The Cause of High Compaction of Barents Shelf Sediments: Ice Versus Sediment Loading
Ebbe H. Hartz1, Sergei Medvedev2, Dani Schmid3, Stephen A. Clark4, and Jan Inge Faleide4
1Det norske oljeselskap, Oslo, Norway.
2Physics of Geological Processes, Oslo University, Oslo, Norway.
3Geomodelling Solutions, Zürich, Switzerland.
4Department of Geoscience, Oslo University, Oslo, Norway.
Conventional models for the Barents Shelf suggest kilometer-scale erosion of Mesozoic sediments in one or several phases during the Cenozoic. These models are based on low porosities/high seismic velocities in the pre-glacial (Cenozoic and older) sediments, subcrop patterns, geothermal gradients, and geochronological data. There are, however, several enigmas with these models. 1) The proposed eroded masses are far larger than the masses proposed to be the depositional products of the erosion. 2) Our 3D isostatic restoration models show that if the proposed eroded masses are placed back, the sediments would be well above sea level at the time of their deposition. However, the large lateral extent of the proposed eroded Mesozoic sediments indicates a submarine origin, however. 3) The large erosion estimates suggest a near-equal thickness of Mesozoic sediments, which raise issues of what could cause such a continental scale, km-thick, late Mesozoic basin. As an alternative to km-deep erosion, we suggest that some of the loading recorded in compaction of sediments was caused by the massive ice-sheet. Independent estimates of Plio/Pleistocene ice cover above the entire the Barents Shelf range from 1 to 2+ km. Ice is about half as dense as uncompacted sediments, and thus 1 km thick ice cover can reduce erosion estimates by ~0.5 km, suggesting that a significant part of the compaction traditionally attributed to sediment loading may instead reflect ice loading. Such an explanation resolves problems related to the mass budget and sedimentation processes, but not the high geothermal gradient, another argument for major erosion. There can, however, be many alternative causes for a high geothermal gradient, and particularly high paleotemperatures. Furthermore, one should acknowledge that the temperature of the system is usually far from static equilibrium and that all measurements of paleotemperature (hydrocarbon maturation, vitrinite, apatite fission track) leave considerable room for debate. We present here geodynamic models of the Devonian to present Barents Shelf, both assuming the conventional dramatic erosion of sediments and with ca. half of the proposed erosion replaced by ice loading. Both models are possible, but the latter resolves some of the geological enigmas listed above, and thus appears more plausible.
AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.����������������������������������������������