Glendonites (pseudomorphs after ikaite) in late carboniferous Marine Dwyka beds in Southern Africa

Abstract

Research Article| September 01, 2001 Glendonites (pseudomorphs after ikaite) in late carboniferous Marine Dwyka beds in Southern Africa I. R. McLachlan; I. R. McLachlan Honorary Research Associate, Bernard Price Institute for Palaeont. Research, PO Box 1174, Parow, 7499, email: mclachlai@petroleumagencysa Search for other works by this author on: GSW Google Scholar H. Tsikos; H. Tsikos Rand Afrikaans University, PO Box 524, Auckland Park, Johannesburg, 2006. *Present address: Department Earth Sciences, University Oxford, Parks Rd, OX1 3PR, Oxford, UK. Search for other works by this author on: GSW Google Scholar B. Cairncross B. Cairncross Rand Afrikaans University, PO Box 524, Auckland Park, Johannesburg, 2006. Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2001) 104 (3): 265–272. https://doi.org/10.2113/1040265 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation I. R. McLachlan, H. Tsikos, B. Cairncross; Glendonites (pseudomorphs after ikaite) in late carboniferous Marine Dwyka beds in Southern Africa. South African Journal of Geology 2001;; 104 (3): 265–272. doi: https://doi.org/10.2113/1040265 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Glendonites are carbonate pseudomorphs after monoclinic crystals of ikaite (calcium carbonate hexahydrate: CaCO3.6H2O). 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°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 19cm 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 accummulated 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.