WHAT DO CHERT FABRICS TELL US ABOUT PROTEROZOIC MICROFOSSILIFEROUS CHERT?
Here we compare petrographic fabrics of the Angmaat Formation chert to other microfossiliferous cherts, including those of the Mesoproterozoic Billyakh Formation and the Neoproterozoic Bitter Springs and Draken formations. Initial observations confirm that the best-preserved depositional microfabrics are characterized by clear rectilinear fabrics. Chert clasts within the Draken Formation, however, show three distinct fabrics: a distinctly rectilinear fabric, a neomorphic fabric with irregular interlocking crystal boundaries, and a chert fabric consisting of uniform, equant crystals. Intercrystalline domains within the latter two fabrics, however, exhibit sweeping extinction similar to that observed in well-preserved spherulitic fabrics. We suggest that observed fabrics represent endmembers in the recrystallization of a primary rectilinear spherulitic fabric during silica maturation. Retention of sweeping extinction further suggests recrystallization under low to moderate water-rock interaction, which may be critical to the preservation of microbial elements.