Peter D.W. Haughton¹, William D. McCaffrey², Maarten Felix³

(1) University College Dublin, Dublin 4, Ireland
(2) Leeds University, Leeds, United Kingdom
(3) Leeds University

ABSTRACT: Origin and Significance of 'Linked' Debrites: a Key Reservoir Heterogeneity in Sandy Turbidite Systems

Sediment gravity flow deposits in many extensional/passive margin settings are dominated by a distinctive type of bed couplet comprising (1) a structureless and/or dewatered sandstone bed capped by (2) a heterogeneous mudclast and carbonaceous-charged unit with complex soft-sediment deformed internal fabrics. Although the linkage has often been explained as either 'rip-down' by sand injection into overlying, finer grained lithologies, or interfingering of unrelated or 'triggered' debris flows with the deposits of turbulent flows, it is now thought the capping divisions represent a hitherto unrecognised class of 'linked' debrite that is an integral part of the sand-emplacement event. This is because unit (2) deposits often contain exotic material and have dispersed sand grains which are coarser than the underlying sand. They also show evidence of emplacement over still consolidating sand. The debris flows may outrun the sand flows to which they are linked. Candidate mechanisms include: (a) the capping clast-rich units may be vestiges of debris flows which only partially transformed to turbidity currents up-slope. The remnant debris flows continued down slope over a carpet of consolidating wet sand laid down by the vanguard turbidity current. This promoted enhanced mobility and competence (b) the debris flows arose by bulking of a lagged tail to sandy turbidity currents; low-density flaky clasts of mud and carbonaceous matter (either incorporated at source during the failure process, or acquired during up-slope erosion) were hydraulically segregated and bulked up to form a following debrite. Preliminary experimental- and numerical-modeling results evaluating the second mechanism are presented.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado