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Sedimentology of the lower cretaceous gates and moosebar formations, northeast coalfields, British Columbia

University of British Columbia

The Lower Cretaceous Gates and Moosebar Formations in the Northeast Coalfields of British Columbia comprise a 350-450 m thick interval of marine and non-marine, coal bearing clastic sediments. The Moosebar Formation and lowermost unit of the Gates Formation (the Torrens Member) consist of between 2 and 4 coarsening-upward marine cycles. In the northern part of the study area, non-marine sediments in the Gates Formation occur interbedded with 3 major marine tongues (the Sheriff member, Falher C and Babcock member) which pinch out towards the south. In the study area south of Duke Mountain, sediments in the Gates Formation above the Torrens Member are entirely non-marine. The coarsening-upward marine cycles in the Moosebar-lower Gates interval were deposited mainly during regressions when the shorelines prograded northwards. The cycles are comprised of up to 3 main facies interpreted as offshore, transition zone and shoreface-beach (and locally distributary channel) deposits. Thin transgressive deposits are present at the base of the Sheriff member. Lower Gates shorelines are interpreted as high energy and wave dominated. Shoreline trends for the Torrens, Sheriff and Babcock Members are approximately E-W, and approximately NW-SE for the Falher C. Non-marine deposits in the Gates Formation are interpreted as forming in lagoons, fluvial channels and overbank environments within a coastal plain setting. The channels are mainly braided river types with anastomosing or straight (non-braided) and meandering river channels also present. The rivers flowed towards the northwest, north, northeast and east with northeasterly directions most common. Up to three separate very coarse fluvial conglomerates, interpreted as proximal braided river-alluvial fan deposits are present in the south near Mount Belcourt. Overbank sediments were deposited in levee, crevasse splay, lacustrine and well to poorly drained swamp environments The upper Gates marine unit (Babcock member) contains both transgressive and regressive deposits. Three types of transgressive deposits are recognised: 1. Thick (maximum 40 m) estuary mouth |shoal retreat massif) sandstones and conglomerates preserved in linear belts approximately 0.4-2 km wide trending NW-SE and N-S. 2. A thin (maximum 90 cm) but laterally extensive marine lag. 3. Lagoon-intertidal deposits. Upper Gates regressive deposits include shelf to shallow marine sandstones overlain by estuarine subtidal channel and shoal deposits which in turn are overlain locally by tidal flat and coastal plain deposits with thin coal seams. Upper Gates shorelines appear to be strongly tidally influenced. Thick (maximum 10 m), laterally extensive coal seams occur in the lower part of the Gates Formation. Coal seams in the upper Gates are thin (generally <1.0 m). With the exception of few very thin seams, the Gates coals appear to be entirely autochthonous and to have accumulated as peat in swamps in a coastal plain depositional setting. Some of the coals in the lower Gates were deposited in swamps which extended inland for at least 75 km from the shoreline. The maximum coal development in the Gates Formation (28 m total coal and 6 seams >1.5 m thick) occurs in the Foothills south of Kinuseo Creek, near the boundary between transitional marine and non-marine facies belts. Cross-sections based on closely spaced borehole and outcrop sections illustrate the occurrence and distribution of coal seams in the Duke, Honeymoon, Babcock, Frame and McConkey Pits. Thinning and pinchout of coal seams occurs adjacent to fluvial channel and splay deposits and near the landward pinchout of the Sheriff member. Draping of coal seams over fluvial channel deposits causes rapid variations in interseam thickness which may lead to correlation problems in the early stages of exploration and affect the potential mineability of coal seams. The main detrital components in the Gates sandstones are quartz and chert with siliceous rock fragments, carbonate rock fragments, clastic sedimentary and metasedimentary rock fragments, igneous rock fragments and feldspar present in lesser amounts. Mesozoic to Cambrian sedimentary rocks of the Rocky Mountain Front and Main Ranges are interpreted as the principal source of the detrital components.

Text

2010-05-02

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http://hdl.handle.net/2429/24275

Geological Sciences

University of British Columbia

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