New Scottish Oil field discovered (470 million years too late)

Posted on 7 January, 2014 by Metageologist

Scottish oil is topical. Most of Britain’s oil and gas deposits sit under the seabed around Scotland but the revenues are shared with the whole of the United Kingdom. If Scotland decides to become an independent state (there’s a vote in 2014) then that wealth will be all theirs. So I was very interested to read about a new Scottish oil field that has been discovered. There’s only one reason you’ve not read about this in the papers: all the oil was boiled off 470 million years ago.

Easdale Slate sample. From British Geological Survey, sample P519560

Oil deposits form from dead critters – buried organic matter. Bury this carbonaceous material deep enough (often in black mud) and it heats up and enters the ‘oil window’. These ‘source rocks’ then produce oil which seeps away. In ideal conditions it enters a rock rich in holes (the reservoir) and is prevented from rising further by impermeable rock layers above (the seal).

Most oil deposits are found in more recent rocks, from the last 541 million years where traces of life are everywhere (the Phanerozoic). We know from rare fossils and geochemical evidence that life was abundant before this time, it was just mostly microscopic bacteria or algae. The ‘Cambrian explosion’ is rightly celebrated for the creation of new lifeforms, but its impact is partly due to innovations like hard shells and burrowing in sediment that made ancient life much more visible. It doesn’t necessarily represent a step-change in the raw *volume* of life. Before the Cambrian, there was lots of carbon being ‘fixed’ and sinking into sediment – many oil deposits are found from the next oldest period, the NeoProterozoic (1,000 to 542 million years ago).

Scotland contains sediments of this age: the Dalradian Supergroup. Some clever chaps from the University of Aberdeen thought to look in them for evidence of oil. In their recent paper Timothy Bata and John Parnell focus on rocks from the Argyll Group – the Easdale Slate and the Scarba Conglomerate.

Figure 1 from Bata & Parnell 2013

The Easdale slate is a dark rock that even today contains up to 6.3% organic carbon by weight -it was a good candidate for a source rock. The Easdale sediments formed in deep water and the Scarba Conglomerate was the equivalent shallow water deposit. As a coarse pebbly sandstone it would have contained many small holes, up to 11% by volume, and so was a good candidate for a reservoir rock. Today it is strikingly dark in colour because it contains abundant solid hydrocarbon residue – it is a fossil oil reservoir. The residue is found within pore spaces and is associated with pyrite crystals which they interpret as forming from Precambrian bacteria attacking/eating the oil.

These rocks are found across Scotland and Ireland – our authors estimate they could have contained over 6 billion barrels of oil. This find isn’t going to affect the vote for Scottish independence in September though. The Iapetus ocean these sediments were deposited on the edge of is long gone and so is the oil. It wasn’t extracted by cunning trilobites but was destroyed along with the ocean. Around 470 million years ago the sediments were buried and heated to high temperatures – the Easdale source rocks were converted from muds into slates useful in roofing. Only useless degraded hydrocarbons remain, the rest would have been returned to the surface as gas.

Rocks equivalent to the Dalradian might be expected to have similar deposits and these are found from Greenland to North America. Other Precambrian fossil oil reservoirs are there to be found – if you live on lightly metamorphosed Neoproterozoic sediments in eastern North America or in Norway, you might be sitting on the ghost of an oil-field.

Bata T. & Parnell J. (2014). A Neoproterozoic petroleum system in the Dalradian Supergroup, Scottish Caledonides, Journal of the Geological Society, DOI: 10.1144/jgs2013-087

Dalradian – a Celtic Supergroup

Posted on 4 February, 2013 by Metageologist

Geology is such a great thing to study because it involves making so many connections through time and space, switching scales from the cosmic to the atomic. This means that challenge for this series of posts about the geology of the west of Ireland is going to be managing scope. So. Although I could start with the big bang, I won’t. I’ll start with the Dalradian.

A thick package of sediment, the Dalradian Supergroup, or just Dalradian, to its friends was was named in the 19th century after Dál Riata a minor kingdom in 6th and 7th century Scotland. Dál Riata was an Irish colony within Scotland, appropriately, as Dalradian rocks are found over much of Highland Scotland and NW Ireland.

