Monthly archives of “September 2014

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The Kai Point Coal Mine – Late Cretaceous vegetation treasure-trove

The lowlands south of Dunedin (New Zealand), used to be almost impassable wetlands. The local Maoris were incredulous when the first Western explorers of the mid 19th century insisted on making their way tediously directly through it. There were other routes south. The wetlands were a mixture of flax, raupo, sedges, and some small patches of real podocarp forest. But drained, the wetlands made excellent pasture – and so, over the years, it was mostly done. One man went the other way – Hori Sinclair (a cousin of my grandfather). Over a lot of protest, he stopped draining his 315 Ha farm and went the other way – trying to ‘rewild’ a rich wetland. I’m not that sure of Sinclair’s green credentials – I suspect the venture had more to do with his love of duck-shooting than biodiversity! But all the same, ‘Sinclair Wetlands‘ became the largest privately owned wetlands in the New Zealand and earned Sinclair a gong – an MBE.

If you had travelled to the same general region about 70 million years ago (the Late Cretaceous period), you would have again struggled through wetlands – but of a very different character. Through a quick of geology, what is now the elongate lowland containing the Sinclair Wetlands was the high ground, and what is now the ridge separating the lowlands from the coast, was the coastal lowlands. Rivers meandered their way across this lowland to the nearby sea – Late Cretaceous limestone with belemnites occurs at Brighton, just south of Dunedin, and Late Cretaceous ammonites at Burnside, more or less in the city.

The Late Cretaceous Kai Point Mine, New Zealand. The Kaituna Seam is in the foreground. In the distance, and overlying it, is the Barclay Seam.

The Late Cretaceous Kai Point Mine, New Zealand. The Kaituna Seam is in the foreground. In the distance, and overlying it, is the Barclay Seam.

Between the river channels huge thickness’s of peat accumulated – the product of forested wetlands. This peat has now been compacted to coal and is exposed in the Kai Point Coal Mine, at Kaitangata. Two main seams are being mined there – the upper Barclay and below that, the Kai Tuna. The mine is the best source of Late Cretaceous plant fossils in New Zealand, and perhaps the best in the Southern Hemisphere. In contrast to other localities, leaf cuticle at Kai Point is well-preserved and the material relatively easy to prepare.

A possible Nothofagus fossil leaf in-situ on a mudstone bedding surface. From Kaitangata, New Zealand

A possible Nothofagus fossil leaf in-situ on a mudstone bedding surface.

A possible fagacean fossil leaf that has been peeled off the sediment. Kaitangata, New Zealand.

A possible fagacean fossil leaf that has been peeled off the sediment.

One of the most common Angiosperm leaf fossils at Kai Point Coal Mine,New Zealand. Affnities unclear.

One of the most common Angiosperm leaves at Kai Point. Affinities unclear.

Palynology has established the Late Cretaceous age of the Kai Point Mine (Couper 1953; Browne 1986; Browne & MacKinnon 1989; Crouch 1994; Raine 1994). It has also highlighted Nothofagus, Podocarpaceae, Proteaceae and the Pteridophytes as being important elements in the vegetation. Browne (1986) found 18 species of Podocarpaceae pollen, and given that pollen tends to distinguish more at the generic level than the species – indicates a truly phenomenal biodiversity of this group in the region. The leaf and shoot fossils emphasise the probable importance of Nothofagus and conifers – although they suggest that in a biomass sense, the Podocarpaceae were secondary to the Araucariaceae (Pole 1992, 1995; Pole and Douglas 1999; Cantrill et al. 2100; Pole 2014). The absence of any sign of Proteaceae in the macrofossil record, compared with the 24 species of Proteaceae pollen found by Browne, is though-provoking. The distribution of macrofossils also suggest that at least parts of the peat vegetation was dominated by conifers – araucarian leaves are abundant in some of the muddier leaves in the coal. On the other hand, angiosperms may dominated the clastic soils nearer the river channels – they predominate in the grey muds in between the coal seams, which were probably open bodies of water. Leaves of the plants growing along the river channels would have fallen off and into the water, travelled downstream, and then deposited in quiet lakes. The shape of the angiosperm leaves suggests that many of them may have been deciduous. We have little idea what many of the angiosperms represented by the fossil pollen and leaves actually were – suffice to say that most of the wetland plants so common in the Sinclair Wetlands did not exist at the time.

Earlier research suggested that New Zealand lay at high latitudes in the Late Cretaceous – above the Polar Circle. However, more recent work suggests higher latitudes than today, -say 50-60S, but not polar (Wright et al., 2013). With a potential polar latitude in mind I suggested (Pole 1995) that the modern boreal forest – the ‘taiga’, may be a suitable analogue for the Kai Point vegetation. I suspect the broader patterns – distinct conifer and angiosperm dominated areas, swamps and fire, still make this useful, but the climate is likely to have been very different.

