T239 Life's Innovations from the Early Earth to the Search on Modern Mars III: Honoring the Career of Andrew H. Knoll

Wednesday, 13 October 2021
1:30 PM - 5:30 PM
Portland Ballroom 255 (Hybrid Room)

WHAT DO CHERT FABRICS TELL US ABOUT PROTEROZOIC MICROFOSSILIFEROUS CHERT?

Paper number: T239-3
Program: Technical Sessions
Time: 1:55 PM – 2:10 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-369151

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.