Layer-bound compaction faults in fine-grained sediments

Abstract

This paper describes examples of a recently recognized type of soft-sediment deformation associated with early compaction of fine-grained sediments. This type of deformation was originally described from the North Sea Basin, where Paleogene slope and basin-floor claystones are deformed 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 development of this regionally extensive polygonal fault system has been attributed to volumetric contraction during early compactional dewatering on the basis of detailed strain measurements carried out using high-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 examples of layer-bound faults are that the deformed units are only found in marine depositional settings, are dominantly composed of ultrafine-grained smectitic claystones or carbonate chalks, and are characterized by high porosity and extremely low permeability. Other factors such as sedimentation rate, organic carbon content, age, depth of burial, methane content, and pore-fluid chemistry are not systematically correlated with this deformational response. The correlation between distribution of deformed units and ultrafine grain size suggests that the deformation mechanism is related to colloidal properties as part of this type 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 faulting is an expression of the process of syneresis, whereby pore fluid is expelled from sedimentary gels under the spontaneous action of osmotic or electrochemical forces.