The Barclay and Kaituna coal seams were formalised as part of a regional coal field stratigraphy by Harrington (1958) that proposed 17 successive seams . With a more modern understanding of how coal fields tend to form, there is scepticism now that the stratigraphy is that simple. Barry Douglas has made a start to this by interpreting the succession of facies at the Kai Point Mine in terms of fluctuating levels of the nearby sea (Douglas and Lindqvist, 1987; Douglas, 2001; and in Lee et al. (2003).

Since Harrington’s time many of the old coal pits in the area have disappeared, either grown over or intentionally rehabilitated. This is basically a good-thing – the area around the town of Kaitangata positively screams for the development of some walking/cycling trails – with longer connections to Dunedin in the north and Catlins in the south. But the obliteration of all outcrop is frustrating for those who want to know more about New Zealand’s plant fossils and the environment they lived in. When the mining moves on, it would be nice to keep some of the outcrop as a research and educational resource. With some foresight a trail may link two treasure-troves, past and present, Kai Point and Sinclair Wetlands.

References

Links will take you to my Academia.edu page where you can download a pdf.

Browne, K. W. (1986). The palynology of the Kaitangata Coalfield, south east Otago. Master of Science, Geology Department, University of Canterbury.

Browne, K.W. & MacKinnon, D.I. 1989: Palynological correlations at Kaitangata Coalfield. Energy Research and Development Report RD 8819.

Cantrill, D. J., Wanntorp, L. & A.N., D. 2011: Mesofossil flora from the Late Cretaceous of New Zealand. Cretaceous Research 32: 164-173.

Couper, R. A. 1953: Upper Mesozoic and Cainozoic spores and pollen grains from New Zealand. New Zealand Geological Survey Palaeontological Bulletin n.s. 22: 77.

Crouch, E. M. (1994). Kaitangata coalfield samples for palynological assessment (FRST Contract DON301). Palynology Section report EMC 2/94. Institute of Geological and Nuclear Sciences, Lower Hutt.

Douglas, B.J. 2001. Field guide to mine geology and operations at Kai Point opencut coal mine (Kaitangata), L & M Waikaka alluvial gold mine and Milburn Lime Quarry. Field Trip Guide, Dunedin Conference, The Australasian Institute of Mining and Metallurgy.

Douglas, B.J. and Lindqvist, J.K. 1987: Late Cretaceous – Paleocene fluvial and shallow marine deposits, Kaitangata Coalfield: Taratu and Wangaloa Formations. Field Trip Guide, Dunedin Conference, Geological Society of New Zealand Miscellaneous Publications 37B.

Harrington, H. J. 1958: Geology of Kaitangata Coalfield. Wellington: Department of Scientific and Industrial Research. New Zealand Geological Survey bulletin 59.

Lee DE, Lindqvist JK, Douglas BJ, Bannister J, Cieraad E 2003. Paleobotany and sedimentology of Late Cretaceous-Miocene nonmarine sequences in Otago and Southland. Field trip guide, Dunedin Conference. Geological Society of New Zealand Miscellaneous Publication 116B.

Pole, M. S. 1992: Cretaceous macrofloras of eastern Otago, New Zealand: angiosperms. Australian Journal of Botany 40: 169-206.

Pole, M. S. 1995: Late Cretaceous macrofloras of Eastern Otago, New Zealand: Gymnosperms. Australian Systematic Botany 8: 1067-1106.

Pole, M. S. & Douglas, B. J. 1999: Plant macrofossils of the Upper Cretaceous Kaitangata Coalfield, New Zealand. Australian Systematic Botany 12: 331-364.

Pole, M. 2014:  The distinct foliar physiognomy of the Late Cretaceous forests of New Zealand — Probably deciduous. Gondwana Research: 1-7  http://dx.doi.org/10.1016/j.gr.2014.02.009.

Raine, J. I. (1994). Palynology of Kaitangata outcrop samples.a preliminary report. Institute of Geological and Nuclear Sciences Report, April 1994, File H46/774.

Wright, N., Zahirovic, S., Müller, R.D., Seton, M., 2013. Towards community-driven paleogeographic reconstructions: integrating open-access paleogeographic and paleobiology data with plate tectonics. Biogeosciences 10, 1529–1541. http:// dx.doi.org/10.5194/bg-10-1529-2013.

 

Fossil Eucalyptus leaf from New Zealand Miocene
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Eucalyptus fossils in New Zealand – the thin end of the wedge

Eucalyptus (aka ‘gum-tree’) is the quintessential Australian tree. There are about 700 species of them today (depending on who you ask), all of them restricted to Australia, except for a couple that are in New Guinea. They range from cold mountains in Tasmania and Victoria, to tropical lowlands of the Northern Territory. They are in the wettest parts of Australia, the dry ‘outback’, but absent in the arid core. In some way or another they are probably all related to fire, and are absent from true rainforest vegetation.

