Bed-material and floodplain sediments of the Ngarradj (Swift) Creek catchment, Jabiluka, Northern Territory

Department of the Environment and Heritage, 2003

Internal Report 413
Erskine WD, Saynor MJ & Webb AA
Supervising Scientist Division

About the report

Shallow stratigraphic data on the fluviatile sediments associated with the channels and floodplains in the Ngarradj (Swift) Creek catchment obtained by the authors as part of other work have been compiled in this report. The bed sediments at eriss’s Swift Creek gauging station were sampled to a maximum depth of 1 m at three cross sections. Most of the bed sediment is slightly granular medium sand. The maximum gravel size measured was 36 mm but the mean maximum gravel size was 7 ± 0.7 mm (standard error of estimate). The bed sediments at eriss’s upper Swift Creek gauging station were sampled to a maximum depth of 1.4 m at three cross sections. Bedrock or weathered bedrock was encountered between 0.35 and 1.4 m below the river bed in every hole. Sediments were texturally variable ranging from medium and coarse sand to medium sandy pebble gravel. The maximum gravel size measured was 32 mm but the mean maximum gravel size was 16 ± 3 mm. The bed sediments at eriss’s East Tributary gauging station were sampled to a maximum depth of 2.8 m at four cross sections. Sediments were variable because of occasional high gravel and mud contents. The dominant textures were slightly pebbly medium sand to pebbly medium sand and slightly granular medium sand to granular medium sand. However, slightly granular muddy medium sand and coarse sandy pebble gravel represented the extremes. The maximum gravel size measured was 17 mm but the mean maximum gravel size was 10 ± 1 mm. The bed sediments on the lower gully of Tributary North (Erskine et al 2001) were sampled to a maximum depth of 1.5 m at five cross sections. Surficial sediments were generally slightly granular medium sand or slightly granular coarse-medium sand. Subsurface sediments varied between slightly granular sands, granular sands and slightly pebbly sands. Sand fractions covered the Wentworth range from very fine to coarse sand. At depth, the sediments in all holes were slightly granular very fine-fine sand. The maximum gravel size measured was 18 mm but the mean maximum gravel size was 7 ± 0.5 mm. The bed sediments on Tributary Central were sampled to a maximum depth of 2.8 m at three cross sections. Weathered sandstone bedrock was reached at the bottom of two holes. Sediments were variable ranging from slightly granular fine sand to medium sand/muddy medium sand to pebbly coarse sand to coarse sandy pebble gravel. The maximum gravel size measured was 35 mm but the mean maximum gravel size was 11 ± 2 mm.

Floodplain sediments were described at the three eriss gauging stations (Swift Creek, upper Swift Creek and East Tributary) and were found to comprise sandy fining upwards sequences deposited by laterally stable channels whose banks were well protected by monsoonal vine forest with extensive root systems and root mats. Sandy sediments were also described along the lower gullied reach (Erskine et al 2001) of Tributary North.