AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Comparison of Lower Cambrian Carbonate Facies and Halokinetic Sequences in Minibasins Developed on Opposite Sides of Wirrealpa Diapir, Central Flinders Ranges, South Australia
(1) Department of Geology, New Mexico State University, Las Cruces, NM.
(2) Institute of Tectonic Studies, Las Cruces, NM.
Wirrealpa diapir is flanked by the Woodendinna and Donkey Bore minibasins, which contain different thicknesses and facies of Lower Cambrian carbonate strata, permitting assessment of the role of sediment supply on the formation of composite halokinetic sequences (CHS). Both minibasins contain primarily tapered-CHS, which form when sedimentation rate outpaces diapir rise rate. However, the upper Woodendinna minibasin strata form a thin tabular-CHS, which is not present in the Donkey Bore minibasin. Tabular-CHS form when diapir rise rate outpaces sedimentation rate. In the Donkey Bore minibasin, the Wilkawillina and Mernmerna formations have a combined thickness of 1200m, thinning to 250m near the diapir over a distance of about 700m, whereas in the Woodendinna minibasin their combined thickness is 3000m, thinning to 600m near the diapir over a distance of about 1000m. Sedimentation was twice as fast in the Woodendinna minibasin, causing thinning and halokinetic drape folding within CHS to form over a broader area.
Facies variation between minibasins reflects differential sedimentation rates. The Wilkawillina Fm. in the Donkey Bore minibasin comprises a tapered-CHS composed of windward-side Archaeocyathid bioherm facies with abundant diapir-derived debris flows that accumulated more slowly than the age equivalent tapered-CHS in the Woodendinna minibasin, which comprises leeward-side interbedded ooid shoal and digitate cyanobacterial facies. The overlying Mernmerna Fm. comprises deep-water carbonate turbidites derived from a regional source to the northwest, placing the Donkey Bore minibasin updip of the Woodendinna minibasin. The Mernmerna in the Donkey Bore minibasin forms a tapered-CHS whereas in the Woodendinna it forms a thin tabular-CHS followed by tapered-CHS. The tabular-CHS contains thick diapir-derived debris flows, indicating sediment starvation downdip of the diapir coincident with maximum shelfal transgression. Mernmerna highstand deposition produced tapered-CHS in both minibasins.