06_underhill 2000.pdf

The Role of Fault Interaction and Linkage in Controlling Synrift Stratigraphic Sequences: Late Jurassic, Statfjord East Area, Northern North Sea1 Nancye H. Dawers and John R. Underhill2

ABSTRACT Examination of well-constrained three-dimensional seismic data demonstrates the role of fault interaction and linkage in controlling the nature of synrift sequences on the hanging wall of the Statfjord East fault, a typical Late Jurassic structure in the northern North Sea Brent province. Although now a single fault, the Statfjord East fault originally consisted of several en echelon segments, each of which defined individual subbasins. Structural and stratigraphic evidence, both along and across fault strike, indicates that the fault resulted from segment propagation, interaction, and linkage. Facies architecture, thickness variations, and the internal character of synrift formations are temporally and spatially related to the subbasin geometry. Variations in displacement along the fault segments exhibit characteristics of interacting en echelon faults, including anomalous displacement gradients in regions of segment overlap. We attribute the observed shifts in depocenters to local enhancement of displacement rates, resulting from the interaction of neighboring fault segments. The results have far-reaching consequences for synrift plays in the northern North Sea because they ©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved. 1Manuscript received August 3, 1998; revised manuscript received April 22, 1999; final acceptance June 30, 1999. 2Department of Geology and Geophysics, University of Edinburgh, Grant Institute, The King’s Buildings, West Mains Road, Edinburgh EH9 3JW, Scotland, United Kingdom; e-mail: [email protected]; [email protected] This work was funded by the UK Natural Environment Research Council (NERC), under the Realising Our Potential Award (ROPA) scheme (no. GR3/R9521), and Norsk Hydro Research Centre. Seismic interpretation was undertaken using Schlumberger™ GeoQuest IESX software at facilities in Edinburgh supported by the Centre for Marine and Petroleum Technology, Norsk Hydro, Shell Expro, and Esso (UK) Ltd. We thank Gunn Mangerud and Pål Skott for their support and efforts in releasing data; Randi Jordan and Rolf Helland for help with well data; and Anker Berge, Kjell-Owe Häger, Cai Puigdefàbregas, Arvid Nøttvedt, Tom Dreyer, Roald Færseth, Anne Otelie Eide, Gunn Mangerud, Pål Skott, Sarah Prosser, Paul Milner, Patience Cowie, Sarah Davies, Sanjeev Gupta, Aileen McLeod, and Jon Turner for discussions. Bulletin reviewers Bruce Trudgill, Manuel Willemse, and Al Lacazette provided very helpful reviews; Juan Contreras, Patience Cowie, and Simon Kattenhorn provided additional helpful comments. Gerard White assisted with figures and Chung-Lun Lau provided computer support.

AAPG Bulletin, V. 84, No. 1 (January 2000), P. 45–64.

imply that only from the perspective of fault growth and linkage can the Late Jurassic structure and stratigraphy be fully understood. INTRODUCTION Field studies show that en echelon arrangement of normal fault segments and the nature of the displacement distribution along them result from segment interaction and subsequent linkage (e.g., Peacock and Sanderson, 1991; Trudgill and Cartwright, 1994; Dawers and Anders, 1995; Cartwright et al., 1995, 1996). A number of modeling studies show that these spatial patterns result from stress perturbations due to fault slip (Segall and Pollard, 1980; Bürgmann et al., 1994; Willemse et al., 1996; Willemse, 1997; Crider and Pollard, 1998). Recent modeling results also show that fault interaction and linkage lead to marked temporal variability within an evolving fault array (Cowie, 1998). Temporal variability in activity along segmented normal faults has been well documented in studies of neotectonic faults, i.e., faults active during the Quaternary, such as the Wasatch fault in Utah (Machette et al., 1991). The behavior of segmented faults over longer time scales, however, is less well known. Because normal faults control the creation of accommodation space for syntectonic deposition in rift basins (Leeder and Gawthorpe, 1987; Schlische, 1991; Gawthorpe et al., 1994), the displacement history of normal faults should be recorded in the synrift stratigraphy. In many continental areas, synrift stratigraphy is poorly preserved or incompletely exposed due to burial. Subsurface data, especially three-dimensional (3-D) seismic surveys, thus provide an important source of detailed information on the long-term structural evolution of normal fault systems and on the development of rift basins; however, little attention has been given to integrating the synrift stratigraphic patterns with data on displacement variations along the basin-bounding normal faults. A number of subsurface studies in the Upper Jurassic 45

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Figure 2 summarizes the relevant stratigraphy of the East Shetland Basin. The Upper Triassic–Lower Jurassic Banks Group (Richards et al., 1993) [or Statfjord Formation of Deegan and Scull (1977) and Vollset and Doré (1984)] and the Lower Jurassic Dunlin Group were deposited in a thermally subsiding basin following Permian–Triassic rifting (see Underhill, 1998). The overlying Middle Jurassic Brent Group, except for the Tarbert Formation, represents a major deltaic complex, which advanced and retreated in response to thermal doming and subsequent deflation (Underhill and Partington, 1993). The onset of extension during the Middle Jurassic (Bajocian) is marked by the drowning of the Brent delta and subsequent deposition of the shallow-marine

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GEOLOGICAL SETTING The study area is located in the Brent hydrocarbon province of the East Shetland Basin, on the western flank of the Late Jurassic Viking Graben of the northern North Sea (Figure 1). The East Shetland Basin formed as a result of distributed continental extension, which is estimated to be approximately 15% (Yielding, 1990; Roberts et al., 1993a). The Middle Jurassic (Bajocian) through Lower Cretaceous (Ryazanian) extension is accommodated predominantly on large, east-dipping normal faults (e.g., Lee and Hwang, 1993). Most of the significant hydrocarbon fields within the Brent province are situated in the uplifted footwalls (Figure 1). The Statfjord East area lies along an alignment of fault-controlled hydrocarbon fields, including Strathspey, Brent, and Statfjord (Figure 1). The Statfjord East area is somewhat unique in that the hanging wall also has been explored. There are several exploration wells in hanging-wall locations, and 3-D seismic coverage extends across all of the hanging-wall basin.

