SAND INJECTION FEATURES AT THE CRATER OF DIAMONDS STATE PARK, MURFREESBORO, ARKANSAS

During the early summer of 2002, excavation occurred north of the Crater of Diamonds Visitors Center and immediately east of the Prairie Creek lamproite diatreme. The excavations exposed sand injection features as well as the contact between the surface unit (Lower Cretaceous Trinity Group) and the Cretaceous igneous diamondiferous rocks. Grab and auger samples of the injection features and surrounding units, digital images, and GPS coordinates were collected. Sieve analyses of the injection sands showed the majority of the sand to be in the fine sand to silt range. Examinations of the fractions show the sand to be sub-angular to sub-rounded quartz grains with a trace of novaculite and heavy mineral grains. The unit that served as the host for the sand injections, the Paluxy Sand member of the Trinity Group, is represented at this locality by brick-red clay. Sandstones were noted in two separate injection features. In one of the 5 injections, several sand nodules were recovered, being barite cemented. In another, the sandstones appeared to be a slurry type deposit at the base of the injection feature and are silica cemented. We feel the injection features were a result of overpressuring of water-saturated sands lower in the host formation as a result of the explosive intrusion of the lamproite diatreme that was emplaced 95-100 ma. Being the first injection features described from the Cretaceous units of southwest Arkansas, they appear quite unique.