Seismic delineation of the Orion South kimberlite, Fort à la Corne, Canada

ABSTRACT The Orion South (140/141) kimberlite is a diamondiferous, multiphase volcano-sedimentary complex within the Cretaceous Fort à la Corne kimberlite field of Saskatchewan, Canada. A network of seven high-resolution 2D seismic reflection profiles were acquired to test the utility of using seismic methods in defining the subsurface architecture of the kimberlite complex, and to provide auxiliary constraints on interborehole geologic correlations. The seismic data were interpreted in conjunction with a set of drillhole geologic logs and associated geologic sections and models, and physical rock properties from core measurements and geophysical logs. Core-determined average density values for the kimberlite suite are low (2240±16  kg/m3), but with some kimberlite facies having significantly higher values (2330 to 2540  kg/m3). Sonic velocities for the kimberlites are 2500−3500  m/s as compared with ∼2000  m/s and 3800−4500  m/s for the enveloping shale and sandstone units, respectively. The kimberlite is imaged to depths of ∼400  m as a lenticular-shaped body lying beneath, Quaternary glacial till and a mantle of Cretaceous marine sediments. The base of the kimberlite is variably imaged due to faulting and lateral changes in the seismic contrast with the underlying Cantuar sandstone. The comparable magnitude of seismic contrasts across inter- and intraeruptive phase boundaries hampers interpretation of the individual eruptive phases within the kimberlite complex, although the conformal nature of internal reflections with the interphase boundaries is useful in mapping these boundaries away from the drillholes. Multiple feeder vents at the base of the complex and their associated vertical faults are clearly identified by truncations of subhorizontal reflectivity. This study demonstrated how a network of relatively low-cost seismic profiles could potentially be useful in guiding detailed delineation drilling of kimberlite deposits.