The Ordovician metasedimentary rocks in the Caledonian nappes in Mid Norway were deposited in a back-arc setting, following ophiolite obduction, uplift and erosion, related to continental uplift and changing directions of subduction during this period. Deep-water conditions were established in the basin and this accommodated a thick succession of variously coarse-grained, conglomeratic and argillaceous sediments. The conglomerates typically display the well-rounded clast material of variable igneous, volcanic, sedimentary and carbonate origin and alternate between being clast-supported and matrix-supported with interbedded sandstone beds. The rounded clasts suggest significant reworking and long-term storage of sediments in the source area before transport into the deeper part of the basin. The basal surface of conglomerates always shows an erosive contact with underlying shales or coarsening-upward sandstone successions. The basal surfaces probably represent sedimentary fairways, where several conglomerate units separated by erosional contacts are stacked on top of each other. The conglomerate successions likely result from episodic release of large quantities of sediments. Between each burst of release, sediments accumulated in alluvial fans, fan deltas and fluvial delta systems along the basin shoreline and carbonate reefs established on the outer shelf. Laterally accreting channel sandstone beds were probably important in setting up sedimentary fairways for the succeeding conglomerates. In the inferred deepest part of the basin, smaller channellised features and thin-bedded turbidites seem to dominate. Channel features and hummocky cross-stratification in shelf to upper slope sandstone beds are alternatively interpreted to result from unidirectional flows and constitute an integral part of the turbidite system. By including all the observed facies within the turbidite system we invoke a depositional model which does not require a eustacy-driven sequence-stratigraphic model for explanation. We rather suggest a tectonic model involving an elevated source area, either volcanic or continental, where erosional products are stored and reworked in the alluvial accommodation zone. These sediments are built up to a threshold level and flushed to the sea by a sudden availability of large amounts of water. The actual cause of this sudden availability of water remains uncertain, but the consequent mixture of water and sediments resulted in a super-concentrated high-energy density current.
Turbidite and conglomerate successions in an Ordovician back-arc basin, Mid-Norwegian Caledonides: a result of long-term staging followed by catastrophic release of sediments.