Joe Cartwright

Fault Growth by Segment Linkage - An Explanation for Scatter in Maximum Displacement and Trace Length Data from the Canyonlands Grabens of SE Utah

Maximum displacement (D) and trace length (L) data for a population of 97 normal faults from the Canyonlands Grabens region of SE Utah are presented. Values of L range from 100 to 6500 m, and of D from 1.5–155 m. The data exhibit a scatter between D and L of about half an order-of-magnitude. This is comparable to that exhibited in other published single population datasets. This magnitude of scatter cannot be attributed either to measurement errors or to variation in mechanical properties. We propose that a scatter of this magnitude can be explained by a general model for fault growth by segment linkage, whereby large incremental increases of length attained as fault segments link temporarily exceed incremental increases in displacement. This results in deviations from an idealized growth path, and a step-like evolution expressed between D and L.

Seal bypass systems

We present an interpretational framework for the analysis of a diverse set of geological structures that breach sealing sequences and allow fluids to flow vertically or subvertically across the seal. In so doing, they act as seal bypass systems (SBS). We define SBS as seismically resolvable geological features embedded within sealing sequences that promote cross-stratal fluid migration and allow fluids to bypass the pore network. If such bypass systems exist within a given seal sequence, then predictions of sealing capacity based exclusively on the flow properties (capillary entry pressure and hydraulic conductivity) of the bulk rock can potentially be negated by the capacity of the bypass system to breach the grain and pore network. We present a range of examples of SBS affecting contrasting types of sealing sequences using three-dimensional (3-D) seismic data. These examples show direct evidence of highly focused vertical or subvertical fluid flow from subsurface reservoirs up through the seal sequence, with leakage internally at higher levels or to the surface as seeps.We classify SBS into three main groups based on seismic interpretational criteria: (1) fault related, (2) intrusion related, and (3) pipe related. We show how each group exhibits different modes of behavior with different scaling relationships between flux and dimensions and different short- and long-term impacts on seal behavior.

Layer-bound compaction faults in fine-grained sediments

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.

High resolution fault displacement mapping from three-dimensional seismic data: evidence for dip linkage during fault growth

Abstract Detailed mapping using high resolution three-dimensional seismic data has revealed a number of sub-horizontal anomalies in the distribution of vertical displacement (throw) on the planes of growth faults in the Gulf of Mexico. Recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled, because they represent local decreases in throw which are confined to small, discrete parts of the fault planes. The distribution of the anomalies is inconsistent with model displacement fields of quasi-elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip-line propagation. An alternative model is proposed, whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction—‘dip linkage’. Overlap and linkage of fault tips in the dip direction results in relay structures that are sub-parallel to fault strike and therefore displacement minima that are sub-horizontal on normal and thrust faults. However, since they are orthogonal to the main slip direction on the fault, these structures have a low preservation potential and are therefore unlikely to be well resolved on cross-sectional seismic profiles.

Episodic basin-wide fluid expulsion from geopressured shale sequences in the North Sea basin

A proposed general model for the episodic dewatering of thick shale successions is based on the recognition of a pervasive polygonal extensional fault network developed in the dominantly fine grained lower Tertiary of the North Sea basin. Seismic data show that the faults are arranged in stratigraphically bound structural units (tiers) that are delimited vertically by almost undeformed condensed sections, and are restricted in distribution to the lowest permeability slope and basin-floor facies. I propose an episodic three-stage mechanism to explain the fault genesis, involving (1) the development of basin-wide overpressured compartments, (2) a density inversion between the overpres-sured units and the overlying seal, and (3) natural hydraulic fracturing, pressure bleed-off, and resealing of the pressure compartment leading to a repeat of the cycle.

Fault growth by linkage: observations and implications from analogue models

Abstract Using time sequence analyses of extensional fault models we demonstrate the pivotal role played by fault segmentation in the accumulation of displacement and length during the growth of faults. Experiments are described in which incremental steps during the development of individual faults have been reconstructed from time-lapse photographs taken during model deformation. These records confirm the composite segment hierarchy of fault structure, a pattern that is frequently recognised in many natural arrays. They reveal the progressive enlargement of individual faults to be the product of a repetitive cycle of tip-line propagation, overlap and linkage between nearest neighbours. By contrasting the displacement patterns of successive increments during growth convincing evidence is also presented to suggest that individual segments of faults may remain kinematically independent once they are physically linked. This behaviour is shown to be responsible for the characteristic saw-tooth patterns often recognised in strike-parallel fault displacement profiles. Such patterns are believed to arise where relict segment boundaries remain preserved as asperities to slip, so that displacement is confined to discrete parts of a fault plane surface. Growth in this way also causes the maximum displacement (D) and surface length (L) of faults to continually change by different proportions. Incremental displacement records presented here corroborate field evidence which shows that linkage between fault segments during growth is responsible for a significant component of the spread of values often recorded in D versus L compilations. Finally, we speculate that linkage between fault segments also accounts for transient irregularities recorded in the frequency distribution of the fault length populations of each model.

