Salt Tectonics, Sediments and Prospectivity: an Introduction

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Salt tectonics, sediments and prospectivity: an introduction

STUART G. ARCHER1*, G. IAN ALSOP1, ADRIAN J. HARTLEY1, NEIL T. GRANT2 & RICHARD HODGKINSON3 1Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK 2ConocoPhillips UK Ltd, Rubislaw House, North Anderson Drive, Aberdeen AB15 6FZ, UK 3Bowleven plc., 1 North St Andrew Lane, Edinburgh, EH2 1HX, UK *Corresponding author (e-mail: [email protected])

Salt: an introduction rafting. Salt influences syn- and post-kinematic sediment dispersal patterns and reservoir distri- Salt is a crystalline aggregate of the mineral halite, bution and can therefore be important for the cre- which forms in restricted environments where the ation of stratigraphic traps. It can also form top hydrodynamic balance is dominated by evaporation. and side seals to hydrocarbon accumulations and The term is used non-descriptively to incorporate all act as a seal to fluid migration and charge at a evaporitic deposits that are mobile in the subsurface. more regional scale. Salt may also dramatically It is the mobility of salt that makes it such an inter- affect the thermal evolution of sediments due to its esting and complex material to study. As a rock, salt high thermal conductivity. A thick layer of salt is almost unique in that it can deform rapidly under cools sediments that lie below it while heating geological conditions, reacting on slopes ≤0.58 dip sediments above it. This effect cannot be underesti- and behaving much like a viscous fluid. Salt has a mated as it helps provide the favourable conditions negligible yield strength and so is easy to deform, for source rock maturation in the deepwater Gulf of principally by differential sedimentary or tectonic Mexico and Santos basins, even though sedimentary loading. Significant differences in rheology and overburden may be 5 km or more in thickness. Salt behavioural characteristics exist between the indi- can also impact reservoir quality. The role of salt in vidual evaporitic deposits. Wet salt deforms the diagenetic history of reservoirs through its largely by diffusion creep, especially under low control on hydrothermal pore waters is a crucial strain rates and when differential stresses are low. element in the risking of the deepwater Palaeo- Basins that contain salt therefore evolve and gene play of the Gulf of Mexico, for example. Salt deform more complexly than basins where salt is continues to kinetically evolve through time, not absent. The addition of halokinetic processes to only by the classical roller-diapir-pedestal-canopy/ the geodynamic history of a basin can lead to a collapse progression but also with varying rates of plethora of architectures and geometries. The rich deformation, in response to changing sedimentation variety of resultant morphologies have considerable rates and patterns. The relative timing of salt move- economic as well as academic interest. ment and its impact on source, reservoir, trap, seal Historically, salt has played an important role in and timing often governs the prospectivity in salt- petroleum exploration since the Spindletop Dome related basins. Beyond the realm of petroleum, salt discovery in Beaumont, Texas in 1906. Today, is also used as a resource for potash, gypsum and much of the prime interest in salt tectonics still nitrates and has the potential to be employed as a derives from the petroleum industry because many repository for radioactive waste or a top seal to of the world’s largest hydrocarbon provinces sequestered CO2. reside in salt-related sedimentary basins (e.g. Gulf of Mexico, North Sea, Campos Basin, Lower Congo Basin, Santos Basin and Zagros). An under- Salt tectonics, sediments and prospectivity: standing of salt and how it influences tectonics and a conference summary sedimentation is therefore critical to effective and efficient petroleum exploration. Within rift basins In January 2010, The Petroleum Group of the Geo- in particular, salt is seen to orchestrate the petroleum logical Society in conjunction with the Society for system. Through halokinesis it creates structural Sedimentary Geology (SEPM), convened an inter- traps, counter-regional dips on continental margins, national conference at Burlington House in and it can carry or entrain adjacent lithologies via London entitled Salt Tectonics, Sediments and

From:Alsop, G. I., Archer, S. G., Hartley, A. J., Grant,N.T.&Hodgkinson, R. (eds) 2012. Salt Tectonics, Sediments and Prospectivity. Geological Society, London, Special Publications, 363, 1–6. http:// dx.doi.org/10.1144/SP363.1 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