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of the central and northern North Sea have described stratigraphic patterns that indicate spatial and temporal variability of activity on normal faults (e.g., Underhill, 1991a, 1991b; Prosser, 1993; Nøttvedt et al., 1995; Ravnås et al., 1997; Ravnås and Bondevik, 1997). In this paper, we discuss the relationship between synrift stratigraphic patterns and normal fault development observed in a 3-D subsurface data set to better understand processes of normal fault growth and rift basin development in time and space. Our results have implications for tectonicstratigraphic models of rift basins, and show that temporal variability of stratigraphy in rift zones on a time scale of a few million years can be explained by processes of fault interaction and linkage.

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Figure 1—Location map of the East Shetland Basin and study area. The Statfjord East fault lies along a large array, including faults bounding the Brent and Statfjord fields.

Tarbert Formation and overlying Viking Group mudstones (Graue et al., 1987; Helland-Hansen et al., 1992). Transgression continued with the deposition of Bathonian–Oxfordian mudstones of the Heather Formation and the subsequent shale-dominated Draupne Formation (Vollset and Doré, 1984) [which is synonymous with the Kimmeridge Clay Formation in UK nomenclature of Deegan and Scull (1977)]. The Draupne Formation is coeval with numerous late Oxfordian–Ryazanian sedimentary wedges throughout the North Sea. A thin carbonate at the base of the Cromer Knoll Group marks the transition from active extensional faulting to a phase of postrift thermal subsidence (Glennie and Underhill, 1998). Transgression during the Late Jurassic in the northern North Sea enabled sea level to reach its maximum height regionally during the middle Kimmeridgian (sensu anglico) and early Volgian (Hallam, 1969; Sneider et al., 1995). Sea level then fell through the remainder of the Volgian and Early Cretaceous (Hallam, 1969; Rawson and Riley, 1982). In this paper we focus on the seismic stratigraphy of the synrift interval extending from the top of the Brent Group through the base of the Cromer Knoll Group. The base of the Cromer Knoll Group generally is referred to as the base Cretaceous

Dawers and Underhill

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conclusions. Differential compaction between footwalls and hanging walls of large faults also may effect throw patterns; however, the throws here are not large enough for this to be a cause of great concern.

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Figure 2—Stratigraphic framework and tectonic events of the East Shetland Basin. BCE = base Cretaceous event (see text), MCU = middle Cimmerian unconformity.

unconformity despite its Early Cretaceous age and the fact that in some structurally deep settings there may be no stratigraphic omission (Rattey and Hayward, 1993). This prominent, regional seismic marker results from a sharp acoustic impedance contrast between carbonates of the Lower Cretaceous Cromer Knoll Group and underlying formations. We refer to it here as the base Cretaceous event (BCE) because no stratigraphy is missing in the Statfjord East hanging wall. The top of the Banks Group has been mapped as a prerift marker horizon for fault displacement analysis. Although the Jurassic rifting may have reactivated some earlier Permian–Triassic structures (see Færseth 1996), the Statfjord East fault is interpreted as being Late Jurassic–Early Cretaceous in age (Nyberg, 1987; Dahl and Solli, 1993; Færseth, 1996). DATA SETS The data sets include four overlapping 3-D seismic surveys and data from 32 exploration wells located within Norwegian Block 34/7 and the adjacent part of Block 33/9 (Figure 3). Biostratigraphic zonation, well correlation, and facies analysis of cored intervals within the Statfjord East hanging wall are described in Häger and Smelror (1997), Nøttvedt et al. (in press), and Dawers et al. (1999), and are used here to place temporal constraints on the structural evolution of the area. For the purposes of this paper, the seismic interpretation has not been depth converted. Although velocity variations in detail may affect the patterns observed in synrift thickness and fault throw, well control is sufficiently closely spaced to suggest that these variations will not change our general observations and

STATFJORD EAST FAULT The Statfjord East fault is an approximately 18-km-long, north-northeast–striking fault (Figure 4). Figure 4 shows the geometry of several regional layers that are offset by the fault. On the level of the BCE, the fault has an overall continuous trace with two left-stepping bends (Figure 4A). At the northernmost bend, a fault splay extends basinward from the principal fault, i.e., the fault surface where most of the displacement is taken up (Figure 4A). On the deeper horizons, such as the top of the Heather Formation (Figure 4B) and the top of the Brent Group (Figure 4C), a series of northeast- to north-northeast–striking intrabasinal splays extend from the principal fault surface into the hangingwall basin. At a more regional scale, the Statfjord East fault is partially overlapped by the Statfjord fault (Figures 3, 4A), which becomes the more dominant fault toward the south, outside our study area (Figure 1). Footwall The uplifted footwall of the Statfjord East fault is relatively simple in profile (Figure 5). A thin capping of Heather Formation occurs along the central portion of the footwall. In well 33/9-12, the Tarbert Formation also is present, as is a thin drape of upper Volgian Draupne Formation shale. South of this well, the Draupne Formation thickens across a significantly eroded part of the Statfjord East footwall, along which the entire Brent succession and the upper part of the Dunlin Group are missing (Figure 5B). The hanging-wall geometry, described in the following paragraphs, is more complex. Hanging-Wall Structure and Stratigraphy The intrabasinal faults, which extend from the principal fault surface into the hanging-wall basin (Figure 4), define several subbasins within the overall half graben. The subbasins are evident in the geometry of the horizons shown in Figure 4 and are illustrated in the along-strike seismic section shown in Figure 6. The along-strike section shows that the subbasin bounding faults cut progressively younger strata toward the north (Figure 6). Only the southernmost intrabasinal fault is associated with

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Figure 3—Structural framework of the study area; faults shown are Middle Jurassic and younger. Sections AA′ and BB′ are shown in Figures 5 and 6, and sections aa′ and bb′ are shown in Figure 7. Also shown are the wells used to constrain the seismic interpretation, outline of area shown in Figure 4, oil fields, and outlines of threedimensional surveys. Survey line, CDP (common depth point) spacings are e86: 18.75 m, 12.5 m; sg9201: 12.5 m, 12.5 m; sg8431: 25 m, 12.5 m; ge83: 50 m, 25 m.

