This Post is part of a Series on some fascinating stones locally called “fossilised worm cast stones”, which I have found on Gemstone Beach as well as along the Te Waewae Bay coast and the beaches of Riverton Aparima in Southland. I now tend to refer to them as “trace fossil stones” (four recently polished ones are in the photos above, another four in photos below). About three years ago, I learned that they can be found elsewhere in New Zealand as well, but they seem to be especially plentiful in the south. The first Post in the Series is entitled “Initial Identification”, published in July 2019. Part Two examines trace fossils in general, and Part Three looks in detail at the trace shapes found in the stones. I then conducted research into the origin and age of these trace fossils. I was fortunate enough to get some information from someone in the Department of Geology at the University of Otago who knew something about trace fossils. Part Four (September 2019) reports on that response and its implications. I also note there that the most interesting and attractive traces in these stones belong to the ichnogenus (trace fossil family) “Protovirgularia”. Protovirgularia is a group of traces made up of an often sinuous line of chevron shapes – these are the ones I keep my eyes open for on the beach, and the eight recently polished stones above and below all contain such traces.
In mid-2020, I compiled a small photo-book “The Trace Fossil Stones of Gemstone Beach”. Part Five in the Series was then planned but only partially written and never published (up until now, over four years later). It was intended to look in detail at information included in Geoff Chapple’s book “Terrain: Travels Through a Deep Landscape” (published in 2015). But at the same time I started searching for information on what kind of animals left these traces, being dissatisfied with the simple answer of worms or marine worm-like animals. This proved an interesting but indecisive search for a number of years, while I kept on collecting, polishing and writing about trace fossil stones. In a 2023 Post, “I is for Ichnogenus Protovirgularia”, made originally on a Facebook Group, I referred to my research in the following terms: “It is thought that the trace is usually the product of locomotion (travel movement) produced by small bivalves, a burrow resulting from the rhythmic action of a muscular cleft-foot. On the southern coast, these traces are called worm-casts – it is thought that the casts were left as the worms tunnelled through mud and compacted the (excreted?) sediment behind them. It is not yet clear to me what left the trace on this Gemstone Beach stone.” Also in 2023, I briefly noted the information from Chapple’s book at the beginning of a Post in my “Southern Sojourn” Series, “The Tattooed Rock, The Trace Fossils… Revisiting Gemstone Beach’s Trace Fossil Stones”. In August 2023, a trace fossil stone became the fourth member of the TumbleStone Hall of Fame.
I have now found a couple of very good sources on the animal producers of the Protovirgularia trace fossil but want to do justice to Chapple’s information first. So this Post is kind of what was envisaged back in 2019, and incorporates information already used in the following: the Saturday 19 June 2021 entry about Mokomoko Inlet from this Post in a Facebook Group Series, the 18th Post in my “2023 Southern Sojourn” Series, “The Tattooed Rock, The Trace Fossils… Revisiting Gemstone Beach’s Trace Fossil Stones”, and the TumbleStone Two Post “Gemstone Beach and Its Stones: An Introduction for the Passing Motorist – Part 7A, Green Argillite Stones”. The next Post in this Series on fossil worm cast stones will report on my most recent findings.
This Post is longer than my usual ones. It covers seven main topics in the following sequence: A) Geoff Chapple’s book “Terrain” and its chapter on Southland; B) Chapple’s companion in Southland, Nick Mortimer, and their visit to trace fossils on the coast east of Tihaka Beach; C) “Mokomoko”, a term used by Chapple for the trace fossils; D) An introduction to “terranes” and their role in New Zealand’s geology; E) The Brook Street Terrane in Southland; F) Argillite and the Brook Street Terrane; and G) Argillite and trace fossils in the context of the main rock types along the coast between Tihaka Beach and Riverton Aparima.
