THE INFLUENCE OF DEPOSITIONAL SETTING AND FLUCTUATING ACCOMMODATION SPACE ON KIMBERLITE EDIFICE PRESERVATION: IMPLICATIONS FOR VOLCANOLOGICAL MODELS OF DIAMONDIFEROUS KIMBERLITES AT FORT À LA CORNE, SASKATCHEWAN, CANADA

Kimberlites are typically reported from continental, stable cratonic settings i.e. regions of inherently low accommodation space. Thus, examples of preserved extrusive pyroclastic kimberlite are rare. Diamondiferous kimberlites in the Fort à la Corne kimberlite field (FALC) in central Saskatchewan, Canada are exceptional in their relative completeness including preservation of volumetrically significant tephra cones and extra-crater pyroclastic deposits. Preservation of extra-crater pyroclastic kimberlite at FALC is a function of the background depositional environment and available accommodation space, which is related to both volcanic style and local tectono-stratigraphic setting.

The FALC kimberlites erupted in both subaerial coastal plain and shallow marine settings on the margin of the Lower Cretaceous (Albian) epeiric Moosebar-Clearwater Seaway. Regional tectono-stratigraphic setting influenced both the eruptive processes that governed the nature of individual kimberlite volcanic events and post-eruptive kimberlite preservation.

Initial kimberlite volcanism in the area occurred coeval with deposition of the Early-Middle Albian Cantuar Formation. Kimberlite body geometry and the presence/thickness of extra-crater pyroclastic kimberlite in the Cantuar Formation is largely a function of fluctuations in fluvial base level and concomitant variation in available accommodation space. Thick, laterally extensive deposits of pyroclastic kimberlite, as well as variably sized tephra cones, first appear in the marginal/shallow marine Middle-Late Albian Pense Formation and are common in the shallow/deep marine Late Albian Joli Fou Formation.

Volcanically induced faulting (ring-faults and half grabens) resulted in the creation of local accommodation space proximal to kimberlite feeder vents. Preservation of pyroclastic kimberlite, including tephra cones, is related in part to increased accommodation space resulting from a regional marine transgression (i.e. expansion of the lower Cretaceous Moosebar-Clearwater Seaway) and encasement of kimberlite deposits in marine shale. Availability and variation of local and regional accommodation space must be addressed to accurately assess the eruptive histories and develop detailed volcanological models of FALC kimberlites.