A Celtic Supergroup

To fans of 1970s Prog Rock, a supergroup is a band formed from musicians who made their names in other bands. For geologists, it means a bunch of groups that are bound together, a group being a large recognisable package of sediment. The groups usually sit on top of each and represent a period of time during which sediment was deposited. For both types of supergroup, the individual members often have distinctive personalities.

The Dalradian Supergroup is made up of the Grampian, Appin, Argyll and Southern Highland groups.

The Grampian is the solid foundation of the Dalradian, fairly calm and ordinary: the bass guitar player of the supergroup. It is a 7-8 kilometre thickness of sandstones and muddy sandstones*. In Ireland, Grampian Group sediments are found only in North Mayo.

The Appin is the lead singer: shallow, good looking and gets lots of attention. Its made up of sandstones, limestones and more muddy sediments all deposited on a marine shelf in shallow water. Now somewhat changed, it contains some gorgeous rocks.

Glencoe Quartzite, Appin Group

Famous ‘Connemara marble’ which is from, er, Connemara in Ireland. Picture credit.

The Appin group is found in the heart of Connemara, also across Mayo and Donegal.

The Argyll is the lead guitar. It’s deeper than the Appin and has been through some pretty intense times – brushes with death in fact. It contains the familiar sandstones and muddier sediments, but also more exotic sediment types.

Twice in the Irish Dalradian there occurs an odd pattern of rock-types. First there is a layer of sediment filled with angular fragments with a wide range of sizes, called a diamictite. A number of subtle features show that it is sediment left behind by a glacier – it is a tillite. Immediately above is a layer of carbonate, a limestone or a dolomite. Such rocks usually form in nice warm parts of the earth so the association with glacial deposits is odd. The first of these tillilte-carbonate pairs can be traced across the Dalradian, indeed across the world. These are ‘snowball earth’ deposits. Some believe that at this time the entire earth was covered in ice, with glacial deposits forming near the equator. The overlying ‘cap carbonates’, contain unusual carbon isotope compositions suggesting to some that life on earth nearly died at this time. This happened not just once, but twice in the Argyll.**

Picture of Argyll group sediments making up the Twelve bens mountains of Connemara. Image from Guilhem Boyer on Flickr under Creative Commons.

The Argyll is rounded off with molten lava spilling into the sedimentary basin. Dangerous to be around for sure, but the Argyll makes the supergroup as a whole much more interesting and ‘edgy’.

The Southern Highland group is the drummer: completely crazy. The ground’s falling away under your feet, with sediments pouring into deep water and there’s another snowball earth episode***, there are big lumps of serpentinite, plus molten lava keeps coming out of the ground: madness. There’s a problem with ‘groupies’ as well. Lots of areas of sediment that say they’re ‘with the band’ but no-one is quite sure. Some of these, rocks along the Highland Boundary fault in Scotland or in Clew Bay in Ireland are worryingly young as well – Cambrian or even Ordovician. They’ve caused a lot of arguments, as you can imagine.

Song about a break-up

The Dalradian sing sad songs, about break-up: continents who once ‘were as one’ but who gradually drifted apart. It was a ménage à trois, so it was bound to end badly.

The song starts with the continent of Rodinia about 730 million years ago. This already had a rich and complex geological history which led to the creation of a large supercontinent consisting of pieces that now make up Scandinavia (Baltica), North American and NW Ireland and Scotland (Laurentia) plus Amazonia. Of course at the time these distinctions didn’t exist – it was simply a large single continent.

Rodinia 750 million years ago. From Cocks & Torsvik 2005

Cracks were starting to show, however. The Grampian Group formed in a rift basin, where Rodinia was starting to break-up. Early variations in the thickness of sedimentary layers suggest that faults were active during deposition. Times when the sedimentary basin became dramatically deeper also suggest tectonic involvement – active rifting was stretching the basin. There wasn’t a full break-up, however. Rifting ceased and a 30 to 40 million year period of calm saw the Appin group deposited in shallow waters under stable conditions.

