Chronology of Pleistocene weathering processes, southeast Queensland, Australia

Abstract

40Ar/39Ar dating of Mn-oxides from the Upper Kandanga (UKA) Mn deposit, southeast Queensland, Australia shows that the weathering profile overlying the deposit is younger than 1 Ma. Seventy plateau ages obtained for 81 grains from 17 samples fall into three groups: I. 1000–800 ka (n = 2), II. 630–510 ka (n = 2), and III. 400–50 ka (n = 66). The last age group can be further resolved, using the mixture modelling solution by Sambridge and Compston [Sambridge, M.S., Compston, W., 1994. Mixture modeling of multi-component data sets with application to ion-probe zircon ages. Earth Planet. Sci. Lett. 128, 373–390.], into two major age clusters (sub-groups, IIIa and IIIb) with peak values at 313 ± 4 ka (n = 43, with an outlier) and 213 ± 7 ka (n = 22), respectively. The results suggest that a supergene blanket in a Mn deposit may develop relatively fast if weathering and erosional conditions are appropriate. The age peaks measured in this study correspond to warm periods identified in SPECMAP (oxygen isotope records of deep-sea sediments) and local paleoclimatic data. The correlation of weathering age peaks with oceanic climatic indicators suggests a climatic control on continental chemical weathering, where warm and humid conditions are conducive to increasing weathering rates. The clustering of supergene Mn-oxide precipitation ages suggests that weathering rates vary with time, supporting an episodic weathering history for the profiles investigated.