- Email: [email protected]
Phanerozoic sea-level changes
316
BOOK Rf'~VIEWS
tonic control, more and better figures would be welcome. There is also a certain Cymric bias in assigning a Late Silurian resp. Late Carboniferous age to the " m a i n " Caledonian and Variscan deformations (Ager, though of English antecedents, became an enthusiastic South Welshman when he was nominated to the Swansea chair). More important movements occurred earlier, with the closure of the Iapetus (or Iapeti) and of the Lizard-Giessen ocean. As already mentioned, stratigraphy can be tedious. D e r e k Ager has done more than anybody else to show how exciting it can be when the fundamental questions, on the significance of rocks and fossils, are asked. His small, easily readable book has become a classic--the ironic fate of some provocative m a n i f e s t o e s - - a n d should be read by every geologist concerned with the principles of the science. While writing this review, I heard, to my great distress, that Derek Victor Ager had died on February 8th, 1993. He will be missed by many people in many countries, not only as an outstanding and original scientist, but also as an intelligent and sensitive human being. R. TROMPY (Kiisnacht, Switzerland)
SSDI 0037-0738(93)E0082-Q Phanerozoic Sea-Level Changes. Anthony Hallam. Perspectives in Paleobiology and Earth History, Series 3, Columbia University Press, New York, N.Y., 1992, 266 pp., cloth $50, paperback $24. Recently developed models of sequence stratigraphy have caused a revolution in modern stratigraphic geology. These models envisage the response of s h e l f / s l o p e sedimentation to changes in relative sea level. They are an outgrowth of the industry's need to construct a "process-oriented" framework within which the intricate sedimentary history of continental margins can be deciphered to aid exploration geology. This has resulted in a conceptual framework that attempts to explain the downdip and lateral changes in facies in terms of processes that spawn them, and it allows the organization of sediments into time-con-
strained genetic packages. This has led to a revolution in the way we look at the sedimentary rock record. Sequence-stratigraphic concepts owe their origin to seismic stratigraphy, but the new models are equally applicable to subsurface (seismic and well-log) as well as outcrop data. The documentation of sea-level changes in sedimentary sections in different parts of the world led to a new generation of sequence-stratigraphically constrained sea-level curves for the Mesozoic and Cenozoic (Haq et al., 1987, also known as the Exxon sea-level curves). These curves represent a significant improvement in resolution over the earlier seismic-stratigraphically based curves (Vail et al., 1977). The Exxon curves were, however. presented as working models based on global data of varying quality. Further improvements in this chrono-eustatic framework are to be expected as it is independently tested and as better stratigraphic and chronologic information accumulates. Tony Hallam's very readable recent synthesis on the topic of geological record of sea-level change comes across largely as an extensive critique of the Exxon sea-level curves. Hallam is one of the most distinguished stratigraphic geologists in the world who has himself contributed significantly to our understanding of factors governing eustasy and to the documentation of sea-level changes, particularly those in the Jurassic period in NW Europe. It is therefore puzzling to this reviewer that instead of choosing a direct, more positive, structure for his book, he chose a somewhat less unequivocal format in the form of an extended criticism. As such it tends to be a very personal view of the author. Nevertheless, it still represents a welcome introduction to the topic, and a fairly up-to-date synthesis. Hallam describes a range of commonly used techniques for the study of sea-level changes of the past, e.g. mapping ancient shorelines, areal spread of marine deposits, alternation of marine and non-marine sediments, and the use of seismic and sequence stratigraphies. He highlights the importance of considering hypsometric data in determining the accurate extent of interior seaways. Seismic- and sequence-stratigraphic metho-
BOOKREVIEWS dologies and the sea-level curves are given a critical treatment. Under common complaints are included: lack of published supporting data for the Exxon model, implied chronostratigraphic significance and limits of resolution of seismic data, inapplicability of sequence-stratigraphic models to cratonic interiors, and potential underestimation of tectonic effects. However, the discussion is sometimes incomplete. For example, in discussing seismic resolution, Hallam would have done better if he had not ignored the fact that the 1987 Exxon curves are based not only on seismic lines but also on extensive outcrop and well-log data whose resolution goes well beyond seismic resolution. In the discussion of the inapplicability of the sequence-stratigraphic models to cratonic interiors, Hallam seems unaware of the " r a m p " model that is particularly applicable to the shallow epicontinental seaways. Also conspicuous by its absence in this chapter on techniques is the discussion of the use of oxygen isotopic data as a proxy for sea-level change, which can be applied at least as far back as the Oligocene when the earth entered its present ice-house mode. The discussion of the Paleozoic, Mesozoic and Cenozoic sea-level fluctuations forms the main body of the synthesis. It is obvious that Paleozoic sea-level history is least well known and quite sketchy. A broad similarity between the Paleozoic sea-level record from North American sequences and that derived from global paleogeographic reconstructions is noted. The dearth of seismic and widely applicable biochronologic data hampers meaningful inter-regional correlations in the Paleozoic. For the Mesozoic the quantity of data improves markedly but the quality of intercontinental correlations does not. Hallam uses the Exxon curves as global yardsticks to compare the Triassic, Jurassic and Cretaceous sea-level curves from various regions. H e highlights both the points of departure as well as broad similarities and concludes that tectonics may have altered the eustatic records locally, and the magnitude of sealevel changes as depicted on the Exxon curves may have underestimated the influence of North Atlantic tectonics. He concedes, however, that even on active continental margins during times
