Sarsen stones at German Hill, Central Otago, New Zealand, and their potential for in situ cosmogenic isotope dating of landscape evolution

Be-10 concentrations measured in silica-cemented Tertiary sandstones in Central Otago have yielded minimum exposure ages of up to 1400 ka, some of the oldest ever recorded outside the dry valleys of Antarctica and some arid deserts. The silica-cemented sandstones outcrop as boulders in a region where their exposure is caused by the growth of anticlinal ridges above blind reverse faults. Initial studies using a combination of in situ cosmogenic isotope measurements and geomorphological observations on one range have demonstrated the potential of this technique to investigate Late Cenozoic fault growth and erosion rates. The best-exposed and preserved occurrence of the quartz-rich boulders is at German Hill on North Rough Ridge, where their stratigraphic context and their sedimentary and diagenetic origin, together with their method of emplacement and preservation on the modern land surface, can be studied in detail. These are all features that affect their suitability for cosmogenic isotope studies of landscape development and are discussed in this article. 10Be concentrations in the German Hill rocks yield minimum exposure ages of 600-1300 ka, and it is possible that these concentrations are not in saturated steady state with respect to erosion. These ages imply that the uplift of North Rough Ridge by its underlying fault is a Quaternary phenomenon. This conclusion has implications for the likely earthquake recurrence on the fault, which with an estimated average long-term slip rate of 0.5 mm yr(-1), is expected to be infrequent.