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Abstract

Abstract Devonian continental redbeds in Norway are of two distinct facies of the Old Red Sandstone: (1) coarsegrained fluvial sediments deposited in a series of separate intramontane basins, and (2) fine-grained fluvial sediments deposited on a broad extramontane alluvial plain. The intramontane facies is characterized by surprisingly thick accumulations of Lower and Middle Devonian breccias, conglomerates, and sandstones deposited as thick coalesced alluvial-fan complexes in structurally formed grabens and half-grabens. Penecontemporaneous uplift of surrounding provenance areas supplied abundant and varied detritus to the intramontane basins, including clasts of former eugeosynclinal metasedimentary and mefavolcanic rocks, high-grade metamorphic schists, gneisses and amphibolites, and diverse mafic and felsic intrusive rocks. The largest of the structurally formed basins covers an area of approximately 2,000 sq km and has a continuous Devonian stratigraphic section with a maximum thickness of approximately 5,000 m.

The extramontane facies is characterized by the accumulation of as much as 1,250 m of red sandstones and siltstones of Late Silurian to Early Devoniani?) age. These sediments were deposited on a broad alluvial plain located approximately 250 km southeast of the intramontane basins, and they thin toward the southeast, grading laterally into intertidal sediments.

The intramontane basins developed in the former eugeosynclinal part of the Caledonian geosyncline, whereas the extramontane sediments were deposited on the former foreland. The geosyncline underwent major orogeny and uplift during the Late Silurian and Early Devonian, resulting in the formation of a major NE-SW-trending mountain system extending from Great Britain to Spitsbergen. Continued tectonic activity in the Devonian resulted in the formation of the intramontane basins, continued uplift of source areas, and subsequent folding and faulting of the Devonian sedimentary rocks. The Devonian rocks locally have been thrust over surrounding older rocks; the original basin margins were probably high-angle normal faults, although strike-slip faulting is suggested locally. Paleocurrent patterns suggest transport of sediment from the surrounding highlands toward the central part of the intramontane basins, where longer rivers probably flowed parallel with the basin axes. The extramontane alluvial plain received sediments from the mountain chain on the northwest.

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