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Martin P.A. Jackson1, D. D. Schultz-Ela1
(1) Bureau of Economic Geology, Austin, TX

Abstract: Why roho, why counterregional?

In 1990, Shell geoscientists presented two end-member systems created by evacuation of allochthonous salt sheets: roho systems and stepped counterregional systems. Since then, to our knowledge, no publication has explained which end member forms from a composite salt-sediment glacier.

In roho systems, seaward-dipping normal faults yield high extension, growth wedges thicken landward, the rollover monocline steepens landward, salt evacuation is incomplete, and the basal weld is a fault weld. Conversely, in counterregional systems, subsidence folding yields little extension, growth wedges thicken seaward, the rollover monocline steepens seaward, salt evacuation is more complete, and the basal weld is a salt weld.

Numerical modeling and analysis of reconstructions suggest that the following factors favor roho over counterregional systems. In roho systems, exposed salt is long enough to absorb significant gravity spreading, the shelf break remains landward of the roof wedge tip, the top of salt dips gently landward, local depositional slope is gentle, aggradation is slow, and sediments are weak. Several of these factors typify mud-prone environments.

During the Heel Stage, normal faults at the landward end of the system allow the roof wedge to extend into emergent salt. During the Toe Stage, the roof wedge becomes too thick at its landward end to extend and so thick at its seaward end that it buttresses extension. Significantly, both systems and their hybrids evolve through Heel-Transitional-Toe Stages as space for gravity spreading is absorbed. The extensional Heel Stage, however, dominates roho systems, whereas the subsiding Toe Stage dominates counterregional systems.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana