Juha Köykkä - Oulu

Res Terrae, Ser. A 32, J. Köykkä, Sedimentology of the Mesoproterozoic Telemark basin-fills, South Norway: implications for
sedimentation processes, depositional environments and tectonic evolution
during the syn-rift stage. The sedimentation basin subsidence was caused by tecton-
ism, sediment-volcanic loading and crustal densification. The basin entered the late
syn-rift stage gradually, where the connection between the rift structure and shallow
marine waters was established and the volcanism terminated. At the late syn-rift stage,
the transgression and associated erosion reworked the underlying sediments, and a
newly formed bay may have acted as a sediment sink for clastic detritus transported by
alongshore currents in a shoreline. This was followed by tidal sedimentation, and the
cooling of the lithosphere eventually caused the basin subsidence to slow. The basin
then entered its post-rift stage. The crust, including the basin, continued to subside on
a larger scale. The fluvial incision and sedimentation indicate a base level drop, the
filling of the basin and a shoreline advancing further to the sea. At the end of the early
post-rift stage and the late post-rift stage, the shallow marine conditions returned and
covered wide areas accompanied by base level transgression and the landward migra-
tion of the shoreline. The late syn-rift stage was characterized by subtidal and high-
energy nearshore sedimentation patterns in an overall trangressive setting, which was
studied in Paper III. A delicate balance between sediment supply and accommodation
space creation was achieved, which resulted in sediment accumulations several kilo-
meters thick. The whole rift was buried under shallow shelf deposits.
When comparing the Rjukan Rift Basin to, for example, the East African Rift
(Frostick et al., 1986) and mid Jurassic Alp rift basins (Loup, 1993), notable differenc-
es in sedimentation patterns appear, including a lack of deep water (turbidites, olisto-
stromes, and megabreccias) and carbonate sediments. The lack of a deepwater sedi-
mentation prism and oceanic crust as well as the non-evolution to passive margin set-
ting could indicate that the rifting in the Rjukan Rift Basin failed to complete the
break-up of the continental crust. The high input of terrigenous siliciclastic sediments,
possible cold-climate conditions, and other factors prevented the ignition of the carbo-
nate factor in the Rjukan Rift Basin. The erosional unconformity (sub-Heddersvatnet
unconformity), clastic sedimentation starvation and voluminous bi-modal volcanism in
the syn-rift stage indicate an active rift model (Sengör and Burke, 1978; Frostick and
Steel, 1993). In an active rifting setting, the surface deformation is associated with the
impingement of a thermal plume or sheet on the base of the lithosphere. Although the
dip of bounding faults is difficult to estimate, the thick accumulation of sediments (ca.
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