THE ORIGIN OF CLASTIC DIKES IN SOUTHWEST MISSOURI

Clastic dikes within the Cotter Dolomite in southwest Missouri are puzzling geologic features with unknown origins. To determine how these features formed, the source of sediment within the dikes must be established. One possibility is that the dikes are lithified injectites, i.e. a mixture of fluid and unconsolidated sand forced through a new or pre-existing fracture of a host rock by intense pressure. In this case, faulting and ensuing earthquakes prior to complete lithification of the source and host rock likely provided the pressure to liquefy and inject the sand into carbonate beds of the Cotter Dolomite. Alternatively, the structures may be karst fills within solution-widened joints. To evaluate these possibilities, petrographic and grain-size analyses of the dike materials can be compared to those of the overlying Bachelor Formation and the underlying Swan Creek (sandstone) member of the Cotter Dolomite. If the dike material is identical to that within the overlying Bachelor Formation, the dikes are most likely karst fills. If, however, the dike material matches that of the underlying Swan Creek member, the dikes are certainly injectites. Initial thin sections from a dike at Saddlebrooke, MO show a strong resemblance to the Swan Creek member and distinct differences from the Bachelor Formation, implying an injectite origin. Further analysis has shown the presence of similar heavy minerals in both the Saddlebrooke dike and Swan Creek member. However, the Bachelor Formation lacks this heavy mineral, but includes microfossils not observed in the dike nor the Swan Creek member. Physical characteristics of the dike resemble descriptions of injectites found in different parts of the world, which supports the hypothesis that the Saddlebrooke dike was formed by local or regional tectonics quickly followed by the injection of fluidized sand.