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Turbidite-Debrite Couplets in Deep Sea Fan Systems – Evidence for Flow Partitioning and Transformation from Subsurface and Outcrop Examples

 

Davis, Christopher E.1, Peter Haughton1, William McCaffrey2 (1) University College Dublin, Dublin, Ireland (2) University of Leeds, Leeds, United Kingdom

 

The deposits of turbidity currents and debris flows dominate the record of deep water fan systems and are commonly ascribed to separate remobilisation events However, beds suggesting repeated coupling between a forerunner turbidity current and an ensuing debris flow are now recognised across a range of systems. Such composite event beds dominate the record of the lateral and distal Forties fan system in the northern North Sea, and the early fill of Carboniferous deep water toughs in NW Europe. In the case of the Forties system, outer fan event beds comprises a m-scale transitional association between massive and dewatered sandstone (=turbidity current), commonly overlain by banded sandstone (=transitional flow) and then a clast-rich and variably carbonaceous argillaceous sand (=debris flow). Most events are capped by a thin (cm-scale) laminated and graded sand to silt cap (= trailing dilute turbidity current). The composite beds are found both close to and remote from salt highs and therefore do not reflect local remobilisation on distal slopes. They are instead interpreted as products of inner fan incision. Analogous m-scale turbidite-debrite couplets in the Carboniferous Pennine Basin were common during establishment of sand supply to deep water. Large cliff exposures at Mam Tor show similar associations of massive sandstone and clast-rich sandstones and establish that the upper debrite divisions can extend laterally for 100's of metres, highlighting their potential as baffles to fluid flow in the subsurface. The dominance of such beds during establishment of the fan system implies significant disequilibrium and up-slope erosion.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California