A) Geoff Chapple’s “Terrain”
I initially borrowed Geoff Chapple’s “Terrain: Travels Through A Deep Landscape” from a library and found it so interesting that I bought a copy for myself in 2019. Geoff Chapple is a New Zealand author, journalist and playwright who founded Te Araroa, the New Zealand-long walking track that opened in 2011. His book, “Terrain”, is based on a series of visits by him to eight different regions of New Zealand. In each region he walks and talks with geologists about some key geological aspects of the area. He also meets with other people who have some connection to the geology, such as jewellers and sculptors. “Terrain” is fascinating and interesting, though at times technical, and I highly recommend it – here is a review from “The New Zealand Herald” and here is a review published in “New Zealand Geographic”. This Post relates to Chapple’s material on the eighth region he writes about, Southland, which includes comments on the trace fossils I find so interesting.
B) Chapple, Mortimer and Trace Fossils
Chapple’s chapter on Southland begins with an introduction to the GNS geologist Nick Mortimer. English-born, Dr Mortimer had worked as a government geologist in New Zealand for more than 30 years, and had been part of the regional geology team at Dunedin GNS since 1994. Mortimer was one of the main geologists promoting the case for “Zealandia” as the Earth’s eighth continent (see here for an introduction to Zealandia and here for Nick Mortimer’s 11 minute video discussion of its discovery). Mortimer was also a key champion for the adoption of the idea of “terranes” as geological entities – fragments of Earth’s crustal material that have broken from one tectonic plate and attached itself to another with an entirely different geological history (see below for more information on terranes). It is Mortimer’s work on terranes and how it applies to the southern coast that Geoff Chapple is mainly interested in.
Chapple and Mortimer drive into the Longwood Range which lies just inland from the coast on which Orepuki’s Gemstone Beach lies (see first map, below left). They go there to look at and discuss aspects of the Brook Street Terrane in the south (it reappears on the other side of the alpine fault way up further north in Nelson). Then they drive to Colac Bay Ōraka and on to Tihaka Beach, looking for further signs of the Brook Street Terrane. Leaving their vehicle, they walked along the route of the Te Araroa Trail from Tihaka towards Riverton Aparima.
Along the way, they come across trace fossils in the bedrock. The following passage is from pages 252-253 of “Terrain: Travels Through A Deep Landscape”:
We walked across long-shore drifts of orange pebbles from the batholith granites further west, over a grassy headland, and as we descended to the coast again, I waited for the Brook Street Terrane to impress me, volcanic, and harsh with it. What I didn’t expect was the silky blue rock that flowed out from beneath the sheep paddocks overhead in platforms dimpled with smooth potholes, platforms that stepped down in poetic shapes to the surf that swirled at their feet.
I walked along one of the platforms and Nick called up from below.
“You’re standing on a two hundred and seventy million-year-old sea floor. You can do the same with any sedimentary rock, of any age, but Permian is getting back there in geological time. It’s fairly respectable.”
We looked for the tattooed rock, the trace fossils that Maori call mokomoko, and it took some time but we did find them, wetting down the face of a rock layer to reveal 270 million-year-old traces of burrowing worms that took the purple of their starting layer down into the pale depths beneath, or dragged their pale layer into some purple darkness below, working their primitive palette, thousands of small finger painters out of the Permian.
There are four things I want to highlight from this passage. First, trace fossils can be found in the bedrock along this coast, not just in stones. I had mentioned in an earlier Post that it had been reported that the trace fossil stones on Gemstone Beach had most likely been brought down the Waiau River from further north. They had been seen along the shores of Lake Te Anau and in the Eglinton River, which is indeed where they may have come from. However, it is now clear that one of their sources could in fact be a lot closer to the southern coast. Back in November 2019, after having read the above passage in “Terrain”, I found trace fossils in the rocks at the east end of Tihaka Beach – I had abandoned a fossick on Gemstone Beach due to high winds whipping up the sand and dropped in on Tihaka Beach on the way back to Riverton Aparima. The traces found by Chapple and Mortimer would have been further east, where the pasture comes down to the rocky shore.
A second point that can be taken from the “Terrain” extract above is the confirmation of the age of the trace fossils – Permian, about 270 million years old.
C) “Mokomoko” and Trace Fossils
Thirdly, Chapple notes that Māori refer to the trace fossil rock as “mokomoko” and uses the phrase “the tattooed rock”. The Te Aka Māori Dictionary states that “mokomoko” is a general Māori term for lizard, skink and gecko. It is interesting that the sinuous line of the Protovirgularia trace fossil is very similar to the shape of a lizard’s curled tail (see photos below).