While Eucalyptus species display a variety of leaf shapes, one of the most distinctive is an elongate, falcate shape. These are linked to the habit of the leaves hanging downwards, an adaptation to open vegetation, rather than closed, such as a rainforest. Eucalyptus fruits form distinctive ‘gumnuts’, that have a characteristic flat rim formed where a kind of lid detaches to let the seeds out.

Eucalyptus is one of the distinctive Australian biota that has suggested that Australia developed in isolation from the rest of the world.

New Zealand lies across the ‘ditch’ (the Tasman Sea) from Australia – relatively nearby, but its vegetation is markedly different. It does not have native Eucalyptus (although there are many imports growing about the country now) and probably as a consequence, lacks the all-important fire ecology of Australia.

However, Eucalyptus pollen was suggested to be present in some New Zealand plant fossil deposits a few decades back. Either the name appeared in lists with no data to back it up (Mildenhall 1980), or suggestions were made that some fossil pollen species might have been produced by Eucalyptus. Pollen is often not distinctive at the generic level, and I did get told once that a certain form of pollen would be unhesitatingly called Eucalyptus – if it was found in Australia. But if the same thing turned up in New Zealand, it would most likely be called Metrosideros. So without being able to nail the identification definitively, and at the risk of being jumped on by outraged Australians (national pride was at stake), claims that Eucalyptus once existed in New Zealand were kept low-key. Pioneering New Zealand palaeobotanist Aline Holden (1983) claimed Eucalyptus leaf fossils from near Roxburgh at a conference. I wasn’t there, but I understand the reception was not good, and no formal publication ensued. It didn’t help that at the time Australia seemed to be suspicious of its own record of Eucalyptus leaf fossils.

In the early 1980s I was lucky enough to find numerous fossil leaves in an Early Miocene deposit along the Kawarau River, near Cromwell. The leaves were elongate, falcate, had a clear intramarginal vein, and some were covered with the little spots of oil glands that almost invited a scratch and sniff test. The clincher was a single ‘gumnut’ consisting of three capsules with flattened rims. Together this seemed to be the critical mass to accept them as Eucalyptus (Pole 1983).

Eucalyptus leaf fossil from the Early Miocene of Kawarau River, New Zealand. The leaf is shown in a 'conventional' orientation, with the petiole end at the bottom, but in life this leaf probably hung the other way.

Eucalyptus leaf fossil from the Early Miocene of Kawarau River, New Zealand. The leaf is shown in a ‘conventional’ orientation, with the petiole end at the bottom, but in life this leaf probably hung the other way.

Detail of a well-preserved Early Miocene Eucalyptus leaf fossil from Kawarau River, New Zealand. The numerous oil glands are clearly visible.

Detail of a well-preserved Early Miocene Eucalyptus leaf fossil from Kawarau River, New Zealand. The numerous oil glands are clearly visible.

Early Miocene Eucalyptus 'gum nut' fossil from the Early Miocene of Kawarau River, New Zealand.

Early Miocene Eucalyptus ‘gum nut’ fossil from the Early Miocene of Kawarau River, New Zealand.

The presence of Eucalyptus in New Zealand virtually demands a significant  ‘fire-regime’ was operating. Indeed, palynologists had already recognised that much charcoal was in the general rocks containing the Eucalyptus leaves (Mildenhall 1989). So this much seemed to making be making sense. New Zealand’s paleobotany was suggesting that at times or places, the ecology may have been much more ‘Australian’ than some would care to admit.

How long had Eucalyptus been in New Zealand? In 1994 I published descriptions of some fossil leaves and fruits collected by Jonathan Aitchison from Livingstone, near the Danseys Pass in North Otago. The leaves include some with a remarkably elongate shape, sometimes falcate, and despite being very leached, an intramarginal vein could clearly be seen on some. Due to the poor preservation I was less confident and published them as ‘aff Eucalyptus sp.’ (Pole 1994). Associated fossil fruits were also comparable to Eucalyptus, but to other Myrtaceae genera as well. The Livingstone fossils are of early Middle Eocene age – about twice that of the Kawarau River specimens.

One of the <em>Eucalyptus</em>-like fructifications from the Eocene of Livingstone (finger gives scale).

One of the Eucalyptus-like fructifications from the Eocene of Livingstone (finger gives scale).

