Deep-burial brecciation in the Devonian Upper Elk Point Group, Rainbow Basin, Alberta, Western Canada

Brecciation is a common diagenetic fabric in subsurface dolomitized sequences of the Upper Elk Point Group in western Canada While not generally associated with hydrocarbon production from these sequences, breccias were a product of the same deep-burial diagenetic processes responsible for creating other secondary pores from which production occurs. Several relationships demonstrate conclusively that brecciation and other associated styles of dolomite dissolution were deep-burial in origin, having formed coincident with, or after, pressure solution in these rocks. These breccias, therefore, are an example of deep-burial “karstification.” Upper Elk Point breccias are invariably associated with fractures and burial replacement anhydrite, both of which were related to local faulting. They are always associated with dolomites and show no preference for development along depositional cycle breaks or formation tops. The common presence of stylolitic clasts, rotated at all angles to each other and the horizon, demonstrates that solution collapse occurred after the onset of pressure solution at depth. Contrary to popular models, brecciation is not unique to near-surface processes such as freshwater karstification or leaching of evaporites. For the Upper Elk Point Group, to invoke these processes as explanations for the observed brecciation is to totally ignore the stratigraphical, petrographical and geochemical attributes of these sequences. Our case study shows that given the right tectonic and diagenetic settings, impressive deep-burial dissolution can occur in buried carbonate sequences, resulting in creation of substantial secondary porosity and brecciation.