QUATERNARY GEOLOGIC INTERPRETATIONS FROM FLANK MARGIN CAVES, KANGAROO ISLAND, AUSTRALIA

Flank margin caves have been observed in Quaternary Bridgewater Formation eolianites on Kangaroo Island, South Australia. High wave energies on exposed coasts have stripped sufficient eolianites from the underlying Kamantoo Group basement rocks, where present, that flank margin caves have also been removed. In embayments and stream outlets, protected conditions have resulted in flank margin caves being preserved. At Snake Lagoon on the west coast, flank margin caves are found in horizons at approximately 25, 30 and 35 m elevations. These cave horizons demonstrate tectonic uplift in the 30 m range (given prior glacioeustatic sea-level positions of up to +6 m), and that such uplift was partially episodic. A wave-cut notch at Hanson Bay on the south side of the island, at 35 m elevation, also supports this cave interpretation. Admirals Arch at Cape du Couedic on the southwest tip of the island, previously presented as forming solely by wave erosion, is a flank margin cave breached and modified by wave erosion working upward along the slanting Bridgewater Formation/Kamantoo Group contact. Point Ellen on the southeast side of the island, contains a Pliocene subtidal carbonate unit, the Point Ellen Formation. Wave-rounded boulders of the underlying Kamantoo Group basement rocks in the lower beds of the Point Ellen Formation indicate that the unit formed within the reach of wave base. Bridgewater eolianites overstep a cliff in the Point Ellen Formation, with a paleotalus in between. Elsewhere in the outcrop, a well-developed epikarst and terra rossa paleosol separate the Point Ellen and the Bridgewater Formations. These features demonstrate that the Point Ellen Formation was uplifted, cliffed by wave processes, and then was karstified before being buried by Bridgewater Formation eolianites, and that the paleokarst surface marks the Plio-Pleistocene transition. A possible flank margin cave at 3 m elevation here supports interpretations of last interglacial notching of the nearby coast, and therefore no tectonic uplift in the last 125 ka. The granite tafoni of Remarkable Rocks on the southwest coast contain interior cuspate bedrock forms, presenting a cautionary note on the use of cave wall characteristics as proof of dissolutional origin.