Cyclic volcanic stratigraphy in a Late Ordovician marginal basin, west Norwegian Caledonides

The well-preserved extrusive sequence of the Solund-Stavfjord Ophiolite Com plex (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 e xtrusive sequence and show stratigraphic and structural evidence for a cycl ic development. The first stage in a volcanic cycle is characterized by hig h extrusion rates yielding sheet flows., commonly with the thickest flow un its at the base. Sequences of sheet flows can be correlated laterally for a t least 6.5 km. Pillow lavas succeed the sheet flows later in a volcanic cy cle with progressively smaller pillows forming at decreasing extrusion rate s. Volcanic breccias occur towards the end of a volcanic cycle, but may als o occur at lower stratigraphic levels. They are made generally of pillow br eccias and hyaloclastites. The extrusive sequence of the SSOC oceanic crust was constructed through seven volcanic cycles that resulted in stratigraph ic units with thicknesses ranging from 40 to 225 m. This architecture is co mparable to sequences in in situ oceanic crust developed along slow- to int ermediate-spreading ridges.