bluuebunny/crossref_metadata_2025 huggingface dataset:
A geophysical investigation of saline intrusion and geological structure beneath areas of tidal coastal wetland at Langstone Harbour, Hampshire, UK
Authors: McDonald Robert J. , Russill Nicholas R.W. , Miliorizos Marios , Thomas Jonathan W.
Published: April 2008

Experiments are described that test how tide and geological structure affect saline intrusion beneath an area of coastal wetland in Hampshire, southern England. Resistivity tomography and time-dependent ground conductivity surveys were carried out at two closely located survey sites. Resultant maps and sections show clear geophysical anomalies that can be attributed to tidal saline intrusion and to the position of geological structures. Results suggest that migration of saline groundwater is, in places, out of phase with the tidal cycle, and is concentrated along sub-surface tidal channels, gravel aquifers and depressions in Chalk bedrock topography. We conclude that the seawater-fresh-water boundary in the studied wetland environment is complicated by shallow-level saline flow along geologically definable routes within the Quaternary deposits above the less permeable Chalk bedrock.
Evidence for rapid groundwater flow and karst-type behaviour in the Chalk of southern England
Authors: MacDonald A. M. , Brewerton L. J. , Allen D. J.
Published: April 2008

With the growing importance of groundwater protection, there is increasing concern about the possibility of rapid groundwater flow in the Chalk of southern England and therefore in the frequency and distribution of ‘karstic’ features. Pumping test data, although useful in quantifying groundwater resources and regional flow, give little information on groundwater flow at a local scale. Evidence for rapid groundwater flow is gathered from other, less quantifiable methods. Nine different strands of evidence are drawn together: tracer tests; observations from Chalk caves; Chalk boreholes that pump sand; descriptions of adits; the nature of water-level fluctuations; the Chichester flood; the nature of the surface drainage; geomorphological features; and the presence of indicator bacteria in Chalk boreholes. Although the evidence does not prove the widespread existence of karstic features, it does suggest that rapid groundwater flow should be considered seriously throughout the Chalk. Rapid groundwater flow is generally more frequent close to Palaeogene cover and may also be associated with other forms of cover and valley bottoms.
Water transport through the unsaturated zone of the Middle Chalk: a case study from Fleam Dyke lysimeter
Authors: Jones H. K. , Cooper J. D.
Published: April 2008

The role of fractures in transporting water in the unsaturated zone of the Chalk has been a source of controversy for many years. There is clearly a potential for fractures to transport water and contaminants rapidly to the saturated zone, although direct observations are rare. The evidence from well hydrographs, solute profiles and soil water measurements is ambiguous concerning the importance of by-pass flow.

This paper examines drainage data from several years’ operation of a 5 m cube undisturbed monolith lysimeter at Fleam Dyke, Cambridgeshire, for evidence of by-pass flow and investigates the factors responsible for its onset. Evidence from measurements of unsaturated hydraulic conductivity of the Chalk at the site and
in situ
measurements of soil water are also presented. Both the lysimeter and soil water observations agree that flow through fractures accounts for approximately 30% of the total drainage from this site, significantly higher than has been inferred at other Chalk sites. By-pass flow was observed to occur above a threshold of approximately −50 cm H
2
O matric potential, or above a flow rate of approximately 1 mm day
−1
. During periods of no soil water deficit, about 50% of drainage occurred via fractures. In general, there was a delay of three or four days between rainfally and drainage at the base of the lysimeter. On once occasion, intense summer rainfall satisfied the existing soil moisture deficit and initiated fracture flow, the peak in drainage through the lysimeter occurring within 24 hours. Drainage from the lysimeter was consistently higher than MORECS estimates of hydrologically effective precipitation during the period after the onset of soil water deficits.
Recharge through the drift: a study of contrasting Chalk catchments near Redgrave Fen, UK
Authors: Soley R. W. N. , Heathcote J. A.
Published: April 2008

Redgrave and Lopham Fens lie at the headwaters of the Rivers Waveney and Little Ouse in East Anglia, UK. The Fens are supported by groundwater discharge from the drift-covered Chalk underlying the area. This discharge and the ecological value of the Fens have been derogated by public water supply abstractions from a nearby borehole.

Aspects of the investigations undertaken to construct a groundwater model to assess options for relocating the borehole away from the Fens are described. Particular attention is given to the value of using basic river flow data analysis to identify contrasts in the hydrological characteristics of the two catchments and constrain estimates of recharge.

The Waveney catchment is flashy and dominated by run-off whereas the Little Ouse receives a more significant groundwater baseflow contribution. These catchment contrasts reflect subtle variations in boulder clay thicknesses and topography. They also reflect the contrasting effectiveness of the two rivers as drains on the groundwater system.

