Abstract

The Ediacara Member of the Rawnsley Quartzite in South Australia is characterized by exceptional preservation of soft-bodied organisms as casts and molds in sandstones. To understand the timing of silica cementation and the relationship between fossilization and the broader diagenetic history of the Ediacara Member, we used a combination of electron backscatter diffraction (EBSD) analyses, cathodoluminescence (CL) imaging and in situ analysis of oxygen isotopes (delta 18O) by secondary ion mass spectrometer (SIMS) of detrital quartz grains and authigenic quartz overgrowths in sandstone samples hosting Ediacara Biota macrofossils. We find that the mean delta 18O value of overgrowth quartz (OQ) from analyzed samples of the Ediacara Member is 18.48 +/- 2.5 %o (1SD, with a range of 7.5-26.7 %o) and quartz overgrowths are systematically zoned, with decreasing delta 18O values outward from the quartz grains, consistent with progressive and continued cementation. These results suggest an early diagenetic initiation of silica cementation adjacent to the quartz grain boundaries, followed by later stages of cementation more distal from grain boundaries and likely under the influence of higher-temperature burial diagenetic processes. These data support the idea that early silica precipitation from silica-rich seawater may have played a critical role in the fossilization of Earth's earliest animal ecosystems, and better elucidate the diagenetic history of a key Ediacaran fossil deposit that has uniquely shaped understanding of the ecology, diversity, paleoenvironment, and preservation of early complex, macroscopic life.