Predictive Diagenetic Clay-Mineral Distribution in Siliciclastic Rocks Within a Nonmarine Sequence Stratigraphic Framework: The Coalspur Formation, West-Central Alberta

Ahmed Khidir and Octavian Catuneanu
University of Alberta, Edmonton, AB

Studies of sandstone diagenesis can be combined with the methods of sequence stratigraphy to enhance our understanding of the parameters controlling the distribution of early authigenic clay minerals and to predict the reservoir quality of sandstones. The study of the upper Cretaceous - lower Tertiary fluvial deposits of the Coalspur Formation in the Foothills region of west-central Alberta reveals that the distribution of early authigenic kaolinite is not only dependent on paleoclimatic conditions, but also has a well-defined relation to the sequence stratigraphic framework. In this context, it has been observed that kaolinite content increases in sandstones lying in the vicinity of subaerial unconformities, which mark the most significant stratigraphic hiatuses and hence the sequence boundaries in fully fluvial successions. The increased abundance of authigenic kaolinite immediately below sequence boundaries was caused by the infiltration of meteoric water during times of subaerial erosion, which was accompanied by the dissolution of unstable minerals (e.g., micas and feldspar) and the formation of kaolinite and secondary porosity. This study therefore suggests that the change in clay mineral assemblages and the clay's chemistry in the stratigraphic section is strongly dependant on the position of the analyzed sandstone samples relative to the sequence boundaries. In a larger context, this method of delineating depositional sequences in nonmarine successions based on authigenic clays needs to be adapted on a case-by-case basis, as the diagnostic early diagenetic minerals underlying the sequence boundary may change as a function of paleoclimate and also as a function of late diagenetic processes.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005