HALOKINETIC DRAPE-FOLD MODEL FOR CAPROCK IN DIAPIR-FLANKING AND SUBSALT POSITIONS

Existing genetic models for diapiric caprock are largely based on observations from excavations or drill-hole data in near-surface or outcropping caprock developed on the crests of vertical diapirs. In these models, caprock develops in a top-salt position during a long-lived, relatively continuous accretionary process of halite dissolution by crossflow of undersaturated waters, with concomitant anhydrite accretion by underplating and subsequent alteration of anhydrite to carbonate in the presence of anaerobic sulfate-reducing bacteria. Caprock in diapir-flanking and subsalt positions has been interpreted to form by the same dissolution process, but with undersaturated waters flowing upward along the salt-sediment interface.

Data from the Neoproterozoic Patawarta diapir in the Flinders Ranges, South Australia and Permian and Mesozoic Castle Valley and Moab Valley diapirs in the Paradox Basin, Utah indicate an alternative origin for diapir-flanking caprock. Patawarta diapir contains a laterally extensive (>10km), 3-100m thick, dolomite caprock assemblage preserved in a diapir-flanking position. The near-vertical to overturned subsalt caprock parallels steeply dipping strata of the Neoproterozoic Bunyeroo Fm, which forms a tapered composite halokinetic sequence (CHS). The caprock terminates upward at an angular unconformity (Bunyeroo-CHS upper boundary). The overlying Wonoka Fm onlaps the CHS boundary and contains clasts of caprock-derived detritus indicating that the caprock formed in a crestal position prior to deposition of the Wonoka Fm. We observe similar relationships in caprock and adjacent strata in the Paradox Basin, Utah.

We propose a 4-step model for fromation of diapir-flanking caprock: 1) caprock forms in diapir crestal position by cross-flow waters; 2) continued diapiric rise and minibasin subsidence cause drape folding of roof strata and competent caprock, which detaches from the underlying halite-dominated diapir and rotates off the diapir top into a flanking position; 3) diapir inflation and concomitant erosional truncation of the underlying CHS and flanking caprock; 4) onlap of the CHS boundary forming an angular unconformity.