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Salt tectonics: a global perspective
Book reviews
Salt Tectonics: a Global Perspective Edited By M. P. A. Jackson, D. G. Roberts, and S. Snelson. Published by the American Association of Petroleum Geologists, 1995. AAPG Memoir 65, 454 pages. ISBN number O-89181-344-6. Price $119 dollars to non-members and $79 to AAPG members. This is a an important and timely book which consolidates many new ideas which have developed in salt tectonics in the last decade. It is mainly based on interpretation of reflection seismic data from a wide variety of sedimentary basins affected by salt tectonics. The papers stem from a highly-successful AAPG Hedberg Research conference held in Bath, England in September 1993, which was organised by the editors. The strength of the book lies in the careful documentation of the case histories which have been beautifully illustrated with numerous seismic sections and many coloured diagrams. Jackson introduces the book with a review of the conceptual breakthroughs that have been made in the understanding of salt tectonics in the last hundred and fifty years. He makes the important point that our knowledge of how overburden rocks deform above salt is still rudimentary. There are outstanding descriptions of the Gulf Coast of Mexico salt structures by Diegel et al., Fletcher et al., Peel et al., Schuster, and Rowan, which summarise a huge amount of exploration data, and demonstrate how complex the interaction between regional tectonics, sedimentation and salt movement can be. Up to three individual allochthonous salt sheets have developed at successively higher stratigraphic levels, which presumably flowed out at the surface in deep water whilst covered by a thin veneer of sediment. Diegel et al. point out that much of the huge thickness of Cenozoic strata accumulated in the Gulf Coast area is due to accommodation space created by the escape to surface and partial dissolution of up to 4 km of Louann salt. Counter regional fault development was discussed from the passive margins in southern Gabon (Liro and Coen) and the Cabo Frio area in southern Brazil (Mohriak et al.). The latter suggested the faults could have grown by prograding sediment wedges displacing a salt body downslope producing an apparent heave of up to 50 km on a single fault. Cobbold et al. (Brazil) and Heaton et al. (Yemen) describe clear examples of contractional zones produced at the down dip edge of continental margins containing salt horizons. Red Sea salt structures were described for the first time in detail by Heaton, which was the only new area covered in the book. The vexed question of whether basement faults control the location of diapirs is discussed by Remmelts in the southern North Sea and Koyi et al. in the Nordkapp Basin, who both argue for a strong basement control, whereas Nilsen et al. do not identify a direct link in the latter basin. They state that where the salt is thick and the fault movement in the overburden
90
Marine
and Petroleum
Geology
1997 Volume
is slow, then the salt effectively decouples the extending layers above and below it. Analytical work also features strongly, with significant contributions on the uses of balanced sections in salt tectonics. A pragmatic approach to balancing is described by Hossack; where the overburden is balanced, and the gaps which are left are filled by salt. Mathematical modelling and observations of salt glaciers by Fletcher et al. indicate that the up-dip extension of sedimentary cover does not need to balance the down-dip contraction in the cover rocks as the latter can be taken up by salt extrusion. Talbot develops an interesting simple approach to the examination of the salt movement rate and the sedimentation rate on salt diapir geometry which treats the overburden as a stiff material which molds around the salt body. De Ruig describes anhydrite diapirism in the Betics of southern Spain, where the diapiric material is more dense than its cover rocks, yet it has still moved upward to fill space created by extension, being less viscous than the overburden. Inversion of salt structures was reviewed by Letouzey et al. and an interesting case study of the Nordkapp basin by Nilsen et al. suggested that diapirs had been reactivated and squeezed due to gravity sliding, without leaving clear evidence of compression. Coward and Stewart also appealed to gravity sliding to explain the salt pillows and diapirs produced below thick Bunter Shale in the southern North Sea. Hooper and More presented some beautiful examples of 3-D seismic data from the southern North Sea where up to four separate salt horizons can act as detachments. Faults abutt onto these detachments at high angles, rather than soling out as smoothly-curved listric faults. Harrison presented a well-documented account of the Parry Island Fold Belt in Arctic Canada where the folds and thrusts are symmetric. In contrast Sans and Verges indicate that the thrusts and folds are asymmetric and foreland vergent in the southern Pyrenees, because the salt thickens northward towards the mountain belt. The editors and publishers are to be congratulated on a splendid job. There are very few mistakes and the overall quality of the diagrams is high. This book will serve as an important source of reference for many years to come, and I can strongly recommend it to both industry and academic geologists. The price is very good value, considering the number of pull-out figures, coloured diagrams and overall quality. Ian Davison Department of Geology, Royal Holloway, University of London, UK
