Polygonal Fault Research Articles

Display and enhancement of volumetric fault images

Fault picking is a critical, but human-labor-intensive component of seismic interpretation. In a bid to improve fault imaging in seismic data, we have applied a directional Laplacian of a Gaussian operator to sharpen fault features within a coherence volume. We computed an [Formula: see text] matrix of the second moment tensor distance-weighted coherence values that fell within a 3D analysis window about each voxel. The eigenvectors of this matrix defined the orientation of planar discontinuities, whereas the corresponding eigenvalues determined whether these discontinuities were significant. The eigenvectors, which quantified the fault dip magnitude and dip azimuth, defined a natural coordinate system for smoothing of the planar discontinuity. We rotated the data to the new coordinate system and applied the sharpening operator. By comparing the vector dip of the discontinuity to the vector dip of the reflectors, we could apply a filter to either suppress or enhance discontinuities associated with unconformities or low-signal-to-noise-ratio shale-on-shale reflectors. We have revealed the value and robustness of the technique by application to two 3D data volumes from offshore New Zealand, which exhibited polygonal faulting, shale dewatering, and mass transport complexes.

Mesozoic tectonostratigraphic evolution of the North Carnarvon Basin unlocked using regional 3D seismic

Interpretation of regional scale merged 3D seismic data sets covering the North Carnarvon Basin has for the first time enabled a detailed description of Mesozoic stratigraphic and structural features on a basin scale. Isoproportional slicing of the data enables direct interpretation of Triassic depositional environments, including contrasting low-stand and high-stand fluvial channel complexes, marginal marine clastic systems and reef complexes. Channels vary dramatically between sinuous-straight single channels within low net:gross floodplain successions, to broad channel belts within relatively high net:gross fluvial successions. The latter can be traced from the inboard part of the basin to the outer areas of the Exmouth Plateau. 3D visualisation and interpretation has demonstrated the huge variety of structural styles that are present, including basement-involved extensional faults, detached listric fault complexes, polygonal faults, and regional scale vertical strike-slip faults with flower structures. Fault trends include north–south, north–northeast to south–southwest, and northeast–southwest, with deformation events occurring mainly between the Rhaetian and Valanginian. Extensional and compressional deformation has created multiple horsts, three-way fault closures, fold belts and associated four-way anticlinal traps. Wrench tectonics may also explain pock-mark trains with the interpreted transfer of over-pressure from Triassic to Early Cretaceous levels. The use of regional scale merged 3D seismic data sets is now shedding light on tectonostratigraphic features on a basin scale that were previously unrecognised or enigmatic on 2D seismic or local 3D seismic data sets.

Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin, South China Sea

Based on 3D seismic data, the polygonal fault system (PFS) discovered in the southern slope of the Qiongdongnan (QDN) Basin was studied through fine seismic interpretation and coherent cube analysis. The results show that: ① the PFS is mostly composed of small plate-type faults, with fault length of 1–3 km, maximum fault throw of 20–40 m and dip angle of 25°–45°; ② the PFS is mainly observed in the Huangliu Formation, deposited in the early stage of fast slope depression; ③ the PFS is layer-bound and can be separated into two tiers by T31′ interface with obvious channel incision features; ④ the PF of tier2 exhibit “y” structural style, including two parallel faults, master fault and associated fault, dipping in opposite directions. Their intersection relationship can be divided into three classes, and 14 sub-classes based on the fault intersection relationship of master fault and its associated fault; ⑤ the PFS is dominated by two strikes: NE60° and NW150°, indicating that it was controlled by tensile stress produced by differential settlement between the Xisha uplift and QDN basin; and ⑥ its genesis was jointly controlled by the syneresis of clay minerals and overpressure cyclical dehydration.

