Paper No. 217-6
Presentation Time: 9:00 AM-6:30 PM
FLANK MARGIN CAVE DEVELOPMENT AND TECTONIC UPLIFT, CAPE RANGE, AUSTRALIA
Cape Range, Australia, on the northwest coast of the continent at 21o S, 113o E, is a NNE striking anticlinal ridge 315 m high, 100 km long, and 20 km wide extending into the sea, consisting of Miocene carbonate rocks with a series of coastal terraces of Pliocene and Quaternary carbonates and siliciclastic dunes. Inland escarpments, representing former sea cliffs, and deep valleys cutting the limbs of the anticlinal ridge host many cave entrances at a variety of elevations. The lowest unit, the Mandu Formation, a chalky and marly limestone, contains many tafoni (pseudokarst) caves with simple, single chamber plans and widths up to 15 m or more, and height up to 10 m. The higher, purer Miocene limestones, and the younger Pliocene and Pleistocene coastal terrace limestones, host numerous flank margin caves from 300 m elevation in the Miocene rocks to sea level in the Quaternary rocks. These caves have entrances up to 30 m wide and heights of 6 m, with single chamber caves being common but complex chamber caves also present. Some caves are entered by small entrances that lead to large phreatic chambers, which eliminates both sea cave and tafoni as possible explanations. The close association of these caves with sea cliffs and incised valleys argues against a deep hypogene origin, which would leave a cave pattern unrelated to the surface configuration. Miocene uplift tapered off into the Pliocene; the flank margin caves in the paleo sea cliffs represent the outcome of the interplay of tectonic activity and glacioeustasy over a 300 m vertical range, with lowstands causing valley incision; highstands raised the fresh water lens and allowed cave development in the valley walls. Cave development began with the first tectonic-driven subaerial exposure in the Miocene and continued through to the last Pleistocene interglacial.