The Argyll group saw rifting start-up again. By the end of the Argyll (600 million years ago) it became clear the split was going to be permanent. The stretching of the crust allowed the underlying mantle to melt, producing a ‘bimodal magmatic event’ with the intrusion of granites and the eruption of basaltic lavas. By Southern Highland group times, the sediments were forming in really deep water. Large bodies of serpentinite found in Ireland suggest the crust was stretched so much that material squeezed out from the underlying lithosphere.

This final extreme stretching marks the opening of a new ocean, called Iapetus. The opening of Iapetus created sedimentary basins all along the Laurentian margin. The Fleur de Lys Supergroup in Newfoundland and the Eleonore Bay Supergroup in Greenland were deposited in adjacent, equivalent basins. Further south in the US Appalachian belt, sedimentation associated with the opening of Iapetus starts only in Cambrian times.

Sing something simple?

I’ve told a nice simple tale, based on current scientific consensus, but it used to be much more complicated. The problem is that these are no longer sediments. Key concepts in both metamorphic and structural geology were developed on these contorted, baked and squashed rocks. Seemingly simple things like identifying the base of the Dalradian is extremely difficult as the rocks below are often also metamorphosed sediments, first transformed before the Dalradian and then deformed and heated again alongside it. Drawing a line between two types of schist requires extremely careful analysis.

The Dalradian is intruded by many granites. Most are older than the deformation, but some are younger, intruded into deep sediments in the Dalradian basin while sediment was still settling on the surface above. A date of 590Ma for one of these ‘Older Granites’ caused years of academic chaos in the 1990s as the granite was initially (incorrectly) thought to post-date the deformation, meaning that any younger sediments couldn’t belong to the Dalradian.

Geological histories hinge on tiny facts. The age of single zircon grain, a fabric wrapping an andalusite crystal: great geological narratives are built from or destroyed by such tiny pieces of evidence.

Evidence of mountain building episode during the Dalradian. From Hutton & Alsop 2004 GSL.

There is one inconvenient fact that might bring the whole of the simple story crashing to the ground. Respected researchers working in Donegal (Donny Hutton and Ian Alsop) have mapped an unconformity within the Argyll group. This could be consistent with our story – unconformities can form within sedimentary basins – but they interpret it as an orogenic unconformity. Based on various lines of evidence, including a tectonic fabric within sedimentary clasts above the unconformity, they infer a entire episode of mountain building took place during the gap in sedimentation shown by the unconformity.

So is the Dalradian a single package of sediments formed during the opening of an ocean, or two packages separated by a previously unrecognised period of mountain building? I’ve no idea, but hopefully time will tell.

Regardless, everyone agrees on what happened next. Iapetus no longer exists: it opened and then it closed, moving the Dalradian from a sedimentary basin into the core of a mountain belt. That’s where we’re going next.

——————————————————————————————————————-

*they’re not sandstones any more, but we’ll come to that

** the first, the “Port Askaig Tillite” is correlated with the Sturtian global event at c. 700Ma. The second ‘Stralinchy diamictite’ with the Marinoan at c. 635Ma

*** the Inishowen and Loch na Cille beds are correlated with the Gaskiers global event at c. 580Ma

References

L. Robin M. Cocks, & Trond H. Torsvik (2005). Baltica from the late Precambrian to mid-Palaeozoic times: The gain and loss of a terrane’s identity Earth-Science Reviews DOI: 10.1016/j.earscirev.2005.04.001

D.H.W. Hutton, & G.I. Alsop (2004). Dalradian Supergroup of NW Ireland
Evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland Journal of the Geological Society DOI: 10.1144/0016-764903-094

David Stephenson, John R. Mendum, Douglas J. Fettes, & A. Graham Leslie (2013). The Dalradian rocks of Scotland: an introduction Proceedings of the Geologists’ Association DOI: 10.1016/j.pgeola.2012.06.002

A bit of Scotland in an English playground

Posted on 29 December, 2012 by Metageologist

There is a park near my home that my children like. As is the way of things, this means I stand around it a lot, ready to rub bruised knees or produce biscuits or push ‘faster!’, but otherwise redundant. My attention often wanders to the big blocks of stone in the park – they are worth looking at.

To start with, here’s some ‘granite’.

