Print this pageROWAN, MARK G.
A tripartite classification scheme for allochthonous salt is proposed based on the geometry of the base salt. Salt-tongue, salt-stock, and salt-nappe systems are end-member models, and mixtures and composites also exist. The base-salt geometry of each type of system exerts fundamental control on characteristic evolutionary histories that impact structural geometries, the amount of extension and contraction, sedimentation patterns, and fluid flow in both supra- and subsalt environments.
Salt-tongue systems are subhorizontal sheets formed by a single salt tongue or amalgamated salt tongues extruded penecontemporaneously from basinward-leaning feeder diapirs. The base salt has minimal structural relief except for lows in the immediate vicinity of deep feeders. Sheet evacuation leads to roho systems, with families of basinward-dipping growth faults, and/or stepped counter-regional systems, with leaning secondary diapirs connected by counter-regional faults.
Salt-stock systems form by salt spreading radially from bulb-shaped diapirs over long periods of time. Amalgamation of salt from multiple feeder diapirs may occur prior to stock collapse (true salt-stock canopy) or as a result of salt evacuation and extrusion (apparent salt-stock canopy), yielding different age relationships between supra- and subsalt strata. In either case, deep minibasins replace the original salt stocks as salt is extruded into sheets and secondary diapirs located over structural highs in the base salt.
Salt-nappe systems form by the gradual advance of allochthonous salt such that the base of salt climbs basinward in a series of ramps and flats over a large distance. The nappe is sourced from a series of diapirs located at its landward edge; there are no local feeders beneath the main body of the nappe. Lateral spreading and early loading of the salt results in elliptical minibasins separated by a polygonal pattern of reactive diapirs. Subsequent evolutionary patterns are unknown because mature, evacuated equivalents of salt nappes have not yet been documented.
The development of different types of salt systems is probably controlled in part by the distribution and thickness of the source layer, whether autochthonous or allochthonous, and in part by spatial and temporal variations in sedimentary loading.