Friday, 12 December 2008
Friday, 7 November 2008
Here's a layer beginning to break up into the boudin-like structures.
You can see a couple of the boudin-like structures here, to the right of the notebook.
This is within a slump unit, and you can see that there are fragments of fold hinges and aerofoil-shaped fold fragments that have become detached from the rest of their parent layers. I think what happens is that these isolated fragments then get rolled up as slumping continues, and you end up with something like this:
A polished section of one of the boudin-like structures, showing internal folding.
This one has a sheath-fold, if you look in the top left of the image. Not sure about this, but perhaps along-boudin extension occurs as the "boudin" gets flattened and rolls during slumpiing, analogous to rolling out a piece of plasticine.
Thursday, 6 November 2008
Almost a month ago now, I wrote a short post on a paper that has been accepted for publication by the Journal of Structural Geology, and said I would write a bit more "probably next week". Better late than never.
The paper is entitled "Structural geology and 4D evolution of a half-graben: new digital outcrop modelling techniques applied to the Nukhul half-graben, Suez rift, Egypt" and the authors are myself, Dave Hodgetts, Frank Rarity, Rob Gawthorpe and Ian Sharp. It looks at a well-exposed extensional half-graben (the Wadi Nukhul half-graben) in the Suez rift, Egypt. We have collected a large amount of terrestrial LIDAR data from this half-graben, and used it to accurately map the structure and stratigraphy of the study area. By combining those data with our structural and sedimentological analysis, we can get an idea of how the fault system evolved over time, by looking at how sedimentation responds to the evolving structure.
There have been a number of studies looking at the evolution of normal fault systems, most of which have used seismic data or conventional field data and have looked at relatively simple fault systems. Seismic data, particularly 3D seismic, is great for looking at these questions because it has perfect 3D coverage. However, the maximum resolution tends to be on the order of 20 to 40 m. In the Wadi Nukhul area, the stratigraphic units we are interested in looking at are quite thin (up to 100 m or so), and more importantly the variations in thickness are quite subtle. Thickness variations in the strata that were laid down during active faulting tell you about the fault activity, so we need to be able to map out those thickness variations as accurately as we can.
Essentially, there are two hypotheses as to how normal fault systems develop. The first is that a large number of small faults initiate. Depending on their spatial relationship to neighbouring faults, these might die out, or they might grow in size (and displacement) and link to neighbouring faults. So there is a progression from a large number of low-displacement faults (in the "rift initiation" phase to a small number of large displacement faults, which have formed by the linkage of fault segments that were initially isolated from each other (in the "rift climax" phase). So faults increase their length gradually through time by growing laterally and linking to other faults. The second hypothesis is that faults establish their length very early in their history, and then accrue displacement through time without significantly increasing their length. This seems more likely to be applicable to reactivated faults. There is more on this in Morley et al. 2007.
Here's a simplified map of the study area:
For this summary, we're interested in the Nukhul fault. This is the main fault that controls deposition of syn-rift strata (Abu Zenima and Nukhul formations) in its hanging-wall. You can see that in plan view the fault is segmented: different portions of the fault have differing strike. Now, here's a graph showing how throw (vertical displacement) varies along the fault, taken from a horizon that we mapped on the LIDAR data :
There are a couple of things to notice here. Firstly, the maximum throw is approximately 1 km and occurs where the Nukhul fault intersects with the Baba-Markha fault to the south; throw then decreases towards the fault tip towards the north. Superimposed on that larger pattern, it can be seen that where the fault strike changes, there are minima in throw values, while in the centre of the fault segments there are maxima in throw values.
The interpretation of this is that the fault segments we can see at the present day, which have differing strike orientations, were initially isolated from each other. The variations in throw are then a kind of hangover from when the fault segments were isolated: the throw minima represent palaeo-fault tips at which displacement was zero before the fault segments became linked.
