GEOSCIENCECANADAThe Jurassic Laberge Group in the White-horse Trough of the Canadian Cordillera:Using Detrital Mineral Geochronology andThermochronology to Investigate TectonicEvolutionDawn A. Kellett1and Alex Zagorevski21Geological Survey of Canada-Atlantic, Natural Resources Canada1 Challenger Drive, Dartmouth, Nova Scotia, B2Y 4A2, CanadaE-mail: dawn.kellett@canada.ca2Geological Survey of Canada-Central, Natural Resources Canada601 Booth Street, Ottawa, Ontario, K1A 0E8, CanadaSUMMARYThe Laberge Group is an Early to Middle Jurassic sequence ofmostly siliciclastic sedimentary rocks that were deposited in amarginal marine environment in the northern CanadianCordillera. It forms a long narrow belt with a total thickness of3–4 km extending for more than 600 km across southernYukon and northwestern British Columbia. These sedimentaryrocks overlap the Yukon-Tanana, Stikinia and Cache Creek ter-ranes that form the main components of the Intermontanesuperterrane. The Laberge Group contains a record of theerosion of some of these terranes, and also offers some con-straints on the timing of their amalgamation and accretion tothe Laurentian margin. The Laberge Group was depositedwith local unconformity on the Late Triassic Stuhini Group (inBritish Columbia) and correlative Lewes River Group (inYukon), both of which are volcanic-rich, and assigned to theStikinia terrane. The Laberge Group is in turn overlain by Mid-dle Jurassic to Cretaceous clastic rocks, including the BowserLake Group in B.C. and the Tantalus Formation in Yukon.Clast compositions and detrital zircon populations within theLaberge Group and between it and these bounding units indi-cate major shifts in depositional environment, basin extent anddetrital sources from Late Triassic to Late Jurassic. During theEarly Jurassic clast compositions in the Laberge Group shiftedfrom sediment- and volcanic-dominated to plutonic-dominat-ed, and detrital zircon populations are dominated by grainsthat yield ages that approach or overlap their inferred deposi-tional ages. This pattern is consistent with progressive dissec-tion and unroofing of (an) active arc(s) to eventually exposeTriassic to Jurassic plutonic suites. Detrital rutile and mus-covite data from the Laberge Group indicate rapid cooling andthen exhumation of adjoining metamorphic rocks during theEarly Jurassic, allowing these to contribute detritus on a morelocal scale. The most likely source for such metamorphic detri-tus is within the Yukon-Tanana terrane, and its presence in theLaberge Group may constrain the timing of amalgamation andaccretion of the Yukon-Tanana and Stikinia terranes. Ther-mochronological data also provide new insights into the evo-lution of the Laberge Group basin. Results from the U–Th/(He) method on detrital apatite suggest that most areasexperienced post-depositional heating to 60°C or more,whereas U–Th/(He) results from detrital zircon show thatheating to more than 200°C occurred on a more local scale. Indetail, Laberge Group cooling and exhumation was at least inpart structurally controlled, with more strongly heated areassituated in the footwall of an important regional fault system.The thermochronological data are preliminary, but they sug-gest potential to eventually constrain the kinematics and timingof inversion across the Laberge Group basin and may alsohave implications for its energy prospectivity. In summary, the Laberge Group is a complex package ofsedimentary rocks developed in an active, evolving tectonicrealm, and many questions remain about the details of itssources and evolution. Nevertheless, the available informationdemonstrates the potential of combined geochronological andthermochronological methods applied to detrital minerals tounravel links between regional tectonics, basin developmentand clastic sedimentation.Volume 4920227Geoscience Canada, v. 49, https://doi.org/10.12789/geocanj.2022.49.183 pages 7–27 © 2022 GAC/AGC®ARTICLE