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substantial thickening in the Heather Formation, suggesting that the intrabasinal faults to the north were much less active during the deposition of the Heather Formation, or perhaps had not yet formed. The geometry created by the intrabasinal faults produces apparent fold structures transverse to the trend of the overall hanging-wall basin. Figure 7 shows two seismic sections across the basin. In the deepest part of the basin, adjacent to the southernmost part of the Statfjord East fault, both the Heather Formation and Draupne Formation are distinctly wedge shaped in cross section (Figure 7A). The internal nature of the Draupne Formation consists of an upper shallow-marine sequence and a lower shale sequence separated by a prominent, top Kimmeridgian reflector that onlaps the Heather

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Formation (Dawers et al., 1999; Nøttvedt et al., in press). Several reflectors onlap the top Kimmeridgian reflector (Figure 7A). The erosion of the Statfjord East footwall here (see also Figure 5) suggests that these internal reflectors, observed only in this part of the basin, are related to scarp degradation. Overlying these are more continuous reflectors that overstep the eroded footwall (Figure 7A). These, in turn, are overlain by discontinuous internal reflections that are truncated by the BCE. The thin drape of upper Volgian Draupne Formation shale in well 33/9-12, located just north of the most intensely eroded footwall area, suggests that this portion of the Statfjord East fault had become relatively inactive by the late Volgian. Note, however, that the Statfjord fault, which lies

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49

Figure 4—Map-view geometry (color-contoured in twoway traveltime) of horizons offset by the Statfjord East fault. (A) Base Cretaceous horizon, (B) top of the Heather Formation, and (C) top of the Brent Group.

west of and overlaps the Statfjord East fault, shows a small but clear displacement of the BCE (Figure 7A), suggesting that displacement slowed or ceased on this portion of the Statfjord East fault in the latest Jurassic, and by the Early Cretaceous displacement was being taken up on the main Statfjord fault. Figure 7B is a traverse through wells 33/9-12, 34/7-24S, 34/7-21A, and 34/7-21 (see Figure 3). Both the Heather Formation and Draupne Formation are wedge shaped (Figure 7B), although in this subbasin the Heather Formation does not thicken as drastically as it does farther south (see Figure 7A). Shown in parentheses on Figure 7B is the thickness of the Tarbert Formation encountered in wells 33/9-12, 33/9-21, and 33/9-21A but not completely drilled through in well 33/9-24S. The hanging-wall thickness of the Tarbert Formation is roughly double that of the footwall. This ratio is consistent with previous observations suggesting that the Tarbert is an initial synrift unit (e.g., Yielding et al., 1992; Ravnås et al., 1997; Underhill et al., 1997). Overlaps Between the Statfjord East Fault and Intrabasinal Splays Figure 8 shows the cross sectional geometry of the zones of overlap between the principal fault surface, i.e., the fault with the greater displacement, and the intrabasinal fault splays (Figure 8A). Both the principal and intrabasinal faults clearly displace the BCE at the northern overlap zone (Figure 8B). The middle intrabasinal splay does not displace the BCE but does displace the top of the Heather Formation (Figure 8C). The southern intrabasinal does not displace the BCE and only locally displaces the top of the Heather Formation (Figure 8D, also see Figure 4B). The southern and middle intrabasinal faults appear to have been active early relative to the principal fault surface, manifested by synrift thickness variations. Across the southern zone, thickening is observed in the Brent Group and Heather Formation across the intrabasinal splay (Figure 8D), whereas thickening is observed in the Heather and Draupne formations across the middle intrabasinal fault (Figure 8C). Note also that the Heather Formation here is on average thinner than that farther south, whereas the Draupne Formation is overall of similar thickness here and toward the south. Across the northern overlap zone, the only synrift unit thickening toward the faults is the

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50 Faults and Synrift Sequences

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Figure 6—(A) Along-strike hanging-wall traverse BB′ (see Figure 3 for location) and (B) line drawing of the seismic section. Intrabasinal highs are associated with the hanging-wall splays. Thickness variations suggest the southernmost splay was most active during Heather deposition, whereas those toward the north were more important during Draupne deposition. TWTT = two-way traveltime.

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Dawers and Underhill 51

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Figure 7—(A) Seismic line aa′ (see Figure 3 for location) and line drawing of the seismic section across the southern part of the Statfjord East fault showing the eroded footwall block between the Statfjord East fault and the main Statfjord fault to the west. (B) Traverse bb′ (see Figure 3) and line drawing across wells 33/9-12, 34/7-24S, 34/7-21A, and 34/7-21. TWTT = two-way traveltime.

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52 Faults and Synrift Sequences

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Figure 8—(A) Fault structure (at top of the Heather Formation) and section locations for parts B–D; (B) northern overlap zone; (C) middle overlap zone; (D) southern overlap zone. The principal fault surface accommodates most of the displacement across all three overlap zones. Intrabasinal fault activity youngs progressively from south to north, as evidenced by thickness variations and lack of offset of top Heather (D) and top Draupne (C). TWTT = twoway traveltime.

Draupne Formation (Figure 8B), but it is possible that the wedge-shaped character of the Draupne Formation here in part may be the result of erosion. The relationships described in this paper and illustrated in Figure 8 indicate that, initially, displacement occurred along the intrabasinal faults, first along the southern fault (Figure 8D), then along the middle fault (Figure 8C), and finally on the northernmost fault (Figure 8B). We observe a pattern of activity and then cessation of activity on the intrabasinal faults that progresses from south to north with time.

In the case of the southern and middle splays, the cessation of activity on the intrabasinal splay was coupled with displacement localization on the overlapping principal fault surface toward the west (Figure 8B, C). This scenario is inferred from the synrift thickness variations and the lack of offset of the upper synrift layers on the southern and middle splays, which were noted in the discussion of Figures 4 and 6. These relationships suggest that the principal fault surface evolved through the breaching of the overlap zones (Figure 8A).

Dawers and Underhill

Figure 9—(A) Along-strike throw profile for the top of the (prerift) Banks Group. (B) Map view of the fault structure on this datum. Summing in the throws of the splays reduces the magnitude of local throw minima along the principal fault surface, a finding that is consistent with the hanging-wall splays representing remnants of en echelon segments that became inactive as breaching structures formed in the footwall. TWTT = two-way traveltime.