“Moko” is a well known Māori term for tattoos, which could be the source for Chapple’s phrase “tattooed rock”. Despite some online searches, I have not come across any other references to trace fossils as “mokomoko”, but have found a link to a place, Mokomoko Inlet, in three sources. In 2021, I bought a book “Trilobites, Dinosaurs, and Moa Bones: The Story of New Zealand Fossils” by Bruce Hayward (published in 1990). At the end of the book is a map which includes a reference to Mokomoko Inlet near Bluff where trace fossils can be found (see first photo below). Also in 2021 I came across the second source in the bookshop of the Riverton Aparima museum. Lloyd Esler’s 2013 publication, “Omaui and the New River Estuary”, has two pages (103 and 104) on the rocks of the Mokomoko Inlet (see second and third photos below). He comments on the trace fossils that can be found there: “Fossil worm casts are a feature of these rocks. The casts were left as worms tunneled through mud and compacted the sediment behind them. The mud solidified to rock and on exposure, the softer part of the rock eroded leaving the raised worm casts…Smoothed pebbles with the characteristic chevron pattern of the worm casts are common on Southland beaches.” The third source is an academic geology article, David Mossman and Lucy Force’s “Permian fossils from the Greenhills group, Bluff, Southland, New Zealand” published in 1969 in the “New Zealand Journal of Geology and Geophysics”. There, on page 669, Mokomoko Inlet trace fossils are referred to as “faecal pellets secreted by worms – the relative compaction may be an expression of the extent to which the material has been digested”. The fourth and fifth photos below come from that article.
If the trace fossils were indeed called “mokomoko”, as suggested by Chapple, and given that significant specimens of trace fossils can be found at Mokomoko Inlet, then was the inlet named after the trace fossils? Or does the origin of Mokomoko Inlet’s name lie elsewhere and the trace fossils were named after the Inlet? I don’t know.
Note: My Google searches on “mokomoko” also turned up the information that the Te Whakatōhea chief Mokomoko was one of five Māori unjustly executed on 17 May 1866 for being involved in the murder of the missionary Carl Völkner at Ōpōtiki – his innocence was formally recognised in 1993 and 2011. I have not come across any connection between this person and trace fossils. Some of his descendants carry the Mokomoko surname. Mokomoko was also a Māori kāinga – village and area of cultivation – on the Manawatu River near today’s city of Palmerston North.
The fourth point to be taken from Chapple’s extract from “Terrain” (above) is that the trace fossils have been left by “burrowing worms”. Such a view is echoed in the extracts we have viewed on the Mokomoko Inlet trace fossils. This may be too narrow, and is a point we will come back to in following Posts.
Now for some background on New Zealand’s terranes and especially the Brook Street Terrane, partly to put the age of these trace fossils into perspective but also to deepen our understanding of their geological context on the south coast between Tihaka and Riverton Aparima.
D) Terranes and New Zealand’s Geology
As Wikipedia puts it, terranes are “large slices of crust with different geological histories”. A number of terranes were “brought together by tectonic activity (subduction and strike-slip faulting) to form New Zealand”. Peter Ballance’s excellent “New Zealand Geology: An Illustrated Guide” was published in 2017 (available for free download here). There, on pages 49-52, he presents a good account of how terranes were part of the development of the super-continent of Gondwana (from which New Zealand originated, as did Australia, India, Africa and South America). The first map below (centre) shows the “Gondwanan phase of terrane accretion, 505–110 million years ago”. Gondwana broke up during the period of 50 to 100 million years ago and the continent of Zealandia emerged, with New Zealand eventually taking shape over the past 25 million years. This means that the basic terranes of New Zealand actually came together in Gondwana, well before New Zealand as we know it was formed.
The second map below (right) comes from page 42 of Ballance’s publication and shows the complex make-up of New Zealand’s basement rocks (note the Brook Street Terrane in the north and south). In 2004, in an article in the academic journal “Gondwana Research”, Mortimer identified the basement rocks of New Zealand as consisting of “nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths”. A batholith is an exposed area of reasonably continuous plutonic rock that covers more than 100 square kilometers – plutonic rock is magma that has cooled under the earth’s surface (Wikipedia).
