Matheson Bay Fossils

At Matheson Bay the rocks tell us about an Early Miocene rocky coastline and rapid submergence as the land subsided. At the west end of the beach, the contact between the hard sandstone and mudstone "basement" rocks and overlying much younger conglomerate, breccia limestone and sandstone is exposed a metre or two above high tide. Round the small point at the east end of the beach, boulder breccia is overlain by conglomerate with many fossils - oysters, coralline algae, barnacles, bryozoa, echinoid (sea egg) spines, bivalves and brachiopods. Deep water marine mudstone which overlies the conglomerate can be seen in the cliffs a little further to the east. The rocks and fossils tell of an eroding rocky coastline which rapidly sank to great depths as the Waitemata Group sedimentary basin formed in Early Miocene time.

The most common fossil types - oysters, coralline algae and echinoid spines - suggest deposition in shallow seas. Walk on the modern beach and you will find modern oysters, coralline algae and echinoids (kina). Is the modern Matheson Bay and nearby offshore rocky coast similar to the Early Miocene Matheson Bay environments? There are no reef corals growing in modern New Zealand seas but there are rare lumps of fossil reef coral at Matheson Bay - see if you can find one. For corals to grow, Early Miocene Matheson Bay must have been warmer than the present day Matheson Bay.