RÉSUMÉLe groupe de Laberge est une séquence du Jurassique inférieurà moyen composée principalement de roches sédimentaires sil-icoclastiques qui se sont déposées dans un milieu margino-marin, dans le nord de la Cordillère canadienne. Il forme unelongue ceinture étroite d’une épaisseur totale de 3 à 4 km s’é-tendant sur plus de 600 km à travers le sud du Yukon et lenord-ouest de la Colombie-Britannique. Ces roches sédimen-taires chevauchent les terranes Yukon-Tanana, Stikinia etCache Creek qui forment les principales composantes dusuperterrane Intermontagneux. Le groupe de Laberge contientun enregistrement de l’érosion de certains de ces terranes, etoffre également certaines contraintes sur la datation de leuramalgamation et de leur accrétion à la marge laurentienne. Legroupe de Laberge a été déposé avec une discordance localesur le groupe de Stuhini du Trias supérieur (en Colombie-Bri-tannique) et le groupe corrélatif de Lewes River (au Yukon),tous deux riches en volcans et attribués au terrane de Stikinia.Le groupe de Laberge est à son tour recouvert de roches clas-tiques du Jurassique moyen à Crétacé, comprenant le groupede Bowser Lake en Colombie-Britannique et la formation deTantalus au Yukon. Les compositions de clastes et les popula-tions de zircons détritiques au sein du groupe de Laberge etentre celui-ci, et ces unités limitrophes indiquent des change-ments majeurs dans l’environnement de dépôt, l’étendue dubassin et les sources détritiques du Trias supérieur jusqu’auJurassique supérieur. Au cours du Jurassique inférieur, les com-positions des clastes du groupe de Laberge sont passées d’uneprédominance sédimentaire et volcanique à une prédominanceplutonique, et les populations de zircons détritiques sont dom-inées par des grains qui donnent des âges qui se rapprochentou chevauchent l’âge présumé de leur déposition. Ce modèleest cohérent avec la dissection progressive et le dévoilementd’un ou plusieurs arcs actifs pour éventuellement exposer lessuites plutoniques du Trias au Jurassique. Les données sur lerutile détritique et la muscovite du groupe de Labergeindiquent un refroidissement rapide puis une exhumation desroches métamorphiques adjacentes au cours du Jurassiqueinférieur, permettant à celles-ci d’ajouter des débris à uneéchelle plus locale. La source la plus probable de ces débrismétamorphiques se trouve dans le terrane Yukon-Tanana, et saprésence dans le groupe de Laberge peut apporter des con-traintes sur la datation de l’amalgamation et de l’accrétion desterranes Yukon-Tanana et Stikinia. Les données thermo-chronologiques apportent également de nouveaux éclairagessur l’évolution du bassin du groupe de Laberge. Les résultatsde la méthode U–Th/(He) sur l’apatite détritique suggèrentque la plupart des régions ont été soumises à des conditions detempérature post-dépôt de 60°C ou plus, tandis que les résul-tats U–Th/(He) sur zircon détritique montrent que des condi-tions de température de plus de 200° C se sont produites à uneéchelle plus locale. Dans le détail, le refroidissement et l’ex-humation du groupe de Laberge étaient au moins en partiestructurellement contrôlés, avec des régions plus fortementchauffées situées dans le mur d’un important système de faillesrégionales. Les données thermo-chronologiques sont prélimi-naires, mais elles suggèrent un potentiel pour éventuellementcontraindre la cinématique et le moment de l’inversion à tra-vers le bassin du groupe de Laberge et peuvent également avoirdes implications sur sa capacité énergétique.En résumé, le groupe de Laberge est un ensemble com-plexe de roches sédimentaires développées dans un domainetectonique actif et en évolution, et de nombreuses questionsdemeurent quant aux détails de ses sources et de son évolution.Néanmoins, les informations disponibles démontrent le poten-tiel de la combinaison des méthodes géochronologiques etthermo-chronologiques appliquées aux minéraux détritiquespour démêler les liens entre la tectonique régionale, ledéveloppement du bassin et la sédimentation clastique.Traduit par la TraductriceINTRODUCTIONThe Cordilleran Orogen of northwestern Canada developedthrough the successive accretion of discrete late Paleozoic