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JOHNSON, KIT and KERRY GALLAGHER, Imperial College of Science,Technology and Medicine

Abstract: The Onshore Mesozoic Erosion History of the NE Greenland Margin and Its Implications for Offshore Basin Evolution

The continental margin of NE Greenland is perhaps the least studied of the North Atlantic margins. This margin is characterised by a long history of post-Caledonian rifting, commencing in the Late Devonian and ending with the onset of sea floor spreading at ~55 Ma. This tectonic history resulted in a distribution of the syn-rift sediments that was well established before the formation of the continent-ocean boundary (COB). Therefore, sediments derived from erosion of one of the margins of this part of the North Atlantic may now reside in basins attached to its conjugate partner. Given the proximity of the COB to the present-day coast, this is likely to be particularly true of the East Greenland margin between 72°-74°N. Prior to break-up, three of the most important frontier regions in the NE Atlantic, the Vøring, Møre, and Faroes-Shetland Basins, were adjacent to the NE Greenland continental margin between latitudes 66° and 75°N. These basins may now be the repository for substantial volumes of Jurassic and Cretaceous sediments that were derived, not from the Norwegian margin, but from East Greenland Consequently, the distribution of reservoir quality sands in the currently distal parts of these basins is likely to be controlled primarily by the nature of the eroding source region on the east Greenland margin during the Mesozoic and Tertiary.

The spatial and temporal distribution of onshore erosion can be constrained with apatite fission track thermochronology. A suite of 200 rock samples has been collected from NE Greenland between 68° and 75°N from the coast to 200 km into the continental interior. The sampling strategy was designed specifically to characterise the pre-break-up denudation chronology. In particular, 20 vertical profiles have been sampled, taking advantage of the relief created by glacial erosion.This very recent erosion has exposed samples from different levels in the upper few kilometres of the crust. Samples from individual vertical profiles clearly will experience similar thermal histories, with the samples now at lower elevations being exposed to higher temperatures as they resided deeper in the pre-glacial crustal section. Further sampling localities were selected to maximise the constraints on pre-break-up movement of fault blocks. The new data has been integrated with regional scale structural data and what is known of the record of Mesozoic and early Tertiary sedimentation to provide a detailed picture of the onshore erosion and offshore deposition of the North Atlantic margins prior to break-up.

Results indicate that maximum temperatures in the Devono-Carboniferous basins were reached at the end of the Carboniferous to early Permian.This timing lends support for the contention that a significant section of the East Greenland "Devonian" basin is Carboniferous in age. Maximum paleo-temperatures of presently outcropping rocks are estimated to range between 100° and 150°C. Temperatures of this magnetude imply cumulate erosion from these basins of 3 to 4 km depending on the chosen values of thermal conductivities for the Devono-Carboniferous sediments and mean early Permian Earth surface temperature. Following these maximum temperatures, regional cooling occurred during the Permian. This is linked to well recognised Permian erosion and the development of a widespread mid-Permian peneplain.

The next important phase of cooling commenced in the Early Jurassic (Pliensbachian), predating the Middle Jurassic marine transgression and deposition of the Vardekloft Formation. Cooling is interpreted as recording erosion resulting in initial deposition in depocentres.presently located in Early Jurassic basins to the east of the present-day onshore Jurassic outcrops. The onshore margin to the north of Kong Oscar Fjord and east of the Post-Devonian Main Fault was exposed at this time, and the erosion produced a peneplain of Permian and Triassic sediments and Caledonian basement.

A third phase of cooling in the Late Jurassic is linked to the break-up of the relatively wide fault blocks of the earlier Jurassic rift system (~90 km) into the narrower blocks seen today (5-30 km). Erosion related cooling is spatially linked to fault movement on the Gauss Halvø Fault system.

A final phase of Mesozoic cooling occurs in the interval 107 to 86 Ma (mid-Cretaceous). This cooling is related to renewed rifting in the Albian and is best resolved in those sections that lie in the footwall of the Gauss Halvø Fault system.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England