East Greenlan geology plays role in Northwest European margin

Re-interpreting the continent-ocean boundary

A wide spectrum of hydrocarbon provinces is present on the Northwest European margin. They range from mature areas in shallow water, where production chall-enges predominate, to new high-risk exploration targets in deepwater areas along the outer shelf. There is a corresponding spectrum of geological data required by industry to locate and extract hydrocarbons in these varied settings. - View of Steensby Bjerg, northern Hold with Hope, East Greenland. Pale Jurassic and Cretaceous sandstones rest unconformably on pink Triassic strata and are overlain by mudstones at the crest of a tilted fault block. The succession is capped by 1 km of subaerial plateau basalt. A 60-meter thick basalt sill is intruded into the base of the succession.

Onshore exposures of equivalent strata provide an important (and cheap) additional information source to supplement information collected offshore. However, in most areas of Northwest Europe, exposures of relevant strata are a rare commodity. Furthermore, new exploration areas are at such great distances from potentially useful exposures in Europe that it begs the question as to whether European outcrop studies would really help.

After many years experience of prepa regional syntheses of North Atlantic geology, combined with 12 field expeditions to East Greenland, it is clear that the most relevant information is not in Europe at all. But, it can be found on the opposite side of the North Atlantic.

East Greenland forms the conjugate margin to the entire Northwest European shelf, from the Barents Shelf to the Rockall Trough. Continental separation occurred around the Paleocene-Eocene boundary (Chron 24R), the last part of the Atlantic Ocean to open. There are two particular aspects of this separation process, which make East Greenland a unique source of information for exploration and production activity on the Northwest European margin. We will look at these two aspects, and their consequences, in turn.

Uplift creates exposure

The passage of the Iceland plume (Offshore, September 1999, page 30) beneath the East Greenland margin du continental breakup produced significant underplating. This resulted in the permanent exhumation of sedimentary basins along the coastal strip. A major fjord network in East Greenland produces near-continuous exposure across the west-dipping fault blocks that contain the sediments. Not only is the quality of exposure excellent, it is the only significant onshore Mesozoic-Cenozoic exposure in the entire northern North Atlantic rift system. Successions are available for all the main time intervals, allowing a detailed picture of basin evolution to be established. The high quality of exposure in East Greenland ensures that there are many useful analogues available for direct study, a point that is best illustrated by examples. - Pre-drift configuration of the northern North Atlantic showing the discrepancy between the axis of Mesozoic rifting and the line of final continental separation.

In the Traill

Another useful analogue is provided by exposures of Cretaceous-Paleocene strata in the region between Traill

Counter-intuitive play

The second aspect of continental breakup that enhanced the significance of East Greenland is the location of breakup itself. The line along which Cenozoic continental separation took place does not coincide with the axis of the precursor Mesozoic rift system: along much of its length, the line of separation was much closer to East Greenland than the former rift axis. There are therefore many areas of the deepwater outer shelf of Northwest Europe underlain by Greenland-derived Mesozoic sediment. - Offshore aeromagnetic data (positive areas elevated, illuminated from NW) combined with approximate location of the continent-ocean (C-O) boundary (red line). Note that north of the Jan Mayen Fracture Zone, the aeromagnetic data clearly indicate that oceanic crust approaches much closer to the coastline than previously recognized. Purple line indicates new C-O boundary location.

This creates an important counter-intuitive play: potential reservoir units on the outer part of the Northwest European margin should become coarser, the further offshore one goes. Significantly, this is true for the outer Vø Basin region of the Norwegian margin and the western part of the Faroes-Shetland Basin, both areas of active hydrocarbon exploration. In these cases, therefore, exposures in East Greenland can provide direct information on sediment distribution, composition and transport pathways.

Reconstructions

To use any information from East Greenland effectively, accurate pre-drift reconstructions are essential to establish the detailed paleogeographic and structural correlations between the conjugate margins. With high-resolution reconstructions, it should be possible to tie individual sediment bodies on the Northwest European margin to specific sediment sources in East Greenland. However, there are several sources of inaccuracy in existing reconstructions, which make such detailed correlations problematic. - Pre-drift structural correlation between the Vøring Basin and East Greenland, showing main faults in red. Line of continental separation in orange. The two blue dashed lines bound the Jameson Land/Liverpool Land block in East Greenland and the potentially correlative structural elements in the Vøring Basin.