The real bomb-shell came in 2011 with the publication of undoubted Eucalyptus leaves and fruits from the Early Eocene of Patagonia, South America. Not only had Eucalyptus once extended far beyond the Australasian region, but the Patagonian material was the oldest Eucalyptus known. This record makes it much easier to accept the New Zealand plant fossil records, and I’m now more confident that the Livingstone fossils are Eucalyptus. Not only that, but rare leaves from the earliest Early Eocene of Kakahu, in South Canterbury, are likely Eucalyptus as well. Added to the Australian Eocene records, it seems like a very similar vegetation may have occurred around the Southern Hemisphere at the time.

The biogeographical significance of these fossils are part of growing evidence, the thin end of the wedge, that the distinctive Australian biota is due not to evolution in isolation, but because its relictual – stuff became extinct everywhere else.

References

Gandolfo, M. A., Hermsen, E. J., Zamaloa, M. C., Nixon, K. C., C.C., G., Wilf, P., Cúneo, N. R. & Johnson, K. R. 2011: Oldest Known Eucalyptus Macrofossils Are from South America. PLoS ONE 6: 1-9 e21084. doi:10.1371/journal.pone.0021084.

Holden, A. M. 1983: Eucalyptus-like leaves from Miocene rocks in Central Otago. Abstracts, Pacific Science Association 15th Congress:: 103.

Mildenhall, D. C. 1980: New Zealand Late Cretaceous and Cenozoic plant biogeography: a contribution. Palaeogeography, Palaeoclimatology, Palaeoecology 31: 197-233.

Mildenhall, D. C. 1989: Summary of the age and paleoecology of the Miocene Manuherikia Group, Central Otago, New Zealand. Journal of the Royal Society of New Zealand 19: 19-29.

Pole, M.S. (1993) Early – Middle Miocene flora of the Manuherikia Group, New Zealand. 7. Myrtaceae, inculding Eucalyptus. J. Roy. Soc. N. Z. 23, 313-328.

Pole, M. S. 1994: An Eocene macroflora from the Taratu Formation at Livingstone, North Otago, New Zealand. Australian Journal of Botany 42: 341-367.

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Four Degrees of Climate Change in New Zealand – should we care?

New Zealand might be a relatively lucky position as regards global warming. We mostly have a moderate, ‘maritime’ climate. Not too hot, too dry, and except for the dirty dribbles of ice we make so much of, no ice-sheets to melt. We’re not like Australia, where the ‘normal’ run of extreme heat, drought, and fire is likely to become catastrophic in all cases.

If you’ve been paying attention, you’ll know there has been talk around the world of trying to limit global warming to two degrees. Really paying attention means you may know that were dead on track for four degrees if not six…. But tell this to the average person, and they’re likely to wonder what the big deal is. Two, degrees, four degrees, six – not much. Hell, we have much more variation that that most days. So I thought I’d have a crack at trying to show this – and I’ve chosen Cromwell as the example. Cromwell is a town in Central Otago, New Zealand. This is the most ‘continental’ part of our basically maritime country. It’s in a low area surrounded by mountains, so its fairly dry and the winters are frosty. This is good for stone fruit, grapes, and there is a growing trend to grassing areas as grazing for dairy cows.

I checked out the National Climate Database and downloaded the minimum and maximum temperatures for each day in 2013. I averaged each one of these and made a histogram. This shows the number of days experiencing a certain average temperature for the year. You can see the most common temperature was 17C (any temperature from 17 and less than 18C). There was one day with an average of 26C, and one with -1C. The average temperature for the whole year was 12C (marked in yellow). Keep in mind this was for one year only – if I had averaged a whole run of years, the ‘bumps’ would tend to smooth out.

We can then ask ourselves what 4C of warming might mean. Four degrees of global warming may not mean 4C in New Zealand (in polar areas it is likely to be much more), but to keep things simple, let’s assume it will be. Based just on the 2013 histogram we could then expect that we might get one day each year of 27, 28, 29 and 30C – something that didn’t happen in 2013. Days with an average temperature of less than zero may no longer occur. The average will now be about 16C.

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Actually this sounds good to me! I could do without the freezing days, and hey, a day at 30C would be perfect for beer, BBQ and swimming. But as for the statistician that drowned in the lake with an average depth of a foot, its the extremes you have to watch out for. I went through the same process for the daily maxima for 2013.

PrintIn 2013 there were 2 days where the temperature hit 34C. But with a 4C increase, there are now two days where the maxima hit 38C. There are now 19 days where the maximum temperature exceeds anything experienced in 2013. The increase in really hot days is the pointy end of climate change. After a 4C rise the wine makers may have given up on pinot noir and switched to shiraz, but spare a thought for the dairy farmers. When the temperature goes above about 27C, dairy cows start to get heat-stressed, and production goes down. After a 4C rise, there might be 3-4 days per year with an average in the heat-stress range. In 2013 there were 42 days where the maxima fell into the cow-stress range. But get this: with a 4C warmer climate, there may be 110 days in the year where the level of 27C is surpassed and dairy cows are put under stress. Significantly more hot days are likely to mean an increase in drought and a heightened risk of fire.

