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Whitehorse Trough

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Fluvial channel sandstones intercalated with floodplain and crevasse splay deposits, Tanglefoot formation, Robert Campbell Highway east of Carmacks.

 

 

 

 

 

 

 

Whitehorse trough is a frontier intermontane basin located in south-central Yukon, south of the Tintina fault. The trough is an overlap assemblage, with strata unconformably overlying both the Stikine terrane (Lewes River arc), and also the Quesnel terrane locally. The trough originated as a forearc basin (based on structural geology and detrital clast content – minor HP/UHP minerals are consistent with such a setting), but progressively evolved to become a synorogenic piggy-back basin sometime after the end of the Pliensbachian (Colpron 2014). The basin straddles the Yukon-British Columbia border, with its northernmost margin in the Carmacks area, and covers an area of approximately 2.44 million hectares. Its geology is characterized by an approximately 3000m thick deformed Jurassic sedimentary succession (the Laberge Group), underlain by a depositional basement of Triassic sediments (the Lewes River Group), and capped by Cretaceous and Neogene volcanic rocks.

In 2004, two 2D seismic lines, acquired in the northern part of the basin, provided approximate stratigraphic thicknesses and subsurface visualisation of the structural fabric within the basin (White et al. 2012). A recent 3D geologically-constrained inversion of magnetic and gravity data (combining new bedrock geology maps – Colpron 2011, rock property and seismic survey data) has since provided higher-resolution evidence for dome structures and ‘crested anticlines’ in the Carmacks area, together with modelling the base of the trough as shallower than previously estimated (Mira Geoscience 2014).

The Upper Triassic Lewes River Group (Stikine terrane) represents the depositional basement to Whitehorse trough. It consists of the Povoas Formation (andesites and basaltic-andesites, together with minor volcaniclastics and carbonates), overlain by the Aksala formation (slope sandstones, conglomerates and mudstones of the Casca member, microbial reef-forming carbonates of the Carnian-Norian aged Hancock member, and peritidal? red mudstones and sandstones of the Mandanna member). The unconformity separating the Lewes River and Laberge groups is estimated at greater than 20 m.y. (Colpron 2014), and is based on detrital zircons taken from a Mandanna member tuff (201 Ma) and Laberge Group sandstones that have a youngest depositional age of ~180 Ma.

The Lower to Middle Jurassic (late Hettangian-Bajocian) Laberge Group is currently subdivided into the Richthofen (submarine fan complex mass-flow conglomerates and deep-water turbidites) and Tanglefoot formations (laterally equivalent shallow marine, deltaic, fluvial and associated coal deposits). The Richthofen and Tanglefoot formations are the same age (Sinemurian-Bajocian), but the former is restricted to the southern half of the basin, whereas the latter occurs in the northern half (Lowey 2008). Intercalated Nordenskiold unit dacitic tuffs are dated at ca. 188-184 Ma (Colpron 2014), and have been recognised at several distinct horizons within both the Richthofen and Tanglefoot formations. They are therefore considered to provide excellent correlative potential to aid in establishing the Jurassic stratigraphic architecture of the basin in future studies. The Laberge Group is unconformably overlain by the Tantalus Formation, an exclusively terrestrial sequence of coarse-grained fluvial deposits with associated coal-forming swamps and small lakes (Long 2005).

The evidence for the presence of conventional (and unconventional – see Hayes and Archibald 2012) hydrocarbons in Whitehorse trough is compelling, and mean in-place assessed volumes are sufficiently substantial to support additional exploration and assessment work in the basin (Hayes 2012), although it should be recognized that the range of possible values around quoted means varies tremendously as a result of our limited knowledge of the basin. Prospectivity is currently assigned to nine plays, all of which are prospective for gas and three of which have oil potential as well. The Tanglefoot formation is the most prospective, with conventional hydrocarbons expected to occur in both structural and stratigraphic traps. Speculative unconventional targets include coal bed methane in both the Tanglefoot and Tantalus formations, and tight/shale gas in the Richthofen formation (Hayes 2012).

