Early 2024 Fossicking Trip – Part 32, Bryozoan Fossils in Chalcedony, Kakanui’s Seadown Beach, Saturday 23 March

On Thursday, two days ago, I had my final Gemstone Beach fossick for this trip. Friday was spent packing up and driving to North Otago, a trip of more than four hours. I arrived in Kakanui, a few kilometres south of Oamaru, just before dark, staying at a holiday home booked through AirBnB. This morning, I spent two hours at Seadown Beach, a couple of kilometres north of Kakanui village. I had called in at this beach on 16 February, on my way south.

It was a very foggy morning though the sun eventually came out. Previously, a lot of sand had been covering most of the stones here. There’s still a lot of sand but the sea has removed a depth of about two metres of it across the south end of the beach, leaving a small cliff stretching for about 50 metres there. The removal of this sand has exposed a 100 metres stretch of stones close to the waves.

The type of stone I keep the sharpest eye out for on this beach is one I first discovered in mid-2022 (see here, last part of Post). It is a dark brown colour, similar to the limonite prase found along the North Otago coast. Jocelyn Thornton describes limonite prase as an “ochre-yellow jasper” (page 34 in “Gemstones”). Two examples of limonite prase:

In June 2022, I had found a stone at Seadown Beach that reminded me of limonite prase, was a dark brown colour but did not have the typical black patches. Looking at it more closely, I noticed some very faint tiny markings in the stone. Taking a photo of it later, I zoomed in on the markings and discovered a wonderland of tiny figures:

I posted these photos on the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. John Taylor, an Englishman who lives in Scotland but often visits New Zealand and is a member of the Group, commented that he had found a very similar rock in Europe, a “Danish Fossil Okkergelber Hornstein”. He pointed out that this rock is chalcedony and contains many tiny bryozoan fossils. When John visited me in 2023, he gave me a slice of a rock he had found at Waabs in northern Germany, just south of the Danish border, on the shores of the Baltic Sea:

I took photos of John’s slice, wet and in bright sunlight – the tiny bryozoan fossils can be seen in the close-ups:

I have since found more of these stones on Seadown Beach. Three from visits in 2023:

The following is what I have found from some initial research into bryozoa (the term “bryozoans” is also used). In her book “The Field Guide to New Zealand Geology” (2003), Jocelyn Thornton describes bryozoans as one-celled animals (zooids) which live in colonies (page 52). Most marine bryozoans have a partially calcified, hard, body wall, which is subject to the process of fossilisation. There are many species and many different types, including branching twig-like forms, lacy forms and fan-like forms. A 2022 news article notes they are often known as “moss animals”, because the bushy-looking live ones look like moss, the literal meaning of bryozoa from the Greek, bryon meaning moss, zoön meaning animal. They are also known as “sea mats” or “lace coral”, depending on the variety and the form they take. The excellent NIWA identification guide, “Bountiful Bryozoans”, available free online, refers to individual zooids as ranging in size from 0.3 to 1.5 millimetres long, only just able to be seen with the naked eye “if you look very carefully”. New Zealander Dennis Gordon is a world authority on bryozoa and co-author of a field identification guide. In a 2003 “New Zealand Geographic” article entitled “Living Lace”, he notes  that 960 of the 1500 living species of bryozoa occur in the New Zealand region, the highest regional “bryodiversity” in the world.

Worldwide, there are about 15,000 named fossil species. According to Wikipedia, mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period (470 to 485 million years ago). The website “Fossil Finds in New Zealand” includes images showing the different forms that fossil bryozoa may take (see first photo below). Gordon reports an abundance of bryozoan fossils in New Zealand limestone: “It has been estimated that Te Kuiti limestones, including those in the Waitomo area, are 50 to 80 per cent bryozoan remains, and Oamaru stone 70 per cent.” The second photo below is of a sample of Oamaru limestone. Ernst Haeckel’s drawing of bryozoa skeletons, published in 1904, are dramatic renderings of their tiny appearance (see third photo below). He also produced a drawing of live bryozoa (Plate #23 in his “Art Forms in Nature”, published 1899 – fourth photo below).

Haeckel (16 February 1834 – 9 August 1919) was an influential German zoologist, naturalist, and artist, who discovered, described and named thousands of new animal species (he is also controversial due to some of his political and racial views). Haeckel’s published artwork includes over 100 detailed, multi-colour illustrations of animals. Modern scanning electron pictographs of bryozoa fossils provide a similar close-up view of their tiny skeletons.

I have yet to look more carefully at the tiny figures in the Seadown Beach stones I have found, to see how they match up with different varieties of bryozoa. And there may be other types of tiny fossils in them. I have also yet to confirm whether the stones are chalcedony or not.

To return to my visit to Seadown Beach today. I found two stones that clearly are the same as my previous finds and very similar to John Tayor’s slice:

Two other finds are less clear. They are similar in colour and both have tiny forms in them, but the forms may not be bryozoa:

Six other finds from this morning of a roughly similar colour but they are either limonite prase or yellow-brown stones of different compositions but no bryozoa:

Among the other interesting stones I found today was a piece of seam agate.

In March 2021, I found a piece of seam agate a few kilometres south near Moeraki Village – see the second half of the Day 20 entry in this Post.

Other finds this morning included a number of quartzites, reasonably common on this beach:

My final five finds today follow. The last two are too soft and crumbly to tumble-polish but are specimens of Kakanui Mineral Breccia. As one source 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.” There is international geological interest in the breccia for a variety of reasons. More information on Kakanui Mineral Breccia can be found in this Post, about halfway through, in the section on Kakanui Beach.

The next Part of this Series reports on my last fossick at Kakanui for this trip. The Series Index is here.