Lacustrine sedimentation – ancient rift fill tectonic control on alternating lacustrine and coal

Date: 31J anuary – 4 February 2026
Cost (NZD inc 15% GST): Approx $1,000 per person
Leader: Kari Basset

The Greymouth Basin on the West Coast of the South Island of New Zealand formed during the Cretaceous rifting that separated Zealandia from Gondwana.  The basin is part of the larger West Coast – Taranaki Rift System and has spectacular coastal outcrop of the conglomerates deposited along the faulted basin margin. The depocentre of the basin is filled with organic-rich lacustrine mudstones that transition through large mires to meandering axial rivers, a cycle repeated 3 times in the basin’s history.  The final 4^th phase opens the basin to the sea allowing deposition of limestones during a regional transgression.  Features included in this trip include lacustrine mudstones and muddy deltaic channel sandstones and coals deposited in the basin centre, as well as fan delta polymictic conglomerates with large scale soft sediment deformation and tight packing of clasts delineating subaqueous delta slope and mouth bar deposition.  The distribution of facies and the cyclic alternation of basin axis facies indicates deposition in a half-graben rift basin that progressively widened over time.  The spectacular coastal outcrops allow a detailed look at fan delta deposition into a large deep rift lake. We will also visit the bioclastic limestones formed during the regional transgression which host modern blowholes that are active at high tide.

Fitness level needed is moderate to high with one outcrop access being a steep path down to the beach.  Sturdy footwear is necessary. We will fly into Christchurch and then drive over the mountains to Greymouth where we will spend 4 nights.

Geotourist stops on the drive over will showcase active tectonic, mountain building sedimentology and geomorphology.  On the drive back we will visit a deep lake analog.