Soils “lie at the interface between earth, air, water, and life” (Needelman, 2013). They are constantly evolving in response to what is around them – such as the climate. For this reason, some scientists have made the study of fossil soils one of their specialties. One of these is Greg Retallack, who has written many scientific papers, either overtly about fossil soils, or that include some descriptions of them, a very readable book on fossil soils (e.g. Retallack, 2001 as well as some related papers. One: ‘Effect of soils on the taste of wines’ (Retallack and Burns, 2016), is, of course, of particular interest (If I had continued to teach American students into 2008 I was going to slip that very subject into the course).
Retallack hails from Australia, but much of his PhD research involved Triassic of New Zealand. His blue-covered thesis was never far away when I was doing my own. What impressed and intrigued me, was that Retallack always tried to put his plant fossils into the context of a broader landscape – and its soils (it helped that Retallack has artistic skills, that I lack). Even in 1977 he was writing “The swamp woodland grew on humic gley (fibrist) soils between the Voltziopsetum and a large coastal lagoon at a time of high base level” (Retallack, 1977).
So how to apply that kind of research to the Jurassic fossil forests at Curio Bay? There, at least ten fossil forest horizons can be traced out in the cliffs (Pole, 2001). By definition, each of these represents a ‘fossil soil’. The stumps and associated leaf-litter are the ‘Surface A horizon’, or ‘Topsoil’. Given that soils were obviously present and developing at Curio Bay during the Jurassic – it raises the question of – ‘specifically, what type of soil?’
Some of Curio Bay’s fossil stump horizons are associated with a reddish zone (these have indistinct upper and lower bounds) and sometimes the ‘redness’ appears to be amorphous, or microcrystalline – a ‘chert’. The ‘main’ Curio Bay fossil forest, visible from the viewing platform, exhibits some of this ‘redness’. However, some of the best examples are on the headland to the north, past the campground, above the Waikawa Estuary (see the featured image). There, layers of red, with the ghostly outlines of logs and other pieces of wood, contrast strongly with the more yellowish weathered layers of sediment.
This red coloration and chert development provides a clue to identify and place the Jurassic fossil soils into the modern soil classification systems.
A further clue is that Curio Bay’s forests grew on a lowland plain, dominated by river floods. But the broader environment was volcanic – the forests were growing on volcanic material deposited by river flooding, and at least some of the forests had aerial volcanic ash dumped on them.
Volcanic ash is rather unstable, and can weather quickly to release silica, which can then accumulate within the soil profile (or within wood – that’s how the tree stumps and logs became ‘petrified’). In technical terms, this is ‘pedogenic silica accumulation’. The reddish colour come from iron oxides (like hematite/goethite), that were also formed by weathering of the volcanic material.
If Curio Bay’s reddish, cherty paleosols were found developing somewhere today, they would probably be classed as ‘Ferric Durisols’. There are various systems of classifying soils, but this term follows the ‘World Reference Base’ (IUSS Working Group, WRB. 2022).
A Durisol is defined by a ‘duripan’ within its profile — a horizon cemented by secondary silica (that’s the chert), and ‘Ferric’ because of local iron enrichment giving the reddish colour. Initially (before much weathering), these soils were probably ‘ Andisols’. These form on volcanic ash, but once the silica/chert horizon starts to form, the soil has essentially evolved from an Andisol into a Durisol, or perhaps a ‘Petroduric Andisol’.
As well as the amorphous silica, weathering of the original volcanic material would form clay – like kaolinite. But one of the features of Curio Bay is a general absence of clay, or mud. Curio Bay appears to have been a well-drained environment, and presumably, the fine clay was washed out of the system, and deposited elsewhere.
A few kilometers to the west of Curio Bay, on the coast of the Fortrose-Otara area, Jurassic fossil forests are exposed in outcrop. There is much clay and some coal (coal is absent at Curio Bay), and therefore this represents quite a different system. The coal was originally peat – and thus was the upper horizon of a very organic soil. In terms of the World Reference Base classification – a Histosol or Organosol.
Coal, carbonaceous mud, and grey, ‘gleyed’ mud in the Jurassic of the Fortrose-Otara coast, New Zealand.
Associated with the coals are greyish, or even greenish, clays – sometimes riddled with fossil plant roots. These may be ‘gley soils’ (or Gleysols in the ‘World Reference Base’), or ‘gleyed’ horizons within Histosols or Organosols. Both form in waterlogged, anaerobic conditions, and both are probably present along the Fortrose-Otara coast. Instead of becoming oxidised and reddish, as at Curio Bay, the iron compounds are reduced and give the gley soils their typical (not always) grey-green colour.
Jurassic grey, gleyed mud, riddled with fossil roots, in the Jurassic of the Fortrose-Otara coastline, New Zealand. 79739
Yes, the words ‘grey’ and ‘gley’ do sound similar! But ‘Gley’ comes from Ukrainian ‘глей’. It’s the everyday word there for ‘sticky muck/gloop/gunk’, or in this context, ‘gluey clay’ (As an aside, it’s not an everyday Russian word, although is used in the context of soil science). ‘Gley’ is not cognate with English ‘grey’ – a proto-Germamic colour term.
So what does all this mean? Ferric Durisols require that the silica that weathers out of volcanic ash, to be mobilised in the soil-water, and then re-precipitated in the upper profile. This is a clue that the Curio Bay landscape was seasonally dry, not ever-wet. That tallies with the lack of coal there. In contrast, the sediment along the Fortrose-Otara coast was probably deposited at a different time, under a wetter climate (I haven’t seen Ferric Durisols there).
It’s unclear to me if there are any good examples of them forming today. The ‘silcretes’ in the very dry parts of Australia and southern Africa are much too developed to be directly relevant. Perhaps the Jurassic environment – with a much higher carbon dioxide content of the atmosphere, led to soils that don’t form today.
There will be much more that these fossil soils can tell us about the Jurassic of New Zealand – but here’s a start.
References
IUSS Working Group, WRB. 2022. World Reference Base for Soil Resources. International soil classification system for naming soils and creating legends for soil maps. 4th edition. International Union of Soil Sciences (IUSS), Vienna, Austria.
Needelman, B.A. 2013. What Are Soils? Nature Education Knowledge 4:2
Pole, M.S. 2001. Repeated flood events and fossil forests at Curio Bay (Middle Jurassic), New Zealand. Sedimentary Geology, 144: 223-242
Retallack, G.J. 1977. Reconstructing Triassic vegetation of eastern Australasia: a new approach for the biostratigraphy of Gondwanaland. Alcheringa, 1: 247-277.
Retallack, G.J. 2001. Soils of the past. An introduction to paleopedology. Second Edition. Blackwell Science.
Retallack, G.J., and Burns, S.F. 2016. Effect of soils on the taste of wines. GSA Today, 26: 1-8. doi: 10.1130/GSATG260A.1
