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Hydrothermal alteration of Britain's oldest palaeosols; saddle dolomite and smectite at the Lewisian-Torridon Group (early Neoproterozoic) unconformity, NW Scotland

George E. Williams
Hydrothermal alteration of Britain's oldest palaeosols; saddle dolomite and smectite at the Lewisian-Torridon Group (early Neoproterozoic) unconformity, NW Scotland
Scottish Journal of Geology (April 2015) 51 (1): 63-68

Abstract

Weathering profiles 2-3 m thick mantle a palaeoplain formed on the Lewisian Complex and are overlain unconformably by fluvial conglomerates of the c. 980 Ma Applecross Formation (Torridon Group) in NW Scotland. Geological and palaeomagnetic data indicate that the profiles formed penecontemporaneously with deposition of the Applecross Formation and constitute the oldest known palaeosols in Britain. However, their degree of later alteration and burial depth are contentious. The weathering profiles are characterized by the presence of mixed-layer illite-smectite and smectite, which is anomalous for Precambrian palaeosols, raising doubts regarding a Precambrian age for the clay suite. Estimates of burial depth range from <1.5 km, based on the apparently limited burial alteration with the preservation of smectite, to c. 6 km and possibly c. 12 km following Caledonian tectonism. Thin (1-10 mm) veins of saddle (baroque) dolomite are present in weathered amphibolite < or =1 m below the Lewisian-Torridon Group unconformity and also in overlying conglomerate 15 km SSW of Cape Wrath, indicating the flow of heated (c. 90-160 degrees C) saline fluids. The anomalous occurrence of mixed-layer illite-smectite and smectite in these Precambrian palaeosols implies 'retrograde diagenesis' of diagenetic illite and chlorite likewise induced by hydrothermal fluids. Alteration of the palaeosols is attributed to hydrothermal fluid flow along the Lewisian-Torridon Group unconformity that was related to fluid movement in the Moine Thrust zone during Silurian or Permian regional tectonism. Radiometric dating of the illite may illuminate the timing of alteration of the palaeosols and their burial history.


ISSN: 0036-9276
EISSN: 2041-4951
Coden: SJGEAX
Serial Title: Scottish Journal of Geology
Serial Volume: 51
Serial Issue: 1
Title: Hydrothermal alteration of Britain's oldest palaeosols; saddle dolomite and smectite at the Lewisian-Torridon Group (early Neoproterozoic) unconformity, NW Scotland
Author(s): Williams, George E.
Affiliation: University of Adelaide, Department of Geology and Geophysics, Adelaide, South Aust., Australia
Pages: 63-68
Published: 201504
Text Language: English
Publisher: Geological Society Publishing House for the Geological Societies of Edinburgh and Glasgow, Bath, United Kingdom
References: 32
Accession Number: 2015-049829
Categories: Stratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch map
N57°00'00" - N57°00'00", W05°00'00" - W05°00'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from The Geological Society, London, London, United Kingdom
Update Code: 201523
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