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Upper Devonian to Carboniferous (Yukon Geological Survey; T. Fraser, T. Allen) |
Upper Devonian to Carboniferous strata in the Peel Plateau consist of clastic rocks interpreted to have a northern orogenic source. This clastic wedge consists of the Imperial, Tuttle and Ford Lake Shale formations. The Imperial is an Upper Devonian turbidite sequence that is conformably overlain by the Upper Devonian to Carboniferous Tuttle and its basinal equivalent, the Ford Lake Shale.
The Tuttle Formation which crops out on the eastern flank of the Richardson Mountains occurs almost exclusively in Yukon, extending slightly east into the Northwest Territories. The Tuttle has been an exploration target for oil and gas with minor amounts of gas recorded from drilling activities in the 1960s and 1970s. Recent field investigations included measuring detailed stratigraphic sections on Trail River, Road River and Tetlit Creek (NTS mapsheet 106L). Visits were also made to Tuttle exposures north of the Road River and south of Trail River on a north-south ridge. At these localities, samples were collected for analysis of lithology, age determination, source rock potential, porosity, permeability and thermal maturation studies.
The Tuttle Formation consists of alternating packages of coarse-grained and finer grained strata. Coarse-grained intervals are resistant in nature and comprise chert- and quartz-rich sandstone with minor conglomerate. Locally, the chert is weathered to a porous, chalky white material and is more commonly observed as variable-coloured clasts (e.g., grey, black, yellow and green). Sandstone textures range from fine to very coarse grained, with sorting mainly poor to moderate. The fine-grained intervals consist of interlaminated siltstone, shale and very fine-grained sandstone beds.
The coarse fraction of the Tuttle Formation has a wide range of grain sizes ranging from fine-grained sandstone to small pebble conglomerate. Forty-six field samples, divided into six grain size classes, were analyzed for porosity and permeability. Porosity ranges from 2 to 26%, with permeability ranging between 0 and 127 mD. For the Tuttle Formation, reservoir potential based on porosity and permeability varies from negligible to very good. Thin section analyses of Tuttle sandstones from outcrop show the presence of intergranular porosity as well as intragranular porosity within weathered chert grains. Oil-stained chert grains and what are likely small oil accumulations can be found in the pores of some Tuttle samples. In samples with lower porosity, the quartz to chert ratio is higher, and primary porosity is often restricted by quartz overgrowths and infill by what appear to be clay minerals.
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Photo gallery
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Outcrop of the Tuttle Formation on Trail River. Covered intervals to the left and right of the outcrop are interpreted to be recessive siltstones, mudstones and shales.
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Typical Tuttle sandstone with dominantly medium-sized chert and quartz grains.
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Fining-upwards sequence in Tuttle sandstone interval near its lower contact with the Imperial Formation. Note that overall the rock is poorly sorted and has an abundance of vari-colored chert. Grain size in this sample is coarser than was generally observed in the field.
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Close-up of granule sample in the Tuttle Formation. Note the poor sorting and the abundance of chert (white, grey, black, yellow and green).
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Plant fossil (Lepidendron?) found in Tuttle Formation on Road River.
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Allen, T.L. and Fraser, T.A., 2008. Hydrocarbon potential of Upper Paleozoic strata, eastern Richardson Mountains, northern Mackenzie Mountains and Peel Plateau, Yukon. In: Yukon Exploration and Geology 2007, D.S. Emond, L.R. Blackburn, R.P. Hill and L.H. Weston (eds.), Yukon Geological Survey, p. 91-114. Download now.
Fraser, T. and T. Allen, 2007. Field investigations of the Upper Devonian to Lower Carboniferous Tuttle Formation, eastern Richardson Mountains, Yukon. In: Yukon Exploration and Geology 2006, D.S. Emond, L.L. Lewis and L.H. Weston (eds.), Yukon Geological Survey, p. 157-173. Download now.
New Rock Eval-TOC data for 18 wells in the Peel Plateau and Plain area in Yukon (YGS Open Files 2008-1) are available online. Download now.
Preliminary Investigations of the Upper Devonian to Lower Carboniferous Tuttle Formation, East Richardson Mountains, Yukon; poster by Allen and Fraser at Yellowknife Geoscience Forum (November 21-23, 2006) and Whitehorse Geoscience Forum (November 27-29, 2006). Download now.
Preliminary Investigations of the Upper Devonian to Lower Carboniferous Tuttle Formation, East Richardson Mountains, Yukon; presentation by Fraser and Allen at Yellowknife Geoscience Forum (November 21-23, 2006) and Whitehorse Geoscience Forum (November 27-29, 2006). Download now.
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