2 S. G. ARCHER ET AL.

Prospectivity. The aim was to bring together indus- 24 km long weld and is thought to be controlled trialists and academics to present a contemporary by the original thickness of the salt wall together view of salt at a global scale and to examine with the amount and direction of subsequent short- its influence on syn-kinematic sedimentation, on ening. Where contraction was normal to the salt basin evolution and ultimately on hydrocarbon wall, then the diapir was locally closed with little prospectivity. further deformation. However, where shortening The conference was one of the best attended in was oblique to the wall then significant post- recent years at the Geological Society and this weld dextral shearing and fracturing took place, volume incorporates 29 papers stemming from the which may affect the sealing capacity of such welds. conference. Salt tectonics obviously continues to Further detailed studies along the La Popa be an important theme within both academic and weld by Andrie et al. demonstrate that fluvial sedi- industrial geoscience communities alike. This col- mentation in a shortening-induced salt-withdrawal lection of papers creates a broad thematic set that basin displays distinct up-section changes in encompasses much of the recent research into salt fluvial facies distribution and the geometry of halo- and sediment dynamics. It is hoped that this kinetic folding. Fluvial channels in the lower part of volume will act as a valuable modern reference the sequence are typically thin, broad and display and a springboard for future studies. This overview variable palaeocurrents, while the upper parts of outlines the key findings and summarizes the key the sequence are marked by thick, stacked channels concepts that are presented more fully in the with weld-parallel palaeocurrents. In addition, papers in the main body of this special publication. halokinetic folding of sediments also intensifies This is the first Geological Society Special Publi- and becomes narrower up the sequence, reflecting cation (GSSP) dedicated to salt tectonics and associ- reduction in sedimentation rates compared to salt ated sedimentation since Alsop et al. (1996) more rise rates. than 15 years ago; it therefore represents a timely Kernen et al. provide a stratigraphic analysis of addition to this expanding field. Neoproterozoic sequences adjacent to an inferred The volume is separated into five main themes allochthonous salt sheet in the Central Flinders covering a variety of geographical and process- Ranges of South Australia. Sediments adjacent to linked topics relating to salt tectonics, sediments the salt represent a progradational sequence from and prospectivity. These include halokinetic- wave-dominated shelf deposits through to coastal sequence stratigraphy, salt in passive margin set- plain sediments. Halokinetic-sequence boundaries tings, Central European salt basins, deformation may be pronounced and marked by up to 508 of within and adjacent to salt and salt in contractional angular truncation. settings and salt glaciers. Smith et al. consider the fracture-controlled palaeohydrology of a secondary salt weld in the Halokinetic-sequence stratigraphy La Popa Basin. Isotopic and fluid inclusion analyses suggest that veins were formed after salt was evac- Halokinetic-sequence stratigraphy involves the uated and are more abundant near a bend in the application of sequence stratigraphic principals weld. In addition, such analyses also indicate that directly to salt-influenced sedimentary strata. This the weld acted as a vertical fluid conduit and a section opens with a paper by Giles & Rowan horizontal baffle. This has clear implications for that investigates concepts in halokinetic-sequence hydrocarbon sealing potential, which may also be stratigraphy and deformation. Hook geometry influenced by factors such as bends in welds and halokinetic sequences are narrow unconformity- amounts of shortening across such structures. bounded zones of deformation with pronounced angular (.708) discordances and abrupt facies Salt in passive margin settings changes. Wedge geometry halokinetic sequences display broader zones of folding, low-angle trun- Mohriak et al. discuss salt structures in Brazil, cations and gradual facies changes. Composite ranging from intracratonic Palaeozoic basins to halokinetic sequences may also form and reflect divergent margin basins formed during the Mesozoic variations in the ratio of diapiric rise to sediment break-up of Gondwana. Seismic reflection data accumulation rates. together with magnetic and gravity data across Rowan et al. describe the anatomy of an exposed Mesozoic rifts of the South Atlantic suggest a thick vertical salt weld from the La Popa Basin of North- autochthonous salt layer that becomes pronounced ern Mexico. Halokinetic folding coupled with local above the transition from continental to oceanic unconformities suggest a precursor salt wall that crust. Recent discoveries suggest that pre-salt plays was subsequently squeezed during regional contrac- in deepwater settings will make an important contri- tion to form a vertical salt weld. The degree of defor- bution to future hydrocarbon production from the mation varies significantly along the length of the region. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