317 of tectonic quiescence, e.g. along the Andes in the late Jurassic, the local curve accurately reflects the global picture. He also notes that the value of 250 m for the highest seastand in the late Cretaceous on the Exxon sea-level curve may be close to being accurate, as both empirical and theoretical considerations suggest. Widespread extensional tectonics during the most recent breakup phase of Pangaea that began in the early Mesozoic may indeed have complicated the eustatic record. This tectonic activity was most likely responsible for the large-scale (second-order) sea-level variations of Haq et al. However, the lack of correspondence between the eustatic curves and regional curves of the Mesozoic can be partly ascribed to our inability to make precise inter-regional correlations due to a lack of universal biostratigraphic markers. Ammonites are the most commonly used biostratigraphic tools for much of the Mesozoic and their endemism makes wider correlations often very tenuous. It is not surprising then that such correlations improve for the late Cretaceous and Cenozoic when more widely distributed biostratigraphic markers become available. For the Cenozoic both the quality and the quantity of database is improving significantly. Planktonic microfossils that are less provincial and more widespread allow better inter-regional correlations. Because of improved correlation potential, it is easier to test the trends in the Exxon sea-level model. According to Hallam, independent studies provide general support for the major changes on the global curve, but minor changes are more difficult to test. For at least the last 30 m.y. glacio-eustasy provides an effective mechanism for worldwide sea-level changes and global correlations of third-order cycles become more convincing. Hallam also explores the relationship between sea-level change and biotic extinctions and radiations and their distribution. H e concludes that over the long term sea-level falls do correlate with increased extinctions as Newell had suggested in the 1960s. Sea-level rise, on the other hand, may lead to increased diversity due to greater niche availability. Transgressions and regressions affecting spread of epicontinental seas
318
may be responsible for controlling the degree of endemism in marine biota and in affecting the biogeographic distribution of terrestrial organisms. Hallam has done a good job of summarizing the well-known potential causes of sea-level changes, but offers no new insights into the reason for pre-Neogene third-order cyclicity. The two first-order cycles of the Phanerozoic seem to be related to the two cycles of convergence and dispersal of continental masses. Second-order cycles (10-20 m.y.) may be caused by major tectonic events on the seafloor, including changes in the seafloor spreading rates, volume of the mid-ocean ridges, and mid-plate volcanism (e.g. in the mid Cretaceous). Since the Neogene the genesis of third-order cycles (1-10 m.y.) is quite readily explained by the waxing and waning of glacial ice. But the origin of pre-glacial third-order cycles is still a mystery. Various potential mechanisms are discussed. For example, sequestering and releasing of ground water can change sea-level by about 50 m, not enough to explain many of the thirdorder cycles. Geoidal anomalies could cause significant sea-level changes, but they occur at a very slow pace making their effect negligible. In-plate stress is another intriguing mechanism that could produce sea-level changes of the right magnitude. However, in-plate stress models fail to consider effects of variation in the crustal
BOOK REVIEWS
structure that accompanies basin deformation. This is an area of obvious promise and the relationship of in-plate stress and stress-induced lithospheric density to third-order cycles needs to be explored further. Testing of the Exxon sea-level curves is the central theme of Hallam's book. These eustatic curves were meant to be working models, open to improvement as better information became available. Because of the prime importance of sea-level change to stratigraphy, it is not surprising that testing these models has become a favorite pastime of many sedimentary geologists. However, there are two prerequisites to effectively test the eustatic models in any area. First, the sea-level signal needs to be separated from the regional tectonic overprint. But more importantly, meaningful chronostratigraphic correlative criteria need to exist that can be used to effectively compare the local record to the eustatic model. With the existing state of knowledge these requisites cannot be met for much of early Cretaceous and older strata. Several recent studies that have attempted to test the eustatic models for older time periods suffer from these shortcomings and have proven little conclusive. Hallam's discussion underscores these limitations. BILAL U. HAQ (Washington, D.C., U.S.A.) SSDI 0 0 3 7 - 0 7 3 8 ( 9 3 ) E 0 1 {}5-0
