A total of 13 kimberlites have been located in a two-county area in northeastern Kansas along the NNE-trending trace of the Proterozoic Midcontinent Rift System, an aborted rift extending from the Lake Superior region southwestward into Oklahoma.
The Kansas Geological Survey drilled three new kimberlites in 1999. They are the Baldwin Creek and Tuttle kimberlites located in Riley County and the Antioch kimberlite in Marshall County. They were discovered as part of a systematic ground follow-up of private aeromagnetic data (200–400-m-line spacing) collected by Cominco American Inc. in the early 1980's and donated to the Kansas Geological Survey in 1999. Six exposed kimberlites were discovered prior to 1970 and another four in the early 1980's, one of which is exposed at the surface.
Cross-cutting major tectonic elements in NE Kansas are ideal for accommodating the intrusion of kimberlites. The major structural elements are regional NNE-trending, high-angle normal and reverse faults associated with the 1.10 Ga Midcontinent Rift System, and regional, older, high-angle normal and reverse, NW-SE striking, cross-cutting faults which offset portions of the rift. The presence of the rift is identified in regional gravity and magnetic data sets and evidenced in core and cuttings recovered from drill holes that penetrate Precambrian basement. Sedimentary rocks up to about 800 m (3,040 ft) thick and ranging in age from Ordovician to Permian overlie the Proterozoic basement.
Ground magnetic surveys were conducted to model the kimberlites and to define targets for drilling and core recovery. These detailed ground magnetic data show that final emplacement of the kimberlite bodies is controlled by N40°W-striking structures, rather than the NNE-trending structures. Modeling of the magnetic anomalies shows that the contacts between the kimberlite bodies and the country rock are steeply dipping.
The kimberlites are believed to be Late Cretaceous in age (about 90 my) and range in character from crater and diatreme facies in Riley County to possible hypabyssal facies in Marshall County. The three new kimberlites are buried under a shallow soil cover ranging in thickness up to 8 m (26 ft).
