Recognition of Passive Salt Diapirism in the Rock Record

Abstract

Passive salt diapirs grow at or near the earth’s surface synchronously with the deposition of surrounding sediments. Recognition of passive diapirism in a basin can be problematic, especially when the salt has been substantially or even completely removed from the system due to dissolution. Yet understanding if, when, and where passive diapirism occurred is critical to structural restoration, burial history, and reconstruction of the tectonic evolution of basins. Additionally, the nature and integrity of hydrocarbon trap elements changes vastly when salt is involved, and passive diapirs greatly influence synkinematic reservoir distribution and quality.

The only “smoking gun” for passive diapirism is stacked halokinetic sequences adjacent to either a salt body or interpreted faults. When in contact with faults, which are in fact welds, halokinetic strata on both sides of the fault/weld dip away from it regardless of fault type. Growth strata associated with salt-cored detachment folds may superficially look similar to halokinetic sequences, but they do not display stratigraphic discordance or stratal truncation with the salt body and have a scale that widens as the overburden thickness increases. Salt-cored detachment folds may become passive diapirs when the fold crest is breached or significantly thinned permitting salt break out, in which case local drape folding is superimposed on longer-wavelength contractional folding. Inclusion of locally-derived detritus that includes non-evaporite clasts sourced from the layered evaporite sequence (LES) or caprock (gypsum/anhydrite or carbonate) in surrounding strata, strongly suggests passive diapirism. Other features that are not exclusive to it, but that hint at possible passive diapirism, are polygonal structural patterns and unusual thrust map traces. Conversely, salt bodies with only structurally concordant and stratigraphically conformable overburden are not passive diapirs.

When developing structural plays in known salt basins, we highly recommend carefully assessing at what stages passive diapirism has played a role. Also, when working basins that have no reported salt, but were associated with the two major periods of continental break-up in Earth’s history (i.e. late Neoproterozoic breakup of Rodinia and early Mesozoic breakup of Pangea), “faults” with unusual flanking stratal geometries or discontinuous igneous bodies along their lengths should be viewed with suspicion.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019