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ALONG-STRIKE DISPLACEMENT PROFILES Figures 9 and 10 show along-strike displacement profiles for the Statfjord East fault. Figure 10 also includes data from the overlapping portion of the Statfjord fault to the west. Displacement data are limited because of partial erosion of the Heather Formation and Brent and Dunlin groups on the southern portion of the footwall; consequently, we show curves for the top Banks Group (Figures 9, 10) and BCE horizons (Figure 10). The displacement measure used is throw, i.e., the vertical component of the total displacement vector. Throw was measured on every tenth line in the e86 data set, which has lines spaced every 18.75 m and oriented parallel to the east-southeast extension direction. The northern tip of the Statfjord East fault lies north of the e86 survey; in the sg9201 survey (Figure 10), throw was measured using similarly spaced traverses parallel to the e86 lines. Variation in the throw of the top Banks Group along the Statfjord East fault is shown in Figure 9A.

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Figure 9B shows the map-view fault geometry on the top Banks Group, which illustrates the variation in the horizontal component, or heave, along the fault segments. Throw along principal segments of the fault, i.e., those segments with the greatest displacement, is plotted as filled squares (Figure 9A). The maximum throw occurs about 2 km north of the southern edge of the e86 survey and tapers off gradually northward. Note that local displacement minima correlate to areas where hanging-wall splays are observed in map view (Figure 9B). Throw across four hanging-wall splays and one footwall splay also is plotted in Figure 9A. Summing the throw across the splays with that measured along the principal fault trace reduces the magnitude of the local throw minima, hence producing a relatively smooth and continuous throw profile (Figure 9A). The pattern of displacement variation observed in Figure 9A is typical of faults that evolve from mechanically interacting en echelon segments (Dawers and Anders, 1995). For initially unlinked

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interacting fault segments, the throw profiles are asymmetric with high displacement gradients toward the region of fault overlap (e.g., Peacock and Sanderson, 1991; Willemse et al., 1996). Following the breaching of the overlap zone (typically across the part of the overlap zone closest to the mutual footwall, as observed here), displacement accrues on the developing fault surface, whereas activity on the previously active overlapping intrabasinal fault diminishes (Childs et al., 1995; Cartwright et al., 1996). The observation that throw on the intrabasinal splays spatially coincides with throw anomalies of similar magnitude along the principal segment indicates that the fault segments are geometrically coherent (Walsh and Watterson, 1991). In other words, the intrabasinal splays together with the principal fault surface form a smoothly varying displacement profile, as would be expected for a single fault (e.g., Dawers and Anders, 1995), and thus act on the large scale as a single fault. Figure 10A compares the total throw profile of the top Banks Group on the Statfjord East and Statfjord faults (upper curves) with that observed for the BCE (lower curves). The difference in the character of the curves for top Banks Group and the BCE horizons is that the BCE horizon represents the youngest synfaulting datum, and thus records only a very small amount of displacement. The important point in Figure 10A is that there is less throw of the Banks Group on the Statfjord fault relative to the Statfjord East fault, whereas the reverse is true of the BCE. At BCE level, only the northern part of the Statfjord East fault exhibits throws significantly greater than seismic resolution (approximately 15 ms). This pattern is consistent with the stratigraphic interpretation that late in the rifting displacement became localized on the Statfjord fault and the overlapping portion of the Statfjord East fault became relatively inactive (whereas the northern part of the Statfjord East remained active). The previously active hangingwall splays, particularly those in the south, do not show clear displacement at the BCE level (Figure 10B, see also Figure 4A). It is not feasible to construct throw profiles using synrift horizons below the BCE for the principal Statfjord East segments because of either erosion or nondeposition on the footwall; however, sufficient data are available for the two northern intrabasinal splays and are shown in Figure 11. Note that for the middle splay, the throw pattern for the top of the Brent Group is similar to that for the prerift Banks Group, whereas the throw profile for the top of the Heather Formation records less accumulated displacement, consistent with this being a synfaulting horizon. Along the northern intrabasinal splay, the throw gradient for the top of the Heather Formation

mimics that of the Banks and Brent groups, which have throw patterns here consistent with that expected for a prefaulting datum; however, the BCE throw pattern along this splay is consistent with a synfaulting interpretation. The conclusion drawn from Figure 11 is that activity localized on the middle splay during Heather deposition, whereas significant activity did not localize on the northern splay until during Draupne deposition. This pattern is consistent with northward growth of the Statfjord East fault by segment linkage; furthermore, this pattern confirms the interpretation of northward migration of activity on the intrabasinal faults based on the cross sectional geometries of the fault overlap zones illustrated in Figure 8. The data shown in Figure 11 do not rule out a component of vertical propagation in addition to lateral propagation through linkage. The basinward dip of hanging-wall reflectors immediately adjacent to the fault segments shown in Figures 7 and 8 is consistent with vertical propagation from depth. This geometry is similar to observations made along synsedimentary normal faults in other parts of the North Sea and in the Gulf of Suez rift, where basinward-dipping strata have been shown to represent the limbs of monoclines formed initially by vertical propagation of blind normal faults (Withjack et al., 1990; Gawthorpe et al., 1997; Gupta et al., 1999); however, vertical propagation alone would not explain the difference in the throw data shown in Figure 11. DISCUSSION The Statfjord East fault is similar to normal faults described from other areas. Studies in the Neogene Basin and Range province in the western United States and Mesozoic rift basins in the eastern United States illustrate hanging-wall subbasins separated by intrabasinal highs or transverse folds adjacent to fault segment overlaps (Anders and Schlische, 1994; Schlische, 1995; Schlische and Anders, 1996), similar to the structures observed here (Figure 6). Along a Basin and Range fault in Utah, Wu and Bruhn (1994) observed that segments stepping toward the footwall became progressively linked from the center of the structure outward. In east Africa, dated volcanic units along Lake Malawi (Ebinger, 1989) and 2-D (two-dimensional) seismic data (Contreras, 1999) indicate that segments along the lake’s western fault system were initially individual subbasins. Sedimentological data from the Suez rift system in Sinai, Egypt, demonstrate that segment interaction controlled Miocene sedimentary dispersal systems there (Gupta et al., 1999). Although these studies illustrated the importance of the fault linkage process, the paucity of hanging-wall data

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Figure 10—(A) Comparison of throw profile for the base Cretaceous event (BCE) with top Banks Group for Statfjord East and main Statfjord faults. Despite less throw of the top Banks Group on the Statfjord fault relative to the Statfjord East fault, the throw of the BCE is greater on the Statfjord fault than on the Statfjord East fault. At BCE level, only the northern part of the Statfjord East fault has throws greater than approximately 15 ms. Thus late in rifting deformation localized onto the Statfjord fault with the southern portion of the Statfjord East fault becoming inactive; the northern part of the fault remained active. (B) Interpretation of faults from a BCE dip-magnitude map. The previously active southern and middle hanging-wall splays do not show clear displacements at BCE level. TWTT = two-way traveltime.

