The Moores Peak Formation, a Cretaceous debris-avalanche deposit in the Antarctic Peninsula Volcanic Group, Livingston Island, South Shetland Islands

Abstract

The Moores Peak Formation occurs on eastern Hurd Peninsula, Livingston Island, part of the South Shetland Island volcanic arc, which was active during Mesozoic and Cenozoic times. The rocks were formerly described as deformed Miers Bluff Formation (MBF) turbidites of early Triassic age. More recent work identified lithogically distinctive sedimentary and volcanic breccias in an ambiguous stratigraphic position, possibly belonging to the underlying turbiditic accretionary complex, or to the overlying volcanic sequence of ?Cretaceous age (Antarctic Peninsula Volcanic Group — APVG), or representing a formation in between the MBF and APVG. New mapping reinterprets the Moores Peak Formation as a c. 150 m thick megabreccia unit unconformably overlying the MBF, and forming pan of the mid- to late Cretaceous APVG. The megabreccia consists of sheared angular clasts and large blocks of lithified, deformed and hydrothermally altered sandstone, distal turbidite, mudstone and conglomerate (derived from the MBF), and volcanic breccia and tuff and lava clasts (typical of the APVG), supported in a matrix of pulverized sedimentary and volcanic material, with relatively abundant epidote and clinozoisite. Jigsaw-breccias, irregular colour domains and intra-block shears indicate that the breccias formed as subaerial debris-avalanches from an area of uplifted pre-volcanic basement (MBF) overlain by volcanic rocks. Hydrothermal alteration contributed to weakening of the uplifted rocks. Collapse of large volcanic edifices results in sudden decompression of underlying basement and contained hydrothermal systems, resulting in solution boiling, hydraulic brecciation, and mineral deposition. The MBF hosts a laterally extensive quartz vein and breccia system 3 km west of Moores Peak, which may have formed as a result of a stratovolcano collapse event similar to that represented by the Moores Peak Formation.