Average recharge to the Chalk through the boulder clay-covered interfluves is estimated at around 36 mm a
−1
(which is similar to other values quoted in the literature). Recharge may locally fall to less than 10 mm a
−1
where the boulder clay is thick and the Chalk is strongly confined.
Assessment of groundwater vulnerability in England and Wales
Authors: Palmer R. C. , Lewis M. A.
Published: April 2008

Groundwater represents around 35% of water distributed for potable supply in England and Wales and is also utilized by industry and for private supplies. It is of high quality and requires little treatment before use. It is important, therefore, that a robust policy of groundwater protection is available to protect the resource from pollution, as rehabilitation of polluted groundwater is expensive, and often may not be technically feasible. One way to protect aquifers is to control activities which pose a threat to groundwater quality, especially in areas where they are naturally poorly protected. Areas where groundwater is at risk of pollution from surface activities are indicated on maps by characterizing the properties of the underlying soils and rocks. The rocks are classified on the basis of their permeability, into Major, Minor and Non-Aquifers. The soils overlying aquifers are then categorized on their physical and chemical properties into leaching potential classes. The Groundwater Protection Policy of the Environment Agency, of which the groundwater vulnerability maps are part, provides guidance on those types of development which require controlling if groundwater quality is to be maintained in the long term. The relevance of these maps to other environmental issues is discussed.
Is all groundwater worth protecting? The example of the Kellaways Sand
Authors: Mather John , Halliday Dawn , Joseph Jeremy
Published: April 2008

Many formations treated as Minor Aquifers for the purposes of resource protection do not meet accepted definitions of aquifers. The Kellaways Sand is a typical example, and modern waste-disposal licences require the installation of an engineered containment system to protect groundwater within it. However, the formation is clay-rich and only 4 to 5 m in thickness, and has a transmissivity of around 1.3 × 10
−1
m
2
day
−1
. Contained water quality is poor with salinities well above maximum admissible concentrations for drinking water. As the Kellaways Sand does not yield water to pumping wells, the only rational reason for protection is that the formation might contribute to baseflow in the River Ouse to the northeast. However, hydraulic gradients are such that seepage rates to the river will not exceed more than a few cubic metres per year over the length of outcrop adjacent to the river. It is concluded that the installation of engineered mineral liners at the base of brickpits in the Lower Oxford Clay do nothing to reduce the risks associated with waste disposal. Resources spent on protecting the Kellaways Sand could be better used in other areas of environmental protection.
Giant fields, petroleum systems and exploration maturity of Algeria
Authors: Macgregor Duncan S.
Published: April 2008

The giant petroleum fields of eastern Algeria can be divided into six significant petroleum systems. More than 60% of the giant fields and 80% of the reserves within them lie in a specific setting in reservoirs immediately below or above the Hercynian unconformity, and below the Liassic salt seal. The second main reservoirs are Early Devonian sandstones. Timing of generation and preservation potential, as controlled by tectonic history, seal type and source rock burial, are key controls on the distribution of petroleum in these giant fields, particularly for oil. Field size distributions within the six petroleum systems can be related to structural style and to differing levels of exploration maturity within each system. Comparisons can be made between the productive trap types and petroleum systems of Algeria and analogous but more heavily drilled basins elsewhere in the world. Such comparisons suggest that further giant fields lie undiscovered in Algeria, particularly in subtle forms of traps, and in the 250–500 MMBOE (million barrels oil equivalent) reserves range. The Ghadames petroleum system may be the system which carries the highest yet-to-find, as predicted from a range of criteria, including discovery trends, drilling density, field size distribution and source rock supply analysis. The challenge to the oil industry in Algeria is to identify the locations of the missing giant fields suggested by this analysis.
Controls on hydrocarbon occurrence and productivity in the F6 reservoir, Tin Fouyé-Tabankort area, NW Illizi Basin
Authors: Alem N. , Assassi S. , Benhebouche S. , Kadi B.
Published: April 2008

Several oil accumulations of various sizes have been found within the F6 reservoir of Upper Silurian-Lower Devonian age in the Tin Fouyé region of the Illizi Basin, about 1500 km from Algiers. Lithologically, the F6 consists of interbedded sandstones and shales and is subdivided into units: M1, M2, A, B1, B2, C1, C2 and C3, the upper units often being missing through erosion. These reservoirs, which were deposited in offshore coastal bars and tidal channels, show a high degree of lateral and vertical facies variation. Reservoir quality is maximized in medium- to coarse-grained clay-poor bar sands within units C1 and B2, which contain significant secondary porosity. Hydrodynamic activity is very important in controlling the distribution of hydrocarbons. Study of the hydrology of the F6 reservoir shows that connate water and oil have been frequently flushed by fresh water in the most permeable units, particularly C1 and B2. This finding is supported by hydrochemical data, by the inclination of oil-water contacts and by a high observed hydraulic gradient. This mechanism has led to the flushing of some oil from the Tin Fouyé Est (TFE) structural trap and a concentration of oil in an accumulation with a tilted contact on the Tin Fouyé Nord structural nose. Many kinds of traps have been recognized in this region. These include a simple structural trap at Tin Fouyé, a combination hydrodynamic-stratigraphical-structural trap at Tin Fouyé Nord, a hydrodynamic-structural trap at TFT-2 and a pure hydrodynamic trap at TFT 8. Hydrodynamic activity is not favourable for the accumulation of hydrocarbons in low-amplitude structures but can lead to significant accumulations in favourably orientated structural noses, even when these lack closure.
Sedimentological evolution of the Givetian-Eifelian (F3) sand bar of the West Alrar field, Illizi Basin, Algeria
Authors: Chaouchi Rabah , Malla M. S. , Kechou F.
Published: April 2008

