Degradation of compressional fold belts:Deep-water Niger Delta

摘要

Three-dimensional seismic interpretation of the toe-of-slope region of the west Niger Delta reveals a range of erosional and depo-sitional features that are the result of the degradation of thrust-propagation folds and include(1)backlimb and forelimb failures that cause debris flows and deposits no more than a few tens of meters thick;(2)large slumps with evidence for internal deformation;(3)more enigmatic ovoid depressions,probably formed as a result of slumping and bottom-current erosion;and(4)degradation by deep-water channel erosion and channel-margin slumping.Thrust-propagation folds in the study area have up to 200 m(656 ft)of relief on the sea floor and are at various stages of degradation and burial.The dominant style of degradation of these folds occurs as retro-gradational,small-volume failures that form thin deposits at or below seismic resolution.Slope morphology,sedimentology,and the presence of anisotropies affect the type of failure that occurs.The backlimb of one fold is long and shallow(6 deg ),and the failed sediment masses have longer runout distances than on the forelimb,which has a steeper slope(15 deg ),which abruptly shallows,causing the flow velocity to drop.A generic model for fold degradation in this submarine setting is synthesized,and it is predicted that a significant proportion of the sediment deposited in adjacent minibasins has been recycled from the fold crests and is a nonreservoir.Stratigraphic variability means that along-strike and across-fault correlation of deposits and erosional surfaces is difficult at shallow levels and highly problematic in buried,potentially prospective,degraded folds.