Drainage reorientation in Marlborough Sounds, New Zealand, during the Last Interglacial

Abstract

Regional drainage of the Marlborough Sounds area, northern South Island, New Zealand, discharged water and sediment towards the south until the Last Interglacial. Drainage reversal occurred at that time, and discharge is now northwards. Drainage reversal was facilitated by regional subsidence and tilting, and build‐up of a thick (c. 60 m) sedimentary pile in the former main southward axial drainage channel comprising the Kaituna and Are Are valleys (the Kaituna corridor), before the Last Interglacial. Minor southward flow persisted in the Kaituna valley after the Last Interglacial, but this involved locally derived sediments only. Dating of these latter sediments by OSL techniques constrains the minimum age of drainage reversal to be c. 70 ka. The maximum inferred age of reversal (c. 130 ka) is based on regional correlation of glacial outwash terraces. These age estimates are consistent with each other and with geomor‐phological and stratigraphic observations within the Kaituna corridor. The drainage reversal occurred when ponding of sediment in the south allowed overtopping of a low drainage divide farther north in what is now Pelorus Sound. Subsequent erosion through this divide was probably facilitated by shoreline retreat and associated downcutting during sea level high‐stands of the Last Interglacial.