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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. SnelsonAuthors:
Raymond C. Fletcher, Michael R. Hudec, and Ian A. Watson 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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Fletcher,
R. C., M. R. Hudec, and I. A. Watson, 1995, Salt glacier and composite
sediment-salt glacier models for the emplacement and early burial of allochthonous
salt sheets, in M. P. A. Jackson, D. G. Roberts, and S. Snelson,
eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 77-108. |
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Chapter
5
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Salt
Glacier and Composite Sediment-Salt Glacier Models for the Emplacement
and Early Burial of Allochthonous Salt Sheets |
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Raymond C.
Fletcher
Exxon Production Research
Company
Houston, Texas
U.S.A.
Present address:
Department of Geosciences
New Mexico Institute
of Mining and Technology
Socorro, New Mexico,
U.S.A.
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Michael R.
Hudec
Ian A. Watson
Exxon Production Research
Company
Houston, Texas
U.S.A.
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Abstract
Allochthonous
salt sheets in the northern Gulf of Mexico were emplaced as extrusive "salt
glaciers" at the sediment-water interface. Massive dissolution was suppressed
by a thin carapace of pelagic sediments. During emplacement, several hundred
meters of bathymetric relief restricted rapid sedimentation to outside
the glacial margins. The glaciers acted as sediment dams, influencing the
transport and deposition of sediment from an upslope source. Because of
contemporaneous sedimentation, the base of the glaciers climbed upward
in all directions away from their feeder stocks, and successive sedimentary
horizons were truncated against it. The local slope at the base of the
sheets is equal to the local rate of sedimentation divided by the local
rate of salt advance. Alternating episodes of slow and rapid sedimentation
gave rise to a basal salt surface of alternating flats and ramps, which
are preserved. Many salt sheets have nearly circular map patterns but are
strongly asymmetric. Feeder stocks occur near upslope edges, and base-of-salt
slopes are greater updip of the feeder. The asymmetry is due to more rapid
sedimentation at the upslope edge and to slower advance induced by the
smaller hydraulic head between the salt fountain and the upslope edge compared
to the downslope edge.
Rapid emplacement of the
Mickey salt sheet (Mitchell dome) from a preexisting salt stock took ~4
m.y., as ~1 km of sediment was deposited. A three-dimensional geomechanical
model for the rapid salt emplacement yields the following relationship
for the diapir's downdip radius versus time: R(t) »Mtq»B[(r
- rw)gK3/h]1/8tq,
where M, q, B, and K are constants related to salt supply
into the sheet, r and
rw
are the densities of salt and water, g is the acceleration of gravity,
h
is salt viscosity, and t is a model time extrapolated back to zero
sheet volume at t = 0. The advance history of the Mickey salt sheet
is equally well fitted by two histories of salt supply, corresponding to
values of q = 1/2 and q = 1 in the above expression. The
model requires that the volume of the sheet grew as V »Kt
(for q = 1/2) or V »Kt7/3
(for q = 1). Fits to the advance history can be used to determine
the remaining constants. From the expression for
M, salt viscosities
h»
8.3 X 1018 (q = 1/2) and h»
4.8 X 1018 Pa s (q = 1) are obtained, consistent with
experimental data on salt creep.
Once salt extrusion ceases,
a large fraction of the glacier's topographic relief is lost, but the steep
shoulder at the downslope edge is maintained. Sediment influx concentrated
at the updip edge maintains a sloping surface, and a glacier-like flow
continues within a composite salt-sediment glacier. If a minibasin forms
near the updip edge, further downdip advance can be substantial. Velocities
on the surface of a composite glacier indicate that overburden particles
above the leading edge can move 1.5 times as fast as the sheet advances,
resulting in a tractor tread model for near-toe kinematics. That the sedimentary
carapace of the glacier moves faster than the sheet advances suggests that
extension in the sedimentary veneer generally exceeds salt sheet advance.
Burial of the toe results in cessation of advance, but updip minibasin
deepening and downdip salt diapir growth continue as long as the surface
remains sloped and the finite-strength sediment in and around the buried
sheet does not establish a mechanically stable configuration. Relative
buoyancy between salt and sediment influence late-stage development. |
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