Building Ice-Age Askja: Processes, Products, and Paleoclimate

Austurfjöll is the largest glaciovolcanic construct at Askja Volcano, the best exposed and largest basaltic central volcano in Iceland. The massif records the repeated interaction of basaltic fissure-dominated eruptions with a 600-900 m thick Pleistocene ice sheet in Iceland. The Austurfjöll deposits serve as an important proxy record for ice presence and thickness, supplementing the limited terrestrial glacial record in Iceland. \ud The model of the construction of the 3.62 km3 glaciovolcanic massif is the first to outline in detail and date the growth and evolution of a long-lived polygenetic ice-confined central volcano. The model is based on lithologic descriptions, petrologic investigations, textural studies, unspiked K-Ar dating, volatile saturation pressures based on FTIR analysis of water content in glass, and remote sensing-based mapping. The massif is composed of basal basaltic pillow lava sheets, dominantly subaqueously-deposited vitriclastic deposits erupted from overlapping fissure ridges, and accumulations of gravity-driven deposits in inter-ridge depositional centers. The ridges are locally capped by emergent to subaerial tephra and subaerial lava flows. Detailed textural studies of sequences of in-situ transitions from pillow lavas through breccias to overlying lapilli tuffs are interpreted as examples of phreatomagmatic explosions triggered by initial magmatic exsolution and fragmentation at water depths > 600 m. \ud A stratigraphy for Austurfjöll is established and consists of one interglacial unit, six glaciovolcanic units, and two glaciogenic sedimentary units established through chemostratigraphy and field mapping. Eruptive units are numbered chronologically, with glacial units designated Dm: Unit 1 (A and B), Dm1, Unit 2, Dm2, Unit 3, Unit 4, Unit 5, Unit 6 and Unit 7. Diamictite deposits and emergent facies are described for the first time at Austurfjöll. Two eruptive units were dated radiogenically by unspiked K-Ar methods to 71 +/- 7 ka (Unit 2) and 29 +/- 8 ka (Unit 3). Ice presence is inferred from glacial, subaerial, and subglacial lithofacies including coherent margined volcaniclastic dikes (CMVDs) that are interpreted as the result of basaltic intrusions into ice-cemented sediments. The deposits described from Austurfjöll reflect a history of interglacial, ice-confined subaqueous, subglacial and emergent eruptions with a dynamic Pleistocene ice sheet over at least 40 ka.\u