AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Entrained Section and Encapsulated Minibasins: Complex Allochthonous Salt Geometries in the Northern Gulf of Mexico
(1) Rowan Consulting, Inc., Boulder, CO.
(2) Cobalt International Energy, Houston, TX.
It has long been known that non-evaporative sediment can get incorporated into allochthonous salt canopies at sutures. This entrained section typically comprises coherent carapace of one salt body that gets overridden by another, but can also include slumped carapace of the overriding salt body. These relatively thin intrasalt sedimentary sections may be coherent in immature canopies but get disaggregated into smaller blocks during subsequent flow of salt. At the other end of the spectrum, thick suprasalt minibasins may become enveloped in salt if younger allochthonous salt sourced from adjacent secondary diapirs flows over the top to form a shallow canopy level. Typically, such minibasins are not entirely covered but open up laterally because of inadequate supply of salt.
Subsidence into salt is normally recorded by growth strata that are progressively deposited at the sea-floor at the top of a thickening minibasin. However, if salt (and a thin carapace) is emplaced at the sea-floor above a growing minibasin, further evacuation of the underlying salt results in subsidence of the base of the shallow salt. If there is adequate salt supply, growth of the minibasin is effectively recorded by thickening of the shallow salt body, not by sedimentary strata. If shallow salt supply is too slow relative to subsidence rate, the top of the shallow salt also sinks, triggering growth of a tertiary minibasin above the salt. However, growth of tertiary minibasins may be misinterpreted to reflect late evacuation of the deeper salt if, in fact, evacuation occurred earlier but was recorded in thickening of shallow salt prior to tertiary minibasin formation.
Flow of shallow salt into a growing minibasin can happen at any point in the minibasin history because it may be sourced from beneath a different minibasin, often in a more proximal position on the margin. Thus, the thickness of the entrained section can vary anywhere between a thin carapace and a full minibasin. The earlier the process occurs, the more likely it is that the entrained section will be completely encapsulated within salt because young minibasins are often bounded on all sides by inflated salt. As minibasins grow, however, flanking salt is segmented into discrete secondary diapirs so that individual minibasins are connected to each other via sediment fairways and the minibasin network is more likely to be locally uncovered.