The development of polygonal fault systems by syneresis of colloidal sediments

Polygonal fault systems occur in numerous sedimentary basins worldwide, are generally located on passive margins in onlap fill units and tend to compr ise the finest grained sediments in this geological setting. These fault sy stems have been most thoroughly described in the central North Sea basin an d the detailed structure shows a significant correlation with lithological variations, both vertically and laterally. Extension measured in stacked de coupled tiers of polygonal faults correlates positively with both clay frac tion and smectite content. Lateral facies variations are also observed and indicate that time-equivalent sequences upslope from the smectite-rich poly gonally faulted sediments are coarse-grained, clay-poor and undeformed. Thi s leads us to believe that the structure and geometry of the fault system a re controlled by the colloidal nature of the sediments, and that the volume tric contraction measured on seismic sections can be accounted for by syner esis of colloidal smectitic gels during early compaction. Syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid. This process occurs due to the domination of interparticle attractive forces in marine clays, dependent on environment, and is governed by the change of gel permeability and viscosity with progr essive compaction. The process of syneresis can account for a number of str uctural features observed within the fault systems, such as tiers of faults , the location of maximum fault throw and growth components at upper fault tips. As such, this paper represents the first attempt to correlate microsc ale properties of clay-rich sediments to their macroscale seismic character . (C) 1999 Elsevier Science Ltd. All rights reserved.