Late Quaternary history of the Gumants catchment, Papua New Guinea

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The Gumants River, rising on the eastern flank of the Mount Hagen stratovolcano, forms part of the headwater catchment of the Wahgi River in the central highlands of Papua New Guinea. The late Quaternary history of the catchment was shaped by a massive debris avalanche with an estimated volume >15 km3 that most likely occurred 400 ± 100 ka. The Upper Gumants River, which occupies about one-quarter of the 420 km2 catchment and underlain almost entirely by volcanic deposits including the avalanche, lahars and widespread airfall tephras that ceased falling ca 200 ka, once flowed directly to the Wahgi River through the Guga subcatchment rather than through the rest of the Gumants Basin. The Kuk Swamp part of the Guga catchment, intensively investigated for evidence of agriculture stretching back 10 ka, was added to the UNESCO World Heritage List in 2008. Nearly 40% of the 79 km2 of the Kugimi Basin is underlain by at least 30 m of swamp deposits with all but the uppermost few metres of the varied swamp and alluvial deposits beyond the range of radiocarbon dating. The Muga Basin forms nearly half of the Gumants catchment with about 50 km2 of swamp deposits and colluvial/alluvial sediments, which have been accumulating rapidly in the last few hundred years but are still underlain at a depth of a few metres by ancient sediments and, by inference, debris avalanche deposits that extend downvalley beyond the junction of the Gumants River with the Wahgi River. While there has been speculation for decades that the Wahgi River once flowed westward, it is now clear that the Gumants River has always flowed eastward, and it seems very likely that that is also true of the Wahgi River.