The Rosebrook Member of the Miocene Bunyan Formation is a highly kaolinitic clay-rock of lacustrine origin. It is ultra-fine grained, plastic and of high cation exchange capacity (CEC) of 20–33 c mol(+)kg−1 of clay. Fabrics observed optically and by transmission electron microscopy (TEM) indicate significant post-depositional consolidation and localized shear failure. Sedimentation and TEM analyses confirm its very fine grain size which is consistent with the relatively high CEC.
Tertiary soils and weathering profiles in the catchment of Lake Bunyan developed under relatively moist and cool conditions. Some were gibbsitic with kaolinite and smectite clay minerals. Highly weathered soils (oxisols) on basalt under modern high rainfall environments in the region are dominated by gibbsite and kaolinite. These and other highly weathered soils in the region, formed on a range of parent rocks, have high proportions of very fine grained kaolinite in their subsoils. Such materials are likely to have been abundant in the catchment of Lake Bunyan during the Miocene and would have contributed very fine grained kaolinite directly to the lake by sedimentation. However, the unusually high proportion of kaolinite in the Rosebrook Member and its euhedral crystal form suggest that some of its features developed diagenetically.