So what was the timing of linkage of the fault segments? It is difficult to know this unequivocally, but three lines of evidence suggest that the linkage was early, around the break between deposition of the Abu Zenima and Nukhul formations (which is locally marked by an angular unconformity). Firstly, the Abu Zenima Formation seems to have been deposited along the entire Nukhul fault. In the map above, it can be seen that the Abu Zenima is exposed at the linkage point between segments 2 and 3 of the fault. Secondly, the sedimentary facies in the areas around linkage points is much coarser than elsewhere in late Abu Zenima and early Nukhul time. That suggests that the linkage points were also sediment input points at those times. Thirdly, there is active faulting close to the linkage points prior to deposition of the Nukhul Formation. This suggests that those areas were subsiding rapidly at that time, perhaps because of the linkage of the fault segments. If this is correct, then the linkage of fault segments occurred within the first 2.5 million years of rifting (and perhaps much sooner). The Nukhul fault therefore seems to be an example of a normal fault that established its length fairly early in the rift history. This would make sense, because the orientations of some of the fault segments suggest that they were inherited from regional basement structures.
There is plenty more in the paper, but I'll post a link to a PDF when it becomes available.
Friday, 10 October 2008
A slightly different image this week; this comes from the paper I've just had accepted for publication in the Journal of Stuctural Geology. This is a normal fault plane from the Gulf of Suez (surprise!), modelled in TrapTester software from our LiDAR data, and contoured for throw values (throw is the amount of vertical displacement of strata across the fault). We are in the hanging wall, looking toward the fault plane. As is typical, the throw contours are elliptical, and throw is highest at a roughly central point, decreasing outward. A reasonable assumption is that the fault initiated at the point of highest throw. This occurs at a level above the pre-rift/syn-rift contact, suggesting that this fault initiated within the syn-rift strata relatively late in the history of the rift.
Friday, 3 October 2008
This is a nice illustration of sedimentation and tectonics in the Suez rift. The picture is taken on the footwall of the Nukhul fault, looking west into the syn-rift units preserved in the hanging-wall. There is an approximately east-striking fault cutting the syn-rift units. The Abu Zenima Formation can be seen thinning dramatically onto the footwall of that fault. The thinning is accomodated partly by onlap onto the underlying pre-rift units, and mainly by erosional truncation at the base of the Nukhul Formation. In the hanging-wall, the Nukhul Formation is slightly thicker than in the footwall, and also contains a higher proportion of fluvial conglomerate. The displacement on the fault can be seen to increase downward.
There was sad news for me this week, and for more or less everyone who has done geological fieldwork in Egypt. Sayed Gooda, our driver on our trips to Sinai, has passed away.
Without Sayed, our work would have been a lot more difficult. Sayed didn't just drive us, over bad roads and difficult terrain. He looked after us, kept us out of trouble, made us delicious sandwiches for lunch (the highlight of the Sinai field day), and tried valiantly to teach us Arabic . Practically everyone who has worked on the Gulf of Suez owes something to Sayed. I will always remember arriving in a hot, noisy and fume-choked Cairo for the first time, nerves jangling from an overnight flight from Amsterdam, and being put at ease right away by the big friendly man in the big friendly green jeep.
Sayed was in Saudi Arabia when he passed away, and has been buried there. In the e-mail telling us of his death, his friend Tarek Moustafa wrote "I think he died Happy as this would have been a dream come true to him to die in the Holy lands". We can only hope so. Thanks for everything, Sayed.
Friday, 19 September 2008
Friday, 12 September 2008
It was interesting to see the reasons given for this. According to Paul Nathanail of the University of Nottingham "geology...gives students, at an early stage in their studies, the chance to be part of cutting-edge science. If a student sees a new rock, they can begin to challenge the established way of thinking". While I'd like to think this is true, I suspect it's only really a factor for the brightest and most motivated students. I think that the chance to get out in the fresh air and study something with clear applications is perhaps a bigger factor. Whatever the reason, it's a good thing, and should help geology departments recruit more students.
Monday, 8 September 2008
The oil price is back down to $107.30 today, from a high of $147 in early July. That's a 20% fall in just two months. This has little do with the state of oil production, and everything to do with speculation. Now the OPEC countries have got used to oil at $100+ a barrel, they're ready to take action, by cutting production, to keep oil prices high. And, of course, the west needs relatively high prices to drive investment in technically and/or politically difficult fields, now that most of the easy oil has been found.
It's a strange situation. If there was a cartel that was fixing prices in, say, the British airline industry, something would be done about it. But oil is not like other commodities.
One thing in the article that bothered me a little was this:
The Saudis guard closely their data on the oil reserves and production capability. Why? "Peak oil" theorists argue it's because the big reserves aren't as big as advertised.
