Leader River landslide dam

This rock slide/slump is predominantly within Greta Formation mudstone (including sandstone and siltstone), and the slide surface is assumed to be deep seated (about 80 m below the surface) with some rotation at the head scarp, possibly along bedding. A faulted contact between the Mesozoic aged Torlesse Supergroup and mudstone is also present in the landslide head scarp. Although there is no field evidence to suggest this contact ruptured, it is possible that a fault also ruptured through the source area of this landslide. The landslide is located around the junction of two ruptured faults, the Leader (coming up from the south), and the Humps fault (extending back west into the Waiau Basin and Emu Plains, and east around the front of Mt Stewart).

The geology here includes the Middle Miocene - Pliocene aged (around 2-15 million years old) Greta Formation. These deposits record near-shore marine and some coastal on-land environments as the local region was uplifted out of a shallowing ocean by the ongoing tectonic growth of the Southern Alps (due to tectonic plate collision of the Australian and Pacific plates).

The Mesozoic aged Torlesse Supergroup is one of the key basement rocks of the South Island and records a time of marine turbidite (from a type of underwater sediment gravity flow) deposits down the continental slope, off-shore the super-continent Gondwana.

This is a fascinating place to observe how erosion processes shape the landscape and how water finds a way, especially as a response to a catastrophic event such as an earthquake. Since the river was dammed and Lake Rebekah formed as a result, the river has managed to erode it's way back down around the southern edge of the debris. Four waterfalls have been formed, a significant delta, and two smaller holding lakes. These features are constantly changing! Compare what you see on your visit in front of you with some of the past photos in this GeoTrip! Note also how the triangular shaped landslide scarp itself has weathered over time as loose material has been eroded off and the exposed rock textures more defined.

The Greta Formation is quite easily erodible, and this therefore means that the bulk of the dam material could likely be modified quite significantly over time. If you look at the upper waterfall you will notice a wooden structure that the landowners have constructed to divert water from the lake over the more resistant bedrock to slow down that erosion process.