Monthly archives of “July 2014

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The Marshall Paraconformity – a 30 year geological debate in New Zealand

Cut back to the late 1970s – I was a schoolboy and had found my way into the Burnside Marl Pit, Dunedin (southern New Zealand) and up to the unit of greensand that is exposed on the hill side at the far end. Greensand is an accumulation of grains of the mineral glauconite. This green substance forms on the sea floor, only in areas that have very low rates of sedimentation, even negative ones. Often the only sedimentation in areas where glauconite is forming  – is the rain of dead organisms that falls down from above. This makes it an ideal place to find fossil sharks teeth for instance.

The layer of greensand at Burnside is called the Concord Greensand, and it can be correlated with similar layers of greensand right up the east side of the South Island. It often separates two bodies of limestone.  The east coast preserves  a remarkable series of rocks that record the sea gradually encroaching across the land from more than 65 million years ago (from the east), to a point in the mid Cenozoic where a lot of New Zealand was under water, and what remained was evidently of very low topography. Following this, as hills and mountains started to grow, the process was reversed, and the shoreline expanded back to the east.

I certainly always understood that the unit of greensand correlated with maximal submergence of New Zealand. Maximal submergence is the point of highest relative sea level rise. With little land to erode, and what there was very low-lying because of years of erosion, that was the time of least clastic sedimentation. It seemed to make sense, but as it turns out, things are a lot more complicated. Maximal submergence may have been sometime later, the greensand overlies a break in sedimentation, and the formation of the greensand may have been either coincident or close-to, a large global sea-level drop.  Something of a paradox then.

Attention was focussed on the greensand by Carter and Landis (1972), two geologists in the Geology Department of the Otago University. The broader academic context was a world in which plate tectonics had been accepted for many years and there was now an extensive deep-sea drilling programme that was revolutionising the understanding of ocean sedimentation. A critical finding was the realisation that there were extensive areas of erosion extending across deep and shallow water as a result of strong ocean currents (Watkins and Kennett 1971, 1972). Oil company geologists had also proposed that there was a coherent global history of sea-level rises and falls driving cycles of sedimentation (Vail et al. 1977; Loutit and Kennett 1981).

Carter and Landis (1972) claimed that the base of the greensand was essentially a single unconformity (although it possibly bifurcated in places) that could be correlated, not just up the coast of the South Island, but to Australia and perhaps beyond. They named this the Marshall Paraconformity after Patrick Marshall, who was professor of Geology at Otago in the early 20th century. They interpreted the Paraconformity as part of the bigger picture – the development of the modern Circum-Antarctica Current (West Wind Drift) that meant strong ocean currents scoured their way across the continental, but submarine parts of New Zealand. A “more detailed account”, Carter and Landis promised, was “in preparation”. On the strength of that, they scored a Nature paper of just over a page.

Exposed surface of highly burrowed limestone (Ototara and a little Otekaiki) on the beach north of Kakanui. This is essentially the Marshall Paraconformity with the greensand stripped off.

Exposed surface of highly burrowed limestone (Ototara and a little Otekaiki) on the beach north of Kakanui. This is essentially the Marshall Paraconformity with the greensand stripped off.

The contact between the Ototara Limestone and Gee Greensand (with a little Otekaiki Limestone in between) visible on the beach north of Kakanui. The contact is the Marshall Paraconformity.

The contact between the Ototara Limestone and Gee Greensand (with a little Otekaiki Limestone in between) visible on the beach north of Kakanui. The contact is the Marshall Paraconformity.

 

 

Unconformities became a topic of discussion and a focus of local research to date (e.g. Jenkins 1975). However, the first real data on the Marshall Paraconformity did not come from Carter and Landis – but by Findlay, in 1980, who was less than impressed with Carter and Landis’s concept (Curiously, his paper was nearly eight pages long and oddly classified as only a “Note” in the New Zealand Journal of Geology and Geophysics). Findlay schematised 46 measured geological columns and maintained that the Marshall Paraconformity was “but one of many extensive unconformities” in the region. The key problem was lack of precise dating of the paraconformity and he concluded that it may be “of no more significance to global tectonic, paleogeographic, or paleoceanographic reconstruction than anyone of the many other Tertiary unconformities within New Zealand.”

Carter and Landis’s next paper came in 1982, where, to help clarify some confusion, they nominated a type section in Canterbury for the Marshall Paraconformity. They argued that it dated to the early-mid Oligocene, but that it “may have overlapped in time with the development of at least twodiscrete unconformities on the West Coast”, the upper one being mid-late Oligocene.