andMesozoic terranes against a long-lived active margin. Figure 1shows the regional distribution of these terranes in BritishColumbia, Yukon and parts of Alaska, including the area ofthe research discussed in this paper. Five terranes, togethercomprising the Intermontane superterrane (i.e. Slide Moun-tain, Yukon-Tanana, Stikinia, Quesnellia and Cache Creek ter-ranes) are of particular importance, as their accretion to theLaurentian margin has been considered to represent the initia-tion of the northern Canadian Cordillera orogen (Colpron etal. 2015; Monger and Gibson 2019). The exact geometry, tim-ing and conditions of the accretion event(s) are not well estab-lished, and several outstanding questions are reviewed else-where (Zagorevski et al. 2017, 2021). Understanding the rela-tive and absolute timing of these events is important in devel-oping and improving the economic geology framework for theregion (e.g. Logan and Mihalynuk 2014). The Late Triassic toCretaceous sedimentary rocks in northern British Columbia(B.C.) and southern Yukon provide a record of changes indepositional environments and the evolution of those sedi-mentary basins through the time interval of progressive ter-rane accretion. This paper reviews the broader Late Triassic toCretaceous evolution of this key segment of the orogen (Fig.1) but focuses most of its attention on the Early–Middle Juras-sic Laberge Group of the Whitehorse trough (Fig. 2; Wheeler1961; Eisbacher 1974), and particularly recent and new data onsediment provenance, depositional constraints, and the timingand conditions of basin deformation. The principal tools thatprovide insight into the development of the Laberge Groupare detrital geo- and thermochronology (U–Pb methods usingzircon and rutile, and 40Ar/39Ar methods using muscovite andbiotite) that constrain sediment source regions and low tem-perature thermochronological methods (U–Th/He on zirconand apatite) that illustrate aspects of basin evolution. Thepaper is an extension of a recent Geological Survey of Canadareport (Kellett and Zagorevski 2021) and is intended to illus-trate how diverse techniques can be applied together to under-stand the linked processes of tectonic accretion and basindevelopment.The Laberge Group is mostly found along the eastern flankof the Stikinia terrane, so Stikinia is the most likely source8Dawn A. Kellett and Alex Zagorevskihttp://www.geosciencecanada.ca
region for detritus entering the Laberge Group basin (e.g. Col-pron et al. 2015; Fig. 2). Much of the research on LabergeGroup sedimentary rocks has been focussed on provenanceand identification of terrane-specific detritus that might con-strain the timing of terrane amalgamation. Identification ofdetritus that could be derived from other now-adjacent ter-ranes (e.g. the Yukon-Tanana or Cache Creek terranes; Figs. 1,2) may provide a temporal constraint on their initial juxtaposi-tion with Stikinia. However, the precise timing may beobscured if detritus is multicyclic. For example, sediment thatwas derived by erosion and recycling of an exhumed melangemay significantly postdate the timing of accretion. In additionto timing of accretion, the nature of detritus in the LabergeGroup sedimentary rocks can provide other critical constraintson the evolution of the orogen. The Laberge Group includesdetritus of rock units that are either no longer exposed, orwere removed completely by such erosion, providing ‘snap-shots’ of the geological architecture of the Cordilleran Orogenthroughout the Early Jurassic. Clast composition, such as clastsof high-pressure metamorphic rocks or mineral grains indica-tive of such conditions, can be diagnostic of specific tectonicsettings, and provide insight into tectonic setting of the Inter-montane terranes at a particular time. Overall, the range ofsource materials deposited into the Laberge Group basin pro-GEOSCIENCECANADAVolume 4920229https://doi.org/10.12789/geocanj.2022.49.183Figure 1.Terrane map of the Canadian Cordillera and adjacent regions of Alaska adapted from Colpron et al. (2015).
