--> Abstract: Sedimentology and Diagenetic Features of the Lower Jurassic "Nordegg Member," Western Canada Sedimentary Basin, by C. L. Riediger; #91012 (1992).

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ABSTRACT: Sedimentology and Diagenetic Features of the Lower Jurassic "Nordegg Member," Western Canada Sedimentary Basin

RIEDIGER, C. L., University of Calgary, Calgary, Alberta, Canada

The many sedimentological and diagenetic features observed in "Nordegg" cores aid in our understanding of the depositional environment and diagenetic history of this unit. Planar parallel lamination is typical, as is a lack of bioturbation. Rare faunal remains include mainly pelagic organisms. The sedimentological evidence thus suggests a low-energy, dysaerobic to anaerobic environment of deposition. TOC values of up to 28% and organic geochemical (biomarker) data (e.g., pristane/phytane ratios generally < 1.0; C35/C34 triterpane ratios > 1.0) are consistent with this interpretation.

Phosphate nodules are observed in core, and the apatitic composition of these nodules is confirmed by scanning electron microscope-energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD) analysis. The origin of the nodules is unclear, but the association of the phosphate with laminated, amorphous organic matter, as well as high magnification SEM photographs showing that the apatite occurs as coccoid spheres (1 to 2 um diameter), suggests bacterial mediation during the formation of the apatite.

Displacive calcite beds occur at several stratigraphic levels within the "Nordegg," and the displacive nature of these zones suggests that they formed during early diagenesis, under small overburden loads. Stable carbon and oxygen isotopic analysis agrees with the early diagenetic interpretation based on macroscopic features, and further suggests varying meteoric contribution to calcite formation. The more depleted Oxygen 18 values occur in western regions of the study area, suggesting the existence of emergent areas to the west and constraining paleogeographic reconstructions of the `"Nordegg" basin.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)