340 Sediments, Carbonates (Posters)

Wednesday, 28 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

VARIATION IN MICROFABRIC WITHIN PROTEROZOIC EARLY DIAGENETIC CHERT

Paper number: 340-8
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Early diagenetic silicification is abundant in Proterozoic peritidal carbonate successions and provides a critical window into organic preservation. The origins of early diagenetic chert, however, remain uncertain. Models for the diagenetic replacement of precursor carbonate and evaporate phases have focused on highly evaporative environments, on coastal mixing zones, and on environments of variable-redox. An additional difficulty in determining the origin of authigenic silica results from extensive postdepositional recrystallization that inhibits interpretation of primary depositional fabrics. The original phase of sedimentary-hosted authigenic chert has, thus, been variably inferred to result from devitrification of silica gel, precipitation and recrystallization of opal A or opal CT, precipitation and recrystallization of the hydrated sodium silicate magadiite, and the direct precipitation of microquartz.

In order to better understand the origins of early diagenetic chert, we provide a comprehensive petrographic analysis of microfabrics from microfossiliferous chert of the Mesoproterozoic Angmaat Formation, northern Baffin Island. These cherts preserve extraordinary petrographic variability. Microcrystalline chert is most common within preserved microbial mats that show no evidence of a precursor mineral phase. Microcrystalline chert in these mats also shows variation in crystal size that corresponds to the density of preserved organic material. Chalcedony occurs within primary voids, and occurs as both length-fast and length-slow chalcedony. Chert that has replaced primary carbonate fabrics, such as microlaminated carbonate precipitates, occurs either as elongate lathes that mimic the precursor fabric, or as microspherulites that overprint (yet preserve) original precipitation fabrics. Such variability in microfabric suggests a potentially more complex history to early diagenetic silicification than previously recognized.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-283245

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