Guest post by Phil Pollock - Giant crabs and concretions of Taranaki.

On right hand side is fossilised species of the crab
Tumidocarcinus giganteus, preserved in a 12-14 million
year old mudstone concretion of the Otunui Formation in
North Taranaki. The complete crab is approximately 20cm 
across. On the left is the end of a single claw from a larger
specimen. Source: Phil Pollock (2018).

Single crab claw, with scale, shown in previous photograph.
This gives an indication of the size to which this species could grow.
This was a cold-water species, inhabiting deep waters between
500 and 700 metres. Source: Phil Pollock (2018).

One of New Zealand's most famous concretions,
the Moeraki Boulders. Here the hard erosion resistant
concretion has been exposed as the more erodible sandy mudstones
in the cliff have been eroded. These concretions started
forming around 60 million years ago in sediments as
they became buried on an ancient sea-floor. Source: Károly Németh (2018).

Phil Pollock in one of his favourite spots,
between a rock and a hard place..

Aotearoa Rocks welcomes today's guest writer Phil Pollock. Phil holds a bachelors degree in Earth Science and is passionate about all things geology. Based in New Plymouth under the shadow of Mt Taranaki/Egmont Volcano, he has a particular interest in fossils and the evolution of marine life in Zealandia. He can often be found in the ‘middle of no-where’ hunting and collecting fossils or rock samples to add to his collection (much to the annoyance of his wife). All through university Phil would thrill his classmates with stories and examples of giant crabs found in the sedimentary rocks of Taranaki, to find out more about these ancient giant crabs read on.....

A common type of mudstone in parts of New Zealand, especially the Taranaki and Wanganui regions, is the blue-grey mudstone commonly referred to as “papa”. This poorly cemented mudstone forms steep hilly landscapes notorious for their instability when exposed by deforestation and subject to heavy rainfall. 

"Disastrous results of soil erosion illustrated in four photographs 
from the Wanganui District" Source: Auckland Weekly News (1939). 
Sir George Grey Special Collections, Auckland Libraries, AWNS-19390503-45-2 (2018).

Yet within this soft mudstone exposed by erosion we may find boulder like “concretions”, as hard as rock, and often encasing well-preserved fossils. Carbonate concretions are a common feature found in most sedimentary rocks. Here in New Zealand they are most often found in mudstones and a few sandstones. But what are they?  How do they form? And what’s inside of them?

Above and below: In situ ovoid mudstone concretions at Whitecliffs,
 north Taranaki. Concretions are preserved in the 11-9 million years old
 sediments of the Mt. Messenger Formation. Source: Scott Cook.
 Retrieved from: https://nzfrenzynorth.wordpress.com/h12/ (2018).

New Zealand’s most famous concretions are The MoerakiBoulders on the Otago coast, their distinctive rounded profile attracting thousands of tourists every year, and their spherical forms half buried in the sands of the beach a favourite photography subject. As erosion has eaten into the surrounding sandy mudstone cliffs, the more erosion resistant concretions have been released from the cliffs, to lie scattered on the beach. 

The Moeraki Boulders are exceptional in terms of their average size of two metres diameter. They are a specific type of concretion known as  septarian, referring to the cracks that criss-cross the exterior and interior of the boulders. In many cases these cracks may be filled with minerals such as calcite found at Moeraki. Often at the heart of a concretion you may find an exceptionally preserved fossil, however not all concretions contain fossils. What all types of concretion do share in common is the processes that lead to these natural mudstone “boulders”.

Above: A septarian concretion from the 
Tangarakau Gorge, northeast Taranaki, showing white calcite 
precipitated as "veins" where cracks have been filled 
by the mineral. Source: Phil Pollock (2018).

Moeraki Boulders, showing criss-crossing pattern of veins typical
of septarian concretions. Largest boulder between 1.5 - 2m diameter
Source: Károly Németh (2018).

Interior of concretion at Moeraki Boulders. In places it can be seen 
how the concretion has built up layers from the inside out over time.
A variety of minerals can also be seen where the boulder has split
along the surfaces of cracks. Source: Károly Németh (2018).

 

How do concretions concrete?