SALT AND SEDIMENTS 3

The paper by Davison et al. considers salt depo- Fort & Brun discuss the kinematics of regional sition, loading and gravity-driven ‘drainage’ of salt flow in the northern Gulf of Mexico. Contrary salt in the Campos and Santos basins of southern to previous interpretations that invoke sedimentary Brazil. Evaporites, which are thought to have been loading as the main driving force, they suggest rapidly deposited (,1 Ma) during the latter phase that the primary control on salt tectonics in the of extension, resulted in loading of the basin northern Gulf of Mexico is gliding related to the which thereby created further subsidence and con- overall dip of the margin. sequent flow (or drainage) of mobile salt into these Kane et al. document the halokinetic effects on subsiding basins. Interpretation of seismic sections submarine channel profiles from the Magnolia suggests that downslope flow of salt occurred Field in the Gulf of Mexico. During the growth of before significant deposition of sediments, thereby salt structures, channels may become entrenched resulting in the redistribution of salt load and prob- where their erosive potential is sufficient to outpace able reactivation of faults. topographic growth. However, where flows are less The role of halokinesis in controlling structural frequent, topographic growth may form a barrier to styles and sediment dispersal patterns in the Santos successive flows, causing avulsion of the channel Basin is investigated by Guerra & Underhill. system. Large-scale sequences of salt growth and Deformation in the post-salt sequence is marked withdrawal may therefore result in a cyclic style by gravitational gliding and spreading driven by of submarine channel evolution. halokinesis and sediment inflows. The Cabo Frio Albertz & Ings consider numerical models of fault is a major landwards-dipping listric fault that mechanical stratification in basin-scale passive- controls major sediment depocentres upslope, margin salt tectonics. They present two-dimensional accommodated downslope by shortening within (2D) plane strain numerical experiments to illus- salt-cored folds. Sediment supply from a variety of trate the effects of variable evaporite viscosity directions forms an interference pattern of folds and embedded sediment layers on the style of salt and intervening minibasins. The post-salt sediments flow and associated overburden deformation. Low- move basin-ward relative to the pre-salt sequence viscosity salt may be almost completely expelled due to halokinesis, although restoration techniques from beneath overlying basins while embedded allow key elements to be linked thereby improving sediment layers may partition salt flow, allowing petroleum system assessments. contractional structures to develop during a seaward- Quirk et al. discuss salt tectonics on passive directed salt squeeze. Density differences between margins with examples from the Santos, Campos embedded sediment layers and salt may result in frac- and Kwanza basins. Salt is considered to flow tionation, ultimately resulting in a thick zone of pure rapidly towards the ocean basin immediately after halite. Such a process of buoyancy fractionation may continental rifting due to thermal subsidence. Trans- explain the occurrence of layered salt in autochtho- lation and extension of the salt overburden on both nous salt basins and pure halite in allochthonous seaward- and landward-dipping normal faults mark salt sheets. the early history of inboard areas, whereas outboard Adam & Krezsek analyse basin-scale salt tec- areas are characterized by contractional structures. tonic processes in the Laurentian Basin offshore The processes of thermal subsidence and salt drai- Atlantic Canada by integrating seismic interpret- nage are capable of moving sediment on passive ation with analogue experiments. More than 3 km margins tens of kilometres seaward in relatively of salt was deposited in a late Triassic age, 50– short periods of time. 70 km wide basin structured by interconnected rift- Quirk & Pilcher introduce the concept of flip- related half-grabens. Subsequent sediment input flop salt tectonics which relates to salt walls devel- mobilized salt into a series of domains dominated oped in extensional regimes. Within such settings by salt welds and pillows, extensional diapirs salt walls may display strata truncated against both and canopies, contractional diapirs and folds and flanks, asymmetry related to normal fault growth allochthonous salt nappes. Secondary salt detach- patterns, and unconformities or onlap surfaces sep- ment levels display growth faulting together with arating strata dipping in opposing directions. The mini-basin formation. structure is considered to develop by a normal Ferrer et al. describe the evolution of salt struc- fault detaching down one flank of a salt roller. Salt tures during extension and inversion of the Parentis flows towards the crest in the low-stress area in Basin in the eastern Bay of Biscay. Salt diapirs and the fault footwall, causing it to tilt backwards and walls began to grow during the Late Jurassic as the become unstable. Growth then continues by switch- North Atlantic Ocean and Bay of Biscay opened. ing to a new counter-dipping fault that detaches on Many of the salt structures had stopped growing the opposite margin of the salt body, leading to an by the Mid-Cretaceous when their source layer unconformity and onlap surface. This process may had become depleted. However, during the Late repeat itself several times. Cretaceous–Cenozoic Pyrenean Orogeny, the basin Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

