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Allochthonous Salt Initiation and Advance in the Northern Flinders and Eastern Willouran Ranges, South Australia: Using Outcrops to Test Subsurface-Based Models from the Northern Gulf Of Mexico

Abstract

Outcrops of the Neoproterozoic Callanna Group (∼850-800 Ma) comprise an assemblage of brecciated rocks throughout the northern Flinders Ranges and the eastern Willouran Ranges, South Australia, and represent salt bodies in which the evaporite is now absent. Using field relationships and stereonet analysis, we demonstrate that the breccia bodies formed both steep diapirs and subhorizontal salt sheets. The geometries of Callanna Group breccia and adjacent and subjacent strata are used to define new models of allochthonous salt initiation and to test existing models of allochthonous salt advance, many of which were derived from seismic and well data in the northern Gulf of Mexico. We present twelve examples of steep to low-angle transitions: half represent the initial emplacement of allochthonous salt from primary diapirs and the other half represent ramp to flat transitions within multi-level canopies. Variable stratal geometries adjacent to steep salt include (1) minibasin-scale folding, (2) tabular and tapered composite halokinetic sequences and (3) unfolded strata, whereas strata beneath base-salt flats of subhorizontal salt sheets are always undeformed. Half of the steep salt bodies have preserved roof strata that are 160–1200m wide and 100–480m thick, steeply truncated and overlain by shallowly-dipping allochthonous breccia, whereas the other half have no preserved roof strata. We suggest two models to explain the transition from steep diapirs to subhorizontal salt: salt-top breakout and salt-edge breakout. Both models involve piston-like rise of a diapir, which variably decapitates overlying roof strata, with top-salt breakout occurring inboard of the salt flank, thereby preserving a flap of roof, and salt-edge breakout occurring at the steep salt margin. The majority of our examples of allochthonous salt that initiate from primary diapirs display salt-top breakout, whereas most ramp to flat transitions exhibit salt-edge breakout. In all cases, post-breakout, low-angle salt advance was accomplished by thrust advance or extrusive or open-toed advance, with no evidence for subsalt thrust-imbricates, basal shear or rubble zones. While key differences exist between South Australia and the northern Gulf of Mexico, including evaporite-layer composition, depositional environment and extent of allochthonous salt, the lessons learned from the study of South Australia outcrops complement the data derived from seismic and well data in deepwater settings.