Morphologies, classification and genesis of pockmarks, mud volcanoes and associated fluid escape features in the northern Zhongjiannan Basin, South China Sea

Based on new high-resolution multi-beam bathymetry and multichannel seismic reflection data, two new groups of numerous pockmarks and mud volcanoes were discovered in the northern Zhongjiannan Basin at water depths between 600 and 1400m. Individual pockmarks are circular, elliptical, crescent-shaped or elongated, with diameters ranging from several hundreds to thousands of meters and tens or hundreds of meters in depth, and they often form groups or strings. Crescent pockmarks, approximately 500–1500m wide in cross-section and 50–150m deep, occur widely in the southern study area, both as individual features and in groups or curvilinear chains, and they are more widespread and unique in this area than anywhere else in the world. Conical mud volcanoes, mostly with kilometer-wide diameters and ca. 100m high, mainly develop in the northern study area as individual features or in groups. Seismic data show that the observed pockmarks are associated with different kinds of fluid escape structures and conduits, such as gas chimneys, diapirs, zones of acoustic blanking, acoustic turbidity and enhanced reflections, inclined faults, small fractures and polygonal faults. The mapped mud volcanoes appear to be fed from deep diapirs along two main conduit types: the conventional conduits with downward tapering cones and another other conduit type with a narrow conduit in the lower half and emanative leakage passages in the upper half. Various types of pockmarks are found and a comprehensive pockmark classification scheme is proposed, according to: (a) their shape in plan view, which includes circular, elliptical, crescent, comet-shape, elongated and irregular; (b) their magnitude, which includes small, normal, giant and mega-pockmarks; and (c) their composite pattern, which includes composite pockmarks, pockmark strings and pockmark groups. For the genesis of the crescent pockmark (strings), a 5-stage speculative formation model is proposed, implying possible controlling factors of gravity sliding/slumping, fluid escape activity and sandbodies intrusion. Seismic data suggest that the mud volcanoes have likely undergone two episodes of evolution, which include a widespread drastic formation of mud volcanoes with dome-like structures, a wide range of seepage occurrences and the formation of complex sediments in the first stage, and the appearance of pockmarks, conical mud volcanoes (groups), and the formation of deformed, filled or broken buried complex sediments by continuous fluid flow processes during stage 2, in the Pliocene and the Quaternary. Pockmark gullies are extensively found along the slope, due to the interaction of slope failure and fluid escape processes. The Cenozoic sedimentary cover is thin in the Northern Zhongjiannan basin, which evolved from a Late Cretaceous or Palaeogene–Oligocene rift to the Neogene–Quaternary post-rift thermal subsidence, and therefore intense deep thermogenic fluid flow can easily affect the soft Cenozoic sediments, produce complex sediment deformation, and form numerous fluid flow structures at the seafloor, in particular pockmarks and mud volcanoes that dominantly emerged during the period of post-rift thermal subsidence and neotectonic movement since ca. 5.5Ma.

Polygonal deformation bands

We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1–5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.

Interpretation of the Penobscot 3D seismic volume using constrained sparse spike inversion, Sable sub-Basin, offshore Nova Scotia