The formation of concretions involves complex processes, some of which are still debated. I won’t get too technical here, but give a general description while highlighting some of the more controversial aspects.

The fact that fossils within concretions often show a high degree of preservation and that surrounding bedding structures are often preserved show that they are diagenetic (caused by physical and chemical processes that turn sediments into sedimentary rock), and form after the deposition of sediments. They are essentially areas of sediment that have been cemented more than neighbouring deposits. This makes them more resistant to erosion than their surrounding parent rock and hence they are often seen protruding from an outcrop.

  

In situ round sandstone concretions preserved in the 11-9 million years old
Mt. Messenger Formation at Tongaporutu, north Taranaki.The dark colour on the outer
layers is caused by oxidation of iron minerals (rust).

Source: Scott Cook. Retrieved from: https://nzfrenzynorth.wordpress.com/h11/ (2018).

Formation and cementation of the concretion begins around a nucleus. This may be a fossil, such as a shell or even a fragment of shell, or it may just be a small concentration of coarser mineral grains within the overall sediment. Concretions therefore grow from the inside out. A higher concentration of cement in the centre of the concretion which decreases toward the edge and differing compositional mineral bands are also evidence for this.

The most common cement in marine derived rocks is calcite(calcium carbonate) and sometimes dolomite (calcium magnesium carbonate). In rocks of non-marine origin the cement is often siderite (iron carbonate). Pyrite (iron sulphide) is another mineral commonly found in concretions, although usually not in high concentrations but rather scattered amongst the sediment grains. 

When a sediment is deposited it contains a large amount of water in its pore spaces, in the case of mud this may be up to 80%. As more and more sediment is laid down two things begin to happen. Firstly, the water is squeezed out from the pore spaces which packs grains closer together and secondly, platy minerals (clays, micas etc.) position themselves into a horizontal orientation which directs more water flow in that direction. This second process has a direct bearing on the shape of the concretion and through this process develops what is known as anisotropic permeability (a rate of permeability that varies in different planes or directions). Sandstone concretions with less platy minerals are therefore more spherical than mudstone concretions which tend to take on an oval ‘rugby ball’ shape.

 

Almost perfectly spherical sandstone concretions from the Mt. Messenger 

Formation lie exposed on the beach at Tongaporutu, north Taranaki. 

Source: Scott Cook. Retrieved from:https://nzfrenzynorth.wordpress.com/h11/ (2018).

A typical oval-shaped mudstone concretion viewed from the top down.

15cm across, from the Taumatamarie Formation (23-16 million years ago), 
Awakino. Source: Phil Pollock (2018).

The cementing mineral precipitates from the water and essentially begins to replace the water in pore sites as compaction increases. More often than not this is a relatively quick process to begin with. Cementation begins around the nucleus before the compaction and concretion process has run its complete course, meaning fossils at the centre of the concretion are fully protected.

Well preserved 155 year old ammonite fossils of Kossmatia sp found in Jurassic 

(200-145 million years ago) rocks of the Murihiku Terrane in the Kawhia area. 

The silvery white mineral visible is zeolite and is related to depth of 

burial rather than concretion formation. Source: Phil Pollock (2018).

  Fossillised specimen of  Tumidocarcinus giganteus 

 preserved in a concretion of the Otunui Formation (14-12 million years ago), northeast Taranaki. 

Crabs normally become disarticulated soon after death, however preservation in a

concretion keeps them relatively intact. Source: Phil Pollock (2018).

In order for cementation to take place the water has to be super-saturated with abundant cations (positive ions) such as calcium or magnesium and an anion (negative ion) which is the carbonate. But there is not a sufficient supply of ions in a static body of water to form substantial mineral build-ups. It was once thought that the carbon could be derived from the organic decomposition of fossils within the centre of a concretion. However that hypothesis ignores the fact that not all concretions contain fossils. Furthermore, there simply is not enough carbon in a decaying organism compared to the carbon content found in a concretion. Both cations and anions therefore have to be delivered to the site of concretion growth. 

It is unclear if ion transport is through flow (ions carried through moving groundwater) or diffusion (movement of ions within static water along a concentration gradient). Both mechanisms present problems.  Flow requires a large volume of pore water to be replaced (up to 10,000 pore volumes) and diffusion is a very slow process over distance.

