2024-07-29
An overview of the largest sedimentary devonian basin in western Norway
Written by: Robert
Keywords: geology, tectonics, sedimentology
A large part of the outer crust of western- and mid- Norway is made up of gneiss rock, a metamorphic rock (The western gneiss region). However, at the outskirts of this region, near the coast, there are four relatively small geological bodies, with very different characteristics than the surrounding greiss region. These are made up chiefly of sandstone and conglomerates, and they are formed by tectonics and sedimentary process - The Hornelen basin, Kvamshesten basin, Håsteinen basin, and Solund Basin.
(image: geographic location of the Hornelen Basin)
The largest of these - the Hornelen basin - is still relatively small (2000 sqm.) compared to sandstone basins elsewhere in the world, but unusual for its stratigraphic thickness, up to 25 km [1].
This geological structure is not uniform, but has a special north-south profile, in particular with respect to rock composition:
The coarseness, clasticity, clast size and composition of the rock varies greatly in the north-south direction. The north and south border is characterized by conglomerate rock and large clast size, whereas the middle of the basin (and along the axis) consists mostly of fine-grained sandstone. The strong coarseness of the rock at the margins of the basin is caused by alluvian fans ([2] and [3]). The basin is bounded by tectonic faults in the north and south. Here, alluvian fans were developed from the sharp inclines, bringing rock debris from the hillside of the faults to the floor of the basin, forming the present day composition of conglomerates.
Another interesting feature is the cyclic north-south “steps” across the basin. These are stacked alluvial fans [2], and cyclothems.
(image: cyclic and stacked features, https://www.alfotbreen.no/geologi/)
A sedimentary basin is formed when sediments are deposited in an subsidence, often between or near tectonic faults. The Hornelen basin was formed by materials being carried from east to west in a river-system, with a gradually build-up of layers.
These geological events occured in the Devonian period, after or late in the Siluro-Devonian Caledonian mountain-building cycle. The basin was formed in a extensional tectonic regime, whereby the crust is stretched, or is subject to longitudinal shear movements, giving rise to basin filling between technonic faults.
References:
[1] Steel, R. J. (1976), Devonian basins of western norway - sedimentary response to tectonism and to varying tectonic context
[2] Steel, R. J. (1977), Coarsening-upward cycles in the alluvium of Hornelen Basin (Devonian) Norway: Sedimentary response to tectonic events
[3] Bryhni, I. (1978), Flood deposits in the Hornelen Basin, west Norway (Old Red Sandstone)