Commercial accumulations of hydrocarbons depend on several factors, among which sedimentological conditions are of prime importance. Such sedimentological factors are of particular importance in controlling both entrapment and reservoir quality in the western part of the Alrar field, within the F3 sandstone reservoir of Givetian-Eifelian age. Recent well results have shown facies and reservoir quality distribution to be more complex and generally more favourable than previously predicted. A series of tidally influenced littoral bars are now believed to have existed in the Alrar area at the time of deposition of the reservoir, these being elongated in a NW-SE direction in the West Alrar area, with an interbar area in the vicinity of well DZ-1. The highest reservoir quality is largely confined to this bar facies and understanding its distribution is therefore vital to locating development wells. Four possible sedimentological interpretations of the extension of the F3 reservoir to the south and west have been developed. In most areas, the pinchout seems to be gradual, but in other areas where the transition to lagoonal shale is sharp, fault control may apply. Significant uncertainties however remain in predicting this shale-out in undrilled areas.
Mesozoic and Cenozoic petroleum systems of North Africa
Authors: Macgregor Duncan S. , Moody Richard T. J.
Published: April 2008

The Mesozoic and Tertiary petroleum systems of North Africa have formed in a suite of extensional basins developed along the Atlantic and Tethyan margin. Petroleum is unevenly spread across the region of Mesozoic basin development, with one world-class petroleum province developed (the Sirt Basin of Libya), and several more modest developments of reserves in Western Egypt and offshore Libya-Tunisia. A set of these extensional basins were later subject to compression on the Atlas and Syrian trends, with generally unfavourable effects for the preservation of the petroleum systems affected. The Moroccan passive margin is also a region of currently low exploration success. Productive reservoirs are spread throughout the Mesozoic section, though, with the exception of the prolific Early Cretaceous clastic deposits and the Paleogene carbonates, are generally not seen to correlate between basins. Effective source rock development is heavily concentrated in the Late Cretaceous and Eocene. Recent evidence also suggests an additional contribution from lacustrine source rocks of Early Cretaceous and older age in the southeastern Sirt Basin. Patterns of maturity on these main source levels, and timing of that maturity relative to trap formation, seem to be the main controls on basin success and failure, with preservation an additional key factor in inverted basins. The Mesozoic basins of North Africa are generally lightly explored compared with basins of similar petroleum productivity elsewhere in the world. Exploration to date in the Sirt Basin, for instance, has barely progressed beyond the structural trap stage, with application of analogues suggesting undrilled potential in various types of subtle trap. Further exploration remains necessary in the Atlas trend and Moroccan margin, focused on plays with more favourable timing relationships between generation and structural development than those drilled to date.
Late Permian to Recent magmatic activity on the African-Arabian margin of Tethys
Authors: Wilson M. , Guiraud R. , Moreau C. , Bellion Y. J.-C.
Published: April 2008

Magmatic activity on the African-Arabian margin of Tethys has fluctuated significantly during the past 250 Ma in response to major phases of mantle plume activity and to extensional stresses within the African plate related to periods of continental break-up. A series of maps have been compiled, based upon stratigraphic constraints and K-Ar and40Ar-39Ar age determinations, to illustrate the changing patterns of igneous activity since the Late Permian. These are divided into five stages: I, Late Permian (256 Ma)-Latest Triassic (208 Ma); II, Early Jurassic (208 Ma, Liassic)-Mid Jurassic (157 Ma, Dogger); III, Late Jurassic (157 Ma)-Earliest Aptian (120 Ma); IV, Mid Aptian (119 Ma)-Mid Eocene (42 Ma); V, Late Eocene (42 Ma)-Recent. Extensive tholeiitic basaltic magmatism atc.200 Ma, preceding continental break-up in the Central Atlantic, is attributed to upwelling of the Cape Verde mantle plume beneath the West African craton. Contemporaneous magmatic activity in the Levant is related to the break-up of the Eastern Mediterranean margin. Early Cretaceous alkaline magmatism around the margins of the Equatorial Atlantic and within the Levant may also be plume related. During Neogene-Recent times there was a major change in the convection pattern within the upper mantle, generating extensive magmatism across the entire margin. Magmatism appears to be extremely long lived in a number of areas (e.g. Sudan, Air), with volcanic centres located along major basement fault zones, such as the Guinean-Nubian lineaments.
The influence of platform morphology and sea level on the development of a carbonate sequence: the Harash Formation, Eastern Sirt Basin, Libya
Authors: Spring D. , Hansen O. P.
Published: April 2008

