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Origin of source rocks in the Middle Devonian Keg River Formation, Rainbow and Zama sub-basins, Alberta: Sedimentological and organic petrological evidence
Organic-rich carbonate facies in the Middle Devonian Keg River Formation in the Rainbow and Zama sub-basins are widely recognized as source rocks but have received considerably less attention than the oil-producing reef facies in the formation. An integrated approach using stratigraphy, sedimentology and organic petrology to study these bituminous laminites has facilitated regional stratigraphic correlation and interpretation of the depositional history, paleoenvironments, and paleoecology of the Keg River ramp and reef foreslope successions. This work provides insight into the influence of depth-related anoxia, high productivity and sedimentation rates as primary controls on organic matter accumulation and preservation in Keg River source rocks.
The lowermost bituminous laminite unit in the Lower Keg River Member in the Rainbow Sub-basin (LRL) occurs at the base of a decameter-scale shallowing-upward cycle. The LRL represents the onset of a transgressive event, possibly related to local subsidence along a widespread carbonate ramp. The middle bituminous laminite unit in the Rainbow Sub-basin (MRL) and the lower laminite unit in the Zama Sub-basin (LZL) occur at the base of reefal successions near the top of the Lower Keg River Member and they are considered to be correlative. The MRL and LZL reflect initiation of basin differentiation and the associated relative sea-level rise, creating the Rainbow and Zama sub-basins. All three bituminous laminite units are characterized by intermediate water-depth organic facies B which is distinguished by small, thin-walled alginites and acanthomorphic acritarchs. This organic facies, along with sedimentological evidence, indicates that accumulation and preservation of organic matter in the bituminous laminite units were due primarily to depth-related anoxia associated with transgressive events.
Bituminous laminite units in the Upper Keg River Member of the Rainbow Sub-basin (URL) and Zama Sub-basin (UZL) occur near the base of reef foreslope and off-reef successions and they are considered to be generally correlative. The URL in distal foreslope positions is interpreted as primarily organic facies B with secondary organic facies BL, characterized by dominant large, thick-walled alginites. These organic facies are indicative of organic matter accumulation and preservation due to depth-related anoxia, as well as intermittent, elevated phytoplanktonic productivity (algal blooms) which was overwhelmed by the anoxia signal. In contrast, the URL and UZL in proximal foreslope positions appear to be dominated by organic facies BL suggesting that episodic, high phytoplanktonic productivity was the primary control. High productivity induced ephemeral anoxia due to increased respiration rates and high organic matter flux to the sea floor. Clastic dilution effects were pronounced in proximal foreslope settings.
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