The volcanic and sedimentary evolution of the Faeroe Plateau Lava Group, Faeroe Islands and the Faeroe-Shetland Basin, NE Atlantic

The Faeroe Plateau Lava Group (FPLG) of the Faeroe Islands, NE Atlantic, has been re-examined in order to understand its stratigraphy, structure, environment of eruption and evolution, for both the volcanic and the associated sedimentary lithologies. The FPLG has an exposed and drilled stratigraphic thickness of ca. 6.5 km on the Faeroe Islands and is separated into five formations.

The Lower Basalt Formation (LBF) is ca. 4.5 km thick and is dominated by subaerial tabular-classic facies lava flows, with average thicknesses of ca. 25 m. These were erupted at high effusion rates, travelled significant distances rapidly, and each flow was emplaced in a matter of weeks to a few months. The recognition of the Stapin Vent, Suðuroy, indicates that small point source vents contributed pyroclastic material to the land surface, with the major fissure eruptions most likely located to the west of the Faeroe Islands. The exposed ca. 900 m of the LBF is predominantly composed of prismatically jointed lava flows, which were emplaced into relatively dry environments. Weathering of the lava flow tops in the upper ca. 100 m of the LBF resulted in the formation of significant palaeosols (reddened boles), which implies hiatuses in the volcanic activity of up to 140 kyrs. During these hiatuses, terrestrial environments (fluviatile, lacustrine and swamps) were established and partial erosion of the lava topography ensued. The resulting sedimentary/epiclastic lithologies consist of volcaniclastic conglomerates through to mudstones and coals. The volcaniclastic rocks are composed of reworked palagonitised basaltic tephra and lithoclasts of basalt lava and pre-existing volcaniclastic rocks. The damming of river channels by lavas and the association of columnar jointed flows with fluviatile/lacustrine strata implies that columnar jointing is directly related to lavas that were erupted into wet environments.

Geochemical analysis of the volcanic interval in Well 214/4-1, Faeroe-Shetland Basin, has enabled a correlation to the Lower Basalt Formation of the Faeroe Islands, ca. 240 km to the W. The volcanic interval consists of a ca. 450 m thick sequence of hyaloclastites, which are overlain by a ca. 50 m thick subaerial lava sequence. This volcanic interval is interpreted to have formed at a palaeoshoreline environment, where subaerial lavas flowed from the land surface into a substantial body of water at least 450 m deep (i.e. the Faeroe-Shetland Basin at that time), resulting in the quenching and fragmentation of magma to produce the hyaloclastites. Well 214/4-1 is <50 km to the SE of the Faeroe-Shetland Escarpment, which has previously been interpreted as a hyaloclastite delta, thus implying that there are a number of unrecognised hyaloclastite units within the Faeroe-Shetland Basin and that the coastline was steadily encroaching W/NW, towards the Faeroe Islands during the volcanic interval.

The overlying ca. 10 m thick Coal-bearing Formation (CBF) represents a significant hiatus in the volcanic activity at the end of LBF times. Erosion and subsidence of the lava field led to the development of an expansive lacustrine environment, which resulted in the accumulation of plant material and associated detritus and chemical sediments, mainly ironstones, and the formation of mineable coal seams. Petrographic and geochemical analysis of siderite spherules within the ironstone beds from two localities on Suðuroy have helped to define margin- and centre- of-lake environments, at least 10 km apart. Contemporaneous fluviatile lithologies in West Suðuroy are composed of reworked palagonitised tephra, basalt lava clasts and plant material.

Renewed volcanism marked an abrupt change from the inter-eruption facies of the CBF to the aggrading syn-eruption facies of the Volcaniclastic Sandstone Formation (VSF). The VSF is at least 30 m thick and comprises the initial deposition of laterally extensive, olivine-phyric, vitric tuffs, >8 m thick. This input of volcanic debris to the land surface swamped the CBF fluviatile systems and, possibly combined with high rainfall, produced sheet floods, which resulted in mass flow deposits. The destabilisation of the land surface by the destruction of vegetation by pyroclastic activity also may have aided in the mobilisation of volcanic debris. The resulting volcaniclastic debris and hyperconcentrated flow deposits are characterised by tabular geometries, consisting of volcaniclastic mudstones through to conglomerates, that are poorly sorted and matrix supported. These units are dominated by reworked palagonitised basaltic tephra and minor amounts of basalt.

The ca. 1.4 km thick Middle Basalt Formation (MBF) consists of ca. 20 m thick subaerial compound-braided facies lava flows made up of thinner flow units, <0.5 to 2 m in thickness. The MBF lavas were erupted at lower effusion rates than those of the Lower and Upper basalt formations. The flow units of the MBF are either S-type (spongy) or, more commonly, P-type (pipe-bearing) pahoehoe lava that was emplaced passively by an inflation (endogenous) mechanism through efficient lava tube networks. Calculations based upon the thicknesses of P-type flow crusts, suggest that the flow units were active for periods of a few hours up to a few days, and that the compound lava flows were emplaced over periods of months to years. The relatively rare interlava lithologies deposited during hiatuses in the MBF volcanism consist of fluviatile volcaniclastic sandstones and siltstones, commonly contained within channel structures. The sandstones are composed of reworked palagonitised basaltic tephra and plant material, indicating that the surrounding land surface was vegetated. A re-examination of some of the vents defined by Rasmussen & Noe-Nygaard (1970b), for example, Sundsmunnin, Viðoy, suggests that many of them are, in fact, epiclastic conglomeratic deposits and, consequently, no MBF vents are recognised.

The boundary between the Middle and Upper basalt formations is not represented by a simple major hiatus akin to the CBF, although a thin (ca. 10 m thick) volcaniclastic debris flow, represented by exposures ca. 34 km apart, is recognised. This sedimentary unit is a volcaniclastic conglomerate that is poorly sorted, matrix supported, and has a homogeneous clast population dominated by plagioclase-phyric basalt. These data imply that during early UBF times there was an influx of pyroclastic debris onto the land surface, which was mobilised by surface water and/or high rainfall, producing sheet flood deposits.

The Upper Basalt Formation (UBF) is ca. 900 m thick, although a few hundred metres have been removed by erosion. The UBF is dominated by subaerially erupted, prismatically jointed, tabular-classic lava flows, with average thicknesses between 8 and 11 m. Akin to the LBF, these lavas were emplaced rapidly, in weeks to a few months, from high effusion rate eruptions. The identification of a small vent, dominated by upward-terminating minor intrusions associated with highly brecciated country rock material, at Húsið millum Gjáir on Viðoy, may represent a localised feeder to UBF flows. Hiatuses in the volcanism during UBF times saw the development of fluviatile and lacustrine environments, together with palaeosol surfaces which were commonly vegetated, with the rare preservation of tree moulds within the basal parts of lava flows. The interlava lithologies consist of volcaniclastic sandstones and siltstones composed predominantly of reworked palagonitised basaltic tephra. Some of the lavas that flowed over these strata were brecciated and subsequently agglutinated, implying that they were emplaced into wet environments.