e86 horizon taceous base Cre solved poorly re

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and the erosion of footwall marker horizons in most subaerial settings limits our understanding of the temporal evolution of faults. Because of the 3-D nature of the subsurface data set and the submarine setting, this study provides important insights into the temporal and spatial evolution of normal faults over the time scale of the entire rifting episode and has implications for the tectonic-stratigraphic development of rift basins. Implications for Evolution of Normal Faults Displacement variations observed along the Statfjord East fault (e.g., Figure 9) are consistent with previous studies of fault interaction and linkage

(e.g., Peacock and Sanderson, 1991; Trudgill and Cartwright, 1994; Cartwright et al., 1995, 1996; Dawers and Anders, 1995; Childs et al., 1995; Willemse et al., 1996; Crider and Pollard, 1998;). The relatively smooth throw profile, produced by summing the Banks Group throws across the intrabasinal splays with that observed along the principal fault surface, clearly demonstrates that the intrabasinal faults are associated with the evolution of the Statfjord East fault. Structural and stratigraphic evidence suggests an overall northward migration of fault activity, which is further illustrated in Figure 12. Figure 12 shows isopachs (in two-way traveltime) of the Heather Formation and the upper Oxfordian– Kimmeridgian Draupne Formation shale sequence. During the Bathonian–late Oxfordian, sediment

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Faults and Synrift Sequences

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Figure 11—Throw profiles for middle and northern intrabasinal faults. On the middle fault, the throw pattern for top Brent is similar to that for (prerift) top Banks Group, whereas the top Heather profile records less accumulated displacement, consistent with this being a synfaulting horizon. For the northern intrabasinal splay, all horizons except for the BCE (base Cretaceous event) have gradients expected for a prefaulting datum. Activity occurred on the middle splay during Heather deposition, but significant activity did not occur in the north until Draupne deposition, consistent with northward growth of the Statfjord East fault by segment linkage. TWTT = two-way traveltime.

accumulation was most pronounced along the southernmost part of the fault (Figure 12A). With time, the locus of activity shifted toward the northeast. Isopachs of the late Oxfordian–Kimmeridgian lower Draupne Formation shale indicate that continued subsidence occurred along the southern part of the fault and the development of a new depocenter along a segment toward the northeast (Figure 12B). We suggest that deposition of the Heather Formation and the lower part of the Draupne Formation occurred during a phase of ongoing linkage of fault segments. The structural relationships and the depocenter patterns we observe are consistent with the pattern of stress redistribution due to slip on a nearby interacting fault (Segall and Pollard, 1980; Bürgmann et al., 1994; Willemse et al., 1996; Willemse, 1997). Fault segments in certain positions relative to active segments develop increased displacement rates as a result of frequent stress redistribution following episodes of slip (Cowie, 1998; Gupta et al., 1998). In the case of normal faults, en echelon or co-planar segments experience enhanced activity, whereas faults in the immediate hanging wall and footwall are inhibited from further activity. In the case discussed here, the data suggest that the process of fault growth involved increased activity on one or more segments in

the south. The northern segments, i.e., toward the tip, experienced enhanced activity and eventually became incorporated into the growing structure by linkage. The migration of the linkage events is producing the overall northward propagation of the fault. This study has not attempted to address the early patterns of fault initiation. A recent sedimentological study of the earliest synrift unit in this area, the Tarbert Formation, shows, however, that the intrabasinal faults controlled small depocenters during the Bajocian (Davies et al., 1998). Thus the intrabasinal faults probably initiated early as a simple en echelon array along which the segments later became progressively more active and linked to form the Statfjord East fault. Lateral growth of the overall fault structure by linkage was on the order of 5–10 km in approximately 20 m.y. The time scale of activity increasing on an individual segment and that segment becoming incorporated into the large fault structure probably is on the order of 5 m.y. It is difficult to infer the slip rates for the Statfjord East fault segments because of the absence of synrift units on the footwall. In the case of the intrabasinal faults, the data in Figure 11 imply slip rates on the order of a few millimeters per 1000 yr (assuming reasonable interval velocities for the Viking Group).

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Figure 12—Thickness variation (in two-way traveltime) of shale successions. (A) Heather Formation and (B) lower Draupne Formation. Fault segments that control thickness variation are shown as bold lines; faults that were much less active or had not yet formed are shown as thinner lines. Modified from Dawers et al. (1999) and Gupta et al. (1998).

Implications for Tectonic-Stratigraphic Models of Rifts Previous studies in the North Sea and elsewhere illustrate several tectonic-stratigraphic features that are common to half-graben basins and that can be linked to spatial and temporal variability of normal fault activity. The facies distribution in shallowmarine and lacustrine rift settings shows broadly similar patterns consisting of shallow-marine or fluvial sediments during the initiation of the basin overlain by deeper water shales (e.g., Schlische, 1991; Prosser, 1993). The frequent association of

shale-prone sequences with aggradational stratal geometries indicates high subsidence rates, i.e., displacement rates on faults are creating accommodation space faster than it can be filled with sediment. Prosser (1993) referred to these patterns as the rift initiation, when displacement rates are relatively low, and the rift climax, when displacement rates are relatively high. The rift-basin shale sequences commonly are overlain by a sand-prone sequence as faulting slows and eventually ceases. The stratigraphic framework of the Statfjord East area consists of a similar rift-basin succession, i.e., Tarbert Formation sandstone overlain by shales of

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Further Northward Migration of Faulting and Shoreline Migration

Heather Fm

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Segments Interact, Focusing Shale Deposition

Figure 13—Evolution for the Statfjord East area from the time of (A) Heather deposition, (B) lower Draupne shale deposition, (C) upper Draupne deposition during the Volgian, and (D) earliest Cretaceous.

