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

Abstract

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 IntroductionThe Caledonides of central Norway are especially well known for their fragmented ophiolite assemblages and, in some areas, the overlying, richly fossiliferous, Ordovician to inferred Early Silurian, low-grade metasedimentary rocks (Bergström, 1979(Bergström, , 1997Bruton & Bockelie, 1980;Ryan et al., 1980;Neuman & Bruton, 1989;Neuman et al., 1997). The...

Depositional environment of polymictic conglomerate of the Gadag greenstone Belt, Western Dharwar Craton, south India: An insight for Neoarchean marginal sedimentation

“…The spatial variation in clast lithology can be explained by multiple alluvial fronts with high input from granitic, felsic volcanic, quartzite, BIF provenance in the northern part of the belt, whereas in the southern part of the belt mafic, and felsic volcanic, as well as, BIF sources possibly from both NE‐SW direction of the basin (Chadwick et al, 2003: Krapez et al, 2020). Similar type of basin margin sediments are reported from back‐arc basin turbidite and conglomerate of Mid‐Norwegian Caledonides (Henriksen, Roberts, & Pedersen, 2018); Dhanjori Formation (Alluvial‐Braided) of Singhbhum Craton (Mazumder & Sarkar, 2004); and synorogenic late‐stage basins of the Eastern Goldfields Province, Western Australia (Krapež, Barley, & Brown, 2008; Krapež, Standing, Brown, Mark, & Barley, 2008). Thus, the graded conglomerate, greywacke represents a unique phase of marginal sedimentation.…”

A Silurian age for the metasedimentary rocks of the Ekne Group, Trøndelag, Mid-Norwegian Caledonides: and inferences for a peri-Laurentian provenance

“…In the Åsenfjord district, fragments of gastropods and corals of imprecise Mid to Late Ordovician age occur in metalimestones in higher parts of the subjacent Vuddudalen Group, which is otherwise characterised by deep-marine Nereites trace fossils (Roberts, 1969(Roberts, , 1984Uchman et al, 2005). Details of the sedimentology and facies associations are given in Henriksen et al (2018). In the present contribution, detrital zircons have been separated from (Dickinson & Gehrels, 2009).…”

“…Of these, the most reliable is the mean age of the youngest three or more grains that overlap in age at 2σ 1 in Slagstad et al, 2014). Shortly after their generation in a suprasubduction-zone setting in the Iapetus Ocean, the ophiolites were obducted, deformed, weakly metamorphosed, uplifted and eroded, with their detritus forming a basal conglomerate to a thick, back-arc or marginal-basin, volcanosedimentary succession which has been inferred, but not satisfactorily proven, to extend up into the Silurian period (Vogt, 1945;Henriksen et al, 2018). A depositional model for the succession in the Ekne-Frosta area involved repeated events of longterm staging and reworking of sediments in near-shore areas before sudden, catastrophic release into the basin (Henriksen et al, 2018).…”

“…Shortly after their generation in a suprasubduction-zone setting in the Iapetus Ocean, the ophiolites were obducted, deformed, weakly metamorphosed, uplifted and eroded, with their detritus forming a basal conglomerate to a thick, back-arc or marginal-basin, volcanosedimentary succession which has been inferred, but not satisfactorily proven, to extend up into the Silurian period (Vogt, 1945;Henriksen et al, 2018). A depositional model for the succession in the Ekne-Frosta area involved repeated events of longterm staging and reworking of sediments in near-shore areas before sudden, catastrophic release into the basin (Henriksen et al, 2018). This largely Ordovician, greenschist-facies succession is rich in body fossils of Mid Ordovician (Dapingian to Darriwilian) age, notably in the Hølonda area just southwest of Trondheim (Fig.…”

“…The base of the group is marked by the polymict Hopla conglomerate. The total thickness of the Ekne Group is estimated at 2800 m (Henriksen et al, 2018). In the Åsenfjord district, fragments of gastropods and corals of imprecise Mid to Late Ordovician age occur in metalimestones in higher parts of the subjacent Vuddudalen Group, which is otherwise characterised by deep-marine Nereites trace fossils (Roberts, 1969(Roberts, , 1984Uchman et al, 2005).…”