The structure of shapeless mountain, antarctica, and its relation to jurassic igneous activity

Abstract

Shapeless Mountain is so named because it does not conform to the normal shape of those mountains in South Victoria Land, Antarctica which consist of layer-cake stratigraphy of Beacon sediments and Ferrar Dolerite sills. At Shapeless Mountain, Devonian-Triassic Beacon rocks are often tilted and associated intimately with early Jurassic Ferrar Dolerite and several different breccia types from the early Jurassic Mawson Formation, including peperite and lahar. Blocks of Beacon sediments, now surrounded completely by dolerite were fragmented and tilted by intrusion of the ctolerite.Contacts of Mawson breccia with Beacon sediments are usually irregular and often steep. Contact metamorphism of the sediments is indicative of emplacement at 300-600°C. So me dikes grade from Ferrar Dolerite through basalt to breccia of Mawson Formation. Lava flows and lahars are conformable above Beacon sediments. Mawson Formation debris, reworked by fluvial processes, is intruded by younger Mawson Formation breccia. Early Jurassic tlioleiitic magma encountered water-saturated sediments of the Beacon Supergroup resulting in superheating and phreatic explosions. Brecciation and fluidisation followed, producing the breccias and structural forms now observed at Shapeless Mountain. © 1984 Taylor & Francis Group, LLC.