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Book Chapter

An abiotic model for the development of textures in some South Atlantic early Cretaceous lacustrine carbonates

By
V. Paul Wright
V. Paul Wright
PW Carbonate Geoscience, 18 Llandennis Avenue, Cardiff CF23 6JG, UKNatural Sciences, National Museum of Wales, Cardiff CF10 3NP, UK
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Andrew J. Barnett
Andrew J. Barnett
BG Group, 100 Thames Valley Park, Reading RG6 1PT, UK
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Published:
January 01, 2015

Abstract

The cyclic distribution of various types of carbonates and Mg-clays in early Cretaceous rift-sag phase lacustrine carbonates from the subsurface of the South Atlantic provides an insight into how evolving lake chemistries in highly alkaline settings control facies development. The typically subdecametre scale symmetrical and asymmetrical cyclothems exhibit three main components: mud-grade laminated carbonates, millimetre-diameter spherulites with evidence of having been in a matrix of Mg-silicates, and millimetre–centimetre calcitic shrub-like growths. The laminites contain conspicuous numbers of ostracods and vertebrate remains and were produced by short-lived pluvial events, causing expansion of shallow lakes. Later evaporation triggered Mg-silicate precipitation and calcite nucleation within gels to produce spherulitic textures. When the rate of gel precipitation decreased or ceased, calcite growth, now less inhibited, produced shrub-like calcites resembling those produced abiotically in modern travertines, although still with some evidence of the former presence of some Mg-silicates. Physical reworking of these sediments led to the dispersion of the gels and the concentration of detrital carbonate components. Despite earlier proposals, evidence of microbial processes producing carbonates in these Cretaceous lake deposits is rare and the application of facies models based on modern and ancient microbialite analogues maybe be misplaced.

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Geological Society, London, Special Publications

Microbial Carbonates in Space and Time: Implications for Global Exploration and Production

D. W. J. Bosence
D. W. J. Bosence
Royal Holloway, University of London, UK
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K. A. Gibbons
K. A. Gibbons
Nexen Petroleum UK Ltd, UK
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D. P. Le Heron
D. P. Le Heron
Royal Holloway, University of London, UK
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W. A. Morgan
W. A. Morgan
Morgan Geoscience Consulting, LLC, USA
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T. Pritchard
T. Pritchard
BG Group, UK
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B. A. Vining
B. A. Vining
Royal Holloway, University of London, UK
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Geological Society of London
Volume
418
ISBN electronic:
9781862397163
Publication date:
January 01, 2015

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