The white material is a medium to coarse grained igneous rock of ‘felsic’ composition – granite (loosely speaking). The dark area is a portion of material within the granite. It may be a xenolith, a piece of rock that fell into the magma, but it looks to me like diorite, possibly the result of magma mingling.

There are few blocks of mafic igneous rock:

This ‘gabbro’ above shows both fresh and weathered surfaces. Plagioclase feldspar is colourless and weathers white while the dark minerals (pyroxenes?) sit within it. Note the rusty iron patch at the top.

Another block of gabbro has a slight sniff of layering to it.

Mafic magma is molten from about 1200°C, whereas more normal continental rocks (sediments say) can melt from 700°C. Put the two together, therefore and you expect some melting, producing migmatites.

This block is of high-grade metamorphic rock, with a gneissic foliation. A thin granite vein cuts through and has itself been folded.

Here’s some more metamorphic rock, with a folded foliation and a mica sheen.

Our final type of rock is sedimentary, a conglomerate.

Notice the variety of clast types. We’ve some red sediments, some ‘granite’, vein quartz, quartzite and more. Here’s a closer look.

I’ve no idea where these blocks came from, but I know it’s not nearby. They are part of the park landscaping and so were brought from somewhere else in Britain, on a lorry. The blocks are rounded and weathered, so they are not blasted from a quarry. I think they are glacial blocks. Assuming they came from the same glacial deposit I suggest they are from North East Scotland. There is a series of syn-orogenic mafic intrusions in this corner of the world that sit within the high-grade parts of the Buchan and Barrow metamorphic areas. Granites are two-a-penny in Scotland and the conglomerate looks like the post-orogenic ‘Old Red Sandstone’.

These rocks are very similar to those in my PhD field area, so to have them turn up close to home is rather splendid.

All photos by me. I was hoping for a sunny day to take them, but I’ve given up waiting. The photos give you an authentically gloomy and dark view of rocks from Scotland, at least.

Mantle support of topography – a swell idea

Posted on 12 November, 2012 by Metageologist

Why are some bits of the earth higher than others? Finding mountains near plate boundaries is easy to explain – various forms of plate collision cause the crust to thicken and the surface to rise. What about Southern Africa?

Reaching a high point of 3473m, most of Southern Africa is a high plateau. It is not tectonically active – the sedimentary layers in the photo are still horizontal – but they are more than a kilometre higher than they ‘ought’ to be. Why?

African swells and superswells

In recent GSL paper, Stephen Jones, Bryan Lovell and Alistair Crosby argue that the topography of Southern Africa can be explained by mantle convection pushing the crust upwards – that flow of the mantle has a direct influence on surface topography. They go onto argue that this phenomena also controls patterns seen in ancient sedimentary basins.

A spot of theory first. On the scale of 100s of kilometres topography is controlled by the properties of the lithosphere – the Tibetan Plateau can be explained this way. But over scales of 1000s of kilometres the lithosphere isn’t strong enough. On this scale, variations in height can only be explained either because the lithosphere is thicker, or because it is being pushed up from below.

Our authors look at Southern Africa and Antarctica, areas far from convergent plate boundaries. Studying the patterns of topography and gravity, they argue that the main geographic features can be explained by dynamic support from mantle convection. So the Drakensberg mountains (pictured above) are high because they sit above a portion of the mantle that is rising, pushing the plate (and the surface) up. The low-lying Congo basin, further north sits above a descending part of the mantle.

They paint a picture where “thousand-kilometre scale dynamically supported swell topography is the norm for continental regions that are not influenced by subduction zones“, suggesting that areas such as eastern Australia, peninsular India, central USA, east Africa, eastern USA, the Siberian Shield and eastern Siberia are all affected by similar long ‘topographic swells‘. The African swells they study in detail are around 2000km in diameter and between 1.4 to 2.7km in height.

This is a tremendously important idea, suggesting that the broad patterns of the landscape can be explained by slow movements of rock deep under the surface. Patterns of convection within the upper mantle subtly change the shape of the earth we stand on. More, there is talk of an Southern Africa superswell, over 10,000km in size. A long wavelength warping of the earth’s surface that the other swells sit on and which could be partly supported in the lower mantle.