Early Oligocene initiation of North Atlantic Deep Water formation.

Dating the onset of deep-water flow between the Arctic and North Atlantic oceans is critical for modelling climate change in the Northern Hemisphere and for explaining changes in global ocean circulation throughout the Cenozoic era (from about 65 million years ago to the present). In the early Cenozoic era, exchange between these two ocean basins was inhibited by the Greenland–Scotland ridge, but a gateway through the Faeroe–Shetland basin has been hypothesized. Previous estimates of the date marking the onset of deep-water circulation through this basin—on the basis of circumstantial evidence from neighbouring basins—have been contradictory, ranging from about 35 to 15 million years ago. Here we describe the newly discovered Southeast Faeroes drift, which extends for 120 km parallel to the basin axis. The onset of deposition in this drift has been dated to the early Oligocene epoch (∼35 million years ago) from a petroleum exploration borehole. We show that the drift was deposited under a southerly flow regime, and conclude that the initiation of deep-water circulation from the Norwegian Sea into the North Atlantic Ocean took place much earlier than is currently assumed in most numerical models of ancient ocean circulation.

The development of polygonal fault systems by syneresis of colloidal sediments

Abstract Polygonal fault systems occur in numerous sedimentary basins worldwide, are generally located on passive margins in onlap fill units and tend to comprise the finest grained sediments in this geological setting. These fault systems have been most thoroughly described in the central North Sea basin and the detailed structure shows a significant correlation with lithological variations, both vertically and laterally. Extension measured in stacked decoupled tiers of polygonal faults correlates positively with both clay fraction and smectite content. Lateral facies variations are also observed and indicate that time-equivalent sequences upslope from the smectite-rich polygonally faulted sediments are coarse-grained, clay-poor and undeformed. This leads us to believe that the structure and geometry of the fault system are controlled by the colloidal nature of the sediments, and that the volumetric contraction measured on seismic sections can be accounted for by syneresis 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 progressive compaction. The process of syneresis can account for a number of structural 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 microscale properties of clay-rich sediments to their macroscale seismic character.

Saucer-shaped sill with lobate morphology revealed by 3D seismic data: implications for resolving a shallow-level sill emplacement mechanism

We use 3D seismic data to describe the 3D geometry of a large igneous intrusion, the Solsikke Compound Sill, and address a number of issues related to sill emplacement. The Solsikke Compound Sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer-shaped depressions and linear discontinuities. One of the saucer-shaped sub-elements of the compound sill, the Solsikke Lobate Sill, has a previously unrecognized morphology. It has a basal feeder and consists of a bifurcating network of interlinked lobe-shaped sill segments. We propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments. The lobate pattern indicates that the Solsikke Lobate Sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation. The feeder location is coincident with a fault intersection, suggesting that magma transport from the underlying source exploited the zone surrounding the intersection.

Lateral Displacement Variation and Lateral Tip Geometry of Normal Faults in the Canyonlands National Park, Utah

Abstract The along-strike displacement variation of 20 well-exposed normal faults from the Canyonlands, Utah, is described and analysed. The displacement profiles of these faults are highly variable, and most irregularities can be related to fault segmentation. Many of the profiles are highly asymmetric, and this can be related to mechanical interaction in some cases. Linear displacement tapers are observed towards all the lateral tips, but the percentage of trace length over which this linear taper occurs is highly variable. Three distinct lateral tip geometries are recognised, referred to informally as types A, B and C. Type A tips have a simple Mode III displacement geometry, Type B tips are characterised by a zone of extensional ‘fissures’ surrounding the fault tip, and Type C tips are characterised by the development of a monocline beyond the tip. Lateral displacement variation towards tips was analysed by measuring displacement gradients from systematic positions along the fault trace. Lateral displacement gradients measured for 39 tips exhibit a wide range of values (0.016–0.25). Fourteen of these lateral tips are regarded as ‘active’ since they exhibit signs of recent surface rupturing. These active tips have a similar range of lateral displacement gradients (0.019–0.25) to the overall population. Lateral displacement gradients were correlated with fault parameters such as length, length/maximum displacement (for faults and segments), and proximity to adjacent faults. No positive correlations were found. We suggest that the large range of lateral displacement gradients is mainly due to interactions between neighbouring faults. Additional complexities are likely to have resulted from strength heterogeneities related to jointing, from local variations in remote loading stresses and the frictional properties of the fault surfaces, and from processes related to segment linkage.