Salt Glacier Model for the Emplacement of an Allochthonous Salt Sheet

R. C. Fletcher, M. R. Hudec, I. A. Watson

Allochthonous salt sheets in the Gulf of Mexico were emplaced as extrusive "salt glaciers" at the sediment-water interface. Massive dissolution was suppressed by a thin blanket of a few tens of meters of pelagic sediment. During active emplacement, topographic relief of a few hundred meters over the glacier restricted sedimentation to outside its margin. The glacier acted as a sediment dam, influencing the transport and deposition of sediment from an upslope source. Because of contemporaneous sedimentation, the base of the glacier climbs upwards away from the feeder stock, and successive sedimentary horizons are trucated against it. The emplacement of one sheet took ^approx 4 Ma, as ^approx 1 km of sediment was deposited. The local slope of the base equals the local rate f sedimentation divided by the local rate of advance, at the time the glacier passed. Alternating episodes of slow, shale-prone, and rapid, sand-prone, sedimentation gave rise to the flats and ramps of a stair-stepped basal surface. Because advance is more in the manner of a tank-tread than a rigid, thrusting mass, this constructive form is preserved. Many salt sheets have a nearly circular map pattern, but are strongly asymmetric, with the source diapir located nearer the upslope edge, and with a greater basal slope in the upslope direction. The asymmetry is due to the more rapid sedimentation at the upslope edge and the lower rate of advance, due to the smaller head between the salt fountain and the upslope edge.

A geomechanical model of salt glacier emplacement yields a relation for the radius in the down-dip direction, R(t) = C[(^rgr - r_w)gK³/^eegr]¹8/t, where r and r_w are the densities of salt and water, g is the acceleration of gravity, ^eegr is salt viscosity, t is approximately the time since the inception of rapid spreading, and, as inferred from the nearly uniform slope of the basal surface, the volume of the sheet grew according to V ^approx Kt⁷3/. This relation yields a salt viscosity ^eegr ^approx 4 × 10¹⁸ Pa-s, in good agreement with experimental data on salt creep. Diverging flow from the source diapir results in circumferential extension at the surface of the glacier, giving rise to closely-spaced, radial norm l faults in the thin sediment cover. Once active emplacement ceases, a large fraction of the topographic relief is lost, but the steep marginal shoulder is preserved. Sediment will first prograde across the upslope edge of the sheet. The salt glacier can continue to operate as a composite salt-sediment glacier, resulting in the development of a minibasin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995