Forest burial by large rock avalanche in Miller Stream, Seaward Kaikoura Range, New Zealand, c. 1700 years ago

Abstract

A forest buried at 1630–1730 cal. BP has been exhumed by incision of Miller Stream, a tributary of Clarence River that drains the steep southeast flank of the Seaward Kaikoura Range, Marlborough, New Zealand. The paleofor‐est is exposed along several hundred metres of the modern channel of the south branch of Miller Stream, and comprises over 40 tree stumps in growth position that are up to 2.5 m in circumference and 2 m in height. Just over 2 km downstream, subhorizontal trees of the same age as the paleoforest are buried by c. 14 m of a fining upward, poorly sorted “debris flow” deposit containing clasts as large as 2 m diameter. This deposit is capped by c. 8 m of thickly bedded alluvial deposits. The aggradation that inundated Miller Stream, and buried the forest, was probably caused by a large rock avalanche in its headwaters with an estimated volume of c. 3.7 × 107m3. The triggering mechanism for the Miller Stream rock avalanche has yet to be established; however, given its large size, the depth of its source scar (≥100 m), and its close proximity (within 15 km) to at least five major active faults (Jordan Thrust, Fidget, Kekerengu, Hope, and Clarence Faults), we suggest that an earthquake may have been the trigger.