The Paleocene section of the Eastern Sirt Basin consists of a carbonate succession which developed in response to subsidence and widespread marine transgression following Cenomanian rifting. Within the study area the principal hydrocarbon reservoir is the Upper Sabil Formation, which is sealed by Harash Formation lime mudstones. Facies analysis and seismic mapping indicate that the Upper Sabil Formation consists of a prograding carbonate shelf with a rimmed platform morphology and palaeo-slopes in excess of 5° along the eastern and western flanks of the Agedabia Trough. To the south, the absence of a lagoonal facies and seismically defined shelf edge indicate a ramp morphology with a palaeo-slope of less than 0.5°. The overlying Harash Formation is characterized by its cyclic nature, relatively high terrigenous mud content and a thickness distribution which shows it to be infilling the pre-existing Upper Sabil shelf topography. The morphology of the Upper Sabil shelf coupled with sea-level fluctuations controlled the distribution of the depositional environments within the Harash Formation. A depositional model for the Harash Formation is proposed which subdivides the formation into five carbonate parasequences (named cycles A-E from base to top). In an exploration context the study facilitated the identification of the distribution of Harash Formation sealing and possible reservoir facies within cycles A and B. Furthermore, the study led to the understanding of the cause of anomalous reflections observed on conventional seismic data within the Paleocene section. Work involved the integration of regional well control with conventional seismic and stratigraphic inversion data. The stratigraphic framework and interpretations derived from this study were ultimately tested and confirmed by an exploration well which was cored through the sections of interest.
Critical factors in the exploration of an Atlas intramontane basin; the Western Hodna Basin of northern Algeria
Authors: Mekireche Karim , Sabaou Nordine , Zazoun Reda-Samy
Published: April 2008

The acquisition and evaluation of a substantial integrated surface outcrop and well database over the western Hodna Basin allows the identification of three critical factors (reservoir quality, source maturity and timing and hydrodynamic regime) to which well failures to date can largely be attributed. Reservoir quality is variable in Cretaceous sandstones, because of a high degree of quartz cementation, but improves to the south. Reservoir quality in Eocene limestones is related to the development of vuggy porosity and can now be mapped out from well control. Potential Cretaceous source rocks lie at moderate to high degrees of maturity, with explusion probably preceding the Miocene phases of structure development. Tertiary palaeostress reconstructions from field studies indicate four main structural phases: (1) Late Cretaceous N-S to NW-SE oriented compression; (2) Oligo-Miocene distension; (3) N-S oriented Miocene (pre-Langhian) pull-apart; (4) Langhian compression related to the emplacement of the Tellian nappes. Seismic analysis illustrates some classic late-stage inversion structures. Organic matter seen in outcrops and cores within the Thanetian-Ypresian and lower Lutetian sections is composed mainly of type II kerogen and varies from undermature to marginally mature, leading to the development of shows in many wells that helped in the past to stimulate continued exploration. Study of the hydrology of the formation waters in drilled wells allows large areas of likely reservoir flushing to be delineated. Study of the three critical factors allows a high grading in terms of remaining exploration potential. Based on the interpretations derived from structural, palaeogeography, diagenetic and hydrodynamic studies, the basin can be divided into three areas, which are ranked on the basis of their prospectivity. Each of the factors also needs to be critically assessed in analogous regions of the Atlas, in which such good geological control may not exist.
Introduction to the development, evolution and petroleum geology of the Wessex Basin
Authors: Underhill John R. , Stoneley Robert
Published: April 2008

Despite containing the largest known onshore oilfield in western Europe, the Wessex Basin hydrocarbon province appears to be extremely limited spatially and it currently only consists of three producing oilfields: Wytch Farm, Wareham and Kimmeridge. The main factor which controls hydrocarbon prospectivity in the area appears to be preservation of oil accumulations originally sited in Mesozoic tilted faultblocks. The extensional palaeostructures of Wytch Farm and Wareham are interpreted to have been charged by upwards migration of oil from mature Liassic source rocks situated across the Purbeck-Isle of Wight fault system in the Channel (Portland-Wight) sub-basin prior to, and unaffected by, either significant effects of intra-Cretaceous (Albian-Aptian) easterly tilting or by Tertiary tectonic inversion. To date, only the small Kimmeridge oilfield, which is situated in the core of a periclinal fold created in response to structural inversion, suggests that any hydrocarbon remigration into younger structural inversion structures has taken place.
The geological history of the southern Wessex Basin — a review of new information from oil exploration
Authors: Butler Malcolm
Published: April 2008

