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TitleAn Eocene post-kimberlite maar lake: lacustrine oil-shale crater-fill deposits, Lac de Gras area, Northwest Territories, Canada
AuthorHamblin, A P
SourceGeological Survey of Canada, Open File 7809, 2015, 26 pages, (Open Access)
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatpdf
ProvinceNorthwest Territories
NTS76C/05; 76C/06; 76C/07; 76C/10; 76C/11; 76C/12; 76C/13; 76D/06; 76D/07; 76D/08; 76D/09; 76D/10; 76D/11; 76D/14; 76D/15; 76D/16; 76E/01; 76E/02; 76E/03; 76E/04
AreaLac de Gras
Lat/Long WENS-112.0000 -108.5000 65.2500 64.2500
Subjectsfossil fuels; stratigraphy; Eocene; oil shales; hydrocarbons; hydrocarbon potential; organic materials; organic deposits; peatlands; peat analyses; Cenozoic
Illustrationslocation maps; stratigraphic columns; photographs; cross-sections
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
Released2015 05 28
AbstractSedimentology and organic petrology-geochemistry from a 160-metre core have been integrated to study the characteristics of an isolated pocket of fine-grained, siliciclastic Eocene sediments deposited within a small kimberlite crater basin, Lac de Gras, Northwest Territories, Canada. These sediments overlie the "Giraffe Pipe", a kimberlite occurrence located at about 65°N/110°W, 25 km northeast of the Ekati Diamond Mine. The strata recovered in the studied core represent an overall shallowing-upward succession of lacustrine-peat mire basin-fill, interpreted to portray the deposits of a maar lake, one of the first identified in Canada (and one of the few maars anywhere to be associated with a kimberlitic pipe). Previously-published palynological data suggest a late Early Eocene to early Middle Eocene age, approximately congruent with a previous radiometric date of 47.4 +/- 0>5 Ma. These deposits include three stratal units: a) a Lower Lacustrine Zone dominated by dark, freshwater, organic-rich mudstone and thick occurrences of oil shale (% total organic carbon: TOC = 15-50 %), interpreted as recording low energy, shallow sub-lacustrine deposition within the crater basin; b) a thin Middle Transitional Zone with evidence of very shallow subaqueous deposition and subaerial exposure (TOC = 2-12 %); and c) an Upper Mire Zone characterized by thick subaerial peat deposits (TOC = 39-55 %), with minor sublacustrine mudstones, interpreted to represent accumulation in a primarily continental setting filling the crater basin. Both lower and upper portions of the succession are characterized by shallowing-upward, higher order sequences whose boundaries correspond with marked changes in TOC, and Rock Eval hydrogen and oxygen indices. These are interpreted to represent either cyclic climatic wet/dry phases or episodic kimberlite/diatreme collapse (downward stoping) subsidence/fill phases. The distribution and type of microscopic organic matter reflects the sedimentological observations: the lower, lacustrinedominated zone includes abundant freshwater diatoms, chrysophytes and liptinites (e.g. sporinite, alginite), whereas the upper, peat mire zone consists predominantly of woody/peat macerals with wellpreserved plant macrofossils. Huminites through the section have reflectance values averaging 0.23 %Ro, indicating that the thermal maturity of these ~47 M.Y.-old sediments, remarkably, is only slightly greater than modern peats, and that post-kimberlite burial and thermal alteration have been insignificant, with temperatures no greater than ~ 30°C. An abrupt cooling and the presence of bentonites during the rapid transitional conversion from lacustrine to mire facies, may indicate a previously-unrecognized regional, post-eruptive, uplift phase, which is recorded in both geochemical and sedimentological indicators.
Summary(Plain Language Summary, not published)
Eocene-aged sediments deposited within a crater which overlies the Giraffe kimberlite pipe were studied in a core. The strata display an overall shallowing-upward basin-fill succession of lacustrine-peat mire environments, interpreted to portray the deposits of a maar lake (low-relief volcanic crater filled with water). Despite their ~47 MY age, these deposits have thermal maturity only slightly greater than modern peat, suggesting an abrupt regional uplift and cooling event.