Explosively Erupted Hydrovolcanic Deposits at Minna Saddle, Antarctica: Evidence of Limited Ice-Magma Interaction

Detailed mapping of a series of layered volcaniclastic deposits on a small nunatak at Minna Saddle, Antarctica suggest that the outcrop was emplaced by energetic phreatomagmatic eruptions. Late Miocene-aged volcanic sequences exposed in nearby Minna Bluff cliff faces exhibit alternations between pillow lavas and associated hyaloclastite breccias and strombolian clastogenic lavas, possibly indicating fluctuating syneruptive ice levels of the adjacent Ross Ice Sheet.

Minna Saddle nunatak deposits include palagonitized planar and cross-bedded units, as well as fine-grained layers rich in accretionary and armored lapilli. The majority of the deposits are poorly sorted, and include both dense gray lava lithic clasts and juvenile bombs. The bedding and sedimentary structures are consistent with a phreatomagmatic origin. Deformation on a variety of scales, including faults with 1 to 30 m of offset, affects much of the outcrop. Muddy damage zones along fault planes and soft sediment deformation of the surrounding deposits suggest that the deposit was still wet when faulting occurred. Large spoon-shaped deposits are interpreted as slump blocks of water-saturated material. The characteristics of the deposits and the deformation suggest that water had a significant impact on eruptive and depositional processes during and after the eruption.

The interpretation of an explosive phreatomagmatic eruption is consistent with limited syneruptive ice cover, which provided meltwater to energize the eruption but did not submerge the volcano. The nunatak is heavily eroded and overlain by late Pleistocene erratic boulders. Ongoing research focuses on petrographic documentation of volcaniclastic lithofacies, chemical characterization of the nunatak rocks, and ⁴⁰Ar/³⁹Ar dating of the volcanic sequence.