In this study we investigate the utility of constrained sparse spike inversion (CSSI) applied to a small (87 km2), public domain, 3D seismic volume at Penobscot in the Sable sub-Basin, offshore Nova Scotia. The objective is to establish the effectiveness of this technique at Sable, in mapping absolute acoustic impedance that can be directly and quantitatively related to lithology. The intent is to subsequently apply the workflow to large 3D seismic volumes sub-regionally, with the goal of identifying and mapping, source, seal, reservoir, and overpressured intervals either not known, or partially known, from well control. Absolute acoustic impedance is an incremental parameter that is not explicit in the reflection amplitude cube and is the primary driver for undertaking this inversion. It has quantitative advantages over relative acoustic impedance that can be derived using ‘fast-track’ inversion techniques. CSSI at Penobscot has been completed and provides a valid geological model in the upper Jurassic to lower Cenozoic interval that is constrained by wireline logs at two well penetrations (one of which has 12 m net pay). This acoustic impedance volume facilitates interpretation of: (1) low impedance Cretaceous reservoir sandstones, in both complex confined channel systems and extensive unconstrained marginal marine systems; (2) polygonal fault systems (PFS) in a high impedance, late Cretaceous, chalk; (3) interfingering of low impedance shales and high impedance carbonates at the margin of the Jurassic Abenaki Carbonate Bank.Three other inversion studies have been reported at Penobscot: Ahmad, 2013; Sayers 2013; Qayyum et al., 2014. Relative acoustic impedance is derived in each study, using ‘recursive’, ‘spectral’ and ‘coloured’ inversion algorithms respectively. None of these studies documented the PFS and Cretaceous channel systems described here and there are significant interpretation differences in the Jurassic section, which is only partly penetrated by wells at Penobscot.Based on this work at Penobscot, CSSI is the optimum technique for progressively building large sub-regional scale inversions at Sable where we need to balance work-effort, signal-to-noise ratio (spectral inversion results are very broadband but appear very noisy), reliable quantitative mapping of absolute acoustic impedance, and the identification of stratigraphic features on horizon-slices (for which ‘fast track’ inversions are also be suitable).

Perturbation of polygonal fault propagation by buried pockmarks and the implications for the development of polygonal fault systems

Evidence is presented that constrains the earliest part of the growth history of a polygonal fault tier. Using high resolution, 3D seismic data from the West African margin, we document the interaction between a shallow buried, wedge shaped polygonal fault tier and pockmarks located at the base of the fault tier.Strongly curved patterns of perturbed polygonal faults are mapped in the shallowest portions of a tier overlying a series of pockmark craters. These perturbed patterns differ markedly from the ‘background’ polygonal pattern developed away from areas of pockmark influence. Perturbation is attributed to local stress effects relating to residual topography of the underlying pockmarks. By analysing the thickness variations of mapped layers within the polygonally faulted tier, we demonstrate that any residual topography was most likely healed after approximately one third of the total tier thickness was deposited. Hence the nucleation of the polygonal faults into their perturbed propagation trajectories must have been established within the first 100–150 m of burial.From these observations of early nucleation in the basal third of a tier, we propose a new model of polygonal fault growth in which individual faults nucleate near the base of a tier at an early stage of burial and then propagate preferentially upwards with basal tips restricted by an underlying mechanically competent unit the base of the tier. Continued growth then shifts the locus of displacement, such that final displacement distributions are more axio-symmetrically disposed with respect to the base and top of the tier.

Diffraction imaging of polygonal faults within a submarine volcanic terrain, Maverick Basin, south Texas

Polygonal fault systems are common structural features of intracratonic continental margins. The map-view geometry of these faults became apparent with the use of powerful fault-imaging seismic attributes, such as coherence and curvature. However, these attributes lack the amplitude information necessary for lithological evaluation. We developed a 3D diffraction volume that not only imaged faults but also contained amplitude information. From the unmigrated stack volume, we extracted diffractions that were transformed into amplitude envelope and root-mean-square amplitude volumes. These attributes, together with clay volume ([Formula: see text]) data, were extracted along interpreted horizons and fault planes. Crossplots between seismic attributes and [Formula: see text] enabled linear relationships between the attributes and [Formula: see text], which were used to infer lithological composition within fault zones. Our results found that, although the fault zones were clay filled, some subvertically inclined clay-poor zones that could serve as permeable pathways were present along the fault planes. In map view, images from diffraction volume were comparable with those obtained from coherence and curvature attributes; however, diffraction images appeared to be busy because of the huge number of diffracted waves embedded in the data. In addition, we found that, although [Formula: see text] increases with increasing diffraction energy, no systematic relationship exists between [Formula: see text] and curvature, or between [Formula: see text] and coherence. As such, curvature and coherence cannot be used to predict lithological distribution within fault zones. Furthermore, we observed that the higher the diffraction energies, the higher the fluid saturation, suggesting higher impedance contrast at the diffraction points. Therefore, we determined that by analyzing diffraction data, it was possible to infer likely sediment variations that largely control permeability within fault zones.

