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Stratigraphic style of coal and non-marine strata in a high accommodation setting: Falher Member and Gates Formation (Lower Cretaceous), western Canada

Department of Geology, University of Newcastle, Callaghan, NSW, 2308, Australia

Dale Leckie

Nexen Inc., 801 - 7th Ave. SW, Calgary, AB T2P 3P7

The Lower Cretaceous Falher Member and Gates Formation of the Western Canadian Sedimentary Basin provide an opportunity to investigate high resolution stratigraphic correlation in non-marine to marginal marine rocks. This is due to the large volume of high-quality data available from subsurface cores and wireline logs in the Elmworth area, corresponding outcrops in the adjacent Rocky Mountains foothills, and outcrop and continuous core data from the Bullmoose Mine area in northeastern British Colombia.

A key component of the high resolution interpretation and correlation in this region is the abundant, thick coal seams that occur in fresh mine highwalls and also in cores from the mine sites and the Elmworth area. Using coal petrographic constituent analysis and derived parameters, we are able to identify systematic variations in coal properties that respond to changes in accommodation. In particular, these properties enable us to distinguish two types of peat (transgressive and regressive) characterized by wetting- and drying-upward behaviour linked to variations in the groundwater table. They also enable recognition of a range of non-marine stratigraphic surfaces that record responses to changing accommodation, including accommodation reversal surfaces, flooding surfaces, hiatal surfaces, paludification surfaces and terrestrialization surfaces. A combination of these coal parameters, together with the facies characteristics of the surrounding non-marine and marginal marine rocks, enables recognition of distinctive high-resolution stratigraphic signatures in the rocks. This in turn provides a previously unavailable ability to correlate stratigraphic units from their downdip marine position, through the shoreline zone and into the terrestrial realm.

For the Falher/Gates unit, we recognize nine regionally correlatable cycles over a vertical distance of around 300 m and a lateral distance of 80 km downdip and 150 km along strike. The basis for this detailed correlation is the similarity of the accommodation trends as seen in both the organic and clastic facies. Results show that earlier concepts of parasequences and their flooding surface boundaries in marine rocks need to be significantly modified in the terrestrial realm. Sharp hiatal parasequence boundaries in the marine realm such as flooding surfaces and wave/tidal ravinement surfaces may correlate updip to packages of rocks that pass gradationally from transgressive to regressive units and preserve the transitions between the two. Non-marine sediments may accumulate during and following shoreline regression, and prior to and during shoreline transgression. The exact style and preservation of the non-marine stratigraphic package depends on the local balance between accommodation and sediment flux at the time of deposition. Coals occur in both regressive and transgressive styles and may initiate or terminate parasequences. Coals may also occur as compound coals that span more than one parasequence and contain internal discontinuity surfaces.

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