Field characteristics and erosional processes associated with komatiitic lavas: Implications for flow behavior

Although komatiitic lavas have long been depicted as turbulent flows, espec ially near the vent, field characteristics indicate that many komatiitic la vas did not flow turbulently, or only initially so. The bases of komatiites are commonly conformable with their substrate, including fine pelitic sedi ments, and the margins of komatiites are overwhelmingly coherent, or marked by local quench-fragmented hyaloclastite breccia. Autobreccias are notably missing. These characteristics are not consistent with turbulent flow, but clearly indicate conditions of laminar flow. If komatiitic flows were turb ulent, they should commonly have scoured into substrate sediments through a variety of physical erosion processes, including foundering into underlyin g seafloor sediments, because of density inversion, and turbulence-induced scouring of sediments. These features are not commonly developed, also indi cating that generally komatiites were emplaced under tranquil, laminar-flow conditions. Trough-like structures that commonly host nickel sulfide miner alization have commonly been interpreted to originate by thermal erosion of substrate by the komatiitic lava. The evidence supporting thermal erosion is not strong, and commonly ambiguous. Trough structures at Kambalda, Weste rn Australia, are fault bounded, as noted by several previous investigators . However, there is a common, but not universal, antithetic relationship be tween trough presence and sediment absence. Removal of sediment from trough s could be explained by physical erosion, with an initial narrow, turbulent flow-head scouring a channel in the underlying sediments. As the lava flow s spread laterally, their flow-front velocity decreased, and flow became la minar, so explaining the conformable contacts with substrate and the presen ce of coherent crusts represented by the random spinifex textural zone. The rmal erosion was rare, and could only have resulted beneath sustained lava tubes, within the flow interior, not from the flow head.