Volcanic landscape controls on pre-rift to syn-rift volcano sedimentary systems: the Prestfjall Formation eruptive hiatus, Faroe Islands Basalt Group, northeast Atlantic

Abstract

ABSTRACT The Paleogene lava flows of the Faroe Islands Basalt Group are divided into three relatively thick formations. The oldest, the Beinisvørð Formation is separated from the second lava flow succession, the Malinstindur Formation, by two formations composed primarily of volcaniclastic rocks. The oldest of these, the Prestfjall Formation has been interpreted as a period of eruptive quiescence and linked to changes in mantle melting. It is characterised in the south by the occurrence of coals, while the overlying Hvannhagi Formation is a sequence of primary and remobilised volcaniclastic strata. Field, laboratory, palynology, and photogrammetry studies have been used to investigate variations in facies and architecture within these volcaniclastic formations. The data reveal significantly different depositional systems in the Prestfjall and Hvannhagi formations over the ~40 km from the island of Vágar in the north to the island of Suðuroy in the south. Facies distribution in both the Prestfjall and Hvannhagi formations was found to have been controlled by a complex interaction of regional paleoslope, pre-existing topography, the eruption and local collapse of low-angle shield volcanoes, and minor brittle deformation. Lithological data and photogrammetry have enabled the identification of a > 180 m thick succession of volcaniclastic conglomerates deposited by lahars reworking a low-angle shield sector collapse. Co-occurrence of facies characteristic of the Prestfjall, Hvannhagi and Malinstindur formations indicate that volcanic eruption continued at a lower tempo throughout the Prestfjall Formation interval. Identification of a Beinisvørð Formation low-angle volcano shield northwest of the Faroe Islands alters the previous eruption model for this extensive lava field.