Structural geology and well planning in the Clair Field

Abstract The Clair Field is a giant oilfield located approximately 70 km west of the Shetland Isles, UK. It was discovered in 1977 and brought on stream some 28 years later. Key to unlocking its economic potential was a series of appraisal wells drilled in the early 1990s that identified fractures as the primary production mechanism. Structural geology contributed in several ways to the detailed planning of the development and appraisal wells. In the sandy (Tertiary) tophole section, outcrop analogues and offset wells were used to establish an appropriate standoff from major faults. This was to mitigate the risk of wellbore instability in what is otherwise a relatively benign sequence to drill. The mid-section, Upper Cretaceous mudstone is prone to wellbore instability, believed to be caused by strength anisotropy with respect to bedding. Here, polygonal faulting may contribute directly to wellbore instability. The associated bed rotation also influences anisotropic failure, which depends in part on the wellbore-to-bedding intersection angle. An example is given of how an understanding of the structural evolution of the overburden section impacts well casing placement. Finally, judgement on the nature of the faulted contact between two fault blocks was required for the pressure prognosis of a planned well.

Geostatistical analysis of seafloor depressions on the southeast margin of New Zealand's South Island — Investigating the impact of dynamic near seafloor processes on geomorphology

Abstract Seafloor depressions are widespread on the present-day continental slope along the southeast coast of New Zealand's South Island. The depressions appear to be bathymetrically constrained to depths below 500 m, correlating to the top of the gas hydrate stability zone, and above 1100 m. Similar depressions observed on the Chatham Rise are interpreted to have formed as a result of gas hydrate dissociation, leading to the hypothesis that a similar origin can be applied for the depressions investigated in this study. Our investigation, however, has found limited geophysical or geochemical evidence to support this hypothesis. The objective of this paper is to examine whether a causal relationship can be established between potential mechanisms of depression formation and the present-day seafloor geomorphology. Geostatistical analysis methods applied to multibeam bathymetry and interpretation of 3D seismic data have been used to empirically describe the geomorphology of the seafloor depressions and investigate potential correlations between geomorphology and other processes such as current flow along the shelf and slope in this region and underlying polygonal fault systems. Although the results of our analysis do not preclude that the seafloor depressions formed as a result of gas hydrate dissociation, neither does our geophysical or geochemical evidence support the theory. Therefore, we propose an alternative mechanism that may have been responsible for the formation of these structures. Based on the evidence presented in this study, the most likely mechanism responsible for the formation of these seafloor depressions is groundwater flux related to the interaction of current systems and the complex geomorphology of submarine canyons on the southeast coast of the South Island.

Polygonal faulting and seal integrity in the Bonaparte Basin, Australia

Polygonal fault systems are observed in over 100 sedimentary basins worldwide where they are confined to fine-grained strata and have the potential to impact on seal integrity for CO2 storage and hydrocarbon reservoirs. 3D seismic reflection and well data have been used to characterise a layer-bound polygonal fault system and gas chimneys in the Bathurst Island Group regional seal, Petrel Sub-basin, offshore NW Australia. Segmented fault arrays extend through the entire faulted interval which contains at least 10 tiers of polygonal faults each spanning ∼4 Myrs of sediment deposition. Polygonal fault densities and intersections reach a maximum at 1000–1100ms TWT and in some cases extend into the Sandpiper Sandstone reservoir at >1200–1300ms TWT. Down-dip displacement profiles are symmetrical and increase progressively across tier boundaries towards maxima of ∼25–47ms between 600 and 1100ms. These systematic displacement variations are similar to those of tectonic faults and suggest that fault segments in different tiers develop synchronously as kinematically coherent arrays. Three-dimensional imaging of the polygonal fault system indicates that throughout the Bathurst Island Group the fault network is well connected vertically and horizontally. Polygonal faulting may locally promote gas flow through the seal to the seabed suggesting that, where faulted, the Bathurst Island Group may not be an effective seal for CO2 storage in the study area.

