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GEOREF RECORD

Glendonites (pseudomorphs after ikaite) in late Carboniferous marine Dwyka beds in Southern Africa

I. R. McLachlan, H. Tsikos and B. Cairncross
Glendonites (pseudomorphs after ikaite) in late Carboniferous marine Dwyka beds in Southern Africa
South African Journal of Geology (September 2001) 104 (3): 265-272

Abstract

Glendonites are carbonate pseudomorphs after monoclinic crystals of ikaite (calcium carbonate hexahydrate: CaCO (sub 3) .6H (sub 2) O). Ikaite forms at present within organic-rich marine or brackish sediments that are at near freezing temperatures and decomposes to a mush of calcium carbonate and water when the temperature rises above 5 degrees C. These unusual pseudomorphs were first recognised in southern Africa in 1972 as a rare component in marine shales just above the top of the Dwyka Group in the Great Karoo basin in South Africa. The crystal casts are rhomb-shaped in cross-section, show well-developed pyramid faces and are up to 19 cm long. The glendonites occur in two forms: 1. as hollow or ochre-filled casts of ikaite crystals in concretions in fossiliferous marine Dwyka shales at Blaauwkrantz near Kimberley and also in the Ganigobis shale at Zwartbas on the Orange River. 2. as carbonate pseudomorphs after ikaite in concretions within shaly siltstone in the upper part of the marine glacial boulder mudstone at De Kalk (adjacent to Blaauwkrantz). The concretions are unweathered and the crystal casts are filled with a hard dark brown granular carbonate. Each granule encloses one or more stellate aggregates of zoned calcite crystals with an outer layer of brown carbonate. Electron microprobe analysis shows that the stellate crystals are pure calcite CaO: 55% (by wt.), Mg: 0.19% and the enclosing brown carbonate layer is Fe/Mg/Mn-enriched calcite (FeO+MgO+MnO: 6.18% CaO: 48.5%). The presence of glendonite demonstrates that the fossiliferous host sediments accumulated under near freezing, highly alkaline, orthophosphate-rich bottom water conditions. These stressful conditions help to explain the observed severely limited species diversity of the associated fossil assemblage.


ISSN: 1012-0750
EISSN: 1996-8590
Coden: TGSAA3
Serial Title: South African Journal of Geology
Serial Volume: 104
Serial Issue: 3
Title: Glendonites (pseudomorphs after ikaite) in late Carboniferous marine Dwyka beds in Southern Africa
Affiliation: Bernard Price Institute for Palaeontological Research, Parow, South Africa
Pages: 265-272
Published: 200109
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 12
Accession Number: 2002-064798
Categories: Mineralogy of non-silicatesSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch map
S35°15'00" - S24°00'00", E16°00'00" - E26°00'00"
Secondary Affiliation: Rand Afrikaans University, ZAF, South Africa
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200220
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