Mapping of intervals between the base of the Aylesbeare Mudstone (Permian or early Triassic) and the top of the Penarth Group (early Jurassic) in the Dorset-Isle of Wight area indicates that the deposition of these formations was influenced strongly by northwest-southeast-trending faults, probably related to underlying Variscan elements. There are significant differences between the distribution of the Aylesbeare Mudstone and that of later Triassic formations. An unconformity is recognized within the Sherwood Sandstone Group, separating the syn-rift sequences of Aylesbeare Mudstone, and overlying Budleigh Salterton Pebble Beds and anhydrites from the post-rift Otter Sandstone and Mercia Mudstone (middle-late Triassic). It is postulated that the top of the Sherwood Sandstone Group becomes progressively younger from the Mid Dorset Platform towards the Isle of Wight, steadily replacing the lower Mercia Mudstone. Halite beds are present within the middle Mercia Mudstone of Dorset, although they are absent over much of the Mid Dorset Platform and areas to the east of Wytch Farm. East-west-trending faults became dominant in the Jurassic, and there is strong seismic evidence for syn-depositional movement on these faults during the Lias across the entire Dorset-Isle of Wight area. In Dorset, Many of these east-west faults are listric, hading out into the Mercia Mudstone halite and creating very different structural styles above and below the salt. Major syn-depositional movement is thought to have also taken place along these faults during Kimmeridgian and Portlandian times, creating a series of rollover anticlines in the hanging walls. Seismic evidence indicates that the early Cretaceous sequences of the Portland-South Wight Trough thinned rapidly onto the footwalls of these east-west faults and that very little deposition took place north of the Purbeck Disturbance, either in Dorset or on the Isle of Wight. Several of these east-west faults reversed their throw during compression associated with the mid-Tertiary inversion. Reverse displacement estimates range from 50 m on the Litton Cheney Fault to 1320 m on the Purbeck Disturbance in the Isle of Wight. Offsets of the inversion trends may be related to underlying northwest-southeast-trending Variscan faults which influenced the margins of the Jurassic and early Cretaceous basins, but strike-slip movements may have accentuated some of these offsets during the Tertiary. These offsets appear to be dextral to the west of Wytch Farm oilfield and sinistral in the area to the east. Structural styles suggest that there may also have been strike-slip movements along east-west-trending faults.
Regional uplift in the English Channel: quantification using sonic velocity
Authors: Law Adam
Published: April 2008

Analysis of the substantial well database held by British Gas Exploration and Production Ltd enables regional estimates of uplift for the English Channel area to be derived from anomalous measurements of downhole sonic velocity. Estimates of section uplift, or apparent erosion, can be derived from variations in sonic velocity/travel-time between wells in a basin relative to a function defining the velocity/travel-time and depth relationship for the basin; the normal compaction relationship. These estimates are, however, extremely sensitive to errors in the normal compaction relationship, and to variations in sedimentary facies and/or diagenesis between wells. In this study, these problems have been minimized using regionally persistent, homogeneous thick facies such as shales and by carefully constraining the normal compaction relationship by ‘backstripping’ apparent erosion values to determine ‘residual apparent erosion’.

Apparent erosion has been determined for three intervals from the Mid- and Upper Jurassic, broadly corresponding to the Fuller’s Earth, Oxford Clay and Kimmeridge Clay Formations. Averaging these values and constraining them as above results in robust estimates of total uplift across the English Channel Basin, with uncertainties of around 20%. The results of this study suggest kilometre-scale uplift within the area, with maxima over the Purbeck-Isle of Wight Fault Zone and the Mid Channel High inversion axes. To the north of the Purbeck-Isle of Wight Fault Zone the apparent erosion calculations estimate uplift of 500 m or less, suggesting that this area may have remained relatively stable during basin inversion. When compared to equivalent estimates from AFTA/VR (apatite fission track/vitrinite reflectance) palaeotemperature data, the sonic-derived uplift is similar within errors in inverted areas. However, in more stable areas such as to the north of the Purbeck-Isle of Wight Fault Zone, calculated apparent erosion indicates that little uplift has occurred, whereas uplift estimates derived from AFTA/VR models are still significant.
Inversion tectonics in the Lyme Bay-West Dorset area of the Wessex Basin, UK
Authors: Smith C. , Hatton I. R.
Published: April 2008

Inversion in Lyme Bay and West Dorset was characterized by the reversal of motion on major Jurassic faults, and regional uplift which resulted in the widespread loss of the mid-Cretaceous to Tertiary post-rift cover throughout the area. North-south contraction during the Tertiary resulted in the development of a major anticline in the hanging wall of the Lyme-Portland faults, which was accompanied by widespread folding within the footwall of these faults. Northeast-southwest sinistral and northwest-southeast dextral strike-slip faults were also developed throughout the Lyme Bay-West Dorset area and accommodated much of the north-south contraction. These features are considered to be a consequence of the mechanical effects of an extensive salt interval within the Triassic Mercia Mudstone Group of this area. Major Early Jurassic faults appear to have been directly linked with basement through the salt interval, although smaller faults within the footwall apparently detached on this horizon. The major faults were inverted in a complex manner during the Tertiary, commencing with simple reversal of motion on these faults, followed by detachment of the post-salt carapace as the footwall and hanging wall salt layers were brought into close proximity, thereby resulting in the widespread deformation observed in the footwall of these faults.
The structural development of the Central English Channel High — constraints from section restoration
Authors: Beeley H. S. , Norton M. G.
Published: April 2008