One of the beautifully elegant things about earth sciences is the way we discover that apparently unrelated aspects of the earth are linked. Discovering such a linkage provides insights into both things simultaneously. So, discovering that topography is related to mantle convection gives you new ways of understanding both. For example looking at sedimentary sequences sitting on these African swells allows us to work out how long the swell has existed for (40 million years). This in turn is an insight into the timescales of convection within the mantle hundreds of kilometres under the surface.

To the UK

The concept of dynamic topography is not new to this paper, (there was a conference about it last year), it adds importantly to the debate by quantifying both modern day swells in Africa and ancient swells in sedimentary basins.

The UK, Ireland and associated oceanic basins make up a stable piece of crust with a long sedimentary record. There is a long and successful history of hydrocarbon exploration in the area, so there is lots of data to draw on. Our authors describe four swells from this area in the last 200Ma, active in the Jurassic, early Cretaceous, Eocene and present day (based on Iceland).

Figure 4 from Jones et al 2012. Reproduced under Geol Soc fair use policy

The evidence presented suggests that these ancient swells are similar in dimensions to present day African ones and formed over similar timescales, growing in 5-10Ma and decaying over 20-30Ma.

A fail for Vail?

The effect of these swells on sedimentary sequences is dramatic – a kilometre of uplift turns a sedimentary basin into an area of erosion. Dynamic support is not just of interest to students of the mantle, but also to lovers of sedimentary basins or those who seek the oil and gas found therein.

It’s been long understood that patterns of sedimentation are controlled by tectonic factors (creating holes with extensional or foreland basins) and by sea-level changes. In the 1970s, the work of a group at Exxon led by Peter Vail transformed the study of sedimentary basins. Their new techniques of sequence stratigraphy showed that patterns of sedimentation are best explained by different cycles of changing water depth (relative sea-level). The level of water within a typical sedimentary basin is constantly changing on both high and low frequencies.

Note that this relative sea-level change can be caused either by moving the surface of the earth or moving the surface of the water. Vail ‘chose’ to move the surface of the water and sought to explain local patterns in terms of global “eustatic” sea-level change. During periods of glaciation there is a plausible mechanism for doing this and the model is successful (for example, the Carboniferous of the UK). A longstanding problem is how to create cycles of sea-level if there are no ice-caps to vary in size and periodically change the volume of sea-water.

In his 2010 Presidential Address for the Geological Society of London, Bryan Lovell challenges the Vail hypothesis asserting that “mantle convection provides an alternative, regional, mechanism to eustatic control for explaining medium-frequency to high-frequency sea-level cycles“. Relative sea-level changes can be explained by moving the surface of the earth in a particular area. Over a mantle swell, basins will be shallower or stop being basins even if global sea-level remains constant.

Figure 2 from Lovell 2010. Reproduced under Geol Soc fair use policy

Lovell proposes a mechanism to explain the different frequencies of cycles. Mantle swells, created by convection cells, work over the 10-40 Ma timescale. He proposes that ‘hot blobs’ (grey patches above) move within the wider cells. The blobs cause their own smaller scale (and faster moving) ‘mini-swells’ that explain the higher frequency patterns seen in sediments.

What I like best about the idea of dynamic mantle support of topography is that it is testable. By starting to quantify the time and length scales of mantle swells it becomes possible to recognise them reliably in the stratigraphic record. Changes due to global sea-level variation will be correlatable across many basins whereas mantle swells will not.

Its another way of thinking about the earth, too. As someone living on a tectonically quiescent patch of the earth I’m used to thinking of nothing much happening here until the Atlantic starts to close again. It turns out that “there is no such thing as a stable continental platform” and that a change in the way rocks are flowing deep below could move Britain up or down by a kilometre within as little as 10 million years.

References

Picture of Drakensburgs via Palojono on Flickr under Creative Commons.

Jones, S., Lovell, B., & Crosby, A. (2012). Comparison of modern and geological observations of dynamic support from mantle convection Journal of the Geological Society, 169 (6), 745-758 DOI: 10.1144/jgs2011-118

Lovell, B. (2010). A pulse in the planet: regional control of high-frequency changes in relative sea level by mantle convection Journal of the Geological Society, 167 (4), 637-648 DOI: 10.1144/0016-76492009-127