Sedimentary documentation of the break-up of Rodinia,Offerdal Nappe,Swedish Caledonides

Late Riphean to early Vendian metasedimentary rocks of the Offerdal Nappe, central Scandinavian Caledonides, were studied by field mapping and measured sections. Even though the rocks have been metamorphosed and tectonically deformed, preserved sedimentary structures and outcrop exposure permit the interpretation of a little-known area with respect to the break-up of Rodinia. This work shows that although the primary sedimentary textures are strongly altered during metamorphism the primary sedimentary structures and bed geometries are commonly preserved. Moreover, the sedimentary structures, where identified, can be used to hydrodynamically estimate the primary grain-size of the metasedimentary rocks.Seven sedimentary facies were identified in three tectonostratigraphic units. The lower tectonostratigraphic unit consists of polymict conglomerates deposited in alluvial fans. The lower part of the middle tectonostratigraphic unit consists of channelled turbidite deposits. The upper part of the middle tectonostratigraphic unit consists of a regressive to transgressive succession with southeast to northwest transition from fluvial deposits, to storm-dominated deposits and into offshore mudstones. The lower part of the upper tectonostratigraphic unit consists of a regressive succession with southeast to northwest transition from channelled turbidites into sheet-like turbidites. The upper part consists of fluvial deposits in updip (southeast) areas, whereas in downdip (northwest) areas the sheet-like turbidites reflect continued deepwater deposition.(1) The ～1500 m thickness of the Offerdal succession, (2) the occurrence of >300 m thickness of the pebble- to boulder-bearing continental-basement derived alluvial fan conglomerates, (3) the high sediment input rates, and (4) the high subsidence rates suggest an actively subsiding basin and a relatively uplifted hinterland. Furthermore, the arkosic composition of the Offerdal metasandstones, as well as the Precambrian porphyry, syenite and quartzite clast composition of the lithic conglomerates, recognised from the subjacent continental basement of Baltica, suggest a continental sediment source. The rapid accommodation changes, and especially the simultaneous subsidence of downdip areas and uplift of updip areas in UTU2 suggest syn-depositional extensional block rotation. Collectively, this evidence suggests that the Offerdal succession was deposited in a continental rift basin related to break-up of Rodinia.