Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks

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Authors:Wignall, P. B.; Newton, R.
Author Affiliations:Primary:
University of Leeds, Department of Earth Sciences, Leeds, United Kingdom
Volume Title:American Journal of Science
Source:American Journal of Science, 298(7), p.537-552. Publisher: Kline Geology Laboratory, Yale University, New Haven, CT, United States. ISSN: 0002-9599
Publication Date:1998
Note:In English. 49 refs.; illus.
Summary:The size distribution of pyrite framboids (densely-packed, raspberry-like spherical equigranular, micron-sized crystals) and other forms of pyrite is determined for the Upper Jurassic Kimmeridge Clay formation of Kimmeridge, England. The size distribution of pyrite framboids is calibrated against palaeoecologically-determined, oxygen-restricted biofacies. For example, tiny framboids (< 5 µm) with a narrow size range correlate with euxinic conditions of H2S-bearing, O2-free bottom waters and an absence of benthic and nektobenthic taxa. Tiny framboids with a narrow size range and rare larger diameter framboids correlate with slightly less euxinic conditions (lower dysaerobic, exaerobic, and poikiloaerobic biofacies) found in fissile strata with discrete, benthos-bearing bedding planes. Larger framboids with a larger size distribution occur in upper dysaerobic biofacies. Sedimentary reworking (storms) can remove the smallest size fraction and thus alter original framboid size distributions. Microcrystals and amorphous lumps of pyrite in the black shales with their unrelated size variations are unsuitable for determining ancient redox levels. Formation of pyrite in the water column (framboids), during diagenesis (microcrystals), and possible biogenic (amorphous lumps) complicates assessment of whole-rock S isotope fluctuations. [C.L.H.]
Sections:Geochemistry
Subsections:Sedimentary rocks
Subjects:Anaerobic environment; Authigenic minerals; Biofacies; Black shale; Calibration; Clastic rocks; Crystal form; Crystal growth; Depositional environment; Dysaerobic environment; Framboidal texture; Geochemistry; Indicators; Jurassic; Kimmeridge Clay; Mesozoic; Mudstone; Oxygen; Paleoecology; Pyrite; Sedimentary rocks; Size distribution; Sulfides; Textures; Upper Jurassic; Dorset England; England; Europe; Great Britain; United Kingdom; Western Europe; Euxinic environment; Kimmeridge Bay
Coordinates:N503700 N503700 W0020700 W0020700
Abstract Numbers:99M/2896
Record ID:1998070596
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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