BOOK Rf'~VIEWS
tonic control, more and better figures would be welcome. There is also a certain Cymric bias in assigning a Late Silurian resp. Late Carboniferous age to the " m a i n " Caledonian and Variscan deformations (Ager, though of English antecedents, became an enthusiastic South Welshman when he was nominated to the Swansea chair). More important movements occurred earlier, with the closure of the Iapetus (or Iapeti) and of the Lizard-Giessen ocean. As already mentioned, stratigraphy can be tedious. D e r e k Ager has done more than anybody else to show how exciting it can be when the fundamental questions, on the significance of rocks and fossils, are asked. His small, easily readable book has become a classic--the ironic fate of some provocative m a n i f e s t o e s - - a n d should be read by every geologist concerned with the principles of the science. While writing this review, I heard, to my great distress, that Derek Victor Ager had died on February 8th, 1993. He will be missed by many people in many countries, not only as an outstanding and original scientist, but also as an intelligent and sensitive human being. R. TROMPY (Kiisnacht, Switzerland)
SSDI 0037-0738(93)E0082-Q Phanerozoic Sea-Level Changes. Anthony Hallam. Perspectives in Paleobiology and Earth History, Series 3, Columbia University Press, New York, N.Y., 1992, 266 pp., cloth $50, paperback $24. Recently developed models of sequence stratigraphy have caused a revolution in modern stratigraphic geology. These models envisage the response of s h e l f / s l o p e sedimentation to changes in relative sea level. They are an outgrowth of the industry's need to construct a "process-oriented" framework within which the intricate sedimentary history of continental margins can be deciphered to aid exploration geology. This has resulted in a conceptual framework that attempts to explain the downdip and lateral changes in facies in terms of processes that spawn them, and it allows the organization of sediments into time-con-
strained genetic packages. This has led to a revolution in the way we look at the sedimentary rock record. Sequence-stratigraphic concepts owe their origin to seismic stratigraphy, but the new models are equally applicable to subsurface (seismic and well-log) as well as outcrop data. The documentation of sea-level changes in sedimentary sections in different parts of the world led to a new generation of sequence-stratigraphically constrained sea-level curves for the Mesozoic and Cenozoic (Haq et al., 1987, also known as the Exxon sea-level curves). These curves represent a significant improvement in resolution over the earlier seismic-stratigraphically based curves (Vail et al., 1977). The Exxon curves were, however. presented as working models based on global data of varying quality. Further improvements in this chrono-eustatic framework are to be expected as it is independently tested and as better stratigraphic and chronologic information accumulates. Tony Hallam's very readable recent synthesis on the topic of geological record of sea-level change comes across largely as an extensive critique of the Exxon sea-level curves. Hallam is one of the most distinguished stratigraphic geologists in the world who has himself contributed significantly to our understanding of factors governing eustasy and to the documentation of sea-level changes, particularly those in the Jurassic period in NW Europe. It is therefore puzzling to this reviewer that instead of choosing a direct, more positive, structure for his book, he chose a somewhat less unequivocal format in the form of an extended criticism. As such it tends to be a very personal view of the author. Nevertheless, it still represents a welcome introduction to the topic, and a fairly up-to-date synthesis. Hallam describes a range of commonly used techniques for the study of sea-level changes of the past, e.g. mapping ancient shorelines, areal spread of marine deposits, alternation of marine and non-marine sediments, and the use of seismic and sequence stratigraphies. He highlights the importance of considering hypsometric data in determining the accurate extent of interior seaways. Seismic- and sequence-stratigraphic metho-
BOOKREVIEWS dologies and the sea-level curves are given a critical treatment. Under common complaints are included: lack of published supporting data for the Exxon model, implied chronostratigraphic significance and limits of resolution of seismic data, inapplicability of sequence-stratigraphic models to cratonic interiors, and potential underestimation of tectonic effects. However, the discussion is sometimes incomplete. For example, in discussing seismic resolution, Hallam would have done better if he had not ignored the fact that the 1987 Exxon curves are based not only on seismic lines but also on extensive outcrop and well-log data whose resolution goes well beyond seismic resolution. In the discussion of the inapplicability of the sequence-stratigraphic models to cratonic interiors, Hallam seems unaware of the " r a m p " model that is particularly applicable to the shallow epicontinental seaways. Also conspicuous by its absence in this chapter on techniques is the discussion of the use of oxygen isotopic data as a proxy for sea-level change, which can be applied at least as far back as the Oligocene when the earth entered its present ice-house mode. The discussion of the Paleozoic, Mesozoic and Cenozoic sea-level fluctuations forms the main body of the synthesis. It is obvious that Paleozoic sea-level history is least well known and quite sketchy. A broad similarity between the Paleozoic sea-level record from North American sequences and that derived from global paleogeographic reconstructions is noted. The dearth of seismic and widely applicable biochronologic data hampers meaningful inter-regional correlations in the Paleozoic. For the Mesozoic the quantity of data improves markedly but the quality of intercontinental correlations does not. Hallam uses the Exxon curves as global yardsticks to compare the Triassic, Jurassic and Cretaceous sea-level curves from various regions. H e highlights both the points of departure as well as broad similarities and concludes that tectonics may have altered the eustatic records locally, and the magnitude of sealevel changes as depicted on the Exxon curves may have underestimated the influence of North Atlantic tectonics. He concedes, however, that even on active continental margins during times
