Responses of alluvial and colluvial sediments will reflect the geochemistry and mineralogy of the source rocks or regolith from which they are derived, and modifications due to weathering and sorting of the sediments (Figure 28) (see Jamestown 3D interactive model). Where erosion, transportation and deposition of the sediments are relatively rapid, the gamma-ray response is likely to largely reflect the composition and geochemistry of the bedrock from which the sediments are derived (Figure 29 and Figure 30). Where these processes are relatively sluggish, weathering of the sediments will modify the gamma-ray response. In this context gamma-ray imagery can be used to assess the relative depositional activity of the fluvial system. Changing responses can also reflect varying textural sorting of the sediments (e.g., overbank fines versus sandy bedload deposits). For example, high Th values might be more common in overbank deposits because of Th affinity with clays, whereas relatively high K values might be expected in sandy channel deposits where K is associated with coarse grained K-feldspar grains (see Figure 13). Pickup and Marks (2002) used gamma-ray spectrometric imagery to trace the provenance of floodplain sediments and to recognised changes in the gamma-ray response due to sorting and weathering to reconstruct palaeoflood extents on floodplains in arid central Australia.
Figure 28. Relationship between gamma-ray response to relative rates of weathering and deposition. The steady state is where depositional and weathering processes have equal influence on the radioelement characteristics of the sediment. Relationships are further complicated when considering dust accession and sediments sourced from catchments that are highly weathered. From Wilford et al. (1997)
Figure 29 (above). Ternary image with K in red, Th in green and U in blue over part of the Ebagoola map sheet area in North Queensland. A - Active floodplain and alluvial fan sediments are highlighted in orange to reddish hues. Former drainage lines (B) are shown in blue/green responses. Sediments in these older inactive channels have been leached and contain only small amounts of K-bearing minerals. Scale: foreground approximately 55 km from right to left.
Figure 30 (above). Ternary image with K in red, Th in green and U in blue over the Boulia 1:250 000 map sheet in western Qld. Radioelement characteristics of the sediments can in most places be traced back to their source rocks. A - Highly leached Lower Cretaceous claystone, siltstone and sandstone; B - less weathered Lower Cretaceous sediments; and, C - brown and black soils on extensive depositional plains.