4 S. G. ARCHER ET AL. was mildly inverted. Nearly all of this shortening was are marked by shortening rather than extension, accommodated in the salt structures which were reju- which is contrary to many previous interpretations. venated to form squeezed diapirs, possible vertical salt welds and salt glaciers. Deformation within and adjacent to salt

Central European salt basins This section commences with a paper by Cart- wright et al. concerning strain partitioning in This section opens with a paper by Krzywiec that gravity-driven shortening of a thick multilayered provides an overview of the Mesozoic and Ceno- evaporate sequence from the Levant Basin in the zoic evolution of salt structures within the Polish eastern Mediterranean. Gravity spreading is driven Basin. This Permian–Cretaceous basin forms part by basin subsidence and tilting of the Levant of a system of epicontinental depositional basins margin and progradation of Nile sediments. Four developed in western and central Europe that were separate major detachments within the Messinian filled with siliciclastics, carbonates and (Zechstein) evaporites are interpreted as halite-rich units, evaporites. Salt-related structures that developed which partition flow within the evaporites. Shorten- around the periphery of the basin (where salt was ing profiles suggest Poiseuille flow, with salt flowing thinner) include grabens bounded by listric faults downdip faster than intercalated sediment layers detaching on the salt and salt pillows. In the more and also faster than overburden translation. This is central or axial part of the basin, where salt was the first published use of seismic data to demonstrate thicker, mature salt diapirs developed. These struc- the flow regime within salt on a regional scale. tures were rejuvenated by Late Cretaceous inversion Fiduk & Rowan also analyse folding and defor- of the basin, and some were also reactivated during mation within layered evaporites. Investigation of Oligocene or Miocene subsidence. the Santos Basin (offshore Brazil) reveals a thick Burliga et al. use the analogue and numerical sequence of evaporites comprising three relatively modelling of salt supply to the Klodawa diapiric competent anhydrite ‘beams’ separated by three structure rising above an active basement fault in weaker (halite-rich) detachment layers. This mech- central Poland. Experimentation shows that salt pre- anical stratigraphy responds in different ways to ferentially feeds from the footwall of the basement contractional deformation associated with conver- fault. Subsequent shortening of the model resulted gent gravity gliding/spreading of the margin. Com- in thinning and redistribution of material within petent beams may become highly disrupted and the diapir, together with its stem shifting to the foot- curvilinear folds and sheath folds are also generated wall. Numerical modelling suggests that the magni- in association with the intense non-coaxial defor- tude of the basement fault governs the amount of salt mation that increases downwards due to strain parti- supply to a diapir across the fault and that there is a tioning across internal detachments within the differential salt supply from the hanging walls and evaporites. footwalls with time. Strozyk et al. describe a 3D seismic study of Maystrenko et al. discuss the regional role of complex intra-salt deformation from the Zechstein Permian salt within the Central European Basin of the western Dutch offshore sector. They use System. The Zechstein salt was mobilized in 10 m thick anhydrite ‘stringers’ to study boudinage response to several post-Permian tectonic events, and constrictional folding within the evaporites, resulting in salt walls and diapirs reaching up to which is compared with observations from salt 9 km in thickness. Withdrawal of salt in to these mines and numerical models. Such deformation is structures has strongly influenced deposition and thought to reflect interaction of the layered salt deformation of the Cenozoic sediments. rheologies, 3D salt flow and basement tectonics Brown et al. describe wedges and buffers with coupled with movement of the overburden. new structural observations from the Late Palaeo- Li et al. provide numerical models of displace- zoic partially inverted Dnieper–Donets Basin of ment and deformation within ‘stringers’ embedded the Ukraine. This large intracratonic basin is charac- in down-built diapirs using a case study from the terized by late Devonian rifting when two evaporite South Oman Salt Basin. They investigate via finite sequences were deposited, followed by post-rift element models how differential displacement of thermal sag in the Carboniferous that was associated the top salt surface induces salt flow and consequent in some areas with extension. It is proposed that folding and fracturing of large carbonate stringers basin margin extension may be linked to thick- that represent prospective hydrocarbon plays in the skinned partial inversion of some basement rift Oman salt diapirs. Models suggest that stringers faults. Inversion of basement faults induced mono- may deform soon after the onset of salt tectonics, clines that were possibly further exaggerated during with extension leading to boudinage and fracturing, later (pre-Alpine) tectonic phases. It is suggested while shortening results in folding and thrusting that late Carboniferous to early Permian tectonics of stringers. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

