Formation of the Cross Bedding Structures

The similarity between the compositionally graded layers and features of graded bedding in sedimentary environments are suggestive of flow sorting by traction current activity. Similar processes have been proposed for other layered intrusion such as the olivine-pyroxene layering preserved within the ultramafic rocks of Duke Island (Irvine 1974, p15). However, in the Wog Wog River example, individual layers are defined by biotite and plagioclase concentrations. The layering is considered as the result of two main, interdependent processes: (1) fluctuations in the temperature of the magma initiated by the presence of the ?cold, tonalite xenolith, and (2) fluctuations in magma composition. Both of these processes may lead to the preferential nucleation and crystallisation of a specific mineral phase such as biotite. As the magma crystallises biotite, for example, the magma composition close to the crystallisation front changes, becoming depleted in elements required by biotite, forcing the crystallisation of felsic phases such as quartz and plagioclase. This continues until the magma returns to its initial composition, and a compositionally graded layer is formed. If the process is repeated, then the cycle is repeated and a type of rhythmic layering is produced. This is one of many possibilities such as flow sorting and crystal settling/accumulation, but has yet to be proven for the Wog Wog example. Generation of the cross-beds requires a different process. As mentioned above, the cross beds are structured/ordered in a similar fashion to trough cross bedding observed in sedimentary environments where currents are present. It is envisaged that while the layers are chemically controlled, the cross beds formed by scouring due to current movement or magma flow, causing erosion of the earlier formed layers. This scouring and development of traction currents is extremely unusual may have resulted from the emplacement of an unusually large block (megaxenolith) of solid material into a crystal-poor magma chamber containing only plagioclase and biotite as cumulus minerals.