Lawn Hill, Australian Outback, August 2012

Following the International Geological Congress in Brisbane, Jason Barnes, Jani Radebaugh, Catherine Neish and I took the opportunity to visit the remote Lawn Hill impact structure, which appears to have some morphological similarities with certain impact craters on Titan. From Brisbane we flew up to Cairns (2hrs) and rented 'Pat' (a Nissan Patrol - a good solid vehicle, first I've driven with a 'sub' tank - a spare fuel tank you can switch to on-the-go for those long unprovisioned legs in the outback.) We struck a little south, to first visit the amazing Undara lava tubes (B, arguably the largest on Earth) and then west across the savanna of northern Queensland to Normanton (C) near the Gulf of Carpentaria, and thence SW to Lawn Hill. We spent a couple of nights at Adels Grove, exploring the crater and the Lawn Hill gorge (D, a rather idyllic desert oasis, somewhat reminiscent of Havasupai in the Grand Canyon), before arcing down via the Riversleigh fossil deposit to the huge mining town of Mt Isa (E), and then the long drive back east to the coast at Townsville (F)and then north (stopping at Mission beach) to Cairns. All in all about 3000km in 7 days.

01. Brisbane was a pleasant city. Cool evenings at this time of year (August - winter !). One day a front came through bringing these dramatic clouds to the normally blue sky. 01P8080079 02. Meeting a koala. They're not actually as cute as they look. 02P8070047 03. Near Undara there is a fair amount of exposed granite in addition to the basalt flows. Here Jason stands for scale on this spheroidal boulder. We climbed up one of the many volcanic cones here, saw wallabies and parakeets and other exotic wildlife. 03P8110104
04. The Undara lava tubes are colossal - adding up the branches there are ~200km of them here - some a cavernous 20m tall, even accounting for the dust that has accumulated over the 190,000 years since they formed. Such lava tubes exist on the moon and Mars (as well as e.g. in Hawaii, Washington/Oregon, and Arizona) 04P8110119 05. Forget the poisonous spiders and venomous snakes, what is most likely to kill you in the Outback is a road train - a truck hauling up to 4 heavily-laden trailers. They don't (can't) stop - you just have to keep your wits about you and keep out of the way. Lots of dust in the dry winter - but much of Northern Queensland gets impassable with mud and rivers across the roads in the wet season, hence many vehicles have snorkels. (see Jani's video of us fording - just- a river between Lawn Hill and Mt Isa). 05P8120153 06. After the long drive, a swim at Adels Grove. There are supposed to be archer fish and freshwater crocodiles ('considered harmless, but may bite if provoked') but we didn't see any of them. 06P8130186
07. As we drive into the Lawn Hill crater, Jani plots our GPS position in real time on the ERS-1 Synthetic Aperture Radar image, so we can relate our view of the terrain out of the window to the remote sensing data (and by extension to comparable structures on Titan). The ring of limestone hills is not the rim, but may be resistant deposits formed in a moat around the central uplift (subsequently eroded) after the impact. 07P8130160 08. The limestone hills are only a few tens of meters high, and the vegetation makes the structure quite subtle in optical images. But the roughness at small scales (due perhaps to the fluted texture of solution erosion, as well as the fracturing and brecciation) makes it prominently radar-bright. The limestone beds are rather jumbled in orientation, again consistent with an energetic deposition immediately after the impact. 08P8130197 09. There is a lot of breccia among the limestone. This would be consistent with a resurge in a shallow submarine impact, ripping up the underlying shale and mixing it in with the limestone in a poorly-sorted deposit. 09P8130198
10. Round geological features, upturned beds and breccias are all only weakly suggestive of impact, but here is the smoking gun - lots of lovely shatter cones. These striated textures only form in the severe shock wave of impact (see pictures from my other crater trips - Vredefort in South Africa, Wanapitei in Canada, Waqf as Suwaan in Jordan, etc.) 10P8130208 11. After exploring the crater, some canoeing. We power upriver, with Jason and Catherine in pursuit. 11P8140220 12. The river has cut down through up to 140m of sandstone, forming a steep gorge (again somewhat reminiscent of the Canyonlands in the southwest USA). 12P8140228
13. A rather idyllic swimming spot by some waterfalls. Still no crocodiles. 13P8140252 14. In places the sandstone preserves some (presumably marine) ripples - just below my feet. 14P8140254 15. At the Riversleigh fossil deposit just south of Lawn Hill, Catherine points out some crocodile bones. 15P8140292
16. The crew, by the rather corny dinosaur model in Hughendon on the long drive back. 16P8150317 17. Murray Falls, a little inland from the Pacific coast. 17P8170327 18. Not exactly geology, but an interesting formation nonetheless on the Pacific Coast at Mission Beach. It seems that a very tiny crab rolls up these balls of sand to make its burrow, and the dendritic pattern is just incidental. My foot for scale. 18P8170350

Back to Ralph's home page and pictures from other trips