #precambrian #mesoproterozoic #fossil #fossils #stromatolite #Australia #green #shale #mudstone #earlylife #proterozoic https://www.instagram.com/p/Cc-dI2mNRVF/?igshid=NGJjMDIxMWI=
