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TitreAn Eocene post-kimberlite maar lake: lacustrine oil-shale crater-fill deposits, Lac de Gras area, Northwest Territories, Canada
TéléchargerTéléchargements
AuteurHamblin, A P
SourceCommission géologique du Canada, Dossier public 7809, 2015, 26 pages, https://doi.org/10.4095/296430
Année2015
ÉditeurRessources naturelles Canada
Documentdossier public
Lang.anglais
DOIhttps://doi.org/10.4095/296430
Mediaen ligne; numérique
Formatspdf
ProvinceTerritoires du Nord-Ouest
SNRC76C/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
Lat/Long OENS-112.0000 -108.5000 65.2500 64.2500
SujetsEocene; schistes bitumineux; hydrocarbures; capacité de production d'hydrocarbures; matières organiques; dépôts organiques; tourbières; analyses de la tourbe; combustibles fossiles; stratigraphie; Cénozoïque
Illustrationslocation maps; stratigraphic columns; photographs; cross-sections
ProgrammeCaractérisation des réservoirs de schiste, Les géosciences pour les nouvelles sources d'énergie
Diffusé2015 05 28
Résumé(disponible en anglais seulement)
Sedimentology 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.
Résumé(Résumé en langage clair et simple, non publié)
Les sédiments de l'Éocène qui se sont déposés dans un cratère au-dessus de la cheminée de kimberlite Giraffe ont été étudiés dans une carotte. Les strates présentent une succession de cuvettes et de remblais constitués de milieux de tourbe et de bourbier lacustres s'amincissant vers le haut, qu'on suppose représenter les dépôts d'un maar (cratère volcanique peu accidenté rempli d'eau). En dépit de leur âge d'environ 47 millions d'années, la maturité thermique de ces dépôts est à peine plus grande que celle de la tourbe contemporaine, ce qui suggère un soulèvement régional et un épisode de refroidissement soudains.
GEOSCAN ID296430