Based on over 600 samples taken from the Lewes River Group, Laberge Group and Tantalus Formation analyzed for RockEval, thermal alteration indices of palynomorphs (TAI) and conodonts (CAI), and vitrinite reflectance, the current petroleum source rock potential of this basin is as listed below. The most prospective areas for petroleum exploration based on these data are Five Finger Rapids, Division Mountain and Tantalus Butte in the northern portion of Whitehorse trough.

  • The Povoas formation and Nordenskiold unit tuffs have no source rock potential due to their volcanic/volcaniclastics composition (Lowey et al. 2009).
  • The Aksala formation is a poor source rock (Lowey et al. 2009) that is postmature with no hydrocarbon potential (although, additionally, the Hancock member reefs may provide potential stratigraphic traps for conventional hydrocarbons where capped by Mandanna member mudstones, e.g. Lowey 2011).
  • The Richthofen formation is a poor (Lowey et al. 2009) to fair source rock in terms of quantity, but kerogen quality suggests minor gas potential only, and both Tmax and vitrinite reflectance suggest the rocks are postmature with respect to oil generation.
  • The Tanglefoot formation is a good source rock that is immature to early mature (with respect to oil generation), and is typically gas prone as indicated by kerogen quality data (Lowey et al. 2009).
  • TOC data suggest that the Tantalus Formation is also a good source rock, with kerogen quality and maturity data suggesting an immature to early mature, gas-prone nature (Lowey et al. 2009).


References


• Colpron, M. 2011 (compiler). Geological compilation of Whitehorse trough – Whitehorse (105D), Lake Laberge (105E), and parts of Carmacks (115I), Glenlyon (105L), Aishihik Lake (115H), Quite Lake (105F), and Teslin (105C). Yukon Geological Survey, Geological map 2011-1, 1:250,000, 3 maps, legend & appendices.
• Colpron, M. 2014. Birth of the Northern Cordilleran orogeny, as recorded by Jurassic sedimentation and exhumation in Yukon. Presentation at 2014 Geological Society of America Annual Meeting. Vancouver, Canada, 19-22 October 2014.
• Hayes, B. R. 2012. Petroleum resource assessment of Whitehorse trough, Yukon, Canada. Yukon Geological Survey, Miscellaneous report 6, pp. 1-52.
• Hayes, B. R. & Archibald, H. B. 2012. Scoping study of unconventional oil and gas potential, Yukon. Yukon Geological Survey, Miscellaneous report 7, 1-100.
• Long, D. G. F. 2005. Sedimentology and hydrocarbon potential of fluvial strata in the Tantalus and Aksala formations, northern Whitehorse Trough, Yukon. In: Emond, D. S., Lewis, L. L. & Bradshaw, G. D. (eds) Yukon Exploration & Geology 2004, Yukon Geological Survey, 167-176.
• Lowey, G. W. 2008. Summary of the stratigraphy, sedimentology and hydrocarbon potential of the Laberge Group (Lower-Middle Jurassic), Whitehorse Trough, Yukon. In: Emond, D. S., Blackburn, L. R., Hill, R. P. & Weston, L. H. (eds) Yukon Exploration & Geology 2007, Yukon Geological Survey, 179-197.
• Lowey, G. W., Long, D. G. F., Fowler, M. G., Sweet, A. R. & Orchard, M. J. 2009. Petroleum source rock potential of Whitehorse trough: a frontier basin in south-central Yukon. Bulletin of Canadian Petroleum Geology, 57, 350-386.
• Mira Geoscience 2014. Geologically-constrained inversion of magnetic and gravity data over parts of the Yukon-Tanana terrane and Whitehorse trough. Yukon Geological Survey, Miscellaneous report 10, 1-85.
• White, D., Colpron, M. & Buffet, G. 2012. Seismic and geological constraints on the structure and hydrocarbon potential of the northern Whitehorse trough, Yukon, Canada. Bulletin of Canadian Petroleum Geology, 60, 1-17.