14 Number
1
Salt Tectonics: a Global Perspective Edited By M. P. A. Jackson, D. G. Roberts, and S. Snelson. Published by the American Association of Petroleum Geologists, 1995. AAPG Memoir 65, 454 pages. ISBN number O-89181-344-6. Price $119 dollars to non-members and $79 to AAPG members. This is a an important and timely book which consolidates many new ideas which have developed in salt tectonics in the last decade. It is mainly based on interpretation of reflection seismic data from a wide variety of sedimentary basins affected by salt tectonics. The papers stem from a highly-successful AAPG Hedberg Research conference held in Bath, England in September 1993, which was organised by the editors. The strength of the book lies in the careful documentation of the case histories which have been beautifully illustrated with numerous seismic sections and many coloured diagrams. Jackson introduces the book with a review of the conceptual breakthroughs that have been made in the understanding of salt tectonics in the last hundred and fifty years. He makes the important point that our knowledge of how overburden rocks deform above salt is still rudimentary. There are outstanding descriptions of the Gulf Coast of Mexico salt structures by Diegel et al., Fletcher et al., Peel et al., Schuster, and Rowan, which summarise a huge amount of exploration data, and demonstrate how complex the interaction between regional tectonics, sedimentation and salt movement can be. Up to three individual allochthonous salt sheets have developed at successively higher stratigraphic levels, which presumably flowed out at the surface in deep water whilst covered by a thin veneer of sediment. Diegel et al. point out that much of the huge thickness of Cenozoic strata accumulated in the Gulf Coast area is due to accommodation space created by the escape to surface and partial dissolution of up to 4 km of Louann salt. Counter regional fault development was discussed from the passive margins in southern Gabon (Liro and Coen) and the Cabo Frio area in southern Brazil (Mohriak et al.). The latter suggested the faults could have grown by prograding sediment wedges displacing a salt body downslope producing an apparent heave of up to 50 km on a single fault. Cobbold et al. (Brazil) and Heaton et al. (Yemen) describe clear examples of contractional zones produced at the down dip edge of continental margins containing salt horizons. Red Sea salt structures were described for the first time in detail by Heaton, which was the only new area covered in the book. The vexed question of whether basement faults control the location of diapirs is discussed by Remmelts in the southern North Sea and Koyi et al. in the Nordkapp Basin, who both argue for a strong basement control, whereas Nilsen et al. do not identify a direct link in the latter basin. They state that where the salt is thick and the fault movement in the overburden
90
Marine
and Petroleum
Geology
1997 Volume
is slow, then the salt effectively decouples the extending layers above and below it. Analytical work also features strongly, with significant contributions on the uses of balanced sections in salt tectonics. A pragmatic approach to balancing is described by Hossack; where the overburden is balanced, and the gaps which are left are filled by salt. Mathematical modelling and observations of salt glaciers by Fletcher et al. indicate that the up-dip extension of sedimentary cover does not need to balance the down-dip contraction in the cover rocks as the latter can be taken up by salt extrusion. Talbot develops an interesting simple approach to the examination of the salt movement rate and the sedimentation rate on salt diapir geometry which treats the overburden as a stiff material which molds around the salt body. De Ruig describes anhydrite diapirism in the Betics of southern Spain, where the diapiric material is more dense than its cover rocks, yet it has still moved upward to fill space created by extension, being less viscous than the overburden. Inversion of salt structures was reviewed by Letouzey et al. and an interesting case study of the Nordkapp basin by Nilsen et al. suggested that diapirs had been reactivated and squeezed due to gravity sliding, without leaving clear evidence of compression. Coward and Stewart also appealed to gravity sliding to explain the salt pillows and diapirs produced below thick Bunter Shale in the southern North Sea. Hooper and More presented some beautiful examples of 3-D seismic data from the southern North Sea where up to four separate salt horizons can act as detachments. Faults abutt onto these detachments at high angles, rather than soling out as smoothly-curved listric faults. Harrison presented a well-documented account of the Parry Island Fold Belt in Arctic Canada where the folds and thrusts are symmetric. In contrast Sans and Verges indicate that the thrusts and folds are asymmetric and foreland vergent in the southern Pyrenees, because the salt thickens northward towards the mountain belt. The editors and publishers are to be congratulated on a splendid job. There are very few mistakes and the overall quality of the diagrams is high. This book will serve as an important source of reference for many years to come, and I can strongly recommend it to both industry and academic geologists. The price is very good value, considering the number of pull-out figures, coloured diagrams and overall quality. Ian Davison Department of Geology, Royal Holloway, University of London, UK
14 Number
1