LAYER-BOUND COMPACTION FAULTS IN FINE-GRAINED SEDIMENTS

This paper describes examples of a recently recognized type of soft-se diment deformation associated with early compaction of fine-grained se diments, This type of deformation was originally described from the No rth Sea Basin, where Paleogene slope and basin-floor claystones are de formed over an area of >150 000 km(2) by a layer-bound system of minor extensional faults arranged in polygonal patterns in map view The dev elopment of this regionally extensive polygonal fault system has been attributed to volumetric contraction during early compactional dewater ing on the basis of detailed strain measurements carried out using hig h-resolution three-dimensional seismic data. A comprehensive review of published two-dimensional and three-dimensional seismic data from 27 other layer-bound fault systems from many different sedimentary basins is presented in this paper. The only factors common to all 28 example s of layer-bound faults are that the deformed units are only found in marine depositional settings, are dominantly composed of ultrafine-gra ined smectitic claystones or carbonate chalks, and are characterized b y high porosity and extremely low permeability, Other factors such as sedimentation rate, organic carbon content, age, depth of burial, meth ane content, and pore-fluid chemistry are not systematically correlate d,vith this deformational response. The correlation between distributi on of deformed units and ultrafine grain size suggests that the deform ation mechanism is related to colloidal properties as part of this typ e of compactional response. The restricted distribution of layer-bound fault systems to predominantly pelagic depositional units with often low sedimentation rates is compatible with a recently presented model of volumetric contraction during early burial. We build on this model of fully three-dimensional compaction to propose that layer-bound faul ting is an expression of the process of syneresis, whereby pore fluid is expelled from sedimentary gels under the spontaneous action of osmo tic or electro chemical forces.