SALT AND SEDIMENTS 5

Using an example from the Spanish Pyrenees, of a submarine salt glacier formed in the Cretaceous Quinta et al. use fracture pattern analysis as a passive margin of North Africa. The Triassic salt means of constraining the interaction between appears interstratified between two Cretaceous regional and diapir-related stress fields. Joint and units that display normal stratigraphic younging. fault data collected from within the overburden The salt is considered to have been emplaced as a adjacent to a diapir enables evolutionary models bedding-parallel extrusion along the sediment– for stress fields around diapirs to be developed. water interface during the Cretaceous. Such salt gla- Data suggest that stress fields evolve from being ciers are typically marked by extensional faulting mostly related to regional tectonics to being domi- associated with the presence of a slope and basin- nated by local stresses associated with diapir ward flow of salt, and characterize allochthonous kinematics. salt sheets in a number of passive margin settings. The section closes with a thought-provoking The final contribution by Graham et al. con- paper by Trude et al. who describe potential siders allochthonous salt in the sub-Alpine fold– salt-related structures from the Bristol Channel of thrust belt of SE France. Although the geology is southern England. Although previous interpret- well known, aspects of its structure remain enig- ations of this well-studied area were linked to matic and may best be explained by invoking salt extension, inversion and strike-slip tectonics, this tectonics whereby an inverted Jurassic sequence contribution challenges the established view. Aided represents an overturned flap created when salt by high-quality 3D seismic datasets from areas such was emplaced at higher levels along the sea floor. as the Gulf of Mexico and Brazil, Trude et al. This allochthonous extrusion or salt glacier, which suggest that many of the well-known features in may have been similar to those in the Gulf of the Bristol Channel are actually better explained Mexico, was subsequently exploited by regional as collapse structures associated with salt withdra- contractional thrusting during the Alpine Orogeny wal and diapirism in the Late Triassic and Early to generate the observed complexity. Jurassic.

Current and future directions of Salt in contractional settings and salt research salt glaciers The broad panoply of papers presented here offer The Zagros Mountains provide perhaps one of the insight into the general status of research into salt best places on earth to study the role of salt in a con- tectonics. This current body of research has built tractional setting, and this section opens with a on the many decades of work that has improved paper by Callot et al. on pre-existing salt structures our understanding of salt both from a deformation and folding in the Zagros. Using 4D analogue exper- perspective (e.g. establishing flow laws for salt iments imaged via X-ray tomography, these authors movement) and also how it influences structural investigate the control exerted by pre-existing salt and stratigraphic architectures. structures during subsequent compressive defor- Certain avenues of research (both geographic mation. They suggest that initial diapir shape may and thematic) are clearly more in vogue than control subsequent geometries with vertical pipe- others at this moment. For example, key regions like diapirs being shortened and localizing sharp are currently Brazil, Gulf of Mexico and Central overturned folds, while pillow-like diapirs may act Europe and research themes include the regional to preferentially orientate ramps. Linear salt ridges scale behaviour of salt and its internal mechanical also have a strong effect on the lateral extent and stratigraphy. Much of this research has been orientation of folds and typically form a disconnect driven by, and significant insights gained from, ana- for structures on either side. logue modelling. This continues to be a focus area Baikpour & Talbot study allochthonous Ter- as it offers a relatively inexpensive means of creat- tiary salt nappes in northern Iran via Advanced Syn- ing scaled models of salt structures. Often the aim is thetic Aperture Radar (SAR) images. They use SAR to understand kinematics and to model progressive images to show that regional folds and faults are episodes of deformation. While these models are active south of the Alborz Mountain front, but are becoming increasingly sophisticated, incorporating ‘dampened’ by the presence of allochthonous salt. digital photogrammetry such as Particle Image A linked seismic reflection study shows that faults Velocimetry (PIV) analysis and Computed Axial are longer than anticipated for earthquakes with Tomography (CAT) scanning, discrete and finite magnitudes ,3.5, suggesting that regional strains element modelling offer additional insights into tec- are more aseismic than previously realized. tonic processes. Here computer power is key to Masrouhi & Koyi continue the analysis of developing large-scale sophisticated continuum lateral salt flow by investigating a Tunisian example models for sedimentary basins that contain salt. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

