A 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 is typically interpreted to form by the same dissolution process, but with undersaturated waters flowing along the salt-sediment interface in a deeper subsurface setting.
The Neoproterozoic Patawarta salt sheet in the Flinders Ranges, South Australia contains a laterally extensive (>10km) dolomite caprock assemblage preserved in both subsalt and suprasalt positions along the sheet margins. The caprock is 3-100 m thick and contains massive, laminated, and crackle breccia textures. The near-vertical to overturned subsalt caprock parallels steeply dipping strata of the Bunyeroo Formation in a tapered composite halokinetic sequence (CHS). The caprock terminates upward at an angular unconformity marking the upper boundary of the CHS. Basal strata of the overlying Wonoka Fm onlap the truncated caprock and contain conglomerates of caprock-derived detritus indicating that the caprock formed in a suprasalt position prior to deposition of the Wonoka, which forms another CHS but without caprock. We observe similar relationships in caprock and adjacent Permian and Mesozoic strata flanking the Castle Valley diapir in the Paradox Basin, Utah.
Our model for flanking and subsalt caprock comprises 4 steps: 1) caprock develops in a crestal position by crossflow 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) topography generated by diapir inflation as diapir-rise rates exceed sediment-accumulation rates result in erosional thinning of the roof to create an angular unconformity that truncates the underlying CHS and flanking caprock; 4) increasing rates of sediment accumulation relative to diapir rise lead to onlap of the overlying CHS, which may or may not develop a new crestal caprock assemblage.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California