A further mystery is why some deposits form concretions while others do not. Or why there may be separate layers of concretions in one outcrop. This may be related to rates of sediment supply and deposition. Under constant rates of deposition ions may just migrate up through sediments and any precipitated minerals would be dispersed evenly throughout the sediment. A break in deposition may allow sufficient time for diffusion to occur and concretions to form. So it is possible that concretion formation is a response to a hiatus in sediment deposition.

How to catch a crab in a concretion.

Some spectacular fossils are to be found within concretions. Everything from plesiosaurs to scallops have been found encased in cemented lumps of rock. But how do you know if a concretion contains a fossil? Most of the time you don't, but there are often clues that you can look for.

Above and below: Two well preserved specimens of Tumidocarcinus giganteus  

fossilised in mudstone concretions from the Otunui Formation, northeast Taranaki.

 Source: Phil Pollock (2018)

The most obvious clue to the presence of a fossil is that part it may be exposed at the edge of the concretion. However loose mud and debris may have to be removed to see it. Areas or spots of discoloration (usually an orange colour from oxidation) can also hint at a fossil inside. 

Mudstone concretion from the early Pliocene (5 million years ago) 

Matemateaonga Formation showing the exposed claw of a Leptomithrax fossil 

(a type of spider-crab). Source: Phil Pollock (2018).

 

Close up of a mudstone concretion showing the slightly exposed tips of legs  

belonging to the crab species Tumidocarcinus giganteus. These were

only visible after cleaning the concretion. Source: Phil Pollock (2018).

A classic ovoid-shaped mudstone concretion with Tumidocarcinus giganteus  

claws and leg tips exposed. Even without exposure of the fossil, concretions of this distinctive 

shape would be worth investigating. Source: Phil Pollock (2018).

As noted above shape and lithology can also be helpful. Fossils are rarely found in round sandstone concretions but are more likely in ovoid mudstone ones. There are of course exceptions with some rounded mudstone concretions also containing fossils. Opening a few from an outcrop soon tells you if you should keep looking or move on.

 

Extracting concretions from Jurassic (200-145 million years old) siltstone at an 
undisclosed location in Kawhia. Concretions at this rich site contain bi-valves, 
gastropods, brachiopods, belemnites, ammonites and even small crayfish like 
arthropods that may be a common ancestor to modern crab species. 
Small concretions and spaces made by their removal are visible. 
Source: Phil Pollock (2018).

 

After millions of years in the making, it is no surprise that opening a concretion is not always an easy task. Concretions that have weathered are easier to open than fresh ones. Tapping a hammer around the long axis of the rock may crack them if they are old and weathered but fresh ones often have to be persuaded with a sledge hammer. Again the long axis should be the target and as soon as a crack appears, switch to a lighter hammer and lightly tap along the crack to propagate it. Often parts of a fossil may be broken. It is actually very rare with some types of fossil for this not to happen. However with the right tools, some patience and a little skill it is possible to extract pieces from one half and join them to the other. This especially true for crab legs and ammonites.

Sometimes they play nice and other times they don’t. The crab above was badly 
damaged when the concretion was opened. One more Tumidocarcinus giganteus, 
from the 12-14 million year old Otunui Formation, North Taranaki. 
Source: Phil Pollock (2018).

The ones that make it all worthwhile.This ammonite (Kossmatia sp.) ‘popped’ 

open after a couple of gentle taps. Source: Phil Pollock (2018).

So the next time you pass a papa outcrop, look for concretions and maybe try to collect a few. Digging them out of cliffs is not always necessary as they are often found in the talus pile at the bottom of an outcrop. Over time you will develop a sense of which ones to hit. And you never know, there may be treasures inside. Remember though, at the very least record the date and location of your find, as this gives any fossils scientific relevancy and is important information for any future study of your find. 

If you would like to get up close and personal with one of these spectacular ancient giant crabs, pop into the Ōpōtiki District Library, where Phil has kindly donated one for display.

Unless otherwise credited, all photographs copyright Phil Pollock (2018) or
Károly Németh (2018).

For a great user-friendly guide to New Zealand fossils check out A photographic guide to New Zealand fossils,
available from GNS here