Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Plint, A. Guy

2nd Supervisor

Cheadle, Burns A.

Co-Supervisor

Abstract

The Santonian to early Campanian mudstone-dominated Puskwaskau Formation was correlated throughout a study area of 50,000 km2 in north-central Alberta using 988 well logs. Fourteen informal allomembers, established by previous studies, are bounded by regionally-mappable marine flooding and/or transgressive surfaces that are traceable for hundreds of kilometres within the study area. These laterally continuous bounding surfaces are parallel to very gently converging, and mostly terminate by onlap onto underlying surfaces. Observations in thin section and in SEM revealed ten mudstone microfacies, grouped into five microfacies associations. The facies preserve evidence for repeated storm-generated reworking of the seafloor. The Puskwaskau Fm. exhibits an assemblage of authigenic cements that are mainly intergranular pore-filling phases indicative of early diagenesis. The geometric style of allomember bounding surfaces, combined with microfacies analysis, suggests that the seafloor was a low-gradient ramp that was repeatedly reworked by storms. The physiography of the ramp was maintained by a near equilibrium between the rates of accommodation and sediment supply.

Allomembers are grouped into three tectono-stratigraphic ‘units’, each of which forms a broadly arcuate, wedge-shaped package of rock with a strike length of > 800 km. The thickest part of successive units is laterally offset from the underlying unit, suggesting that the locus of tectonic loading underwent a corresponding shift. The flexural forebulge surrounding unit 2 is exposed in the southern Alberta foothills where 2 m of sandy, bioclast-rich sediment is equivalent to 100 m of mudstone in the unit depocentre. The same forebulge exposed > 250 km distal to the orogen is instead mantled by clay- and organic-rich sediment. Isopach maps of individual allomembers show little evidence of thickening toward the orogen as predicted by flexural models of subsidence. Subsidence patterns are, instead, interpreted to have been governed by episodic movement along four inferred deep-seated faults, that are interpreted to have generated localised horst and graben structures that resulted in localised regions of accommodation. Movement of these faults is interpreted to have been controlled by changes in the magnitude of in-plane stress within the plate, associated with orogen-related flexure.

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