ABSTRACT The Hornelen basin is the largest of several Devonian terrestrial basins in west-central Norway. The basin is filled by alluvial fan and stream deposits eroded from the Caledonian highlands. These deposits form shingled strata uniformly east-dipping with a total accumulation of sediment of ~25 km, and that today stand in positive relief. The little-metamorphosed sedimentary rocks are separated from the underlying Western Gneiss Region (WGR) and Scandian nappes (Lower, Middle, and Upper Allochthons) by the west-directed Nordfjord-Sogn Detachment Zone (NSDZ). The basin’s origin has been debated for more than 50 years. In the 1960s and 1970s several workers ascribed the unusual thickness and longitudinal shingling of the strata to strike-slip deformation comparable to the late Miocene–Pliocene Ridge basin of southern California. However, the recognition of extensional mylonites beneath the basin led others to propose a different model (the scoop or supradetachment model) in which extension and basin filling were due to west-directed displacement on a low-angle normal fault. An examination of kinematic indicators on the brittle fault surface atop the NSDZ reveals consistently N-S motion suggestive of late out-of-syncline thrusting rather than west-directed extension. The purpose of this paper is to make the case for a return to the Ridge basin model for the Hornelen and Kvamshesten basins, overprinted by later north-south shortening. An ~100 km long strike-slip fault, the Bortnen fault, close to the northern margin of the Hornelen basin, may be the structure responsible for the basin’s development.

Abstract Neoproterozoic strata in southeastern Idaho and Utah include the <766 Ma Uinta Mountain Group and Big Cottonwood Formation (Fm.) deposited in an east-trending rift basin and, to the west, the lower part of a westward-thickening rift to passive-margin succession that initiated c. 720 Ma. The latter contains a lower diamictite and volcanic succession, with a complex stratigraphic interval of Cryogenian marine glacial deposits (Pocatello and Mineral Fork formations and correlatives). This is overlain by a mostly terrigenous succession of <667 Ma strata assigned to the upper member of the Pocatello Fm. and Brigham Group in southeastern Idaho, to the Kelley Canyon Fm. and Brigham Group in northern and western Utah, and to the McCoy Creek Group and Prospect Mountain Quartzite in adjacent Nevada. Although the Brigham Group and correlative deposits contain no direct evidence for glaciation, widely developed, though stratigraphically restricted, incised valleys, with erosional relief from a few metres to as much as 160 m, are inferred to represent subsequent times of Cryogenian glacially lowered sea level. Overall interpretations of the stratigraphy and sedimentology of these rocks have changed little in the past 10–15 years. The most important recent advances relate to U–Pb geochronology. In strata that lie unconformably below demonstrable glacial deposits, the lower Uinta Mountain Group (formerly thought to be c. 900 Ma) contains populations of detrital zircons as young as 766±5 Ma. Cryogenian magmatism north of the Snake River Plain in central Idaho is recognized near House Mountain, east of Boise at c. 725±5 Ma, in the Pioneer Mountains Core Complex at about 695 Ma, and in central and east-central Idaho at 685–650 Ma. Clasts interpreted to be from the rift-related Bannock Volcanic Member of the Pocatello Fm. are dated at 717±4 Ma and 701±4 Ma. The overlying diamictite-bearing Scout Mountain Member contains a mafic lapilli tuff near the base (686±4 Ma) and a reworked fallout tuff near the top (667±5 Ma). Strongly negative C-isotope data have been obtained from some of the carbonate rocks, although the latter constitute only a small fraction of the succession. Palaeomagnetic data are available only for the Uinta Mountain Group, and suggest an equatorial palaeolatitude.