Battle-lines consolidated along the Waitaki River. The issue of two unconformites would not be brushed aside, and was central to the attack by Lewis & Belliss (1984). They clearly laid out the case for two unconformites – perhaps both being angular and the result of tectonism, and maintained that regional sea level change may not have been significant at all. To twist the knife deeper, they argued that even the term “Paraconformity” was inappropriate, as it was introduced to refer to a biostratigraphic discontinuity, where there was an “inconspicuous” sedimentological break involving parallel beds. The Marshall “Paraconformity” (in fact, either uncoformity), was none of these. In particular, Carters referencing of his own attempt to place local stratigraphic terms into a broader system (Carter 1977) definitely got up their noses. Much as I like the system, this does seem to have been a bit of a swifty. It was presented in a field trip guide and not really published for another decade (Carter 1988). In addition, the Lewis & Belliss paper includes some the earliest (the earliest?) documentation of Oligocene paleokarst processes in New Zealand. So even around the time of maximum sea transgression, the limestone had been exposed for a period.

Carter’s counter-attack came in 1985. He reiterated that the Paraconformity marked a significant break in New Zealand stratigraphy between the transgressive, shelf-on-lap sediments below, and bioclastic (including greensand) above and represented a break in sedimentation of at least 3 million years. Significantly, its origin was now seen to result from the regional sea level highstand (despite the apparent drop in global sea level at the time) than oceanic currents resulting from continental movements.

A year later the debate then flared up in the pages of the Newsletter of the Geological Society of New Zealand where Lewis et al (1986) wrote a spirited response to Carter.  They wrote that the Marshall Paraconformity was the subject of “considerable dispute amongst New Zealand geologists.”  Among their complaints was the rather parochial view that this should have been argued out in New Zealand journals before being published in international ones and that “overseas readers” might be “misled” by what Carter had written. Carter was given a response, and he addressed several points, but said he was mystified “as to what all the fuss was about”.

Unconformities became a hot topic in New Zealand (Hornibrook 1987; Jenkins 1987; Lewis 1987) with Gage (1988) entering the fray and claiming that because of the “extremely condensed sequences” correlation between various unconformities was currently not possible – and that the term “Marshall Paraconformity” should be dropped. Putting the boot in further, he claimed even the “attachment” of Patrick Marshall’s name to the concept was “inapt”. He didn’t restrict his criticism to Team Carter and Landis– but also voiced his reservations about the Lewis and Belliss (1984)  interpretation of paleokarst.

A further study on the Marshall Paraconformity  (Fulthorpe et al. 1996) concluded that it correlated with “hiatuses in at least two, and possibly three, offshore exploration wells “ and its date meant that it correlated with the opening of the Pacific sector of the Southern Ocean and a postulated mid-Oligocene sea-level fall  (Haq et al. 1987). They further concluded: “Lowering of base level, coupled with cooling and enhancement of current activity, may have caused the temporary cessation of limestone deposition and a regional hiatus. This hypothesis reconciles the apparently contradictory palaeogeographical evidence for a regional highstand.”  Carter had another chance to promote the Marshall Paraconformity when he was lead author for the results of Leg 181 of the deep sea drilling project (Carter et al. 2004) and then matters then rested until another attempt to clarify the issue came from north of the Waitaki (Lever 2007). She concluded that “the available data show a distinct range of unconformity ages in different basins. In general, Oligocene deposits are condensed, but there appears to be no one time in the Oligocene when unconformities developed everywhere.” And then:

“Unconformities in the Oligocene could be caused by global sea-level falls, relative sea-level high-stand, local faulting and volcanic activity, and oceanic current activity. These causes could have formed both local and regionally extensive unconformities. In successions with only one unconformity surface, that surface may represent the occurrence of multiple unconformity-causing processes, further complicating the unravelling of Oligocene geologic history in New Zealand”

Her final words on the subject, were: “In the meantime, it would be wiser to dispense with the confusing term Marshall Paraconformity in the South Island, as this concept represents an oversimplification of a complex story of multiple unconformity development (Jenkins 1975; Findlay 1980; Gage 1988).”

