Salt Glacier and Composite Sediment-Salt Glacier Models for the Emplacement and Early Burial of Allochthonous Salt Sheets
Published:January 01, 1995
Raymond C. Fletcher, Michael R. Hudec, Ian A. Watson, 1995. "Salt Glacier and Composite Sediment-Salt Glacier Models for the Emplacement and Early Burial of Allochthonous Salt Sheets", Salt Tectonics: A Global Perspective, M.P.A. Jackson, D.G. Roberts, S. Snelson
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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[(p - pw)gK3/η]1/8tcl, where M, q, B, and K are constants related to salt supply into the sheet, p and pw are the densities of salt and water, g is the acceleration of gravity, n is salt viscosity, and tis 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 = l/2)orV~ Kt7/5 (for q = 1). Fits to the advance history can be used to determine the remaining constants. From the expression for M, salt viscosities T) ≈ 8.3 × 1018 (q = l/’2) and r| ~ 4.8 × 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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Salt Tectonics: A Global Perspective
The conceptual breakthroughs in understanding salt tectonics can be recognized by reviewing the history of salt tectonics, which divides naturally into three parts: the pioneering era, the fluid era, and the brittle era.
The pioneering era (1856-1933) featured the search for a general hypothesis of salt diapirism, initially dominated by bizarre, erroneous notions of igneous activity, residual islands, in situ crystallization, osmotic pressures, and expansive crystallization. Gradually data from oil exploration constrained speculation. The effects of buoyancy versus orogeny were debated, contact relations were characterized, salt glaciers were discovered, and the concepts of downbuilding and differential loading were proposed as diapiric mechanisms.
The fluid era (1933–1989) was dominated by the view that salt tectonics resulted from Rayleigh-Taylor instabilities in which a dense fluid overburden having negligible yield strength sinks into a less dense fluid salt layer, displacing it upward. Density contrasts, viscosity contrasts, and dominant wavelengths were emphasized, whereas strength and faulting of the overburden were ignored. During this era, palinspastic reconstructions were attempted; salt upwelling below thin overburdens was recognized; internal structures of mined diapirs were discovered; peripheral sinks, turtle structures, and diapir families were comprehended; flow laws for dry salt were formulated; and contractional belts on divergent margins and allochthonous salt sheets were recognized. The 1970s revealed the basic driving force of salt allochthons, intrasalt minibasins, finite strains in diapirs, the possibility of thermal convection in salt, direct measurement of salt glacial flow stimulated by rainfall, and the internal structure of convecting evaporites and salt glaciers. The 1980s revealed salt rollers, subtle traps, flow laws for damp salt, salt canopies, and mushroom diapirs. Modeling explored effects of regional stresses on domal faults, spoke circulation, and combined Rayleigh-Taylor instability and thermal convection. By this time, the awesome implications of increased reservoirs below allochthonous salt sheets had stimulated a renaissance in salt tectonic research.
Blossoming about 1989, the brittle era is actually rooted in the 1947 discovery that a diapir stops rising if its roof becomes too thick. Such a notion was heretical in the fluid era. Stimulated by sandbox experiments and computerized reconstructions of Gulf Coast diapirs and surrounding faults, the onset of the brittle era yielded regional detachments and evacuation surfaces (salt welds and fault welds) along vanished salt allochthons, raft tectonics, shallow spreading, and segmentation of salt sheets. The early 1990s revealed rules of section balancing for salt tectonics, salt flats and salt ramps, reactive piercement as a diapiric initiator resulting from tectonic differential loading, cryptic thin-skinned extension, influence of sedimentation rate on the geometry of passive diapirs and extrusions, the importance of critical overburden thickness to the viability of active diapirs, fault-segmented sheets, counter-regional fault systems, subsiding diapirs, extensional turtle structure anticlines, and mock turtle structures.