THE AGE OF SALT IN THE GULF OF MEXICO BASIN

Abstract

The assumption that all salt in the Gulf of Mexico basin is Jurassic demands a fixed structural framework since mid‐Jurassic time. This assumption has questionable geological support and is challenged.Salt in the Salina Basin of southern Mexico has been presumed Jurassic because shallow salt at Chinameca is overlain by ammonite‐bearing Kimmeridgian — early Neocomian limestones which, in turn, are overlain by Oligocene sediments.Despite numerous salt penetrations elsewhere in the Salina Basin, the Chinameca limestone has not been encountered. Instead, the non‐piercing salt massifs directly underlie Oligo‐Miocene beds, which have also been found intercalated with the salt. Two wells that have traversed the salt found Oligocene sediments below.Field checks in the Chinameca limestone type‐locality reveal that these beds are both strongly folded and heavily brecciated. They do not overlie the salt normally, and are interpreted as slumped masses that slid from elevated fault scarps onto Oligocene evaporites during Oligocene block movements.Geological evidence supports the conclusion that the Salina salt is Oligocene, and was deposited during the worldwide Oligocene eustatic drop. Because the Salina salt is stratigraphically continuous with the salt in offshore Campeche, this also is deemed to be Oligocene.The Challenger salt has been dated as Jurassic on the basis of one palynomorph‐bearing sample cored from the cap rock of the Challenger knoll. However, the Challenger salt is in continuity with the Campeche evaporites, which suggests that it, too, is Oligocene. In view of the wide range of datable detrital material recovered from Salina salt samples, the dating of Challenger salt from a single sample of insoluble residue cap‐rock is considered premature.On the north side of the Gulf, evidence for Oligocene salt obtained earlier from the Belle Isle salt dome is supported by the presence of intra‐Frio evaporites encountered in two deep exploration wells.The probability of large deposits of Tertiary evaporites in the Gulf of Mexico leads to the interpretation that the evolution of the Gulf Basin was governed by a sequential process of deep crustal extension that persists until the present day, rather than an exclusively Jurassic extensional event as is currently accepted.