Lithostratigraphy of the Upper Part of the Tjörnes Sequence in Furuvík, Breiðavík, Öxarfjörður, and Central Tjörnes Mountains, North Iceland

Thick Quaternary sediments of the Breiðavik Group are intercalated between lava flows and tuffs on Tjornes, North Iceland. The present chapter is a compilation of lithostratigraphic and fossil data, combined with an interpretation of the sedimentary and volcanic facies. The vertical column of strata displays a rhythmic character, and a model for sedimentation during glacial-interglacial cycles in a volcanic environment is presented. The Quaternary part of the Tjornes sequence contains evidence of 14 separate glaciations with tillite horizons. The tillites are typically associated with kame and outwash conglomerates, followed at first by glacio-lacustrine and glaciomarine mudrocks and diamictites, and then by deltaic and/or bar-lagoon assemblages, fluvial sediments, and subaerial lava flows. Depositional environments are reconstructed for several steps in the geological history of the Breiðavik Group. The oldest tillite has been dated to c. 2.6 Ma. Several of the tillite beds are succeeded by marine sediments with fossils, especially in the lower part of the sequence. During the first million years of the accumulation, the deposition took place in a transitional environment while the area was subsiding, but the latter part of the geological record reflects terrestrial deposition and increased volcanism in the region.