The Central English Channel High is an east-west-trending asymmetric, southfacing, anticlinal structure which forms the southern boundary of the Central Channel Basin. Upper Jurassic rocks are exposed at seabed in the core of the structure, while Tertiary strata are preserved immediately to the south. Elsewhere in the Wessex-Channel basins structures of this overall form are interpreted as linear inversion structures formed during the mid-Tertiary by the reactivation of Mesozoic extensional faults, themselves formed by the partial reactivation of Variscan thrust ramps. Projection of the eroded sequence over the high using the geometry of the preserved section indicates the original presence of an approximately symmetrical Upper Jurassic-Lower Cretaceous sub-basin formed in the hanging wall of a north-dipping Mesozoic extensional structure, the Central Channel Fault. Seismic velocities from wells over the high are consistent with the implied post-Cretaceous relative uplift of up to 1500 m. The deeper geometry of the Central Channel Fault has been determined using the restored hanging wall geometry and the main aspects of the inversion structure have been recreated by forward modelling. The internal structure of the Central Channel High is consistent with a moderately steep fault near surface which passes relatively abruptly into a lower-angle fault. This is interpreted to indicate the presence of a Variscan backthrust at depth which has been partially reactivated during both Mesozoic extension and mid-Tertiary inversion.
Fault size distribution analysis — an example from Kimmeridge Bay, Dorset, UK
Authors: Hunsdale R. , Sanderson D. J. , Hunsdale R.
Published: April 2008

Fault displacement data were measured over four orders of magnitude for a fault set cross-cutting Upper Jurassic rocks exposed along the Dorset coast. Fault data were subdivided into three data-sets, based on data source and field character. Distribution analysis showed that these faults conform to a power law. The scaling relationship is, however, not constant over the entire displacement range of the faults. Faults with displacement >2m are characterized by a negative power law having an exponent of ≈0.96, while faults with displacement <1m are related by a negative exponent of ≈0.7. The change at the small scale is interpreted as being a product of the influence of lithological heterogeneity on fracture initiation and growth.
Towards an integrated study of the depositional environment of the Bencliff Grit (Upper Jurassic) of Dorset
Authors: Goldring R. , Astin T. R. , Marshall J. E. A. , Gabbott S. , Jenkins C. D.
Published: April 2008

An attempt to establish the depositional environment of the Bencliff Grit and its stratigraphic relationships is made using sedimentology, micropalaeontology, palynology and ichnology. Integration of these techniques demonstrates that the main sand units that characterize the Bencliff Grit represent periodic incursions by unidirectional west- and WNW-directed high-density flows, into water depths of at least 5 m from which biological recovery was relatively slow. An essentially autogenic process with dune migration over a colonized interdune area can probably be rejected. The ‘event’ sand units were not associated with colonized sediment-water interfaces. There are at least eight thin units of heterolithic muds and sands closely associated with two suites of marine trace fossils representing sand (
Diplocraterion habichi
) and mud (
D. parallelum
) associations. All mud samples are characterized by a signature of marine palynomorphs and microfossils. The palynofacies show significant differences which correlate with sedimentary facies. These include a palynofacies indicative of rapid sedimentation (in muds immediately following sands). In muds associated with the marine trace fossils a diagenetically modified assemblage is present. A bay or lagoonal setting with back-barrier washover sands is tentatively favoured rather than the intertidal, shoreface or estuary model previously used. The base of the Bencliff Grit does not
a priori
represent a sequence boundary and there are no parasequences within the formation. A sequence boundary may be present within the top 1 m of the formation, associated with a complex transgressive surface.
The Kimmeridge Bay oilfield: an enigma demystified
Authors: Evans Jonathan , Jenkins David , Gluyas Jon
Published: April 2008

The Kimmeridge oilfield was the first commercial discovery in the Wessex Basin. It was discovered in 1959 and is still producing from a single well. To date over 3 million barrels of oil have been extracted. Production is likely to continue into the next century. Oil is trapped in the fractured Cornbrash Limestones in an anticline close to the main inversion axis. The Kimmeridge accumulation remains the only producing field in the hanging wall to the Purbeck Disturbance, making it an intriguing enigma within the basin. Many myths have been perpetuated regarding the size of the field and the results from drilling elsewhere within Kimmeridge Bay. By presenting data on the field here, we hope to dispel some of these myths.
Oblique simple shear in transpression zones
Authors: Jones Richard R. , Holdsworth Robert E.
Published: April 2008

A mathematical and conceptual framework is presented for extending existing models of transpression and transtension to consider vertical and oblique displacements of the deformation zone bounding blocks. This allows transpression and transtension to be considered in terms of the interplay between four biaxial ‘end-member’ strain components, which are shown graphically as the four vertices of a ‘strain tetrahedron’. Transpressional and transtensional strains represented by the strain tetrahedron generally display triclinic symmetry, although the end-member biaxial strains are orthorhombic or monoclinic. Deformation in which vertical simple shear dominates over horizontal simple shear gives rise to finite strains that tend towards plane strain. Finite strains caused by deformation involving oblique simple shear are generally orientated non-parallel to all three Cartesian reference axes.
Analogue modelling of faulting in zones of continental transpression and transtension
Authors: Schreurs Guido , Colletta Bernard
Published: April 2008

