Deep-Sea Response to Eustatic Change and Significance of Gas Hydrates for Continental Margin Stratigraphy

  1. Henry W. Posamentier2,
  2. Colin P. Summerhayes3,
  3. Bilal U. Haq4 and
  4. George P. Allen5
  1. B. U. Haq

Published Online: 15 APR 2009

DOI: 10.1002/9781444304015.ch6

Sequence Stratigraphy and Facies Associations

Sequence Stratigraphy and Facies Associations

How to Cite

Haq, B. U. (1993) Deep-Sea Response to Eustatic Change and Significance of Gas Hydrates for Continental Margin Stratigraphy, in Sequence Stratigraphy and Facies Associations (eds H. W. Posamentier, C. P. Summerhayes, B. U. Haq and G. P. Allen), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304015.ch6

Editor Information

  1. 2

    Plano, Texas, USA

  2. 3

    Godalming, UK

  3. 4

    Washington, DC, USA

  4. 5

    St Remy les Chevreuses, France

Author Information

  1. National Science Foundation, Washington, DC 20550, USA

Publication History

  1. Published Online: 15 APR 2009
  2. Published Print: 17 NOV 1993

ISBN Information

Print ISBN: 9780632035489

Online ISBN: 9781444304015

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Keywords:

  • deep-sea response to eustatic change and gas hydrates for continental margin stratigraphy;
  • deep-sea record versus eustatic change;
  • models of deep-sea sedimentation and eustatic connection;
  • erosive-corrosive cycle model - corrosive phase during transgression;
  • preservational phase during late highstand of sea level;
  • gas hydrates modifying continental margin sedimentary record;
  • gas-hydrate instability and implications for climate and sedimentary patterns

Summary

Eustatic fluctuations on the continental shelf cause the familiar sequence-stratigraphic depositional patterns that result from shifting depocentres as sea level rises and falls. Sedimentary patterns on the basin floor are also influenced, albeit indirectly, by changing sea level. The erosive–corrosive model of deep-sea response to these changes maintains that erosion on the sea floor takes place largely during the sea-level fall and the ensuing lowstand. On the margins, steepened stream gradients induce shelfal incision, enhanced erosion and increased turbidity. Intensified thermal gradients also may lead to climatic deterioration that contributes to enhanced weathering, adding to the sediment load flux. In the deep sea, increased bottom-water activity reinforces its erosive capability. Moreover, while erosion increases during the lowstand phase, dissolution is reduced on the sea floor due to increased influx of carbonate and siliciclastics to the basins which suppresses the CCD (calcite compensation depth) and enhances productivity offshore due to increased nutrient supply. The corrosive phase occurs largely during the transgressive and early highstand phase of the eustatic cycle when accommodation on the shelves is at an optimum. The sequestering of carbonate and clastics nearshore and depletion offshore elevate the CCD and may cause widespread dissolution on the sea floor. Weakened thermal gradients and climatic amelioration induce reduced bottom-water activity, diminishing its erosive capability. In the late highstand phase when accommodation on the shelves and banks is reduced considerably, carbonate and clastics may once again prograde out or be transferred to the deeper basin (late highstand shedding). The suppressed CCD and increased productivity offshore in the late highstand time translate into high preservation potential due to lack of both erosive and corrosive forces.

These depositional patterns may be altered periodically by another process that is forced by fluctuating sea level. The growth and decay of methane hydrate on the margins may significantly influence the long-term preservability of depositional patterns and complicate the sequence-stratigraphic models, especially during the lowstand phase. A major sea-level fall and reduced hydrostatic pressure on the shelf/slope and rise can effect the breakdown of hydrates, substituting a zone with weakened sediment strength where solid hydrate converts to free gas and water. These zones of weakness would be more prone to faulting and/or slumping. Major slumps can release large amounts of methane into the atmosphere. If the preceding sea-level fall is glacially forced, addition of methane from large natural gas pools in the low latitudes could provide a negative feedback to the cooling trend, eventually reversing the course of glaciation. As high-latitude temperatures ameliorate, additional methane may be added rapidly from near-surface sources of these regions, providing a positive feedback to the warming trend, eventually terminating the glacial cycle. Thus, gas hydrates may be an important factor in climatic change, and as agents of tectonic activity along the margins. At present few empirical data are available on hydrates as the field has been largely ignored. A need for considerable research effort is obvious if we are to learn more about this important, perhaps first-order, forcing mechanism for continental margin stratigraphy and tectonics.