August 2025 Stone Collecting Trip – Part 20, Kakanui Mineral Breccia and 31 Finds, Seadown Beach, Kakanui, Tuesday 26 August

I timed my visit to Seadown Beach today to be at low tide so that I could access some of the small bays in the rocks at the south end. I arrived at 11am, half an hour before low tide – it was sunny, the temperature was nine degrees, and there was a light but cool breeze. There were a couple of divers looking for shellfish in the submerged part of the rock platform at the south end of the beach, and a woman nearby on the seaweed-covered rocks also collecting shellfish.

I spent some time in three of the small bays but did not find much worth collecting. I noticed some of the rocks I was walking over there were Kakanui Mineral Breccia. Page 11 of a guide for a self-drive tour of East and Central Otago geology notes: “The Kakanui Mineral Breccia is a mineral-rich fragmental rock which formed from the contact of an intruding magma with water about 31.6 million years ago (Early Oligocene). The resulting violent eruption totally disrupted the magma components into angular fragments.”

The Kakanui School Centennial History (1969) includes a short chapter “Geology of the Kakanui District” which gives the following about the Mineral Breccia: “The Breccia contains single crystals of garnet (deep red), feldspar (pale cream), hornblende and augite (black), chrome diopside (bright green) and other minerals. Many of the mineral grains attain large size but all are shattered and fractured… This deep-seated material, brought to the surface by volcanic eruption, is unique in the New Zealand region” (page 8). NZ Petroleum and Mineral’s website contains “Mineral Commodity Report 17 – Rare Earths and Related Elements”, and it notes the rare earth elements in Kakanui Mineral Breccia (page 10 of pdf). It was partly for this reason that the Breccia attracted a lot of geological attention in the late 1960s and early 1970s (see Mineralogical Magazine, 1974), with publications on it continuing to the present day.

Sometimes Kakanui Mineral Breccia stones appear on Seadown Beach, such as the two below which I collected last year – they are interesting but too soft to tumble polish.

Today, after spending some time in the small rock bays at the south end, I walked north along the beach, looking through the light scattering of stones at the waves’ edge. It was a less productive fossick than yesterday, and I left just over an hour after I had arrived.

I collected 31 stones this morning, some of them quite small. Fifteen of my finds are featured in this Post. First up, here are four jaspers I came away with. They all show signs of tectonic stress and fragmentation. The first is a small dark red brecciated stone:

A second small dark red brecciated jasper:

This next brecciated jasper is a bit lighter in colour and has quartz through it:

The fourth one, even more lighter in colour, has also come under tectonic stress but has not fragmented as much as the other three, leaving a veined appearance:

The next stone is the right colour to be a candidate for containing tiny bryozoan fossils, but closer examination shows it to be a brecciated quartzite:

I also looked carefully in this stone for bryozoan fossils but again it is a quartzite:

Many of the quartzites on Seadown Beach have interesting hues, often in cloud-like patterns, and are smooth enough to start tumble polishing with a fine grit:

This orange stone has a lot of tiny quartz fragments in it:

There’s a lot going on in this dark brown stone:

This brown stone has quite a different composition, including tiny light-coloured flecks:

The best of my fossiliferous limestone finds – the close-up photos are often revealing of tiny fossils, especially shells and shell fragments:

A similar type of limestone:

I like the bands in this very quartzy stone:

A type of brecciated stone?

This looks like an opaque quartz with some brecciation:

The next Post in this Series covers the third of my three fossicks at Seadown Beach before I continued north. An Index to the Series is here.