Outcrop Examples of Subsalt Structure Related to Allochthonous Salt Breakout, Flinders and Eastern Willouran Ranges, South Australia

Neoproterozoic outcrops of remnant salt sheets and associated subsalt minibasins in the Flinders and the eastern Willouran Ranges, South Australia display subsalt structures related to allochthonous salt advance. Here, we present detailed mapping (1:10,000) of four field areas that contain similar styles of subsalt deformation at ramp-to-flat transitions: Oladdie, Patawarta, Pinda, and the eastern Willouran Ranges. We observe common features in all examples: 1) depositionally thinned, overturned strata beneath the ramp, which represent halokinetic growth monoclines that formerly onlapped the top of salt; 2) strata forming the upper part of each monocline are truncated by the base salt at the ramp-flat transition; and 3) strata underlying the base-salt flat are undeformed. The halokinetic folds are exposed in oblique cross-sectional views and vary in width from approximately 120 m to over 2 km. Both monocline fold hinges are typically preserved beneath the salt, except at Pinda diapir, where the upper hinge is missing. Allochthonous salt breakout in the four field areas was multi-phase and was not time equivalent.

These geometries allow comparison of allochthonous salt emplacement throughout the Flinders and Willouran ranges and permit us to test previously postulated models of allochthonous salt emplacement derived mostly from seismic data in the northern Gulf of Mexico. None of our field observations is compatible with thrust-imbricate or basal-shear models of salt-sheet advance. Instead, a three-stage development is inferred: 1) pinned salt inflation during periods of high sediment-accumulation rate relative to salt-supply rate, which rotates the top salt and onlapping strata into a base-salt ‘mock’ ramp and overturned growth monocline; 2) thrust breakout near the top fold hinge of the monocline, decapitating the thinned upper limb, during a time of increased salt supply relative to sediment accumulation that often corresponds to 3rd-order depositional maximum flooding surfaces and transgressive surfaces; and 3) continued low-angle thrust advance over undeformed minibasin strata. We further suggest that various processes of localized erosional thinning (e.g., tidal or shoreface erosion vs. deepwater mass wasting) of the upper part of the monocline may lead to thrust breakout during different systems tracts in different depositional environments.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California