Gore Bay

The cliffs at the southern end of Gore Bay expose a cross-section through shallow to deep water calcium carbonate-rich ocean sediments. The Oligocene-aged Motunau Group at this location consists of limestone, extensive calcareous (calcium carbonate-rich) mudstone, with greensand (literally "green sandstone") at the base.
The first unit you come across when walking south along the beach are the shallow ocean debris-flow conglomerates in the Greta Formation, also seen at Cathedral Cliffs. Directly beneath the conglomerates lie the mudstones and fine limestone beds of the Motunau Group. In time, these deposits all record the ocean depth decreasing as the New Zealand landmass was lifted out of the water over the last 25 million years.
The greensand is green in colour because it contains high quantities of the mineral glauconite. Glauconite is an interesting potassium-rich mineral that forms in marine sediments and can be influenced by the decaying process of organic matter with high biological activity. It's therefore most commonly found in continental shelf environments with low sedimentation rates.
The controversial "Marshall Paraconformity" (see link) is also exposed at this location at the base of the cliff around the first main headland from the southern end of the beach (WARNING: Not accessible at high tide!), where the Motunau Group sits unconformably (time break and erosion) on Eocene-aged Amuri Limestone.

The cliffs are actually the southern limb of a large syncline (downward closing fold, like a trough) and you can see the same layers repeated sloping the other way at the northern end of Gore Bay!
If you get to the "Marshall Paraconformity", can you identify the burrows and the formation of numerous phosphate nodules that mark this contact? It was in part formed because many small marine animals have burrowed into the top surface of the Amuri Limestone before the greensand and calcareous mudstone (from the Spy Glass Formation at the base of the Motunau Group) was laid down on top.