WHAT DO CHERT FABRICS TELL US ABOUT PROTEROZOIC MICROFOSSILIFEROUS CHERT?

Exquisite preservation of microfossils in early diagenetic chert requires silicification processes on timescales comparable to bacterial decomposition and limited water-rock interaction during silica maturation. Unfortunately, both the pathway of microbial silicification and the processes associated with silica diagenesis are poorly defined. Earlier petrographic analyses on microfossiliferous chert of the 1.1 Ga Angmaat Formation (Dunham, 2018) suggests that exquisite microfossil preservation is associated with silicification that initiates as a silica gel phase with crystallization as 20-50 μm diameter length-fast chalcedony spherules. In the Angmaat chert, spherules often have well-defined morphologies, and a dense network of spherules is readily identified by sweeping extinction under crossed polars, and by a strong rectilinear pattern when viewed under crossed polars with insertion of a 530 nm gypsum plate.

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.