Secular Change in Mineral Composition of A Mesoproterozoic Black Shale Predating the Advent of Terrestrial Life and Soil-Formed Clay Minerals

Abstract

A key transition in the history of the Earth’s biosphere is from the lifeless terrestrial surfaces to the biologically mediated weathering environment of soils. Detrital clay minerals formed in soils that are ultimately deposited in continental margin sediments comprise > 60% of the sedimentary record in the Phanerozoic. The Mesoproterozoic Velkerri Formation provides a contrasting record of continental margin shale before the advent of terrestrial life and soils. Micro X-Ray mineral mapping of black shale intervals within the Velkerri Formation shows that while clay minerals and clay sized grains are of similar abundance, petrographic relationships showing intergrowth of illite-smectite and quartz identify an authigenic and not detrital origin for this fraction that controls the porosity, permeability and brittleness. The detrital minerals that are present include large (> 60 µm) mica flakes, feldspar and lesser quartz grains indicating limited chemical weathering. Organic carbon is dominated by laminated mat-like laminae as well as a significant pyrobitumen fraction found in fractures and shelter porosity. Other authigenic phases include pyrite, apatite (early diagenesis), and kaolinite (late stage diagenesis). The lack of maturity in the detrital mineral assemblage is consistent with the absence of soil chemical weathering influence during physical sediment production. Key rock properties in the Velkerri Formation such as organic carbon enrichment, cementation and porosity are thus dominated by diagenetic processes that modified a distinctly non-uniformitarian initial suite of detrital grains.