The following year there was another relevant paper – discussing the Waipounamu Erosion Surface (Landis et al. 2008). Discussion of the Marshall Paraconformity was quietly avoided, but the authors concluded maximum transgression in the New Zealand region was in the Waitakian – perhaps 4-6 Ma later than Carter’s (1985) Whaingaroan-Duntronian boundary date for the Marshall Paraconformity.

And that, as far as I know, is where a more than 30 year ‘controversy’ stands. I don’t have an opinion, I just think it’s a good example of where an argument stimulates a line of work, and eventually the ‘truth’ comes out. Whether or not that is Lever’s conclusion, may be too early to tell.

References

Carter, R. M. & Landis, C. A. 1972: Correlative Oligocene unconformities in southern Australasia. Nature (Physical Science) 237: 12-13.

Carter, R.M : 1977: Tour Guide for Queenstown to Dunedin, in: Norris, R.J. et al. (eds), Field Trip Guides for the 22nd Annual Meeting, Geological Society of New Zealand: Q-06.

Carter, R. M. & Landis, C. A. 1982: Appendix: Oligocene unconformities in the South Island. Journal of the Royal Society of New Zealand 12: 42-46.

Carter, R. M. 1988: Post-breakup stratigraphy of the Kaikoura Synthem (Cretaceous-Cenozoic), continental margin, south-eastern New Zealand. New Zealand Journal of Geology and Geophysics 31: 405-429.

Carter, R. M., McCave, I. N. & Carter, L. 2004: Leg 181 synthesis: fronts, flows, drifts, volcanoes, and the evolution of the southwestern gateway to the Pacific Ocean, eastern New Zealand. In. Richter, C.  (Ed.)  Proc. ODP, Sci. Results, 181, 1–111 [Online].

Available from World Wide Web: <http://www-odp.tamu.edu/publications/181_SR/VOLUME/SYNTH/SYNTH.PDF>. Leg 181 synthesis: fronts, flows, drifts, volcanoes, and the evolution of the southwestern gateway to the Pacific Ocean, eastern New Zealand.

Findlay, R. H. 1980: The Marshall Paraconformity (Note). New Zealand Journal of Geology & Geophysics 23: 125-133.

Gage, M. 1988: Mid-Tertiary unconformities in north Otago — A review and assessment. Journal of the Royal Society of New Zealand 18: 119-125.

Haq, B.U., Hardenbol, J. and Vail, P. R. (1987) Chronology of fluctuating sea levels since the Triassic Science 235, 1156-1167.

Hornibrook, N. de B. 1987: Mid-Tertiary unconformities in the Waitaki Subdivision, North Otago-a comment. Journal of the Royal Society of NZ 17: 181-184.

Jenkins, D. G. 1975: Age and correlation of some unconformities in the New Zealand region. Geological Society of New Zealand Newsletter 39: 45-47.

Jenkins, D. G. 1987: Oligo-Miocene unconformities in North Otago and the Tasman Sea. Journal of the Royal Society of New Zealand 17: 177-186.

Landis, C. A., Campbell, H. J., Begg, J. G., Mildenhall, D. C., Paterson, A. M. & Trewick, S. A. 2008: The Waipounamu Erosion Surface: questioning the antiquity of the New Zealand land surface and terrestrial fauna and flora. Geological Magazine 145: 173–197.

Lewis, D. W. 1987: Mid-Tertiary unconformities in the Waitaki Subdivision-a reply. Journal of the Royal Society of New Zealand 17: 184-186.

Lewis, D. W. & Belliss, S. E. 1984: Mid-Tertiary unconformities in the Waitaki Subdivision, North Otago. Journal Royal Society of New Zealand 14: 251-276.

Lever, H. 2007: Review of unconformities in the late Eocene to early Miocene successions of the South Island, New Zealand: ages, correlations, and causes. New Zealand Journal of Geology & Geophysics 50: 245-261.

Loutit, T. S. & Kennett, J. P. 1981: New Zealand and Australian Cenozoic sedimentary cycles and global sea-level changes:. Am. Assn. Petrol. Geol. Bull. 65: 1586-1601.

Vail, P. R., Mitchum, R. M., Jr. & Thompson, S., III 1977: Seismic stratigraphy an global changes of sea level.4. Global cycles of relative changes of sea level. American Association of Petroleum Geologists Memoir 26: 83-97.

Watkins, N. D. & Kennett, J. P. 1971: A major sedimentary disconformity as evidence of an upper Cenozoic change in bottom water velocity between Australia, New Zealand and Antarctica. Geological Society of America, Abstr.: 746.

