Motunau

The Motunau Group is the name of a rock sequence found widespread throughout North Canterbury and beneath the mid-Canterbury Plains, as well as isolated fault-bounded basins in the eastern Southern Alps. The Group has a range of rock types but is most easily identifiable as associated with some of the limestones that typify well-known locations such as Weka Pass, Waikari, Waipara and Kaikōura.

There are 4 key Formations that make up the Motunau Group: Greta, Mount Brown, Waikari and Waima (also including the Weka Pass Limestone). These rocks all record a period (roughly 25 through to 2 million years ago) of mainly shallow marine sedimentation that ended progressively northward as New Zealand was uplifted out of the ocean under the ongoing tectonic plate collison. Hence rocks in this group generally get younger to the north.

The younger rocks of the sequence, the Greta Formation, form the main rock type along the eastern headland at Motunau and are what makes up the small island direct out from the main beach. This flat-topped island surrounded by a large intertidal platform is an eroded remnant of the marine terrace along what was the nearby mainland coastal plain. The rocks on the eastern headland are very soft and easily eroded, consisting of fine sandy silts, muds and some debris-flow conglomerates, recording evidence of an ancient underwater canyon that was carved into the Miocene continental shelf. These deposits are a rich source of fossils as the debris flows buried and preserved the healthy canyon ecosystem. A diverse range of crabs and molluscs have been found here in particular. It is also the site of the only Pliocene fossil penguin bones in New Zealand. Other fossils include giant toothed seabird bones.

To the west of the river mouth, the crumbling cliffs consist primarily of undifferentiated Motunau Group (older than Greta Formation) and are a mixture of calcareous sandstones with some limestone beds and greensands.

The Motunau Group rocks are overlain by large scale Pleistocene (less than 2 million years) debris flow deposits. These are shell-rich underwater debris-flows in mid-outer shelf mudstones.

Motunau, like Glen Afric, is well-known for its fossil crabs. They tend to be found amongst the beach cobbles as they get washed out of the original rock beds - so low tide is the best option when hunting for these fossils. The main variety found here are the Spider crabs. Look for well rounded concretions. Break open these concretions with a sharp whack with the straight edge of a hammer or chisel. The best place to look is south amongst the cobbles on the beach below the the cliffs - just be wary of going too close to the cliffs themselves as they are so easily erodible and tend to collapse!

There is a protected geopreservation site below the headland to the east of the river mouth so please avoid taking anything from this area.