vides a record of vigorous erosion and deep incision of ayoung and evolving orogen.TERMINOLOGY AND DATA SOURCESThe Whitehorse troughis a pre-plate tectonic term that was orig-inally introduced to describe a linear belt of volcanic and sed-imentary rocks in southern Yukon and northwestern B.C.,including throughout the Whitehorse area (Fig. 3). Initially, theterm was intended to include part or all of the Triassic LewesRiver and Stuhini groups (located in the Yukon and B.C.,respectively) and the Jurassic Laberge Group (in both areas),while more recent studies have also included the overlyingJurassic–Cretaceous Tantalus Formation in Yukon. Theregional stratigraphic and geological relationships are summa-rized in the time–space diagram of Figure 3 (after Wheeler1961; Hart 1997; Lowey 2008; Templeman-Kluit 2009),although not all components indicated on the figure are specif-ically discussed in this paper. A more restrictive definition ofthe Whitehorse Trough (Hutchison 2017) includes only theLaberge Group.The Laberge Group extends from Carmacks in southernYukon to Dease Lake in B.C. (Fig. 2). Regional mapping hasresulted in two sets of formal nomenclature for geologicalunits that are at least in part equivalent (Fig. 3). In this paper,the B.C. nomenclature is used for B.C. field areas, and Yukonnomenclature is used for Yukon field areas, with the probableequivalence identified where appropriate. This is done to aidthe reader, but it should be noted that this may introduce someinconsistencies as the formal definitions of lithologically simi-lar units are not necessarily identical. The transition from the Lewes River and Stuhini groupsinto the Laberge Group was interpreted in both B.C. and theYukon to represent a Hettangian (or younger) unconformity,above which the sedimentary environment changed markedly,and sedimentation rates increased (e.g. Johannson and McNi-coll 1997; Shirmohammad et al. 2011; Colpron et al. 2015;Hutchison 2017; van Drecht and Beranek 2018). The bound-ary between the Laberge Group and the overlying Late Jurassicto Early Cretaceous Tantalus Formation in southern Yukon isa locally angular unconformity that also marks a significantchange in sedimentary environment, and change in the extentof the sedimentary basin (Fig. 3; Colpron et al. 2015). The var-ied definitions of Whitehorse trough make this term ambigu-ous. In this contribution, rock units are mostly referred tousing specific lithological or stratigraphic terminology, and,following Hutchison (2017), the term Whitehorse trough isused synonymously with the Laberge Group. The Geological Survey of Canada’s Geomapping for Ener-gy and Minerals Program (GEM) included acquiring a range ofnew geochronological and thermochronological data fromsedimentary rocks in the Cordilleran Orogen, under theGEM2 (2013–2020) Cache Creek and Yukon Tectonic Evolu-tion activities. Recent work by the British Columbia GeologicalSurvey and the Yukon Geological Survey has also generatedsignificant new geochronological datasets for sedimentaryrocks across the region shown in Figure 2. These new data addgreatly to existing and new sedimentological (e.g. Hutchison2017; van Drecht and Beranek 2018), biostratigraphic (e.g.Johannson et al. 1997; Golding 2018), and structural (e.g. Eng-lish et al. 2002; White et al. 2006) information. Collectively,GEOSCIENCECANADAVolume 49202211https://doi.org/10.12789/geocanj.2022.49.183Figure 3.Comparative Upper Triassic to Lower Cretaceous lithological sections for Yukon (modified from Hutchison 2017; Sack et al. 2020 and references therein) and BritishColumbia (modified from Souther 1972; Shirmohammad et al. 2011; van Straaten and Bichlmaier 2017; Nelson et al. 2018; Mihalynuk et al. 2018 and references therein). Geo-logic Time Scale from Walker et al. (2018). Note that some units indicated here are not identified individually in Figure 2, and that some minor subdivisions of units referencedin the text cannot be represented in this summary diagram.