Outcrop Analysis of the Counter-Regional-Style, Partly Welded Oladdie Diapir, South Australia

The Oladdie diapir is located on the northwestern edge of the Nackara Arc in the southern Flinders Ranges, approximately 30 km north of Orroroo, South Australia. The Callanna Group (~850 Ma) is the source for diapiric salt at Oladdie and comprises an assemblage of highly brecciated rocks originally interbedded with evaporites that are now absent. Well-exposed outcrops in the Oladdie area display complex salt-sediment interaction in cross-sectional view, with an overall geometry similar to counterregional systems in the northern Gulf of Mexico.

The Oladdie diapir comprises, from bottom to top, a 6 km2 pedestal of breccia at the autochthonous level, an inclined weld <10 m to 100 m thick and 1.2 km long, and a shallow 400 m2 body of breccia that extends laterally into a thin, 1 km long remnant salt sheet. The diapir separates two Late Neoproterozoic minibasins with very different thicknesses and large-scale halokinetic geometries containing generally shallow-water siliciclastics and carbonates. Strata in the southern, proximal minibasin are approximately two times thicker than equivalent strata in the northern, distal minibasin and are rotated downward toward the diapir. The southern minibasin lacks halokinetic folding, suggesting a lack of drape-folded roof on this flank of the diapir during salt rise. The northern minibasin is relatively undeformed away from the diapir but contains several stacked tapered and tabular composite halokinetic sequences (CHS) adjacent to the diapir, showing that a roof existed on this flank. The thickest segment of the inclined weld occurs where strata in the northern minibasin are juxtaposed against a pronounced cusp formed between a lower tabular CHS and an upper tapered CHS. Strata immediately flanking the salt weld contain no evidence of meso-scale faults or significant fracturing.

A growth monocline 1250 m wide in the Elatina Formation comprises the uppermost tapered CHS in the northern minibasin. In addition, an anomalous 50 m thick block of the Elatina Formation is present within the shallow breccia body along its lower edge. This is interpreted as remnant carapace of the Oladdie diapir preserved after being overridden by another allochthonous salt sheet and abandoned at the point of allochthonous salt breakout, a style increasingly observed on seismic data in the northern Gulf of Mexico.
 

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