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,) 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, 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.