Statfjord Fault

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60 Faults and Synrift Sequences

Dawers and Underhill

the Heather and Draupne formations, followed by discontinuous shoreface sandstones in the uppermost part of the Draupne Formation (Figure 2) (see also Dawers et al., 1999). The Tarbert Formation is known from well data to be widespread in this part of the East Shetland Basin and thickest in structural lows (e.g., Underhill et al., 1997). These characteristics are consistent with the Tarbert Formation having been deposited across a region of distributed faulting with little total strain early in the rifting episode (Yielding et al., 1992). The deeper marine, more focused nature and wedge-shaped geometry of the Heather Formation and lower Draupne Formation shale depocenters observed here (Figure 12), and on the larger scale as well (Lee and Hwang, 1993), may be attributed to the enhancement of displacement rates, and hence hanging-wall subsidence rates, as individual fault segments interacted. During deposition of the sand-prone upper Draupne Formation (Figure 13C, D), despite the regional fall in sea level, evidence for local shoreline retrogradation suggests that fault growth led to a relative sea level rise within our study area. Our observations suggest that the structural character of the rift climax is that of a still highly segmented fault (Figures 12; 13A, B). We interpret the intrabasinal splays to represent the remnants of en echelon segments that show a pattern of increased activity and cessation that migrated northward with time. This process of growth by segment interaction and linkage encompasses the time periods recorded by the wedge-shaped shale successions of the Heather Formation and lower Draupne Formation (Figures 12; 13A, B). This evolution contrasts with the view that the border fault is a well-developed throughgoing structure during the period of high displacement rate (e.g., Prosser 1993). Rather than rapid deepening and shale deposition occurring mainly after segment linkage (due to the newly linked fault being “underdisplaced” and inhibiting propagation until a critical displacement-length ratio is reached (see Schlische and Anders, 1996)), our observations suggest that rift-climax shale deposition is closely associated with the interaction of fault segments prior to the formation of a fully linked fault array in the latest Jurassic–earliest Cretaceous (Figure 13D). An implication of this study is that diachroneity in rifting might be best explained through the processes of fault propagation, interaction, and linkage. Our results suggest that individual fault segments along an array may show variable activity over a time scale on the order of 5 m.y. For example, along the southern part of the Statfjord East fault, most of the activity occurred in the early part of the rifting, whereas at its northern tip it appears that fault activity was confined mostly to the latest Jurassic–earliest Cretaceous (see Figures 8, 10, 11). The cessation of

61

activity along the southern part of the Statfjord East fault appears to be associated with the transfer of activity onto the Statfjord fault, which overlaps the Statfjord East structure. Structural evidence of interaction at this larger scale is the skewed nature of the throw distribution on the Statfjord East fault seen in Figure 9. If the Statfjord East structure as a whole were to be considered a single isolated fault, then we would expect the throw distribution to be symmetric (e.g., Dawers and Anders, 1995). This view implies that processes discussed here for the intrabasinal faults, and hence temporal variability, should be observed along the whole of the Strathspey-BrentStatfjord-Statfjord East fault system (Figure 1), and probably on similar time scales. Some evidence of this view exists. The Statfjord fault within our study area appears to have been most active relatively late in the rifting episode, whereas high rates of displacement are inferred to have occurred earlier (approximately late Oxfordian) for the part of the Statfjord segment south of our study area (Roberts et al., 1993b; A. E. McLeod, 1999, personal communication). CONCLUSIONS Results of integrating seismic stratigraphy with fault analysis suggest that the Statfjord East fault of the northern North Sea evolved by fault segment linkage during Late Jurassic rifting. The intrabasinal fault splays in the Statfjord East hanging wall represent inactive tips of earlier formed en echelon segments; thus they represent an integral part of overall fault development rather than being related to other tectonic events or discrete phases of rifting. The synrift stratigraphy of the Viking Group, in particular the thickness variation in the Heather Formation and lower Draupne Formation shale sequences, suggests that the subbasins controlled by these splays young progressively toward the north. This conclusion is consistent with the segments becoming progressively more active and eventually linking in a south-to-north manner. We attribute this pattern of fault activity and depocenter development to the interaction of normal fault segments. The results of this study have several implications for stratigraphic studies and hydrocarbon exploration in rift basins. Because the spatial extent of the fault interaction is determined by the scale of the fault segments, synrift sequences will vary spatially along fault systems. For example, high displacement rates near segment centers may promote rift-climax stratal patterns and facies associations, whereas shallow-marine conditions may persist at fault tips and in overlap zones between unlinked faults. Thus an improved understanding of patterns of fault activity may aid in

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Faults and Synrift Sequences

reconstructing synrift paleogeographies and may help to explain discrepancies between regional and local sea level changes. As a result of the temporal changes associated with fault interaction, the apparent duration of the rift episode may vary if observations are made using a spatially limited data set. The duration of the rifting thus may be significantly underrepresented in the stratigraphic record. Moreover, because stratal patterns are strongly influenced by local fault interactions, variation in synrift stratal geometry alone is not sufficient evidence of temporal variation in the regional strain rate or of multiple rifting events.

REFERENCES CITED Anders, M. H., and R. W. Schlische, 1994, Overlapping faults, intrabasin highs, and the growth of normal faults: Journal of Geology, v. 102, p. 165–179. Bürgmann, R., D. D. Pollard, and S. J. Martel, 1994, Slip distributions on faults: effects of stress gradients, inelastic deformation, heterogeneous host-rock stiffness, and fault interaction: Journal of Structural Geology, v. 16, p. 1675–1690. Cartwright, J. A., B. D. Trudgill, and C. S. Mansfield, 1995, Fault growth by segment linkage: an explanation for scatter in maximum displacement and trace length data from the Canyonlands Grabens of SE Utah: Journal of Structural Geology, v. 17, p. 1319–1326. Cartwright, J. A., C. Mansfield, and B. Trudgill, 1996, The growth of faults by segment linkage, in P. G. Buchanan and D. A. Nieuwland, eds., Modern developments in structural interpretation, validation and modelling: London, Geological Society Special Publication 99, p. 163–177. Childs, C., J. Watterson, and J. J. Walsh, 1995, Fault overlap zones within developing normal fault systems: Journal of the Geological Society, London, v. 152, p. 535–549. Contreras, J., 1999, Tectonic and stratigraphic modeling of the evolution of continental rift basins: Ph.D. thesis, Columbia University, New York, 109 p. Cowie, P. A., 1998, A healing-reloading feedback control on the growth rate of seismogenic faults: Journal of Structural Geology, v. 20, p. 1075–1087. Crider, J. G., and D. D. Pollard, 1998, Fault linkage: threedimensional mechanical interaction between echelon normal faults: Journal of Geophysical Research, v. 103, p. 24,373–24,391. Dahl, N., and T. Solli, 1993, The structural evolution of the Snorre field and surrounding areas, in J. R. Parker, ed., Petroleum geology of northwest Europe: Proceedings from the 4th Conference, London, The Geological Society, p. 1159–1166. Davies, S. J., J. R. Underhill, N. H. Dawers, and A. E. McLeod, 1998, The evolution of early syn-rift depositional systems (abs.): AAPG Hedberg Research Conference Abstracts, p. 33. Dawers, N. H., and M. H. Anders, 1995, Displacement-length scaling and fault linkage: Journal of Structural Geology, v. 17, p. 607–614. Dawers, N. H., A. M. Berge, K.-O. Häger, C. Puigdefàbregas, and J. R. Underhill, 1999, Controls on Late Jurassic, subtle sand distribution in the Tampen Spur area, northern North Sea, in A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of NW Europe: Proceedings from the 5th Conference, London, The Geological Society, p. 827–838. Deegan, C. E., and B. J. Scull, 1977, A standard lithostratigraphic nomenclature for the central and northern North Sea: Bulletin of the Norwegian Petroleum Directorate, v. 1, 36 p.

Ebinger, C., 1989, Geometric and kinematic development of border faults and accommodation zones, Kivu-Rusivi rift, Africa: Tectonics, v. 8, p. 117–133. Færseth, R. B., 1996, Interaction of Permo–Triassic and Jurassic extensional fault-blocks during the development of the northern North Sea: Journal of the Geological Society, London, v. 153, p. 931–944. Gawthorpe, R. L., A. J. Fraser, and R. E. Ll. Collier, 1994, Sequence stratigraphy in active extensional basins: implications for the interpretation of ancient basin fills: Marine and Petroleum Geology, v. 11, p. 642–658. Gawthorpe, R. L., I. Sharp, J. R. Underhill, and S. Gupta, 1997, Linked sequence stratigraphic and structural evolution of propagating normal faults: Geology, v. 25, p. 795–798. Glennie, K. W., and J. R. Underhill, 1998, Origin, development and evolution of structural styles, in K. W. Glennie, ed., Petroleum geology of the North Sea: London, Blackwell Science, p. 42–84. Graue, E., W. Helland-Hansen, J. R. Johnsen, L. Lømo, A. Nøttvedt, K. Rønning, A. Ryseth, and R. J. Steel, 1987, Advance and retreat of the Brent delta system, Norwegian North Sea, in J. Brooks and K. Glennie, eds., Petroleum geology of northwest Europe: London, Graham and Trotman, p. 915–937. Gupta, S., P. A. Cowie, N. H. Dawers, and J. R. Underhill, 1998, A mechanism to explain rift basin subsidence and stratigraphic patterns through fault array evolution: Geology, v. 26, p. 595–598. Gupta, S., J. R. Underhill, R. L. Gawthorpe, and I. Sharp, 1999, Role of fault interaction in controlling syn-rift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez rift, Sinai, Egypt: Basin Research, v. 11, p. 167–189. Häger, K-O., and M. Smelror, 1997, Reworking of dinocysts as a means of obtaining useful geological information: a study of the Upper Jurassic Draupne Formation in the Tampen area, Norwegian North Sea (abs.): American Association of Stratigraphic Palynologists, Annual Meeting Abstracts, p. 20. Hallam, A., 1969, Tectonism and eustasy in the Jurassic: Earth Science Reviews, v. 5, p. 45–68. Helland-Hansen, W., M. Ashton, L. Lømo, and R. J. Steel, 1992, Advance and retreat of the Brent delta: recent contributions to the depositional model, in A. C. Morton, R. S. Haszeldine, M. R. Giles, and S. Brown, eds., Geology of the Brent Group: London, Geological Society Special Publication 61, p. 109–127. Lee, M. J., and Y. J. Hwang, 1993, Tectonic evolution and structural styles of the East Shetland Basin, in J. R. Parker, ed., Petroleum geology of northwest Europe: Proceedings of the 4th Conference, London, The Geological Society, p. 1137–1149. Leeder, M. R., and R. L. Gawthorpe, 1987, Sedimentary models for extensional tilt-block/half-graben basins, in M. P. Coward, J. F. Dewey, and P. L. Hancock, eds., Continental extensional tectonics: London, Geological Society Special Publication 28, p. 139–152. Machette, M. N., S. F. Personius, A. R. Nelson, D. P. Schwartz, and W. R. Lund, 1991, The Wasatch fault zone, Utah— segmentation and history of Holocene earthquakes: Journal of Structural Geology, v. 13, p. 137–149. Nøttvedt, A., R. H. Gabrielsen, and R. J. Steel, 1995, Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea: Marine and Petroleum Geology, v. 12, p. 889–901. Nøttvedt, A., A. Berge, N. H. Dawers, R. B. Færseth, K.-O. Häger, G. Mangerud, and C. Puigdefàbregas, in press, Evaluation of the syn-rift play potential of the Snorre fault block, in A. Nøttvedt, ed., Integrated Basins Studies Special Publication: London, The Geological Society. Nyberg, I. T., 1987, Statfjord Øst, in A. M. Spencer et al., eds., Geology of the Norwegian oil and gas fields: London, Graham and Trotman, p. 351–362. Peacock, D. C. P., and D. J. Sanderson, 1991, Displacements,

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segment linkage and relay ramps in normal fault zones: Journal of Structural Geology, v. 13, p. 721–733. Prosser, S., 1993, Rift-related linked depositional systems and their seismic expression, in G. D. Williams and A. Dobbs, eds., Tectonics and seismic sequence stratigraphy: London, Geological Society Special Publication 71, p. 35–66. Rattey, R. P., and A. B. Hayward, 1993, Sequence stratigraphy of a failed rift system: the Middle Jurassic to Early Cretaceous basin evolution of the central and northern North Sea, in J. R. Parker, ed., Petroleum geology of northwest Europe: Proceedings of the 4th Conference, London, The Geological Society, p. 215–249. Ravnås, R., and K. Bondevik, 1997, Architecture and controls on Bathonian–Kimmeridgian shallow-marine synrift wedges of the Oseberg-Brage area, northern North Sea: Basin Research, v. 9, p. 197–226. Ravnås, R., K. Bondevik, W. Helland-Hansen, L. Lømo, A. Ryseth, and R. J. Steel, 1997, Sedimentation history as an indicator of rift initiation and development: the late Bajocian–Bathonian evolution of the Oseberg-Brage area, northern North Sea: Norsk Geologisk Tidsskrift, v. 77, p. 205–232. Rawson, P. F., and L. A. Riley, 1982, Latest Jurassic–Early Cretaceous events and the “late Cimmerian unconformity” in the North Sea area: AAPG Bulletin, v. 66, p. 2628–2648. Richards, P. C., G. K. Lott, H. Johnson, R. W. O’B. Knox, and J. B. Riding, 1993, Jurassic of the central and northern North Sea, in R. W. O’B. Knox and G. W. Cordey, eds., Lithostratigraphic nomenclature of the UK North Sea: Nottingham, British Geological Society, p. 219. Roberts, A., G. Yielding, N. J. Kusznir, I. Walker, and D. DornLopez, 1993a, Mesozoic extension in the North Sea: constraints from flexural backstripping, forward modelling and fault populations, in J. R. Parker ed., Petroleum geology of northwest Europe: Proceedings of the 4th Conference, London, The Geological Society, p. 1123–1136. Roberts, A. M., G. Yielding, and M. E. Badley, 1993b, Tectonic and bathymetric controls on stratigraphic sequences within evolving half-graben, in G. D. Williams and A. Dobbs, eds., Tectonics and seismic sequence stratigraphy: London, Geological Society Special Publication 71, p. 87–121. Schlische, R., 1991, Half-graben basin filling models: new constraints on continental extensional basin development: Basin Research, v. 3, p. 123–141. Schlische, R. W., 1995, Geometry and origin of fault-related folds in extensional settings: AAPG Bulletin, v. 79, p. 1661–1678. Schlische, R. W., and M. H. Anders, 1996, Stratigraphic effects and tectonic implications of the growth of normal faults and extensional basins, in K. K. Beratan, ed., Reconstructing the history of Basin and Range extension using sedimentology and stratigraphy: Boulder, Geological Society of America Special Paper 303, p. 183–203. Segall, P., and D. D. Pollard, 1980, Mechanics of discontinuous faults: Journal of Geophysical Research, v. 85, p. 4337–4350. Sneider, J. S., P. de Clarens, and P. R. Vail, 1995, Sequence

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stratigraphy of the Middle to Upper Jurassic, Viking Graben, North Sea, in R. J. Steel, ed., Sequence stratigraphy on the northwest European margin: Amsterdam, Elsevier, Norwegian Petroleum Society Special Publication 5, p. 167–197. Trudgill, B., and J. Cartwright, 1994, Relay-ramp forms and normal-fault linkages, Canyonlands National Park, Utah: Geological Society of America Bulletin, v. 106, p. 1143–1157. Underhill, J. R., 1991a, Implications of Mesozoic–Recent basin development in the western Inner Moray Firth, UK: Marine and Petroleum Geology, v. 8, p. 359–369. Underhill, J. R., 1991b, Controls on Late Jurassic seismic sequences, Inner Moray Firth, UK North Sea: a critical test of a key segment of Exxon’s original global cycle chart: Basin Research, v. 3, p. 79–98. Underhill, J. R., 1998, Jurassic, in K. W. Glennie, ed., Petroleum geology of the North Sea: London, Blackwell Science, p. 245–293. Underhill, J. R., and M. A. Partington, 1993, Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence, in J. R. Parker, ed., Petroleum geology of northwest Europe: Proceedings from the 4th Conference, London, The Geological Society, p. 337–345. Underhill, J. R., M. J. Sawyer, P. Hodgson, M. D. Shallcross, and R. L. Gawthorpe, 1997, Implications of fault scarp degradation for Brent Group prospectivity, Ninian field, northern North Sea: AAPG Bulletin, v. 81, p. 999–1022. Vollset, J., and A. G. Doré, 1984, A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea: Norwegian Petroleum Directorate Bulletin no. 3, 53 p. Walsh, J. J., and J. Watterson, 1991, Geometric and kinematic coherence and scale effects in normal fault systems, in A. M. Roberts, G. Yielding, and B. Freeman, eds., The geometry of normal faults: London, Geological Society Special Publication 56, p. 193–203. Willemse, E. J. M., 1997, Segmented normal faults: correspondence between three-dimensional mechanical models and field data: Journal of Geophysical Research, v. 102, p. 675–692. Willemse, E. J. M., D. D. Pollard, and A. Aydin, 1996, 3D analyses of slip distributions on normal fault arrays with consequences for fault scaling: Journal of Structural Geology, v. 18, p. 295–309. Withjack, M. O., J. Olson, and E. Peterson, 1990, Experimental models of extensional forced folds: AAPG Bulletin, v. 74, p. 1038–1054. Wu, D., and R. L. Bruhn, 1994, Geometry and kinematics of active normal faults, South Oquirrh Mountains, Utah: implications for fault growth: Journal of Structural Geology, v. 16, p. 1061–1075. Yielding, G., 1990, Footwall uplift associated with Late Jurassic normal faulting in the northern North Sea: Journal of the Geological Society, London, v. 147, p. 219–222. Yielding, G., M. E. Badley, and A. M. Roberts, 1992, The structural evolution of the Brent province, in R. S. Haszeldine, M. R. Giles, and S. Brown, eds., Geology of the Brent Group: London, Geological Society Special Publication 61, p. 27–43.

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ABOUT THE AUTHORS Nancye H. Dawers Nancye Dawers holds a B.S. degree from the University of Kentucky, an M.S. degree from the University of Illinois at UrbanaChampaign, and an M.Ph. degree and a Ph.D. from the LamontDoherty Earth Observatory of Columbia University. Since 1996 she has been a research associate at the University of Edinburgh. Nancye’s research interests include fault growth, displacement and length scaling in fault populations, neotectonics, and basin analysis.

John R. Underhill John Underhill graduated from Bristol University in 1982 and was awarded his Ph.D. in 1985 from the University of Wales. He subsequently worked for Shell before moving to Edinburgh, where he has recently been promoted to the Chair of Stratigraphy. John has been awarded the European Association of Petroleum Geoscientists’ Distinguished Lecturer award twice, the AAPG Matson Award once, and was one of AAPG’s 1999 Distinguished Lecturers.