Watkins, N. D. & Kennett, J. P. 1972: Regional sedimentary disconformities and upper Cenozoic changes in bottom water velocities between Australasia and Antarctica. Antarctic Res. Ser., Washington (Am. Geophys. Union) 19: 273-293.

 

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Flooded Forests: sea-level rise in the Haast, West Coast, New Zealand

I own a little patch of bush in the Haast, on the wet, West Coast of New Zealand. It was selectively logged a few decades ago, which meant that the large podocarp trees were removed. It then became the edge of what was left of the forest, and farmland.

My place, Westland, New Zealand. A very-poorly draining spot resulting from logging. Flax plantings are taking hold around the edges.

My place. A very-poorly draining spot resulting from logging. Flax plantings are taking hold around the edges.

My place, Westland, New Zealand. An old logging track is now overgrown with grass. Large kamahi (Weinmannia) trees, left by the loggers, poke up above regowth.

My place. An old logging track is now overgrown with grass. Large kamahi (Weinmannia) trees, left by the loggers, poke up above regowth.

I bought it with the intention of restoring the vegetation, and then perhaps sneaking a small hut in to the most unobtrusive spot. Logging activities severely messed up the drainage of what would have been a pretty swampy forest anyway. After heavy rains (frequent in this part of the world!) a large patch of the area becomes standing water, sometimes for weeks at a time. These areas simply remain in grass and sedge – it’s essentially waterlogged, overgrown pasture. One of the big jobs has been to dig small drainage ditches, just deep enough to get rid of that standing water. Not an easy job as there is little drop-off in this particular spot, and the best route ended up being through standing bush. After many visits I’m finally at the point where flax, and just perhaps, some kowhais will survive. At this stage, these are the only two native species that don’t occur naturally on the property, that I will ‘introduce’. I hope to recreate the kind of flax-kowhai wetland found further south at Hapuka.

Logging opened up the ground for invasion of other weeds, mainly gorse. There were several patches of this, and although they weren’t spreading, they needed to go. A hand-saw and paint-on weedkiller did the trick – until, like the proverbial gerbil-thumping, they resprouted from all around.

Then there is the biggest curse in this country – blackberry. I have yet to find their roots and nuke them. Logging also favoured our native Rubus, and the very numerous tree ferns which now form a canopy in many parts. Both of these were there before logging, but their increased numbers are shielding the forest floor from light, and in the case of the tree ferns – dropping a prodigious amount of litter on the ground, smothering seedlings. So much of my effort has been to keep trimming these back to try and stimulate the return of broad-leaved and conifer seedlings, of which there are a lot.

With a lot of work, in 400-500 years – the time it will take the conifer seedlings that I plant to mature, this little patch of land will once again start to look like intact forest. It’s an investment for future generations.

But here’s the rub. My land is quite some distance from the coast – more than 6km. It is (and it differs according to what source I rely on), about 20 m above sea level. Given the talk of global sea level rise, one would think this was a good, long-term bet for stability. Or is it? How might it feel from the perspective off trees that live for 500, even 700 years?

The current rate of sea level rise is about 4 mm per year, or 2 m in 500 years. But one certainty is that that rate will rise. The predictions of the IPPC are that, as a result of increased atmospheric CO2 levels, sea level will rise by 56-200 mm by the end of the century. So by 2100, the scariest scenario would be a 2m rise in mean sea level. Where will that put the limits of the effect of salt water? That’s harder to say. On top of mean sea level there is a tidal range of about 2m on to that, and above that, storm surge. On New Zealand’s West Coast this will mean coastal erosion and a few white-baiter’s huts gone. There are some coastal swamps that will change entirely as they become tidal and saline – the fantastic Hapuka board walk just south of Haast is history. Likewise, the board walk through kahikatea swamp forest at Ship Creek. Also history. See them while you can. But for my little patch of paradise, perhaps things are not too bad.

The Hapuka boardwalk, Westland, New Zealand, at high tide. A small rise in sea-level will overwhelm the marsh and destroy the flax and trees at a slightly higher level.

The Hapuka boardwalk at high tide. A small rise in sea-level will overwhelm the marsh and destroy the flax and trees at a slightly higher level.

The Hapuka boardwalk. A couple of meters sea level rise will flood the boardwalk, and all the swampy vegetation back to the edge of the tall forest in the background.