317 of tectonic quiescence, e.g. along the Andes in the late Jurassic, the local curve accurately reflects the global picture. He also notes that the value of 250 m for the highest seastand in the late Cretaceous on the Exxon sea-level curve may be close to being accurate, as both empirical and theoretical considerations suggest. Widespread extensional tectonics during the most recent breakup phase of Pangaea that began in the early Mesozoic may indeed have complicated the eustatic record. This tectonic activity was most likely responsible for the large-scale (second-order) sea-level variations of Haq et al. However, the lack of correspondence between the eustatic curves and regional curves of the Mesozoic can be partly ascribed to our inability to make precise inter-regional correlations due to a lack of universal biostratigraphic markers. Ammonites are the most commonly used biostratigraphic tools for much of the Mesozoic and their endemism makes wider correlations often very tenuous. It is not surprising then that such correlations improve for the late Cretaceous and Cenozoic when more widely distributed biostratigraphic markers become available. For the Cenozoic both the quality and the quantity of database is improving significantly. Planktonic microfossils that are less provincial and more widespread allow better inter-regional correlations. Because of improved correlation potential, it is easier to test the trends in the Exxon sea-level model. According to Hallam, independent studies provide general support for the major changes on the global curve, but minor changes are more difficult to test. For at least the last 30 m.y. glacio-eustasy provides an effective mechanism for worldwide sea-level changes and global correlations of third-order cycles become more convincing. Hallam also explores the relationship between sea-level change and biotic extinctions and radiations and their distribution. H e concludes that over the long term sea-level falls do correlate with increased extinctions as Newell had suggested in the 1960s. Sea-level rise, on the other hand, may lead to increased diversity due to greater niche availability. Transgressions and regressions affecting spread of epicontinental seas
318
may be responsible for controlling the degree of endemism in marine biota and in affecting the biogeographic distribution of terrestrial organisms. Hallam has done a good job of summarizing the well-known potential causes of sea-level changes, but offers no new insights into the reason for pre-Neogene third-order cyclicity. The two first-order cycles of the Phanerozoic seem to be related to the two cycles of convergence and dispersal of continental masses. Second-order cycles (10-20 m.y.) may be caused by major tectonic events on the seafloor, including changes in the seafloor spreading rates, volume of the mid-ocean ridges, and mid-plate volcanism (e.g. in the mid Cretaceous). Since the Neogene the genesis of third-order cycles (1-10 m.y.) is quite readily explained by the waxing and waning of glacial ice. But the origin of pre-glacial third-order cycles is still a mystery. Various potential mechanisms are discussed. For example, sequestering and releasing of ground water can change sea-level by about 50 m, not enough to explain many of the thirdorder cycles. Geoidal anomalies could cause significant sea-level changes, but they occur at a very slow pace making their effect negligible. In-plate stress is another intriguing mechanism that could produce sea-level changes of the right magnitude. However, in-plate stress models fail to consider effects of variation in the crustal
BOOK REVIEWS
structure that accompanies basin deformation. This is an area of obvious promise and the relationship of in-plate stress and stress-induced lithospheric density to third-order cycles needs to be explored further. Testing of the Exxon sea-level curves is the central theme of Hallam's book. These eustatic curves were meant to be working models, open to improvement as better information became available. Because of the prime importance of sea-level change to stratigraphy, it is not surprising that testing these models has become a favorite pastime of many sedimentary geologists. However, there are two prerequisites to effectively test the eustatic models in any area. First, the sea-level signal needs to be separated from the regional tectonic overprint. But more importantly, meaningful chronostratigraphic correlative criteria need to exist that can be used to effectively compare the local record to the eustatic model. With the existing state of knowledge these requisites cannot be met for much of early Cretaceous and older strata. Several recent studies that have attempted to test the eustatic models for older time periods suffer from these shortcomings and have proven little conclusive. Hallam's discussion underscores these limitations. BILAL U. HAQ (Washington, D.C., U.S.A.) SSDI 0 0 3 7 - 0 7 3 8 ( 9 3 ) E 0 1 {}5-0