6 S. G. ARCHER ET AL.

This avenue of research will continue to develop and salt-sediment research. In the past, the geometry may help characterize strain rates and how specific of salt bodies often had to be inferred. Now that basin geometries and subsidence patterns influence clear images of salt sidewalls and bases are avail- salt tectonics. This has particular application to able, it is possible to revisit models for balancing solving the conundrum: does salt glide or spread and restoring cross-sections through salt both in under a differential sediment load? 2D and, more importantly, in 3D. This enables a A further avenue for modelling research that has better understanding of the geodynamic evolution become increasingly of interest to the petroleum of salt within a sedimentary basin and how the industry is in understanding the geomechanics of surrounding rocks have deformed to accommodate salt. Because salt behaves over long time periods this flow. Additional trap potential may be identified as a fluid, it cannot support a shear stress. The or weak points in structures and pore pressure stress tensor within a salt body is simultaneously cells recognized that delimit or condemn exploration both hydrostatic and lithostatic. One consequence targets. These new data, when made available, will of this stress system is that it perturbs the regional enhance the continued study of salt tectonics and stress state and results in complex interactions how salt both positively and negatively impacts between the salt and the visco-elastic wall rocks. the exploration for hydrocarbons in salt-related This has important consequences for drilling close basins. to salt diapirs and through salt sheets, and for under- Other improved techniques for the remote analy- standing the containment potential of salt as a side sis of salt flows include Interferometric Synthetic seal to petroleum traps. Recent finite element mod- Aperture Radar (InSAR) that continues to develop elling illustrates the effects of the presence of salt on and will provide information on surficial salt flows regional stress tensors. More advanced models over periods of weeks if not days. Similar resolution using more realistic rock properties and variable may also be provided by microseismic analysis with mechanical stratigraphy will further advance our implications for toxic waste and CO2 storage within knowledge of salt geomechanics and hopefully salt. Finally, remote analysis via InSAR, microseis- contribute to a fuller understanding of how salt micity and even widely available Earth imaging promotes wall rock deformation and influences tools can now be used to focus fieldwork and hydrocarbon sealing, trapping and pore pressure outcrop studies better. This fundamental ground development. truthing will continue to provide the scientific reas- Improvements in seismic reflection, acquisition surance that is required to test and further constrain and processing techniques have led to vast advances models of salt tectonics and their associated sedi- in our understanding of salt and sediment inter- ment interactions. actions both along the flanks of vertical or overturned salt margins and in sub-salt plays such as The editors would like to take this opportunity to thank all Tupi offshore Brazil. In addition, seismic resolution those who attended the Salt Tectonics, Sediments and Pro- has now been refined to such a level that internal spectivity conference in 2010. They would also like to structures within salt sheets and salt diapirs are thank all contributors to this special publication. Finally we would like to extend our gratitude to our industrial now being imaged, providing a huge opportunity sponsors, Maersk Oil, Hess and Statoil, for their generous to consider the variability within ‘salt’ and to link contributions which have been used to fund colour printing this directly with resulting structures. The recog- in this volume. nition of partitioned flow within salt bodies may become increasingly significant in determining the behaviour of salt and its interaction with sediment. Reference The improved imaging of salt bodies using Alsop, G. I., Blundell,D.&Davison, I. (eds) 1996. Salt seismic reflection data now offered by enhanced Tectonics. Geological Society, London, Special Publi- processing techniques opens up new avenues for cations, 100, 310.