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Sedimentary organic matter in the Colorado Group (Cretaceous), Alberta

University of British Columbia

The transgressive interval between the top of the Viking Formation and the top of the Second White Speckled Shale in south-central Alberta consists predominantly of mudstones with varying organic richness. The section has been divided into six lithologic "zones" named A through F in ascending stratigraphic order. Existing informal names are used for Zone B (the Fish Scale Zone) and Zone F (the Second White Speckled Shale or Second White Specks). The organic matter in all zones is largely of marine origin. Low TOC values characterize Zone A as a result of destruction of organic matter in a relatively oxidizing environment. All zones above Zone A were deposited under mainly oxygendeficient conditions caused by constriction of the basin due to tectonic movements and by a sea level drop at the onset of Fish Scale Zone time. At peak transgression during deposition of Zone E and the Second White Speckled Shale episodic oxygenation of bottom waters occurred. Relatively high TOC values characterize the Fish Scale Zone, Zone C, Zone D and the Second White Speckled Shale. Low TOC values in Zone E resulted from destruction of organic matter at the sediment/water interface, and possibly as a result of low productivity. The high TOC levels in the Second White Speckled Shale occur in spite of episodic oxygenation of bottom waters and may reflect a global productivity increase at about the Cenomanian-Turonian stage boundary. The present distribution of organic matter in all zones is influenced by a westward increase in thermal maturation. Clay mineralogy is unrelated to organic content. Coarse-grained beds contain less and more refractory organic matter than the finergrained rocks. The amount of silt in mudstones and argillaceous siltstones is unrelated to TOC in all zones except the Fish Scale Zone, in which a increasing silt content correlates to lower TOC. Silt content and oxygen index are related only in Zone D; the correlation is positive. Variations in sedimentation rate derived from thickness variations are correlated in a positive sense for Zone A, the Fish Scale Zone and Zone E. Zone D exhibits a negative correlation, suggestive of dilution. Potential oil exploration targets exist in sandstones in the Fish Scale Zone, at the base of Zone D, and in fractured shales of the Second White Speckled Shale.

Thesis/Dissertation

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2008-12-17

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http://hdl.handle.net/2429/3026

Geological Sciences

University of British Columbia

UBCV

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