Focused fluid flow systems and their implications for hydrocarbon and gas hydrate accumulations in the deep-water basins of the northern South China Sea

The types, influencing factors and their evolutions of focused fluid flow systems in the northern South China Sea (SCS) are deeply studied based on high-resolution 3D and 2D seismic data. These systems include mud volcanoes, mud diapirs, gas chimneys, pipes, polygonal faults and tectonic faults. They are influenced by both tectonism and sedimentation. The gas chimneys, mud diapirs and mud volcanoes may have relationship of unidirectional evolution and we can separate them from whether the sediments are involved and whether the sediments or fluids erupt into the seawater or not. Finally, a development model of focused fluid flow systems in the northern SCS is proposed. Because their great importance for the deep-water resource exploration (hydrocarbon and gas hydrate), they should receive much attention from now on.

3D seismic analysis of complex faulting patterns above the Snapper Field, Gippsland Basin: implications for CO2 storage

Mechanical damage (e.g. faults and fractures) related to tectonic forces and/or variations in formation pore pressures may enable the leakage of fluids through otherwise effective seal rocks. Characterisation of faults and fractures within seals is therefore essential for the assessment of long-term trap integrity in potential CO2 storage sites. 3D seismic reflection data are used to describe a previously unrecognised network of extensive, small Miocene-age faults with displacement of generally <30 m and lengths that vary between ∼300 and 2500 m above the Snapper Field, in the Gippsland Basin. The Snapper Field is a nearly depleted oil and gas field that presents an attractive site for potential CO2 storage due its structural closure and because it has effectively retained significant natural hydrocarbon (including CO2) columns over geological time-scales. Volume-based seismic attributes reveal that this fault system is located within the Oligocene Lakes Entrance Formation of the Seaspray Group, which acts as the regional seal to the Latrobe Group reservoirs in the Gippsland Basin. Detailed analysis of fault lengths and linkages suggests that the Miocene faults are non-tectonic, polygonal faults, although the displacement analysis of fault segments reveals strong correlations with the both the structure of the underlying Top Latrobe surface and normal faults that segment the Latrobe Group reservoirs, suggesting that the development of this fault system has been influenced by underlying structures. The geological evidence for long-term retention of hydrocarbons within the Snapper Field suggests that this fault system has not compromised the integrity of the Lakes Entrance Formation seal, although elevated pore pressures during CO2 injection could potentially lead to reactivation of these structures.

Distributed normal faults in the Niobrara Chalk and Pierre Shale of the central Great Plains of the United States

Normal faults occur in the Niobrara Chalk and Pierre Shale in numerous localities in South Dakota, Nebraska, and Kansas. Lakeshore outcrops provide a window into strata that otherwise outcrop poorly. Various criteria were used to distinguish mass-wasting features from faults that formed at the subsurface. Locally, these faults have variable orientations and throws that vary from centimeters to tens of meters. In the Niobrara Chalk, the fault zones typically display well-developed striae, occasional slickensides, limited to absent damage zones, and dilational jogs with coarse, microvein calcite fill. In places, the faults are associated with gentle monoclines and folds, but elsewhere they occur in horizontal strata. A weathering bias suggests these faults are more common than observed. At any one locality, these faults have been or could be attributed to local tectonism, differential compaction, and/or glacial rebound. If due to tectonism, taken in aggregate, they indicate fairly widespread (albeit low-strain) activity, which is consistent with the idea of a critically stressed continental interior with migrating sites of faulting. An alternate and preferred hypothesis is that these are polygonal faults due to diagenetically driven deformation in fine-grained mud rocks. Subsurface polygonal fault systems have been described from the Niobrara Chalk and Pierre Shale in the Denver-Julesburg Basin area in eastern Colorado. Smectitic clays, associated with polygonal faulting elsewhere, are abundant in these units. The fault kinematics (extension in multiple directions), widespread character, and other traits are consistent with an origin by diagenetically driven deformation.

