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Unravelling the eruption history of the Drammen Caldera: A multidisciplinary study of caldera-forming processes in the Permian Oslo Rift, Norway

Guro L. Andersen1 , Henrik H. Svensen1, Lars E. Augland1, Sara Callegaro1,2, Jack W. Whattam1, Frances M. Deegan3, Heejin Jeon4, Martin J. Whitehouse4

  • Affiliations: 1Department of Geosciences, University of Oslo, Oslo, Norway; 2Department of Biological, Geological, and Environmental Sciences, University of Bologna, Bologna, Italy; 3Department of Earth Sciences, Uppsala University, Uppsala, Sweden; 4Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden

  • Presentation type: Talk

  • Presentation time: Thursday 14:15 - 14:30, Room R380

  • Programme No: 3.11.5

  • Theme 3 > Session 11

Abstract

During the mature stages of the Permo-Carboniferous Oslo Rift, at least 15 calderas formed. Thick ignimbrite deposits are preserved in several of these calderas but remain poorly studied with modern methods. This study focuses on a rhyolitic ignimbrite succession in the 7x7 km Drammen Caldera, located 40 km southwest of Oslo. Here, we present field observations, petrographic results, and geochemical analyses of the deposits. Despite significant glacial erosion and partial Quaternary coverage, we show that over 450 m of ignimbrite stratigraphy is preserved in the Drammen Caldera. In addition, we have identified ashfall and lahar deposits interbedded between ignimbrite units. The ignimbrites range from high- to low-welding grade, and we interpret them to record at least five explosive eruptions resulting in two caldera collapse events. The ignimbrites are altered but retain primary volcanic textures. Cathodoluminescence (CL) reveals complex zoning patterns in quartz phenocrysts, indicating multiple growth and dissolution stages. Phenocrysts are heavily fragmented, with fractures healed by quartz cementation. Oxygen isotopes measured in-situ in zoned quartz phenocrysts suggest assimilation of high-δ18O Lower Palaeozoic sedimentary rocks surrounding the caldera. Ongoing work on silicon isotopes and U-Pb geochronology aims to further refine the timing and petrogenetic processes. We conclude that the ignimbrites preserved in the Drammen Caldera hold information about complex magma reservoir dynamics, leading to several explosive eruptions. We present a six-stage evolution model for the caldera. This study highlights the potential for detailed multidisciplinary ignimbrite studies in other Oslo Rift calderas, as well as other eroded calderas globally.