Its the extremes that get ya….

 

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Beetroot with your Peanut Butter and Marmite? The basic geological structure of New Zealand

New Zealand must be one of the best places on the planet for geology. We are famous among tourists for having so much variety of landscape in such a small area – you don’t have to drive far to see something totally different. This goes for the geology as well.

The basic geological structure became clear in the 19th century – New Zealand was composed of a collage of sharply defined blocks, of differing lithologies and ages. In New Zealand we have a granite-dominated complex of old rocks in the west of the South Island. This was clearly some sort of ‘continental’ core. To simplify things (quite a bit) east of that there is a strip of volcanic island arc rocks (Takitimu Mountains area), and east of that again, a band of sediments deposited in shallow-water, often richly fossiliferous (the Murihiku), and east of that again, a group of mainly deep-water, mostly unfossiliferous rocks (the Torlesse). This could be (and was: Fleming 1970) interpreted as a simple eastward deepening geosyncline, alongside the volcanic arc.

Along with the rest of the world, the paradigm of plate tectonics took hold by the early 1970s and New Zealand’s construction began to be interpreted in that light (Landis and Bishop 1972). Plate tectonics allowed the idea that some bodies of rock in New Zealand had arrived from somewhere else. Coombs et al. (1976) proposed this was the case for the ‘Torlesse’. This, and the other distinct bodies of roick that made up New Zealand were termed ‘terranes’. Perhaps it is safe to say that  the full potential of this mechanism was just starting to be grasped.

One of the earliest (the earliest?) syntheses of New Zealands Mesozoic geology in terms of plate tectonics (Coombs et al. 1976). The Torlesse 'terrane' (blue) is shown as coming from somewhere to the east.

One of the earliest (the earliest?) syntheses of New Zealands Mesozoic geology in terms of plate tectonics (Coombs et al. 1976). The Torlesse ‘terrane’ (blue) is shown as coming from somewhere to the east.

Enter a very simple tool – the QFL ternary diagram. This is a way to quantify the relative proportions of the grains that make up most sandstones: Q=quartz, F=feldspar, L=lithics (you get the basic data for these by counting a thin section of rock under a microscope). By plotting up sandstone composition on one of these it was apparent that the Torlesse was rich in quartz. This showed it was not simply a deep-water equivalent of the of the volcanic-rich sediment adjacent the volcanic island arc. The sediment, in fact the entire body of Torlesse rock, seemed to have come from somewhere else (Mackinnon 1983), as had been foreseen by Coombs et al. (1976).

A QFL diagram showing the different compositions of New Zealand's Mesozoic terranes (Mackinnon 1983).

A QFL diagram showing the different compositions of New Zealand’s Mesozoic terranes (Mackinnon 1983).

I remember a seminar at the Geology Department in the University of Otago, when everything seemed to change. It would have been early 1980s – I cant recall who gave it, a visiting American guru I think. The topic was ‘terranes’. The basic point was: In the past, when explanations were sought to explain how fault-bounded, but adjacent bodies of rock originated, there was a basic assumption that their relative locations had remained the same. At least without strong evidence otherwise.

The new ‘Terrane-concept’ asked for a 180 degree change in mental framework – don’t assume the bodies of rock were always neighbours. Instead, assume they may have nothing at all to do with each other. For a place with such a rich, complex geology as New Zealand, this was powerful stuff. With a bit of leg-work many of our distinct bodies of rock could be considered as ‘suspect terrane’s. Then, with suspicions more or less confirmed, it could be regarded as a full-blown ‘tectonostratigraphic terrane’ (to give it the correct term). These are fault-bounded package of rocks of regional extent that have a distinct geological history from neighbouring terranes. With the known mechanism of plate tectonics, they could have been juxtaposed after travelling from very different locations. Of course, defining the problematic rocks as terranes didn’t solve anything. But it did give geologists the freedom to think more …. laterally. It was clear by this time that the faults the bounded terranes represented a lot of movement – hundreds of kilometres, if not thousands. Very soon, the basic outlines of New Zealand’s tectonostratigraphic terranes were mapped (Bishop et al. 1985), and except for some local, but important additions since then, the map has stood the test of time.

New Zealand's major terranes and a (too) simplistic genetic interpretation. Modified from Bishop et al. (1985).

New Zealand’s major terranes and a (too) simplistic genetic interpretation. Modified from Bishop et al. (1985).