E) The Brook Street Terrane in Southland
The two maps below (top row) show the terranes of the South Island. The first map has labelled the terranes with names, including the Brook Street Terrane in the south. The second map more clearly shows the Brook Street Terrane in dark blue, enabling us to see its presence also in Nelson in the north as a very thin segment (the terrane is named after Nelson’s Brook Street). It is over 400 kilometres long and is thought to be more than 14 kilometres thick (Mawson, 2018, page 2). The Brook Street Terrane started off as an oceanic volcanic arc – see Wikipedia for an account of how a volcanic arc develops due to plate subduction (diagram below, bottom row, left). As Ballance notes on page 277 of “New Zealand Geology: An Illustrated Guide”, the mainly basaltic rocks of the Brook Street Terrane come from the Permian/Triassic periods, being around 250 million years old. They are “made up of a wide range of lavas, dikes, volcanic breccias and sandstones, along with their deeper intrusive equivalents (‘granites’)”. In the south of the South Island, the Brook Street Terrane comprises mostly of lavas and sediments in the Takitimu Mountains, gabbroic plutonic rocks at Bluff, and fine-grained diorites at Riverton Rocks (see text extract below, at right on bottom row).
In the book, “The Natural History of Southern New Zealand” (2003), is a chapter on “Geology” written by Tony Reay. He notes that part of the Brook Street Terrane exposed along the coast between Orepuki and Colac Bay Ōraka differs from other parts of the Terrane in that their rocks contain much larger mineral grains. “This implies they have cooled and thus crystallised much more slowly… at a depth of 6-10 km” (page 6). They are plutonic rocks, remnants of magma chambers that stored and supplied the lava that broke out onto the surface. An account of the Permian rocks in New Zealand [see “Stratigraphy” in the GNS “Geology of New Zealand”, click on “Permian (Y)”] explains the development of volcanic and sedimentary rocks and notes the volcanic origin of the Takitimu Mountains and the plutonic origin of the Longwood Range.
F) Argillite and the Brook Street Terrane
Argillite is the Brook Street Terrane sedimentary rock in which the trace fossils are found. In “Terrain” (page 253-254), Chapple writes the following during his walk eastwards from Tihaka Beach with Nick Mortimer, keeping an eye out for signs of the Brook Street Terrane:
What I didn’t see was any obvious volcanic forms, and Nick agreed. We were walking along the apron that spread out from an eroding volcanic centre, the sandstones, the siltstones and mudstones. At least half of the Brook Street Terrane was that kind of sedimentary rock, but its source was volcanic, its colours derived from basalt. On the way back to the truck, I picked up an interesting pebble, hard and glassy…
– It’s a very fine-grained sandstone (said Nick Mortimer). This was a sedimentary rock, but like many of them around here, it’s now metamorphic. We’re only a few kilometres from the Median Batholith, which was a big hot thing…
– Really? How far can those things transmit heat? (asked Geoff Chapple)
– In round numbers, molten granite and gabbro sits at about a thousand degrees centigrade…Typically you can expect to see thermal effects from one hundred to one thousand metres out from a decent sized pluton.
I picked up another interesting pebble, and passed it across. Nick looked at it, licked it, and looked again through his hand lens.
– Argillite.
The website of the Nelson Rock and Mineral Club notes this about the Brook Street Terrane: “It consists primarily of volcanic rocks – tuffs and some basalts – interbedded with breccia, sandstone, siltstone and mudstone. Most of the sedimentary rocks contain minerals clearly derived from nearby volcanic outcrops, and the sequence seems to have formed in an island arc, fed both by lava flows and ash deposits from active volcanoes, and reworking of volcanic rocks.” I want to take a closer look at volcanic tuff and the mudstone called argillite.
In 2018, Jasmine Mawson completed a Masters thesis in geology at the University of Otago, “Submarine Eruptive Processes in the Brook St Terrane at Colac Bay, Southland, New Zealand”. She looked closely at the rocks along the coast between Tihaka Beach and the southern part of Riverton Rocks (see map below, left). This included geochemical analysis. She then produced a generalised map of the four main rock types on the surface in this area (below centre). The thesis also contains photos of the types of rock. Below right (Figure 11, page 32 of the thesis) is one which shows volcanic tuff overlain by argillite. Both rocks are very familiar to people who spend time around Riverton Aparima. I’ve often looked at the blue tuff and wondered if in fact it was argillite.