This makes 'peak oil' sound like a conspiracy theory. In fact, the peak oil hypothesis says nothing about the size of Saudi Arabian reserves. What it says is that global oil production will follow a similar pattern to sub-sets of global production (such as production from single basin, or a single country): that is, it will increase until roughly 50% of the oil has been produced, and then it will fall. Some people, who you might call early peak oil theorists, think that OPEC reserves are overstated, but that doesn't alter the fact that oil production will peak: it just shifts the timing of the peak.
Friday, 5 September 2008
This week's image is a nice example of a fault zone from the Suez rift, Egypt (There will probably be a lot of images from Egypt in this series). This is the Nukhul fault, juxtposing Cretaceous chalk of the pre-rift Sudr Formation against the syn-rift Miocene Abu Zenima and Nukhul formations. A sliver of Eocene pre-rift Darat Formation is caught between the paired slip surfaces of the main fault zone, and is internally deformed. In the hanging wall of the main fault zone (to the right), a series of minor faults occur in a damage zone about 50 m wide. The minor faults tip out downward, with one fault showing a duplex pattern near its tip as it merges into bedding.
Monday, 1 September 2008
Point cloud dataset, coloured using digital images, with geological interpretation added.
This being a fairly new technology for geological mapping, it has not always been straightforward to use. For example, software for geological interpretation of LIDAR data didn't exist. So we had to create our own: my colleague Dave Hodgetts has been working on this, and the resulting software is now being spun out. And we're still trying to figure out how we can use all the data to its best advantage. But we're getting there, and I hope the first publications from this work will be coming out soon. Watch this space...
Tuesday, 26 August 2008
This post is about abiogenic oil, a collection of hypotheses by which oil could be created in the mantle, rather than in sedimentary basins at relatively shallow depth. A recent(ish) Hedberg Conference of the American Association of Petroleum Geologists (AAPG) addressed this issue, and the results were written up in the AAPG Bulletin earlier this year.
First of all, why is this important? For two reasons. If significant quantities of oil are produced through inorganic chemical processes in the mantle, it is possible that there is far, far more oil available for production than is conventionally thought. The much-dreaded peak oil could be put off for a considerable period of time. Secondly, current exploration is based on the biogenic origin of hydrocarbons; that hydrocarbons are created during the burial of organic material. This hypothesis makes predictions about where hydrocarbons should be found. If a significant amount of oil really is abiogenic in origin, then we need to urgently revisit how we explore for oil and gas resources.
How could abiogenic processes produce hydrocarbons? There are two possibilities: firstly, degassing of the mantle, followed by polymerisation the low-molecular-weight compounds released. Secondly, the serpentinisation of ultramafic rocks combined with a Fischer-Tropsch reaction, in which carbon monoxide and hydrogen are combined, using a catalyst, to form hydrocarbons. What both these hypotheses have in common is that hydrocarbons are produced deep in the crust or in the mantle, and must migrate considerable vertical distances to reach the relatively shallow reservoirs in sedimentary basins where hydrocarbons are normally found.
Unfortunately, the evidence for large accumulations of abiogenic hydrocarbons seems to be somewhat thin. No-one disputes that non-commercial deposits of abiogenic hydrocarbons do exist, but commercial quantities seem to be elusive. The problem is really one of Occam's Razor; where there are suggestions that hydrocarbon accumulations might be abiogenic, they can also be explained within the biogenic paradigm. For example, hydrocarbons have been found in fractured basement rocks such as granite. At first glance, this seems to be contrary to the biogenic model. On closer analysis, though, these occurences can be explained coventionally. The hydrocarbon source rock is subjected to heat and pressure, generating hydrocarbons, which then migrate via fractures. If the horizon they are generated in is overpressured, they can be forced downward. Alternatively, uplifted areas of granitic basement might have source rocks adjacent to them across faults, for example.
Another problem for the abiogenic oil hypothesis is the presence of age-restricted biomarkers. These are organic compounds that are only produced from certain types of organism. For example, a compound called oleanane is derived only from angiosperms (flowering plants). It is therefore restricted to hydrocarbons derived from Late Cretaceous or younger source rocks. Not only that, but the radiation of angiosperms through time should mean that the proportion of oleanane increases as the source rock becomes younger. This is exactly what is observed in real hydrocarbon provinces. This is consistent with the biogenic origin of oil, and not with the abiogenic origin of oil.
