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TitleZebra and boxwork fabrics in hydrothermal dolomites of northern Canada: Indicators for dilational fracturing, dissolution or in situ replacement?
 
AuthorMorrow, D W
SourceSedimentology vol. 61, issue 4, 2014 p. 915-951, https://doi.org/10.1111/sed.12094
Image
Year2014
Alt SeriesEarth Sciences Sector, Contribution Series 20130276
PublisherWiley-Blackwell
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories; Yukon
NTS75D; 75E; 85A; 85B; 85C; 85D; 85E; 85F; 85G; 85H; 95A; 95B; 95C; 95F; 95G; 95H
AreaGreat Slave Lake; Pine Pointe; Presqu'ile; Manetoe Dolomite
Lat/Long WENS-126.0000 -110.0000 62.0000 60.0000
Subjectsdolomites; hydrothermal systems; dolomitization; porosity
Illustrationslocation maps; correlation charts; geological sketch charts; tables; diagrams; photographs; photomicrographs; graphs; histograms
ProgramGEM: Geo-mapping for Energy and Minerals Mackenzie Delta and Corridor
Released2014 02 05
AbstractZebra and boxwork fabrics in Manetoe and Presqu¿ile hydrothermal dolomites provide critical data for discrimination between hypotheses for their origin. In a unique occurrence, sets of weakly imbricated, decimeter-sized curvilinear lenticles of white dolospar are developed within sub-meter-sized tongues of greyish-white dolomitized crinoidal packstone and grainstone. This nascent zebra fabric has developed along wavy to nodular intra-bed sedimentary partings. Primary bedding can be traced through dolomitized masses containing these fabrics into the surrounding undolomitized limestone. All zebra and boxwork fabrics are confined within single beds, consistent with an early burial time of origin. No change of bed thickness occurs across zebra or boxwork fabrics within beds. This indicates a dissolutional, rather than a dilational origin, for the creation of pore space partly cemented with white dolospar. The dolomitized groundmass displays an abruptly gradational or sharp transition to centripetal saddle dolomite cement, which partially, to nearly completely occludes vugs. Circulation of geothermally heated hypersaline Devonian Elk Point basinal seawater brines led to dissolution and replacive dolomitization of limestone adjacent to vugs and near-contemporaneous precipitation of white dolospar within dissolutional vugs,consistent with geochemical simulations. The presence of downward extending galleries of white dolospar-cemented solution-collapse breccia provides further support for an interpretation of regional thermally-driven convection of hydrothermal vaporative seawater brines across a broad area of northern Canada. The absence of gases under pressure in vacuoles within fluid inclusions from Manetoe and Presqu¿ile dolospars is also more consistent with a dissolutional and contemporaneous dolomitization origin for these fabrics, rather than an origin involving dilational fracturing for space creation and dolospar precipitation. The ubiquitous presence of zebra and boxwork fabrics in hydrothermal dolomite reservoirs indicates that they are not confined fault zones and instead occur whereever precursor limestones had relatively greater porosity and permeability prior to dolomitization.
Summary(Plain Language Summary, not published)
Bands of white dolospar inclined to bedding (zebra fabric) and rectilinear networks of white dolospar (boxwork fabric) are well exposed within large masses of hydrothermal (high temperature) dolomite that has replaced Devonian limestones of northern Canada. These exposures demonstrate that zebra and boxwork fabrics developed together by a process of limestones dissolution accompanied by dolomitization and white dolospar precipitation within dissolutional voids. This implies that prospective petroleum reservoir hydrothermal dolomite masses are not confined to fault zones, as previously argued, but rather may be found wherever precursor limestones had relatively high porosity.
GEOSCAN ID293142

 
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