Weathering and sedimentation events in the development of the
Faroe Islands Basalt Group, NE Atlantic
Simon R. Passey
Jarðfrøðisavnið (Faroese Geological Survey)
Brekkutún 1, P.O. Box 3169, FO-110, Tórshavn, Faroe Islands
simon.passey@jfs.fo
The Palaeogene Faroe Islands Basalt Group has an exposed stratigraphic thickness of
ca. 3 km on the Faroe Islands and is dominated by tholeiitic basalt lava flows of the
Beinisvørð, Malinstindur and Enni formations. Hiatuses in the volcanism allowed time for
weathering and erosion of the flows and the deposition of various volcaniclastic lithologies.
These interludes appear to be more frequent in the upper parts of each formation,
representing waning eruption frequencies. Volcaniclastic lithologies deposited during the
hiatuses comprise fluvial and lacustrine sandstones, siltstones and mudstones, together with
coals and the development of palaeosols formed where there was no deposition. The fluvial
and lacustrine deposits are commonly composed of reworked palagonitised basaltic glass
with minor clasts of basalt, derived from the erosion of the lava field. Thick palaeosols
suggest that chemical weathering of the lava flows must have been intense, possibly due to
heightened weathering rates associated with the warmer and wetter climate attributed to
the Palaeocene-Eocene Thermal Maximum (PETM). Saprolitic boles are prevalent within the
Beinisvørð Formation and exhibit a gradation down through highly altered, fine-grained,
basaltic material to unaltered basalt lava flows.
The hiatus between the Beinisvørð and Malinstindur formations was sufficient to
permit the development of the ca. 60-70 m thick Prestfjall and Hvannhagi formations. The
Prestfjall Formation represents fluvial, lacustrine and swamp environments typical of an
inter-eruption period, whereas the Hvannhagi Formation comprises olivine-phyric tuffs and
high discharge debris and hyperconcentrated flow deposits. These mass flow deposits most
likely formed during periods of high rainfall and when large amounts of pyroclastic debris
inundated the pre-existing fluvial systems. Similarly, the hiatus between the Malinstindur and
Enni formations is represented by the maximum 30 m thick Sneis Formation, which was
deposited on the highly weathered upper surface of the Malinstindur Formation. The Sneis
Formation is omnipresent at this level and consists of a basal volcaniclastic sandstone, the
Sund Bed, which is overlain by volcaniclastic conglomerates and planar laminated
volcaniclastic sandstones. The Sneis Formation units were most likely deposited from
hyperconcentrated and debris flow events.
Detailed regional mapping of the Enni Formation has identified other thick
volcaniclastic sequences, such as the Argir Beds, that may eventually lead to further
subdivisions of the volcanic pile. This is also supported by changes in morphology and to
some extent petrology of the basalt lava flows.
All of the volcaniclastic rocks preserved on the Faroe Islands are intraformational
and, therefore, are composed almost entirely of basaltic clasts/fragments. This requires a
more detailed investigation of the textures and subtle variations in clast compositions of the
volcaniclastic lithologies in order to determine their mode of formation, i.e. pyroclastic or
sedimentary. The processes involved in forming these volcaniclastic sedimentary rocks may
help to constrain pathways and environmental conditions under which the lava flows were
emplaced. In addition, the volcaniclastic rocks may act as reservoirs within the offshore
section, opening up other potential offshore plays.
The new stratigraphy for the Faroe Islands Basalt Group makes use of the hiatuses in
the eruption history marked by the various volcaniclastic lithologies noted above. Close
links can also be made with significant changes in the lava facies architecture, resulting in an
integration of observations concerning both volcanic and sedimentation events.