Eruption processes and facies architecture of the Orion Central kimberlite volcanic complex, Fort à la Corne, Saskatchewan; kimberlite mass flow deposits in a sedimentary basin

The Fort à la Corne diamondiferous kimberlite field consists of at least 70 bodies of volcaniclastic kimberlite, hosted within a contemporaneous non-volcanic sedimentary succession. This study of the three-dimensional stratigraphy and facies architecture across the Orion Central kimberlite volcanic complex highlights the variations in upper and extra-vent processes.The sedimentary succession consists of continental to marginal marine quartz sandstones and mudstones, overlain by marginal to deep marine dark mudstone and muddy sandstones and siltstones. Relatively thin conformable volcaniclastic kimberlite packages are interbedded throughout the host rock stratigraphy. Extremely thick (up to at least 211 m thick) discordant to concordant, volcaniclastic packages/series, infill at least three elongate northwest-trending craters (145A, 145B and 219 craters), and contain laterally equivalent conformable extra-crater deposits bound by marine mudstones, indicative of a prevailing dominantly marine environment. The volcaniclastic deposits within the 145A and 145B craters, respectively, are separated by a considerable hiatus, whereas the deposit infilling the 219 crater was formed around the same time as 145B crater deposit.Multiple depositional units of massive to stratified, olivine-rich sand- to pebble-sized volcaniclastic facies infill craters and were emplaced by megaturbidite pulses fed by crystal-rich eruption fountains, which interacted with the crater relief. Stacked, normally graded, thick to very thick bedded matrix-supported olivine-rich facies characterized by brief depositional breaks between some beds represent syn- to post-eruptive turbidite pulses associated with the early eruptive event in the 145A crater. Thin layers of light grey kimberlitic mudstone underlie, or occur near the base of and above the main volcaniclastic packages associated with the 145B and 219 eruptions. Crater-infilling volcaniclastic deposits were later reworked by storm induced currents into thin to medium graded, moderately sorted, fine to coarse olivine-rich sandstone with intercalated discontinuous muddy laminae. Laminated to cross-laminated olivine-rich silt- to sand-sized volcaniclastic facies constitutes a small volume volcaniclastic turbidite, which marks the last preserved kimberlite emplacement event at Orion Central.