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Abstract
Chapter from: M
65: Salt Tectonics: A Global Perspective
Edited By
M.P.A. Jackson, D.G. Roberts, and S. SnelsonAuthor:
Jake Hossack Structure, Tectonics, Paleostructure
Published 1995 as
part of Memoir 65
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved.
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Hossack,
J., 1995, Geometric rules of section balancing for salt structures, in
M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics:
a global perspective: AAPG Memoir 65, p. 29-40. |
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Chapter
2
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Geometric
Rules of Section Balancing for Salt Structures |
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| Jake Hossack
BP Exploration
Uxbridge, Middlesex
U.K.
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Abstract
Restored sections
provide not only a measure of the viability of structural interpretations
but also have the ability to recreate the geometry of the structures through
geologic time. Geologists have known for a long time that section balancing
is more difficult in salt structures because of the ability of the salt
to flow in and out of the plane of section and also to dissolve and thereby
violate constant volume considerations. However, the surrounding sediments
generally deform by brittle-plastic processes and are less able to flow
out of the plane of a properly chosen section. The pragmatic approach is
to restore sections by assuming constant-area conditions for the sediment
structures alone and to leave the salt area as gaps that may change in
area through time. Most restorations of salt structures suggest that throughout
long periods of geologic time, salt remains at or close to the depositional
surface and that volume reductions of up to 50% are possible in nature.
Salt structures usually involve
regional displacements of the salt and its surrounding sediments so that
extension in one place has to be  balanced by basement extension or cover
contraction in another. A key aid to the recognition of contraction and
extension is the regional elevation of reference horizons. Generally, salt
withdrawal and extensional faulting drop reference beds below regional
elevation, whereas salt pillowing , salt sheet formation, and contraction
will raise beds above regional elevation. In the Gulf of Mexico, the updip
extensional growth faulting and salt withdrawal are balanced by the formation
of downdip allochthonous salt sheets and fold and thrust belts, so that
the total linear strain across the sediment cover is zero. The extension
and contraction are linked by a series of salt and fault welds that lie
at several structural levels. |
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