The Hapuka boardwalk. A couple of meters sea level rise will flood the boardwalk, and all the swampy vegetation back to the edge of the tall forest in the background.

The Ship Ck boardwalk, Westland, New Zealand (part of the boardwalk is visible at far-right).

The Ship Ck boardwalk (part of the boardwalk is visible at far-right).

Kahikatea (Dacrycarpus) swamp forest on the Ship Ck boardwalk. A couple of meters sea level rise will probably be enough to kill these trees.

Kahikatea (Dacrycarpus) swamp forest on the Ship Ck boardwalk. A couple of meters sea level rise will probably be enough to kill these trees.

Of course, sea level rise doesn’t stop in a century. The raised level of atmospheric CO2 is a gift that will keep giving. Because it takes so long for the natural ‘sinks’ to take CO2 out of the atmosphere,  the high levels of CO2 will remain and continue to make change for centuries to come. It’s this long-term perspective that means melting polar ice caps can be a very real possibility. The IPCC has made some predictions through to 2300, well-within the life-span of the trees I am planting. The upper range of their predictions are for 4 m “or more” of sea level rise by 2300. So that’s high-seas washing maybe 8 m above today’s mean sea level. My trees are off the hook so-far.

But let’s take this even further – and consider entirely melting the Greenland Ice Sheet. This has become a very real, and to some, inevitable possibility. That will raise global sea level about seven metres. Factoring in tidal range and storm surge, this is the Haast Pub becoming coastal. This is waves sloshing across the Haast bridge. It’s a lot of coastal wetlands becoming lagoons. This scenario is similar to what did happen in the Last Interglacial period (c. 130-115,000 years ago) when sea level rose more than 6.6 m and likely more than 8 m above the present (Kopp et al. 2009). Incidentally, carbon dioxide levels in the Last Interglacial were much less than what they are now,  and consequently the Greenland Ice Sheet only partially melted, and probably contributed less than half of the total global sea level rise  (Stone et al. 2013). There is of course, other ice melting at the same time, as well as thermal expansion of the oceans. An 8 m rise in sea level is about what 4 m of rise by 2300 look like by 2600 – if the rate stays the same.

A line representing the extreme values for two future sea level predictions - 2100 and 2300. I've continued the line on to 2600, at which point I hope many of the trees I have planted will still be alive. If the extreme prediction comes to pass, and the rate of sea level rise doesnt change, sea levels may be 8 m higher then - essentially like the Last Interglacial.

A line representing the extreme values for two future sea level predictions – 2100 and 2300. I’ve continued the line on to 2600, at which point I hope many of the trees I have planted will still be alive. If the extreme prediction comes to pass, and the rate of sea level rise doesnt change, sea levels may be 8 m higher then – essentially like the Last Interglacial.

It is plausible that we will put enough CO2 into the atmosphere to eventually melt all the ice on Earth. Neither melting the Greenland Ice Sheet or Antarctica, I emphasise, is a result of complicated and possibly dodgy climate models. It’s just based on past history. When global CO2 levels hit a certain threshold, polar ice caps melt. Once Antarctica melts,  sea level will be about 70m than it is now. All of lowland West Coast will be inundated. The Haast Valley will become an arm of the sea – a fiord. That will put my wee patch of paradise in open ocean. Tidal mudflats will develop 3-4 km downstream of the junction of the Landsborough River.

How long this will take is quite a different question – that does require modelling, and the answer will be necessarily vague. The figures I’ve read dont seem to get more precise than “thousands of years”. It seems like in all but a nightmare scenario my trees may get to grow old, and perhaps see their offspring grow up. But it might be in a rather different neighbourhood – perhaps with the coast line looking much more like the Last Interglacial.

Map of the Haast region with an 8 m higher coastline (pink line). The Haast Pub and bridge are underwater at high tide. Many of the inland swamps may be salty.

The Haast region with an 8 m higher coastline (pink line). The Haast Pub and bridge are underwater at high tide. Many of the inland swamps may be salty.

References

Kopp, R. E., Simons, F. J., Mitrovica, J. X., Maloof, A. C. & Oppenheimer, M. 2009: Probabilistic assessment of sea level during the last interglacial stage. Nature 462: 863–868.

Stone, E. J., Lunt, D. J., Annan, J. D. & Hargreaves, J. C. 2013: Quantification of the Greenland ice sheet contribution to Last Interglacial sea level rise. Climate of the Past 9: 621–639.