Chapter from: M 65:  Salt Tectonics: A Global Perspective
Edited By 
M.P.A. Jackson, D.G. Roberts, and S. Snelson

Authors: 
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.
 

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) »Mt^q»B[(r - r_w)gK³/h]^1/8t^q, where M, q, B, and K are constants related to salt supply into the sheet, r and r_w 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 »Kt^7/3 (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.