UAV Photogrammetry of Inflated Komatiite Flow Lobes and Submarine Debris Flows in an Archean Bimodal Volcanic Terrane, Yilgarn Craton, Western Australia

Abstract:

The Archean greenstone belts of the eastern Yilgarn Craton in Westermn Australia contain some remarkable outcrops of highly weathered submarine volcanic rocks, where excellent preservation of primary textures is preserved within saprolitic pediments. We have used UAV photogrammetry to identify and map two of these outcrops: one of avalanche and debris flow deposits formed by collapse of a compound dacitic volcano, and the other of simultaneously erupted komatiite lava flows, dacite flows and crystal tuffs. The sequence in both cases was tilted to the vertical on the flanks of a regional isoclinal fold. Both localities form part of the 2705 Ma old plume-related bimodal volcanic sequence of the Eastern Goldfields Superterrane, Yilgarn Craton.

inflation features characteristic of modern pahoehoe flows in Archean komatiites are preserved at the Gordon Sirdar Lake locality near Kalgoorlie. Komatiite lavas here were emplaced within a sequence of dacitic lava flows and semi-consolidated tuffs. Komatiite lava lobes form characteristic lenticular cross sections ranging from 1-6 m thick and up to 20m long, in some cases with lower margins draped over pre-existing dacite flow tops, and in others showing invasive textures implying eruption onto or into wet sediment. Inflation features include tumuli, inflation clefts, breakouts, and terraced margins. Spinifex textures are preserved locally at flow tops and rarely at bases.

The Breakaway locality at Kanowna contains a “mega-breccia” avalanche deposit containing blocks of dacite lava and pillow basalt tens of metres across within a matrix of dacitic breccia, in places containing abundant komatiite clasts ¹. Individual blocks contain fragments of dacite invaded along fractures by komatiite, in a similar style to that seen at the Gordon Sirdar locality.

The outcrops together provide intriguing insight into the anatomy of an Archaean seafloor bimodal volcanic complex, implying simultaneous eruption of komatiitic and dacitic lavas in a paleo-rift. The images obtained attest to the usefulness of high resolution UAV-based photogrammetry in documenting complex volcanological relationships.

¹Trofimovs, J., et al., . Precambrian Research 131, 283-304 (2004).

Image: Dacite block in avalanche deposit at Breakaway. Dips steep, top to the left.