EVOLUTION OF A HALOKINETIC MEGAFLAP: UTILIZING SANDSTONE PROVENANCE TO RECOGNIZE SYNDEPOSITIONAL AND SYNDEFORMATIONAL EXPOSURE OF WITCHELINA SALT DIAPIR, WILLOURAN RANGES, SOUTH AUSTRALIA

Megaflaps are near-vertical to overturned panels of deep minibasin strata that extend multiple kilometers up the flanks of steep salt diapirs and equivalent welds. Megaflaps commonly represent relatively thin roof strata deposited over inflated salt highs and are deformed by halokinetic drape-folding, shortening-related squeezing, or a combination. Our current understanding of megaflap development is based mainly on seismic data, restorations, and modelling, whereas detailed analysis of megaflap sedimentology and stratigraphy is limited. Thus, outcrop studies are imperative to interpret the intimately linked stratigraphic and structural evolution of megaflaps. In the Willouran Ranges, South Australia, outcrop exposures provide an oblique cross-sectional view of a halokinetic megaflap flanking Neoproterozoic Witchelina diapir. We use stratigraphic and structural relationships to refine existing models of formation with a specific focus on sandstone provenance to identify key salt exposure events during deposition of megaflap strata.

The Witchelina megaflap has vertical relief of ~2.5 km and thins upward from 1750 to 90 m with highly convergent internal strata. The lower boundary displays low-angle erosional onlap, while the upper boundary is a diffuse zone of progressive stratal rotation and thinning. Megaflap strata of the Willawalpa Formation and Witchelina Quartzite were dominantly deposited in a regional high-energy barrier bar shoreface during rifting. Provenance studies allow us to distinguish between regional continental basement and local diapir sources. Subarkoses represent typical sediments shed from emergent basement of the rift margins. In contrast, feldspathic litharenites with abundant diapir-derived detritus reflect localized erosional events that stripped the crestal cover above inflated salt and shed diapiric material into adjacent minibasins. These grains are present only at diapir-proximal stratal terminations that progressively onlap higher up the steepening diapir flank. These data indicate that Witchelina diapir formed topographic relief, was periodically exposed and locally eroded, and progressively onlapped during megaflap formation. All observations support an interpretation that the Witchelina megaflap formed by halokinetic drape-folding.