Can polygonal faults help locate deep-water reservoirs?

Polygonal faults are compaction-related normal faults that develop in very fine-grained sedimentary successions. Despite their ubiquity, few studies have highlighted the application of polygonal fault mapping to identifying deep-water sandstone reservoirs. We use three-dimensional seismic and borehole data from the Maloy slope, offshore Norway to demonstrate that the distribution, cross-sectional geometry, and throw characteristics of polygonal faults can be used to locate deep-water sandstone reservoirs. We identify two tiers of polygonal faults in the Cretaceous to lower Paleogene succession. The lowermost tier is stratigraphically restricted to the lower Barremian-to-lowermost Turonian succession and likely formed during the early Turonian. The uppermost tier spans the entire Cretaceous succession and likely formed during the Maastrichtian. An abrupt decrease in the thickness of the upper tier occurs where a 92-m (302-ft) thick, sandstone-rich slope fan is developed in the upper Turonian interval. Furthermore, the lower tips of faults in the upper tier, which are defined by anomalously high throw gradients, cluster at the top of the sandstone, resulting in decoupling of this tier from the underlying, lower Turonian tier. We interpret that faults in the upper tier nucleated above the reservoir across the entire slope and that the slope-fan sandstone acted as a mechanical barrier to downward fault propagation, resulting in abrupt thinning of the tier at the sandstone pinchout. We demonstrate polygonal faults are not simply an academic curiosity; mapping of these enigmatic structures can have practical applications for the delineation of a variety of reservoir types in hydrocarbon-bearing sedimentary basins worldwide.

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt: REPLY

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt: REPLY

Seafloor mounds, craters and depressions linked to seismic chimneys breaching fossilized diagenetic bottom simulating reflectors in the central and southern Scotia Sea, Antarctica

Based on an extensive dataset including swath bathymetry, chirp sub-bottom profiler (TOPAS) and multichannel seismic reflection profiles obtained during four cruises in the Scotia Sea aboard the R/V Hespérides, we report a variety of seismic and morphological structures related to focused fluid flow in the Scan Basin (southern Scotia Sea) and the central Scotia Sea (Antarctica). We show that both positive-relief (mounds) and negative-relief (craters and elongated depressions) seafloor morphologies are associated with deep seismic chimneys that link the deep source zone to the subsurface structures through a network of fractures that progressively breach sub-horizontal bands of anomalously high-amplitude reflections. Based on the recognition that these bands of reflections generally mimic the seafloor topography and locally cross-cut the stratigraphic seismic reflections, we recognize three different bottom simulating reflectors (BSRs). According to the theoretical model for hydrate and silica diagenesis stability conditions in the central and southern Scotia Sea and the calculations of temperature and seismic polarity for the three BSRs, we infer that BSR-2 and BSR-3 are reflections caused by the transformation between Opal-A/Opal-CT and Opal-CT/Quartz, respectively. We thus postulate that the successive diagenetic fronts were caused by significantly high geothermal gradients during the early-middle Miocene. In contrast, the low temperatures calculated for the depth of the BSR-1 event rule out its diagenetic origin but delineate the base of the gas hydrate stability zone (GHSZ).An evolutionary model is proposed to explain the plumbing system and chimney structures that help the focused flow of gas-rich fluids to migrate into the subsurface. Firstly, the formation of silica transformation zones may have acted as reservoir traps during Neogene times. Secondly, the progressive decrease of heat flow during the late Pliocene and Quaternary favored the development of the networks of polygonal faults forming collapses and downward tapering chimneys. Finally, seafloor mounds are formed as a result of the continuous injection of gas-enriched fluids through these networks of fractures; they are transformed into gas hydrates above the present base of the GHSZ and move upwards by buoyancy drive as they lose density and increase their volume. We present these structures as type cases that might represent highly concentrated hydrates around local seafloor fluid venting structures. Furthermore, they may be one of the most important conduits into the ocean–atmosphere system for deep methane in the Antarctic seafloor. The breach of BSRs influenced by global warming may induce the catastrophic release of greenhouse gases to the ocean–atmosphere system and, in turn, impact on the Earth's evolution.