Firstly, continental breakup in the northern North Atlantic was associated with the eruption and intrusion of large volumes of magma related to the Iceland plume. A number of crustal parameters (thickness, velocity structure, gravity, and magnetic signature) are generally required to define the location of the continent-ocean boundary (COB). The voluminous magmatic rocks tend to mask these crustal parameters across transitional regions that may be well in excess of 100 km across. This uncertainty in positioning of the COB can have serious consequences for continental fits.

To illustrate this point, we have recently reassessed a key section of the COB on the East Greenland margin, a reassessment that has important implications for exploration activity in the Vøring Basin. North of the Jan Mayen Fracture Zone, existing interpretations of the East Greenland margin placed the COB at the edge of the bathymetric shelf. This interpretation is also consistent with free-air gravity maps, which show a pronounced gravity high parallel to the shelf edge. Several multi-channel seismic profiles have been shot across this shelf margin, which appears to coincide with the oceanward edge of seaward-dipping reflector wedges.

However, more recent compilations of aeromagnetic data show linear magnetic anomalies to the west of the previously assumed COB, covering an area approaching 30,000 sq km. They can clearly be connected northeastwards into lineations in normal oceanic crust. This segment of crust therefore appears to be oceanic, which brings the COB very close to the present coastline.

Reinterpretation

There are a series of important implications of this new interpretation, which both simplifies and clarifies aspects of the geological evolution. For example, previous interpretations of the East Greenland margin make it difficult to envisage plate relationships during the early stages of spreading. They imply that anomalies 24 and 23 terminate a long way north of the Jan Mayen Fracture Zone on the Greenland side, whereas they do not on the Norwegian side. This would require a phase of highly oblique spreading during initial separation, but there is little evidence to support this model. However, with the new interpretation this requirement for oblique spreading is largely removed.

The reinterpretation of the COB also fundamentally changes the pre-drift fit of key structural elements on the Norwegian and Greenland conjugate margins, bringing the outer Vøring Basin very close to the East Greenland coast north of the Jan Mayen Fracture Zone. There are several important implications for hydrocarbon exploration in the Vøring Basin:

  • The relevance of East Greenland field data is greatly increased.
  • With accurate reconstructions, it should be easier to match individual sediment bodies in the Vøring Basin with specific sediment input points from East Greenland.
  • Prospective traps in the outer Vøring Basin (Gjallar Ridge) lie immediately adjacent to proven, former hydrocarbon traps in East Greenland.
  • NE-SW trending structural elements of the outer Vøring Basin probably had an original southward continuation into onshore areas of East Greenland south of the Jan Mayen Fracture Zone. The Mesozoic tectonic evolution of individual structural elements in East Greenland may therefore be directly correlated with equivalent structures in the Vøring Basin.

Other inaccuracy

The other major source of inaccuracy in pre-drift reconstructions arises from the methodology employed to create them. Most existing reconstructions available for the North Atlantic region are derived from global plate models based on the constraints provided by magnetic anomalies, fracture zone orientation, and paleomagnetism. For these global models to be manageable, large continental areas have to be assumed to behave as single rigid blocks.

While these reconstructions are fine for obtaining a general impression of relationships, their resolution is inadequate when working at a smaller scale. Furthermore, we know that since the end of the Caledonian Orogeny in the northern North Atlantic region, a rift system has developed along the trend of the former orogen, characterized by a series of discrete rift pulses, with intervening thermal subsidence phases. Few attempts have been made to remove the effects of these repeated stretching events from the continental margins, yet understanding the changing relationships between structural elements in the rift system is vital if sediment transport pathways are to be accurately predicted.

Future objectives

We have shown that a relatively minor alteration to the COB location on one section of the East Greenland margin has a large number of implications for hydrocarbon exploration on the Norwegian margin. We are encouraged to reassess reconstructions in the entire North Atlantic region, both in terms of COB location and removal of stretching factors from continental margins. Combined with ongoing field-based research projects in several areas of East Greenland, this will provide a unique insight into the evolution of the Northwest European margin.

To make these reconstructions possible, CASP has developed a plate reconstruction program designed specifically for use with GIS data. This enables us to translate GIS data, and change its shape, while maintaining links with attached databases. This program will be used in conjunction with 2D basin-modeling software that uses an inversion technique to obtain the spatial and temporal variation of strain rate, and hence stretching factors, from subsidence data.

Authors

Dr. Robert A. Scott leads CASP's regional studies in the North Atlantic and Arctic, and Dr. Andrew G. Whitham directs CASP's field-based research in East Greenland. CASP is a not-for-profit research group based at the University of Cambridge, England. Further details can be obtained from the authors.

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