Field Evidence for Debris Flows Beneath from Allochthonous Salt Canopy, Willouran Ranges, South Australia

P. Tyler Hannah¹, Thomas E. Hearon¹, Timothy F. Lawton¹, Mark G. Rowan², and Kaherine A. Giles^1
1New Mexico State University, Las Cruces, NM
²Rowan Consulting, Inc, Boulder, CO

Stratigraphic geometries and sedimentary facies in subsalt Neoproterozoic strata of the Willouran Ranges South Australia, are used to test models of allochthonous salt emplacement. Folding during the Cambrian Delamerian orogeny resulted in cross-sectional exposure of salt structures in map view and provides a rare opportunity to examine subsalt stratal relations. The Callanna Group evaporite began moving immediately after its deposition, ultimately forming a minibasin floored by the autochthonous salt and covered by a multi-level canopy. At the top of the minibasin, strata that record allochthonous salt movement contain slump folds overlain by large olistoliths (up to 250 m thick and 700 m long) in an unsorted pebbly matrix. Both clasts and matrix in these strata include Callanna and overburden lithologies. These beds, interpreted as debris flows deposits and olistoliths derived from the advancing salt sheet, are in turn overlain by remnant salt or equivalent welds and then supra-salt minibasins. Detailed field mapping of the salt-minibasin contact zone and delineation of individual blocks show that there are no shear zones or thrust faults, and that the zone of chaotic debris originated through depositional rather than structural processes. Synthetic geophysical logs based on structural measurements and geologic maps of the debris interval compare favorably with data from the northern Gulf of Mexico, supporting an interpretation of subsalt “gumbo zones” or “rubble zones” as debris flows rather than as products of basal shear or thrust imbrication.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas