Geoscientist Online

It began in the mountains. Generations of structural geologists have cut their teeth on dramatic outcrops of large-scale folds and thrusts in the Alpine ranges of Europe, the Canadian Cordillera and Rockies. But for those who seek definitive structural geometry, the outcrop is too often unsatisfying. Critical parts of the structure necessary to test geometric models remain buried or have been eroded away. And this uncertainty leaves us with many different models competing for the same piece of real-estate, and occupying an expanding volume of literature. In a few cases boreholes may provide subsurface control. But generally, seismic data are sparse and unhelpful. Acquiring 3D seismic data in mountain ranges is hugely complex and very expensive – too expensive - except for those who track down large hydrocarbon fields.

The last decades of the 20th Century saw the search for hydrocarbons step off the shallow continental shelves and into the deeps. Two areas drove this move: the Gulf of Mexico offshore Louisiana and Texas, and the Niger Delta. Both regions are tectonically quiescent – and both have thick sediment accumulations. At the toes of their slopes lie thrust and fold belts.

Being marine, seismic data are far cheaper to acquire and of spectacularly higher quality than on land. Imaging is made more complicated by shallow salt in Gulf of Mexico examples but for the Niger Delta thrust structures are exquisitely displayed in seismic data. This is the place to argue about how compressional deformation works at the kilometre scale – at least in poorly lithified sedimentary successions. Do thrusts cut up simply through the strata or do they relay and splay? How do the large-scale anticlines – the structural targets for the oil of course – relate to displacements on the thrusts? How rapidly do thrust structures vary along their length?

All these questions are now being answered, and it’s taking more than seismic. The well penetrations are encountering steep beds, sub-seismic folds, multiple fault strands which make for complex imaging. These structural complexities increase the risk inherent in assessing the amount of oil in place, and how it will produce. The stakes are high, when wells cost hundreds of millions of dollars each. Putting in the infrastructure to produce these fields runs into billions. So it is worth spending some money to improve the imaging and capturing the range of possible structural models. Unsurprisingly there are lots of structural geologists working the data now – few go into the mountains any more!

The greatest throw on Earth

So what are thrusts doing on passive continental margins? The answer comes not from the spectacular 3D seismic volumes that image the thrusts but from regional 2D data, often patched together from different surveys. For the Niger Delta the thrusts in deepwater appear to detach on a regional tract of over-pressured mud. Back on the delta top, seismic images show large listric normal faults – which have some of the greatest throws on Earth. So the whole delta complex, over 12km of sedimentary thickness, is on the move - slowly creeping into the Atlantic. And it has been doing so since at least the middle Miocene – 15 million years or more. It’s a gravity spreading system that’s wider than the Alps.

Elsewhere along the margin of West Africa the sedimentary wedge is collapsing too, but the style changes. Offshore Namibia, within the Cretaceous strata that accumulated after the South Atlantic opened, lie some of my favourite thrusts. They, too, pass back up-dip into extensional faults; but this system all dips down into the Atlantic. Here the slope failed by sliding. This collapse only happened for a short period (perhaps two or three million years) and is covered by a barely-deformed drape of Tertiary strata. If you look at shallow seismic or bathymetric data, you would never know that the thrust belt was there.

As more and more seismic data are acquired from continental margins around the world, the more we are realising that large-scale gravitation collapse is a common occurrence in thick sedimentary piles. The first decade of the 21st Century has seen a surge in oilfield discoveries driven by this exploration. Understanding how the sedimentary units collapse is generally important for modelling Earth surface processes: whether a margin is slowly creeping down-slope impacts on the long-term sediment flux into the deep oceans. On the scale of the margin, sediment can be ponded in shallow water, preferentially trapped in the extensional basins. On a finer scale sediment pathways are influenced by the faults, folds and gradient changes on the seabed caused by the deformation.

Of course sediments on continental margins can fail in other ways, beside wholesale gravity spreading and sliding. Submarine landslides are another form of gravity collapse. The speed of these need not be measured in millions of years but in a few hours or less. Some are implicated strongly in the generation of tsunamis. Slumps, slides and debris flow deposits are been recognised in outcrops of submarine slope deposits for decades. They have been encountered by ocean drilling and imaged by high resolution bathymetric data.

The biggest of all the modern collapses is the Storegga slide, offshore Norway. Failure on the upper reaches of the continental slope, where thick accumulations of glacial deposits had accumulated during the last ice maximum, drove debris out across the floor of the Atlantic, almost as far as the mid-ocean ridge. The last serious collapse happened about 8200 years ago, probably triggered by a large earthquake in response to glacial rebound, and caused a tsunami that dumped debris along the Northern Isles and on cliff tops along the North Coast of Scotland. Modern understanding of the Storegga Slide has come from a set of investigations by Statoil (and its merged forerunner Norsk Hydro) designed to quantify the risk of further slope failures along this sector of the Norwegian Sea. This is not altruistic. One of Europe’s largest gas fields – Ormen Lange - lies beneath part of the main scar and pipelines drape back up the failure headwall. The prize is worth having, nearly 400 billion cubic metres of gas, representing 20% of the UK’s demand for the next 30-40 years. Understandably, it has been important to understand the near-surface and the potential hazard it poses to seabed infrastructure.

