ANALYSIS OF SMECTITE/ILLITE CLAYS WITHIN THE WHITE RIVER GROUP

In the summer of 2012 we conducted field work at Monroe Creek, NE and Toadstool Geologic State Park, NE, located in northwestern NE. This work was based on studies of fractures and fracture systems within the Oligocene to Eocene age White River Group strata. These fractures may have formed by chemically driven volume changes as the sediments were buried, an understudied phenomenon.

Fracture types include clastic dikes, which are sediment filled fractures, and chalcedony veins. Samples of clay rich strata were collected, spanning the White River Group and including the very base of the overlying Arikaree Group. Prepared samples were analyzed by x-ray diffraction (XRD) for their mineralogy, including the clay minerals, smectite and illite. These two minerals are of particular interest because swelling clays like smectite can change during geologic burial into nonswelling illite producing a volume loss, release of silica, and release of fluids within the rocks, which may produce fractures through syneresis.

The results of the XRD analysis indicate that there were distinct changes in smectite, illite, and mixed smectite-illite content with sample ascent through the White River Group. At Toadstool Geologic State Park mixed smectite-illite vanishes at the very top of the Chadron Formation, a lower unit in the White River Group. At Toadstool Geologic State Park the clastic dikes and chalcedony veins are concentrated in the Chadron Formation. The samples collected at Toadstool Geologic State Park and Monroe Creek, Nebraska were also compared to other sample sets from locations around northwestern Nebraska and South Dakota. Continued research on fractures and fracture systems will aid in understanding how water and other fluids move through sedimentary rocks, reconstructing stresses, and understanding deformation processes.