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Authigenic chlorite in late Triassic sandstones from the Central Graben, North Sea

Published online by Cambridge University Press:  09 July 2018

B. Humphreys ,
S. A. Smith  and
G. E. Strong
B. Humphreys
Affiliation:
Stratigraphy and Sedimentology Research Group, British Geological Survey, Keyworth, Nottingham NG12 5GG
S. A. Smith
Affiliation:
Stratigraphy and Sedimentology Research Group, British Geological Survey, Keyworth, Nottingham NG12 5GG
G. E. Strong
Affiliation:
Stratigraphy and Sedimentology Research Group, British Geological Survey, Keyworth, Nottingham NG12 5GG
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Abstract

Late Triassic sandstones from Quadrant 22 in the North Sea contain two generations of chlorite cement. Grain-coating chlorites consisting of curved, irregular crystal plates formed very early in diagenesis and are largely confined to marginal marine beds. Later diagenetic pore-filling chlorites, consisting of euhedral platy crystals, occur throughout the sequence in fluvial as well as marginal marine beds. Both generations of cement show similar XRD patterns; the 001 basal reflection of 14·0–14·06 Å is broad and of low intensity, whereas the 002 reflection at 7·05 Å is sharp and intense. Slight contraction of the 14 Å basal spacing to 13·6 Å occurs after heating to 600°C. Electron microprobe analyses show the cements to be Fe-rich clinochlores with Fe/(Fe + Mg) ratios varying between 0·41 and 0·50. They are more Mg- and Si-enriched than most other reported North Sea authigenic chlorites. The grain-coating chlorites have an average composition of (Fe3·06Al3·93Mg3·32) [(Si6·76Al1·24)O20] (OH)16. The pore-filling chlorites tend to show more variable compositions. The high Si values probably reflect both minor interstratification with smectite or vermiculite and contamination of the analyses by a closely associated illite/smectite clay. An origin related to alteration of a precursor swelling chlorite or corrensite is attractive on chemical and morphological grounds for the early grain-coating chlorites. However, direct precipitation from porewaters is the favoured origin for the later pore-filling chlorites, with the required ions being supplied by mineral decomposition reactions involving detrital silicate minerals and dolomite cements.

Type
Research Article
Information
Clay Minerals , Volume 24 , Issue 2 , June 1989 , pp. 427 - 444
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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