Flood-flipped boulders: in-situ cosmogenic nuclide modeling of flood deposits in the monsoon tropics of Australia
During the Quaternary, extreme floods along the Durack River, in the Kimberley, northern Australia, dislodged, transported, and stacked massive meter-sized boulders from the underlying bedrock channel floor. Field evidence identified a population of the boulders to have been overturned after detachment. We measured in situ cosmogenic 10Be and 26Al concentrations in six imbricated boulders to constrain the timing of flood events. We present a simple numerical model that simultaneously solves the expressions for the predicted nuclide concentrations from the exposed and hidden surfaces of a flipped boulder to calculate the time since it was overturned. The ability of the model to unequivocally discern whether a boulder was overturned depends on boulder thickness and the site-specific steady-state erosion rate. Of the six boulders sampled, our model successfully determined four finite flip ages, whereas the other two boulders indicated steady state and were either not flipped or flipped sufficiently long ago for the nuclide profile to have returned to steady state. While the two older model ages (ca. 150 ka and ca. 260 ka) are strongly sensitive to assumptions made for the local erosion rate correction, the two younger flip ages, 5.6 ± 1.0 ka and 10.3 ± 1.9 ka, are robust against such corrections. Early to mid-Holocene major floods have been recorded in other parts of northern Australia. We suggest that similar Holocene floods occurred in the Kimberley and that such extreme events may have been widespread in northern Australia in the late Quaternary. Our boulder-flip model can be applicable to similar deposits associated with other extreme events such as paleo-tsunamis.