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt: COMMENT

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt: COMMENT

Are outcrop studies the key to understanding the origins of polygonal fault systems?

Polygonal fault systems (PFSs) are extraordinary features of many sedimentary basins. They consist of vast numbers of small (<100 m throw) normal faults aggregated into complex three-dimensional (3-D) arrays with a characteristically polygonal map pattern, and fault lengths and spacing of the order

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt

Research Article| June 01, 2014 Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt Barbara J. Tewksbury; Barbara J. Tewksbury 1Department of Geosciences, Hamilton College, Clinton, New York 13323, USA Search for other works by this author on: GSW Google Scholar John P. Hogan; John P. Hogan 2Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, Missouri 65409, USA Search for other works by this author on: GSW Google Scholar Simon A. Kattenhorn; Simon A. Kattenhorn 3Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA Search for other works by this author on: GSW Google Scholar Charlotte J. Mehrtens; Charlotte J. Mehrtens 4Department of Geology, University of Vermont, Burlington, Vermont 05405, USA Search for other works by this author on: GSW Google Scholar Elhamy A. Tarabees Elhamy A. Tarabees 5Department of Geology, Damanhour University, Damanhour 22516, Egypt Search for other works by this author on: GSW Google Scholar Author and Article Information Barbara J. Tewksbury 1Department of Geosciences, Hamilton College, Clinton, New York 13323, USA John P. Hogan 2Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, Missouri 65409, USA Simon A. Kattenhorn 3Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA Charlotte J. Mehrtens 4Department of Geology, University of Vermont, Burlington, Vermont 05405, USA Elhamy A. Tarabees 5Department of Geology, Damanhour University, Damanhour 22516, Egypt Publisher: Geological Society of America Received: 08 Dec 2013 Revision Received: 02 Mar 2014 Accepted: 04 Mar 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (6): 479–482. https://doi.org/10.1130/G35362.1 Article history Received: 08 Dec 2013 Revision Received: 02 Mar 2014 Accepted: 04 Mar 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Barbara J. Tewksbury, John P. Hogan, Simon A. Kattenhorn, Charlotte J. Mehrtens, Elhamy A. Tarabees; Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt. Geology 2014;; 42 (6): 479–482. doi: https://doi.org/10.1130/G35362.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Although polygonal fault systems and related features are common in fine-grained sediments in modern submarine basins and have been studied in basins worldwide using three-dimensional (3-D) seismic data, extensive on-land exposures have remained elusive. We report here on the discovery of a polygonal fault system occurring in nearly continuous surface exposure over ∼900 km2 in chalk of the Cretaceous Khoman Formation near Farafra Oasis, Egypt. Field exposures reveal polygon boundaries defined by clusters of dozens of normal faults with strongly grooved fault surfaces and coarse calcite veins along faults with evidence for multiple fluid flow events. Geometric patterns and fault intersections reveal that mechanically interacting normal faults with multiple orientations were active contemporaneously in a horizontal strain field that was essentially isotropic and extensional. We interpret the very steep dips (∼80°) to reflect fault initiation in response to elevated pore fluid pressures. In the uppermost part of the Khoman Formation, a terrain of isolated circular structures displaying shallow inward dips overlies the polygonal fault network. The spatial relationship to the underlying faults is consistent with these small circular basins having formed as fluid escape structures as the polygonal fault system evolved. Outcrops in the Khoman Formation provide an unprecedented look into the 3-D geometry of a polygonal fault system, providing context for the analysis of analogous systems in marine basins and other on-land exposures. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.