Titahi Bay

BY JULIAN THOMSON (GEOLOGIST)
Accessibility: EASY
Titahi Bay Fossil Tree, Photo J.Thomson @ GNS.Science
The south end of Titahi Bay beach has fossil tree trunks that are intermittently exposed depending on the movement of sand by waves and tides. The adjacent shore platform is a good exposure of greywacke bedrock.
Titahi Bay Fossil Tree, Photo J.Thomson @ GNS.Science
The fossil forest floor on the beach has been dated at about 100 000 years old, and was living during the last warm inter-glacial period. It actually extends most of the way along the beach, but is usually buried by sand except for a few tree stumps at the very south end of the beach. Sometimes the fossil tree stumps disappear almost entirely under sand. On rare occasions after big storms, the trees can be exposed over a relatively large area. In the middle of the beach, a black surface can be exposed. This black material is mud-rich soil (paleosol) loaded with masses of black root material.

The wave-cut platform at the end of the beach is a classic erosion feature on a rocky coast. It is a good exposure of the bedrock that underlies the Wellington region.
Titahi Bay Shore Platform, Photo J.Thomson @ GNS.Science
Search for remains of tree stumps, with roots radiating from them. Growth rings are visible on the stumps. Exposure of these features varies a lot. Sometimes they are completely buried.
On the shore platform at the end of the beach there are faults cutting through the vertically tilted layers of alternating sandstone and mudstone. You can see cross bedding if you look carefully, as well as areas with well-developed box weathering. This is a secondary feature related to the differential erosion of joints and cracks in the exposed bedrock.
Directions/Advisory

Accessibility: EASY

A short walk along the beach from the car park at the south end. The fossil forest is usually only seen at low tide.

Features
Sedimentary Fossils
Geological Age
Fossil trees are Pleistocene, 100 000 years old. Zealandia's cover sequence. The bedrock is Late Triassic to Early Jurassic, 230 to 180 million years old.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present
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