Experiments were performed to simulate deformation in zones of continental transpression and transtension. Stratified models consisted of brittle analogue materials overlying a thin layer of viscous material. Oblique deformation was obtained by combining a basal, distributed strike-slip shear component with either transverse shortening (transpression) or transverse extension (transtension). In transpression experiments the imposed ratio of shear strain rate and shortening strain rate exerts an important control on initial fault evolution in the brittle layers of the model. In those experiments with a relatively high strain rate ratio (≥3.6), subvertical, en echelon strike-slip faults develop first, striking at angles of 25–37° to the shear direction. With increasing strain several convergent strike-slip fault zones form displaying positive flower structures. In low strain rate ratio experiments (≤2.7), gently dipping (30–45°), downward converging thrust faults accommodate initial failure. They bound pop-up structures that strike parallel to the shear direction. Increasing strain results in a fault pattern dominated by oblique-slip reverse faults. Partial partitioning of fault motion occurs at late stages of strain when strike-slip faults form within popup structures. The strike-slip faults merge at depth with confining oblique-slip reverse faults, have a curved shape in plan view and a dip direction which changes along strike. Fault patterns can be used as kinematic indicators. En echelon strike-slip faults initially accommodate deformation in a transtension experiment and strike at low angles (6–10°) to the shear direction. With increasing strain, normal faults form parallel to older strike-slip faults. They develop as a result of partitioning of fault motion and gravity failure. There is good agreement between experimental results and natural examples of continental transpressional and transtensional tectonics.
The structural response to evolving plate kinematics during transpression: evolution of the Lebanese restraining bend of the Dead Sea Transform
Authors: Butler R. W. H. , Spencer S. , Griffiths H. M.
Published: April 2008

Structural evolution along continental transform faults may be related to fault zone geometry and to regional variations in plate kinematics. Using a case study of the Lebanese sector of the Dead Sea Transform, the finite geometry of transpression at a restraining bend can be shown to have evolved in time. Relative structural chronologies, calibrated against dated landscape features such as lava-covered palaeosurfaces, coastal erosion surfaces and their incised drainage basins, are used to establish the timing of displacement activity on the major transcurrent faults. The early part of the region’s structural history, up to late Miocene times, was controlled by the geometry of the through-going Yammouneh Fault. Transpression on this right-trending left-lateral structure was accommodated by strike-slip and distributed crustal shortening represented by the initial uplift of Mount Lebanon. For the past 6 Ma the principal active strand of the transform has been the Roum Fault. For much of this period it is presumed to have been a through-going fault which accommodated about 30 km left-lateral displacement. During the Quaternary, the fault zone has become strongly segmented. Although the location of active transcurrent faulting has migrated during the history of the transform, the major site of crustal shortening, the Mount Lebanon-Jabel Barouk structure, has remained broadly fixed. However, the rates of amplification of this structure and the coastal flexure appear to have varied. Continuing uplift of Mount Lebanon and local Plio-Quaternary folding suggest that the offshore continuation of the Roum Fault contains a rightwards, transpressive bend. We relate this multistage history for the Lebanese sector of the transform to an evolving plate tectonic setting: the rotation pole for relative plate motion between Africa and Arabia has migrated through time and the triple junction between the transform and the Tethyan destructive plate margin to the north has moved from onshore SE Turkey to now lie in the NE Mediterranean. Our case study illustrates the transient and evolving nature of deformation in continental restraining bends.
The Trans Mojave-Sierran shear zone and its role in Early Miocene collapse of southwestern North America
Authors: Dokka Roy K. , Ross Timothy M. , Lu Gang
Published: April 2008

The relative motion between the Pacific and North American plates in early Miocene time was not parallel to the overall NW strike of the transform, but was instead oblique and transtensional. It has recently been proposed that in response to this divergence, the western edge of the North American plate east of the transform gravitationally collapsed and moved 100–150 km to the southwest (S50°–60°W). The region of collapse covered an area of nearly 10
6
km
2
and included what is now southern California, southwestern Arizona, and northwestern Mexico. A major structure facilitating collapse between 21 and 18 Ma was the Trans Mojave-Sierran shear zone (TMSSZ). This east-west shear zone linked the classic detachment fault terranes and metamorphic core complexes of the Mojave desert, southeastern California, southern Arizona, and Sonora, Mexico, to the transtensional plate boundary.

