Debris avalanche deposits associated with large igneous province volcanism; an example from the Mawson Formation, central Allan Hills, Antarctica
An up-to-180-m-thick debris avalanche deposit related to Ferrar large igneous province magmatism is observed at central Allan Hills, Antarctica. This Jurassic debris avalanche deposit forms the lower part (member m 1 ) of the Mawson Formation and is overlain by thick volcaniclastic layers containing a mixture of basaltic and sedimentary debris (member m 2 ). The m 1 deposit consists of a chaotic assemblage of breccia panels and megablocks up to 80 m across. In contrast to m 2 , it is composed essentially of sedimentary material derived from the underlying Beacon Supergroup. The observed structures and textures suggest that the breccias in m 1 were mostly produced by progressive fragmentation of megablocks during transport but also to a lesser extent by disruption and ingestion of the substrate by the moving debris avalanche. The upper surface of the debris avalanche deposit lacks large hummocks, and sandstone breccias dominate volumetrically over megablocks within the deposits. This indicates pervasive and relatively uniform fragmentation of the moving mass and probably reflects the weak and relatively homogenous nature of the material involved. The avalanche flowed into a preexisting topographic depression carved into the Beacon sequence, and flow indicators reveal a northeastward movement. The source area is probably now hidden under the Antarctic ice sheet. Sparse basaltic bodies, which were hot and plastic during transport in m 1 , reveal the role of Ferrar magmatism in triggering the avalanche, possibly in relation to the emplacement of large subsurface intrusions. The documented deposits indicate that debris avalanches are among the various phenomena that can accompany the early stages of large igneous province magmatism, despite the common absence of large central volcanic edifices. Where large igneous provinces develop in association with faulting or slow preeruptive uplift accompanied by deep valley incision, there is a high probability that feeder dikes will approach the surface in areas of steep topography, allowing volcano-seismicity and fluid overpressures associated with intrusion to effectively trigger avalanches.