I suspect the argillite in Mawson’s photo (above) is quite weathered as many of the trace fossil stones I have found have a cleaner more-consistent character. The photo shows that the tuff is blue and the argillite is green, a key indicator distinguishing them, though I have also found trace fossils (a little further west at Gemstone Beach) in red and light gray argillites, as well as a wide range of shades of green, including a very dark green.
Geology.com reports that tuff forms from the products of an explosive volcanic eruption. The volcano blasts out rock, ash, magma and other materials from its vent which falls to Earth and is compacted and cemented into rock. There are many types of tuff depending on what it is composed of and how hard it has been consolidated. Mawson’s “lapilli tuff” (map above centre) is a tuff consisting at least partially of “spheroid, teardrop, dumbbell or button-shaped droplets of molten or semi-molten lava ejected from a volcanic eruption” (Alex Strekeisen). It contrasts with the plain tuff which consists of much finer grains. The tuff in Mawson’s study area is very hard and well consolidated.
Mawson describes the argillite of her study area as follows (page 31): The argillite forms large cliffs with distinct bedding. It is pale green on fresh surfaces, weathered to brown and grey in many places, and smooth to the touch. It is very fine grained (silt to very fine sand (3.9-125 μm)), and fractures in an irregular, almost conchoidal manner. Well-formed pyrite and chalcopyrite crystals are scattered throughout; these are usually <1 mm, though at site 30 they range up to 4 mm. In several places these crystals are concentrated in veins. She concludes that the argillite originates from the accumulated erosion of volcanic material, from a main volcanic vent to the south. It was likely formed under the sea at depth, because of its very fine grains.
Mawson notes that the tuffs and pillow lava come from two small volcanic vents within her study area – “Their close proximity and small size renders them likely to be satellite vents on the flanks of a larger stratovolcano” (page 75) – see map and text extract from Mawson’s thesis below. Furthermore: The presence of pillow lavas makes it clear that these volcanic rocks were formed in a subaqueous environment. Abundant turbidites and the absence of any wave-formed sedimentary structures suggest that the deposits formed below wave-base… and the abundance of fine-grained argillite suggests a relatively deep setting. (page 65) Pillow lava looks like a group of rounded plumped-up cushions, and Riverton Aparima (e.g., Back Beach) is well known as an area for them. Pillow lava forms when molten lava, usually basaltic, is pushed up into the sea floor and suddenly chilled. As an article in “New Zealand Geographic” puts it, “They result only when molten rock—at a searing 1200° C—meets the cold embrace of seawater” .
G) Argillite and Trace Fossils East of Tihaka Beach
“The Photographic Guide to Rocks and Minerals of New Zealand”, authored by Nick Mortimer, Hamish Campbell and Margaret Low (2011), describes argillite as a “hardened, slightly recrystallised mudstone” (page 98). University of Auckland Geology calls it a “highly indurated mudstone”. Elsewhere, it has been called a fine-grained “metasedimentary” rock (see Museum of Stone Tools). “Metasedimentary” means that it started out as a sedimentary rock which was then buried beneath other layers of rock and subjected to intense pressures and temperatures, causing the rock to recrystallise and harden. If more heat and pressure were to be applied, it would turn into a metamorphic rock, such as slate and schist, and no fossils, trace or otherwise, would survive. In the case of the argillite between Tihaka Beach and Riverton Aparima, the proximity of the volcanic-derived mudstone to the volcano and its satellite vents, and maybe to the Median Batholith, was enough to produce the extra hardening needed. Prior to that, when lying beneath the ocean surface, the sediments of mud were inhabited by small animals, maybe worms or worm-like, which left traces that were preserved by the hardening process. And so trace fossils arose. All of this happened more than 250 million years ago, offshore from the ancient continent of Gondwana.
The next Post in this Series (not yet available) will look at what kinds of animals may have produced the Ichnogenus Protovirgularia traces in these stones.