A third problem is the existence of oil shales. These can be explained by the biogenic paradigm: they are the strata that contain the organic material that gets buried, matures into hydrocarbons, and migrates into reservoir units of high porosity and permeability. But the shales themselves are of very low porosity and permeability. So how could fluids migrate from deep in the crust or mantle and accumulate in them?
Does all this mean that there is no such thing as commercial abiogenic oil? Not necessarily. If you wanted to find abiogenic oil in exploitable quantities, you would have to look in areas where crustal-scale fault or fracture systems could allow fluids generated deep in the Earth to migrate towards the surface, perhaps in areas of basement where there are no sedimentary basins. No-one has been looking in those places, because the biogenic model works perfectly well. It would be hugely risky to go looking for hydrocarbons using the abiogenic theory as your guide, but it could ultimately be worth it. Especially if oil prices rise much further...
Friday, 14 March 2008
D.C.P. Peacock (2008) provides a useful caution against the unnecessary proliferation of synonymous geological terms. I am entirely in agreement that such proliferation should be avoided. However, I wish to take issue with his example of ‘architecture’. Peacock makes two arguments against the use of the term architecture; firstly, that it implies the existence of a (perhaps divine) architect, and secondly that it is synonymous with the term ‘structure’, and adds nothing but confusion to the literature.
While I would tend to agree somewhat with the first point, it seems that it is difficult to avoid such implications. Peacock mentions the term ‘tectonic’, one definition of which in the Oxford English Dictionary is ‘…pertaining to building, or construction in general; constructional, constructive: used esp. in reference to architecture and kindred arts.’ But a similar problem arises in the use of the term of structure: The Oxford English Dictionary provides two definitions of ‘structure’: i) the action, practice, or process of building or construction, and ii) manner of building or construction; the way in which an edifice, machine, implement, etc. is made or put together. Thus the term ‘structure’ also has connotations of building or construction. Peacock asks ‘Who is the architect?’, but one might just as well ask ‘Who imparted the structure?’ The answer in both cases is that natural processes created the ‘architecture’ or the ‘structure’: I would argue that neither term implies a divine ‘builder’.
It could also be argued that ‘architecture’ does have a usefully distinct meaning from ‘structure’. It has perhaps been poorly defined, but in the study of fault zones ‘architecture’ has been used to refer to the overall arrangement of structural elements (such as gouge zones or subsidiary brittle structures) within the fault zone (e.g. Caine et al., 1996; Heynekamp et al., 1999; Faerseth et al., 2007). It has also been used in sedimentology, in a similar way, to describe the overall arrangement of sedimentary facies elements within a depositional system (e.g. Dreyer, 1994; Boris and Thomas, 2007). In the case of fault zones, using ‘structure’ instead of ‘architecture’ would lead to the same term being used for the fault zone as a whole, for the subsidiary structures within it, and for the overall arrangement of elements within the fault zone. In sedimentology, it would lead to the use of the term ‘structure’ for sedimentary facies elements that are not normally thought of as ‘structures’ by structural geologists. So, I would suggest that replacement of the term ‘architecture’ by ‘structure’ could create as many problems as it would solve. To conclude, use of the term ‘architecture’ in the earth sciences is defensible, as long as it is adequately defined.
Boris, K., Thomas, A. Sedimentary architecture and 3D ground-penetrating radar analysis of gravelly meandering river deposits (
Caine, J.S., Evans, J.P., Forster, C.B., 1996. Fault zone architecture and permeability structure. Geology 24, 1025-1028.
Dreyer, T. 1994. Architecture of an unconformity-based tidal sandstone unit in the Ametlla Formation, Spanish Pyrenees. Sedimentary Geology 94, 21-48.
Faerseth, R.B., Johnsen, E., Sperrevik, S. 2007. Methodology for risking fault seal capacity: Implications of fault zone architecture. AAPG Bulletin 91, 1231-1246.
Heynekamp, M.R., Goodwin, L.B., Mozley, P.S., 1999. Controls on fault-zone architecture in poorly lithified sediments,
Peacock, D.C.P. Architecture, gods and gobbledygook, Journal of Structural Geology (2008), doi: 10.1016/j.jsg.2008.02.003.
Is this sort of thing really worth doing, I wonder?
Update: My comment has now published, and is available (behind a highly ridiculous paywall, unless you or your institution is a subscriber: who would pay $31.50 for this?) here. If anyone wants a PDF, drop me a line.