Geologists with a penchant for classification have saddled the products of submarine slope failure with a lexicon of “slides”, “slumps”, “debris flows” (strictly, “debris flow deposits”) and “olistostromes”. These terms have a long tradition in field geology – each implying differing degrees of stratal disruption and sediment mixing. The advent of seismic data has tended to gather all of these terms under the single banner of the “mass transport complex” (MTC). The Storegga surveys have slipped revealed blocks of strata, hundreds of metres thick and wide that have retained the integrity of their seismic reflectors. These slide blocks are surrounded by chaotic reflectors or regions that are seismically transparent. So the character of deposits resulting from individual failure events can be complicated in space, and presumably evolved in time. So it is useful to have the blanket term MTC.

In the earlier days of imaging, MTCs revealed little of their internal structure. Large slide blocks, more disorganised regions, shears, break-away scars and “pressure ridges” have all been recognised for a long time on high resolution bathymetric surveys; but many shallow, 2D seismic profiles failed to reveal MTCs’ internal structure. Indeed, chaotic or transparent seismic character is generally interpreted as how MTCs manifest themselves in the subsurface. Logically, this reasoning is perhaps curious - based as it is on the absence or obscurity of data, rather than anything positively diagnostic. Chaotic reflections or transparency have generally been thought to represent stratal chaos and intense disruption. It is as if the interpreter is saying that the internal structure of an MTC is too complicated to be resolved, or perhaps even understood.

Modern 3D seismic datasets are changing all that. It still takes some imagination to pick out structure in 2D profiles cut through 3D seismic volumes, but maps and timescales are far more revealing. The other technological advance is to display the seismic data not in terms of amplitude (the standard way) but by using so-called “seismic attributes”. These identify variations in seismic character, such as subtle variations in amplitude strength. Suddenly, all sorts of structures appear – organised fractures and faults, looking like crevasses in glaciers. Other examples show imbricate thrusts, stacking stratal reflectors. If individual structures like these can be identified, it means that the surrounding material must be reasonably coherent – not a jumble of material. It does not take too much to mess up the seismic character seen on profiles – narrow faults every few tens of metres, broad fold structures between. If you came across this in the field, the strata might appear only slightly disrupted – albeit translated along a basal detachment. Rather like a conventional thrust belt in fact! Just because the seismic character in profiles is “chaotic” does not mean the MTC is a debrite.

Why does this matter? Some ancient, heavily-disrupted MTCs represent significant drilling hazards because they have significantly reduced permeability that allows formation pressures to build up beneath. Their very heterogeneity makes drilling a challenge. But the geological community needs to understand them more, because they are such common components of continental margin successions. How many more lie beneath the blanket of seabed sediments, awaiting discovery by seismic surveys? How does internal structure and the propensity for disruption within MTCs reflect emplacement mechanics and especially, rates of translation? We need answers if the geological record on margins is to be useful in providing probabilistic assessments of tsunami risk.

So passive margins are not so passive. Many are in a state of collapse. Deep-water thrust structures can be just as large, and in some cases larger, than the fold-thrust complexes found in mountain belts. Even MTCs can be many hundreds of metres thick. Others have speculated that these gravitational structures will become incorporated into mountain belts as continents collide. So how many of the deformation structures described from ancient mountain belts may have formed, not during that ancient orogeny, but in earlier times? It may be hard to tell.

Conclusions, further reading & acknowledgements

This article has barely scratched the surface of gravitational tectonics on continental margins, but one thing I hope is clear. It is no coincidence that the images illustrating this article all come from the oil and gas industry and the seismic companies that service them. Ninety-nine percent of the geophysical data from margins are collected for these commercial ends – while only a tiny proportion comes from academic research campaigns. If we want to understand and quantify the hazard of submarine slope failure – we need to work with commercial data.

A joint meeting of the Petroleum Group and the Tectonic Studies Group was held at Burlington House in October 2008 - Gravitation Collapse on Continental Margins: Processes and Products. A taste of the issues raised - the importance of excellent seismic data, coupled with carefully directed fieldwork on ancient analogues - can be gauged from the Thematic Set of papers arising from the meeting.

These were edited by Jonathan Turner and myself and appear in the May issue of the JGS. You can also find a great introduction to deep-water fold and thrust belts in Rowan, Vendeville & Peel (2004; AAPG Memoir 84). If you wish to gain an insight into structural interpretation and its potential uncertainties, even in these settings, take a look at the paper by Kostenko and others (2008, AAPG Bulletin). Much of the Storegga description can be found in a special issue of Marine & Petroleum Geology (volume 22, parts 1-2) from 2005. Some of the modern approaches of structural mapping within mass transport complexes made possible by 3D seismic data are provided by Moscardelli, Wood & Mann (2006, AAPG Bulletin), Bull et al. (2009, Marine & Petroleum Geology), and references therein.

The images used in this article are a small sample of those that image gravitation tectonic structures, available on the Virtual Seismic Atlas (www.seismicatlas.org).

*Dr Rob Butler is in the School of Geosciences, University of Aberdeen.