To more fully understand the nature and kinematic significance of the TMSSZ and its role in facilitating early Miocene fragmentation of the North American plate, a palinspastic reconstruction of the Mojave desert was performed to remove the disruptive effects of the TMSSZ and younger tectonic events. Features formed just before movements along the TMSSZ were used as markers to assess the TMSSZ deformation. Our analysis indicates that TMSSZ deformation was distributed across a
c.
90 km wide band; restoration of markers to their original positions implies that >80 km of dextral shear occurred along the TMSSZ. First-order dextral shear deformation within the TMSSZ is expressed internally by clockwise vertical axis rotations of large areas that were facilitated by second-order zones of sinistral shear that separated the blocks. These second-order sinistral zones apparently exploited older transfer zones of the 24-21 Ma Mojave Extensional Belt.
Transtensional deformation in the evolution of the Bohai Basin, northern China
Authors: Allen Mark B. , Macdonald David I. M. , Xun Zhao , Vincent Stephen J. , Brouet-Menzies Christine
Published: April 2008

Extensional basins with an element of strike-slip deformation can form because of a perturbation in a strike-slip fault zone (pull-apart and fault wedge basins), or where extension is oblique to the margins of the deforming zone (transtensional basins). Transtensional basins are characterized by en echelon arrays of normal faults which are individually oblique to the basin margins. The Bohai Basin, northern China, has previously been modelled as either (1) a giant pull-apart between NNE-SSW trending dextral strike-slip faults, or (2) a rift basin caused by WNW-ESE extension, without significant strike-slip deformation. We present a model for the Bohai Basin’s rift history in which the basin formed as a result of dextral transtension. The Bohai Basin is one of a family of early Tertiary extensional basins present within eastern Asia from northeastern Russia to southeast China. The structural grain in this basin was inherited from a phase of late Mesozoic sinistral transpression. Tertiary extension began in the Paleocene. Most half-grabens in the eastern and western regions of the Bohai Basin have master faults with a NE-SW or NNE-SSW orientation. Secondary normal faults strike oblique to the main structures, in en echelon arrays which indicate a component of dextral transtension. The central part of the basin, the Bozhong Depression, became a significant depocentre for the first time in the middle Eocene. It formed when activity on transtensional zones to its east and west created an extensional overlap between them. Thus the basin as a whole resembles a giant pull-apart basin, with the Bozhong Depression as its central depocentre, but dextral transtension rather than simple strike-slip controlled the deformation. The component of dextral deformation in the Bohai Basin is shared by other early Tertiary east Asian extensional basins, and is consistent with the sense of shear implied by the oblique convergence of the Pacific and Asian plates: an east-west convergence vector applied to a NE-SW trending plate boundary. The consistency of this dextral shear along the Asian margin, and the fact that several of these basins pre-date the India-Asia collision, supports an origin by subduction roll-back of the oceanic Pacific plate from Asia. The extrusion model for east Asian basin formation, whereby extension was caused by lateral transport of lithospheric blocks out of India’s northward path following the India-Asia collision, is not applicable to major basins east and northeast of the Red River Fault.
Transpressionally driven rotation in the external orogenic zones of the Western Carpathians and the SW British Variscides
Authors: Gayer Rod , Hathaway Tanya , Nemcok Michal
Published: April 2008

Analysis of two examples of obliquely convergent external orogenic zones, the western part of the Western Carpathians and the northern Variscan margin in southwest Britain, indicates the operation of two dominant stress rotation mechanisms in the transpressionally deformed thrust wedge: (1) the rotation of an inferred stress field; (2) the rotation of a deforming body within a constant stress field. In the thinnest, external parts of the thrust wedge, σ
1
stress trajectory rotations of up to 90° occur with deformation having a relatively small component of pure shear. Towards the hinterland, σ
1
stress trajectories in thicker parts of the wedge are progressively less rotated but develop a larger component of pure shear. Resultant σ
1
trajectories are curvilinear, lying parallel to the orogenic convergence vector in the hinterland but diverging progressively from this direction towards the foreland, where they lie at high angles to the external margin in frontal parts of the thrust wedge. It is argued that balanced cross-sections should be constructed parallel to the curved trace of the σ
1
stress trajectories.
Development of kinematic partitioning within a pure-shear dominated dextral transpression zone: the southern Ellsworth Mountains, Antarctica
Authors: Curtis M. L.
Published: April 2008

The Heritage Range of the southern Ellsworth Mountains, West Antarctica, is composed of Cambrian to Permian sedimentary and volcanic rocks, which were deformed during the Permo-Triassic Gondwanian orogeny. The structural grain of the Heritage Range exhibits a previously unrecognized 18° swing from 333–153° in the north to 315–135° in the south. A change in structural and kinematic style accompanies this strike-swing, with a structure consistent with near-orthogonal shortening present in the south, and a structural style consistent with dextral transpression within the central and northern Heritage Range. Kinematic partitioning is present within the central Heritage Range, where strikeparallel, contemporaneous domains of dextral and reverse shear have developed simultaneously with the regional cleavage. Comparison of the structure and kinematics within both structural domains suggests that the central and northern Heritage Range experienced pure-shear dominated dextral transpression, with an approximate angle of relative shortening (α) of 65–70°. Results derived by integrating field data into a published kinematic partitioning model suggest relatively efficient kinematic partitioning has occurred. However, such efficient partitioning cannot be explained by strain partitioning models based purely on plate boundary conditions. Therefore, it is proposed that pre-existing weak structures were present within the unexposed basement facilitating the apparent high percentage of kinematic partitioning.