Cyclic volcanic stratigraphy in a late ordovician marginal basin, West Norwegian Caledonides

The well-preserved extrusive sequence of the Solund-Stavfjord Ophiolite Complex (SSOC) in the West Norwegian Caledonides enables reconstruction of the uppermost oceanic crust that developed in a marginal basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the extrusive sequence and show stratigraphic and structural evidence for a cyclic development. The first stage in a volcanic cycle is characterized by high extrusion rates yielding sheet flows, commonly with the thickest flow units at the base. Sequences of sheet flows can be correlated laterally for at least 6.5 km. Pillow lavas succeed the sheet flows later in a volcanic cycle with progressively smaller pillows forming at decreasing extrusion rates. Volcanic breccias occur towards the end of a volcanic cycle, but may also occur at lower stratigraphie levels. They are made generally of pillow breccias and hyaloclastites. The extrusive sequence of the SSOC oceanic crust was constructed through seven volcanic cycles that resulted in stratigraphic units with thicknesses ranging from 40 to 225 m. This architecture is comparable to sequences in in situ oceanic crust developed along slow- to intermediate-spreading ridges.