The perfect example of a terrane to have travelled a long way is the Permian Akatarawa Formation in South Canterbury. This is a tiny exposure of limestone, volcanic and sedimentary rocks, within a broader expanse of more typical Torlesse. The limestone contains fusulinids – an extinct group that lived in warmer, more equatorial seas, while the fossils in the surrounding Torlesse indicate distinctly cooler water (Hada and Landis 1995). The limestone is thought to have accumulated in shallow water on top of an isolated volcanic sea-mount in the tropics or subtropics. It was then rafted south on its oceanic plate, until it was sheared off as it encountered a deep sea trench in cooler, higher latitude waters. As it was sliced into the Torlesse sediments – it became another, albeit small, terrane (Bishop et al. 1985). The same history may apply to one of New Zealand’s very few occurrences of Carboniferous rock – a melange with conodont-bearing marble in the Kakahu region of South Canterbury (Jenkins and Jenkins, 1971; Hitching,1979).

Schematic explanation of the origin of the Akatarewa Terrane - as a 'sea mount' that accumulated a carbonate cap in tropical waters, before being rafted into cooler waters and being subducted into the Torlesse Terrane.

Schematic explanation of the origin of the Akatarewa Terrane – as a ‘sea mount’ that accumulated a carbonate cap in tropical waters, before being rafted into cooler waters and being subducted into the Torlesse Terrane.

In the early days fossils were the main clues to  the ages of the terranes. They could show, for instance, that there was not a simple progression of age from east to west. But they were often rare, and when present at all, sometimes enigmatic. The next major breakthrough was the ability to precisely date zircons. These form inside rocks like granite, and from that point they are virtually indestructible. The rest of the rock can decay to clay or dissolve away, but the zircons remain. They can then be ‘recycled’ into a sandstone, and when that erodes, into another one. The immense power here is that if a sandstone is found to contain three ‘populations’ of zircons, say of 360, 210 and 200 million years old – this puts tight limits on whereabouts in the world these could have come from. For the Torlesse, this seems to have been somewhere off the coast of Queensland (Adams 1998, 2010; Adams et al. 1998).

The possible Late Triassic (c. 210 million years ago) locations of what are now New Zealand's main Mesozoic terranes (after Adams et al.1998).

The possible Late Triassic (c. 210 million years ago) locations of what are now New Zealand’s main Mesozoic terranes (after Adams et al.1998).

So there you have it: you could think of New Zealand as a slice of bread with a smear of butter, marmite, peanut butter, a sprinkling of peppercorns and then some cheese, and not to forget the obligatory beetroot.

References

Adams, C. J. 1998: A provenance in northeast Australia and south China for New Zealand Palaeozoic terrane sediments. Journal of African Earth Sciences 27: 218-220.

Adams, C. J. 2010: Lost Terranes of Zealandia: possible development of late Paleozoic and early Mesozoic sedimentary basins at the southwest Pacific margin of Gondwanaland, and their destination as terranes in southern South America. Andean Geology 37: 442-454.

Adams, C. J., Campbell, H. J., Graham, I. J. & Mortimer, N. 1998: Torlesse, Waipapa and Caples suspect terranes of New Zealand: Integrated studies of their geological history in relation to neighbouring terranes. Episodes 21: 235-240.

Bishop, D. G., Bradshaw, J. D. & Landis, C. A. 1985: Provisional terrane map of South Island, New Zealand. In. Howell, D. G.  (Ed.)  Tectonostratigraphic terranes. Provisional terrane map of South Island, New Zealand, Circum-Pacific Council for Energy and Mineral Resources Earth Science Series 1: 515-521.

Coombs, D. S., Landis, C. A., Norris, R. J., Stinton, J. M., Borns, D. J. & Craw, D. 1976: The Dun Mountain ophiolite belt, New Zealand, it setting, consitution and origin, with special reference to the southern portion. Americal Journal of Science 276: 561-603.

Fleming, C. A. 1970: The Mesozoic of New Zealand : chapters in the history of the circum-Pacific mobile belt (22nd William Smith lecture.).  Q. J. Geogr. Soc. Land 125: 125-170.

Hada, S. & Landis, C. A. 1995: Te Akatarawa Formation – an exotic oceanic-continental margin terrane within the Torlesse-Haast Schist transition zone. NZ Journal of Geology and Geophysics 38: 349-359.

Hitching, K. D. 1979: Torlesse geology of Kakahu, South Canterbury. New Zealand Journal of Geology and Geophysics 22: 191-197, DOI: 10.1080/00288306.1979.10424218.

Jenkins, D. G. & Jenkins, T. B. H. 1971: First diagnostic Carboniferous fossils from New Zealand. Nature 233: 117-118.

Landis, C. A. & Bishop, D. G. 1972: Plate Tectonics and Regional Stratigraphic-Metamorphic Relations in the Southern Part of the New Zealand Geosyncline. Geological Society of America Bulletin 83: 2267-2284.

