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A Reversal in the Roles of Salt and Sediment in the Northern Gulf of Mexico: The Rare Effect of Allochthonous Salt Advancement and Inflation on Over-Pressured Sediment

Abstract

In almost every basin in the world that contains evaporites, it is the evaporites which get deformed by sedimentation. A typical simple scenario would be for a) evaporites to be deposited, b) clastic and/or carbonate sediments to be deposited on the evaporites, and c) the evaporites to begin deforming into diapiric and then allochthonous salt features. However, it is possible for the roles of salt and sediment to be reversed. An advancing salt body can provide the load that causes deformation to underlying strata. This unusual reversal of roles has been observed and documented in the deep northern Gulf of Mexico basin. One result of this reversal of roles is the development of extensional keels at the base of the salt canopy. Keels form in the canopy base where sediment underlying the canopy gets displace and salt from the canopy infills the space created. As allochthonous salt advances over deep basin sediments, those sediments may become overpressured if they cannot dewater quickly enough. When overpressuring occurs with a thin overburden of sediment and/or salt, frontal thrusts may develop at the leading edge of the salt canopy. If slow evenly distributed sedimentation occurs prior to canopy advance across an overpressured area, then a thicker overburden will develop between the canopy and the detachment surface. A thick overburden is harder to deform than a thin overburden. Thus, the advancing salt canopy could have to inflate substantially to initiate deformation of the sediments below. Two styles of large-scale sediment displacement/keel development have been observed. Group 1 keels are well delineated by mapping of the base salt canopy. These keels form a trend parallel to the Sigsbee Escarpment but are offset shelfward (updip). This trend extends over 200 kilometers across the Keathley Canyon OCS area and into the Alaminos Canyon OCS area. The canopy over these keels was emplaced in the late Miocene to early Pliocene. Group 2 keels were not previously unrecognized as being salt keels. They lie farther updip of the Sigsbee Escarpment than Group I keels but are basinward of the ascension zones where salt rises from the primary autochthonous salt basin(s). Group 2 keels are not easily delineated by mapping the base salt canopy. However, the sediment displaced by Group 2 keels does form significant high spots on the base canopy map. The canopy in this area was emplaced in the early to middle Miocene. It is possible that Group 1 keels may potentially evolve into Group 2 keels given the proper geological circumstances.