Abstract
AbstractPolygonal fault systems, sometimes termed layer‐bound faults, are extraordinary features of many fine‐grained sedimentary successions and have been described in a significant number of sedimentary basins over the last two decades. Their formation represents an important mechanism by which fine‐grained sediments compact often resulting in a variety of complex patterns for which several controlling factors have been proposed. Here, three‐dimensional seismic data from the North West Shelf of Australia are used to interpret previously undescribed characteristics of layer‐bound fault systems where systematic horst and graben structures are the dominant structural style. Conjugate fault pairs, which form the horsts and grabens, frequently have a systematic spacing with graben‐bounding faults exhibiting a spacing of half that of the horst‐bounding faults. This systematic spacing of fault pairs indicates, (a) the presence of a mechanically weaker layer at the base of the fault system and (b) that the horizontal shortening required by the volume loss due to compaction can be accommodated without reaching saturation with respect to fault intensity. Furthermore, topological analysis indicates that areas with different patterns also have different intersection and branch characteristics, and these differences suggest that the growth of layer‐bound faults is not explained by a single model. The findings have implications for the genesis and growth of layer‐bound fault systems and the potential for cross‐stratal fluid flow.
Highlights
Polygonal faults, sometimes termed layer-b ound faults, are a ubiquitous feature in many sedimentary basins on Earth (Cartwright, 2007)
Fault spacing, and topology are inherently linked to the local geological setting
This is the first time that a systematic spacing and linked variations in topology have been described for layer-bound fault systems
Summary
Sometimes termed layer-b ound faults, are a ubiquitous feature in many sedimentary basins on Earth (Cartwright, 2007) These fault systems are commonly found in fine-grained sedimentary successions and comprise networks of normal faults with orientations that typically form polygonal geometries on bedding planes (Lonergan et al, 1998). The bedding plane geometries of the fault systems may depart from these polygonal forms due to the influence of external stresses such as basin floor slope (Higgs & McClay, 1993), tectonic faults (Hansen et al, 2004) and the influence of stratigraphic features (Ireland et al, 2011) They are an important class of fault because they typically deform fine-grained, low permeability sediments that are. Similar lithologies are encountered in stratigraphically older sequences, and at depth ranges of 200 to 500 mbsf, porosities vary between 30% and 60% (Brereton, 1992)
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