Mackinnon, T. C. 1983: Origin of the Torlesse Terrane and coeval rocks, South Island, New Zealand. Geological Society of America Bulletin 94: 967-985.

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A Glimpse in to the Rise of the Mongolian Gobi Altai

The Gobi is the great desert that extends along southern Mongolia and northern China. Its northern extent is marked by a series of mountain ranges called the Gobi Altai (Altai means ‘golden’). The mountains appear desert-like, with usually nothing higher than small shrubs growing on them, if anything at all, but isolated patches of forest show was was, and what is still possible. The mountains appear as long northwest-southeast chains, separated by much drier basins. The landscape reminds me a lot of my Central Otago home in New Zealand, but with the Gobi Altai being on a much larger scale.

The mountains are made from old, hard rock. The basins are full of sands and gravels that have eroded off the mountains. Dry though it may be, when it does rain in the Altai the rain can be incredibly intense. Streams that are mostly dry become raging torrents of mud and rocks. Camps get washed away, people die. These are classic conditions for the formation of alluvial fans – an integral part of Mongolian geology. Most of the basins are lined with giant fans. They start at a prominent valley – a notch in the mountain chain, and expand out for kilometers. From the outer base of the fan to the head can be hundreds of meters. From a distance they can look so smooth, but up close the fan surface is covered by steep-sided gullies. These make driving across them a sheer Hell and basically a stupid idea. Most tracks in the Altai are around the bottom edge of the fan, avoiding the fine sediment that blankets much of the more central parts of the basins, and makes bogging a serious problem. If travelling round the base of the fan is not an option, the next best bet is to try and get above it.

Deep gullies running down the fans can have nice exposures of the strata that make up fans. These are typically alternations of gravels and finer beds. The gravels were mostly emplaced by mass-flow processes following intense rainfall events. This was far-beyond silt in suspension or cobbles bouncing along the bottom of streams. Instead, water and rocks mixed to form a kind of wet-concrete like slurry which rushed out of the mountains and spread out over the fan, the rocks being dropped as the water finally filtered away. Finer-grained beds of mud are when floods funnelled down gullies and then overflowed, spreading only the smallest particles. In the dry periods dust-storms redeposited this material as a mantle of silt.

The beds in the fan-gullies parallel the surface of the fan – angling up to where the fan starts, as you would expect. However, along the southern edge of the Khantaishir Range – a sort of satellite of the Altai, the basin is rimmed by what appear to be a dramatic series of multi-coloured ‘teeth’. As geologists, these drew myself and colleagues like a moth to a flame. It turns out that the ‘teeth’ are the result of beds of gravel that have been rotated, so they almost stand on their ends, and then eroded. The basic structure of the gravels looks just like that in the fans, and they probably formed in much the same way – as fans on the edge of the Altai. A closer look at them showed they told a story – of the birth of the Altai themselves.

A row of pale 'teeth' along the southern margin of the Khantaishir Mountains, Mongolia. These are up-turned strata that reveal a history of the rise of the Gobi Altai.

A row of pale ‘teeth’ along the southern margin of the Khantaishir Mountains, Mongolia. These are up-turned strata that reveal a history of the rise of the Gobi Altai.

First question, when faced with beds that are nearly vertical – is working out which way is ‘up’. In this case it was pretty simple to look at the pattern of cross-cutting. Then it became apparent that there was a distinct change in lithology from bottom to top. The oldest gravels were dominated by quartz pebbles – and had good, fluvial cross-beds. However,  the youngest beds were dominated by greenschist and cross-beds were absent. The earliest strata seem to record the erosion of a quartz-gravel rich landscape (I’ve seen these in a few parts of the Altai) by genuine river flow. This landscape may date back to the flat ‘peneplain’ that covered the area in the Late Cretaceous-early Cenozoic. The flat-tops of many of the Altai mountains preserve this peneplain. This erosion,probably recording very early uplift of the Altai. Then as the mountains really started to grow, deeper schist became exposed  and eroded, and mass-flow took over. Then the mountain uplift seems to have overwhelmed even this area – and it was tilted. Following this, the fans we see today established themselves.

A quartz-gravel unit within reddish mud, near the base of the sequence. It has been rotated almost ninety degrees. Mongolia.

A quartz-gravel unit within reddish mud, near the base of the sequence. It has been rotated almost ninety degrees.

A tabular cross-bed of very poorly sorted quartz-rich sand and gravel, from near the base of the succession. Mongolia.

A tabular cross-bed of very poorly sorted quartz-rich sand and gravel, from near the base of the succession.

 

Greenschist-rich gravel at the top of the succession. Mongolia.

