Awakino River Mouth

Here, three rock units are exposed. On the left when entering the beach are light grey coloured mudstone of the Manganui Formation approximately 17 million years old. Sometimes these rocks contain gastropod (snail) shells and corals and represent a mud deposited in an offshore marine environment.

The second unit is a grey-coloured sandstone rich in very small broken shells and is called the Managara Sandstone about 13-14 million years old based on the fossils it contains. It likely represents a shallower marine environment than the underlying mudstone. On top of the Managara Sandstone is a 20-30 cm thick layer of boulders which mark an unconformity (see description under "have a look").

Lastly near the river mouth, is the youngest rock unit, about 11 million years old, called the Mohakatino Volcanics. It is made up of sand-sized grains derived from a nearby volcano or series of volcanoes that were erupting offshore. The various layers are the successive beds deposited from these volcanoes.

In this short walk, there are several rock units exposed spanning a period of approximately 7 million years. We know the age of the rocks mostly through the fossils contained in them, but not all the geological time is recorded by rocks. Part of that time is missing and is here marked by a layer of conglomerate or boulders. This surface represents what geologists call an unconformity. When you look at this conglomerate layer you may notice that the rocks above it are inclined at a slightly different angle from the rocks below, and geologists call this an angular unconformity. As best we can tell, the unconformity represents a break of around 3 million years, between about 14 and 11 million years ago.

You may see some fossils in the Manganui Formation such as gastropods and corals, as well as shell fragments in the Managara Sandstone.

At the river mouth the Mohakatino Volcanics have well developed layering called bedding. They form beds up to 50 cm thick which are marked by having dark and coarse-grained sediment at their base, and lighter coloured sediment above. These are called graded beds and an example of these is shown in one of the photos. In this particular case burrowing animals seaching for nutrients in the volcanic sediment left trace fossils in the upper part of the bed.