Greenschist-rich gravel at the top of the succession.

Detail of the greenschist gravel beds, alternating with mud. Mongolia.

Detail of the greenschist gravel beds, alternating with mud.

 

It is a nice little window into Mongolian geology. The Gobi Altai are thought to be related to the collision of India with Eurasia around the Eocene, although they may date back to this time, or perhaps be a late Cenozoic phenomenon (Cunningham et al. 1996; Owen et al. 1999). Unfortunately there seemed to be no fossils to give any clues to the age of the ‘teeth’. Maybe time for a closer look?

References

Cunningham, W. D., Windley, B. F., Dorjnamjaa, D., Badarngarov, J. & Saandar, M. 1996: Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan connection. Earth and Planetary Science Letters 140: 67-81.

Owen, L. A., Cunningham, D., Windley, B. F., Richards, B. W. M., Rhodes, E., Dorjnamjaa, D. & Badamgarav, J. 1999: Timing of formation of forebergs in the northeastern Gobi Altai, Mongolia: implications for estimating mountain uplift rates and earthquake recurrence intervals. Journal of the Geological Society: 457-464.

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Mixed-up with Mongolian Migmatite

When I was a novice geology student, the world appeared kind of simple. Faced with a rock, I could ask myself the most fundamental question – is this rock sedimentary, metamorphic or igneous? And there was some sort of expectation that the question could be answered clearly. It seemed the same in First-year Botany. We were taught about xylem, tracheids and parenchyma cells and shown specimens where they could be easily distinguished under a microscope. It seemed kind of simple. Then came the day when our teacher, Dr Brenda Shore (a very capable scientist, who I much admired), suddenly thundered” “You Lot! You think you know it all – I’ll show you something and you won’t have a clue what it is!!” Or something to that effect. I’m not sure what prompted it, I think the background chat in the class had gone above the acceptable level. But the comment scored a point, because it stuck in my mind ever since. The world does not break down into the categories we want it to.

Back to geology – it’s when you find yourself alone in some place, without a detailed map (you’re the guy making that), without a library of academic papers to get clues from, and you find yourself faced with some rock that makes you think – What the Hell is this? Where’s my Prof to tell me? That is really comes home. The universe doesn’t follow our rules. And just to be perverse, said Universe might even make the rock is deeply weathered, and place you under a tropical rainforest canopy in a storm where its so dark you can barely see anyway. Fortunately Mongolia is not a place where a geologist has to deal with those extra problems. In Mongolia, many rocks seem to look as fresh as the day they were minted, and in the land of the Eternal Blue Sky, there’s generally no problem seeing a rock clearly (absent snow). So it’s perturbing to still come across stuff that leaves you puzzled.

One kind of rock that Mongolia has good examples of is ‘migmatite’. This is a rock that’s part-way between a metamorphic rock (recrystallised as a result of pressure and heat) and igneous (formed from molten rock). Note that it’s not a term for when you are not quite sure whether its a metamorphic or igneous rock and just want some word to cover your ignorance and sound smart. A migmatite is a metamorphic rock that has actually started to melt. So it’s right between a metamorphic and an igneous rock. And there’s the rub – as a geologist facing an outcrop in the field – are you sure that quartz has formed from a melt, or from solution, or as some sort of recrystalisation? You can find your self-confidence waning rapidly. I have seen migmatites where my colleagues were sure of what it was, but I’ve also come across rocks where I’m still stumped (where’s a Prof when you need one?).

A typically highly-folded migmatite in the Gobi of Mongolia. There has been a segregation into light-colored fraction (leucosome) and dark-colored (melanosome).

A typically highly-folded migmatite in the Gobi of Mongolia. There has been a segregation into light-colored fraction (leucosome) and dark-colored (melanosome).

It’s some consolation to learn that the whole field of migmatites is pretty esoteric stuff, and perhaps way beyond a non-specialist in the field with no access to microscopes, thin-sections and so-on. Migmatites are tied up in the debate over the origin of granites – a huge, complex issue by itself. So nice to come across a whole book devoted to beautiful photographs of migmatites – an ‘Atlas of Migmatites‘ (Sawyer 2008). I find there’s nothing like a nice, visual guide. But as well as that, Sawyer’s Atlas concisely explains what to look for, the different types of migmatite, and what they mean. Migmatites are not just ‘mixed-up’ rocks. For the geology of Mongolia, migmatites are a critical part of understanding how Mongolia was put together. They are clues to where and when ancient terranes collided to form the geologically complex land that is Mongolia. The Atlas of Migmatites is a large, pricey tome, but boy, I wish I had this sitting on the bookshelf in the ger…..

Reference

Sawyer , E.W. 2008. Atlas of Migmatites: v. 9: The Canadian Mineralogist Special Publication.