The Benton Fault is a

geological fault Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

in southern

Pembrokeshire Pembrokeshire ( ; ) is a Principal areas of Wales, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and otherwise by the sea. Haverfordwest is the largest town and ...

in southwest

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

that was active as a normal fault during the deposition of the

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

during the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

period, forming a thick

half graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...

. During the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

, the Benton Fault was strongly inverted as part of the

Variscan Orogeny The Variscan orogeny, or Hercynian orogeny, was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Varis ...

Geological history

The Benton Fault probably originated during the early

Palaeozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma at the start of ...

era (over 400 million years ago) as a boundary fault of a horst—a raised block of land between faults. During this period, it formed part of the margin of a raised block of

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

igneous rocks. Following the

Caledonian orogeny The Caledonian orogeny was a mountain-building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events tha ...

(mountain-building event) in the late

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

period, the Benton Fault became an active normal fault during the deposition of

sediments. As a normal fault, it allowed the land on its southern side to drop down relative to the northern side. At its maximum, the fault achieved a downthrow to the south of at least 1,500 metres. This movement helped create space for thicker sedimentary deposits to accumulate on the southern side of the fault. The fault's

displacement Displacement may refer to: Physical sciences Mathematics and physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

varied along its length, with evidence suggesting that movement increased eastward. This differential movement contributed to the varying thickness and composition of sedimentary rock units across the region. By the

period, the Benton Fault had become inactive, as evidenced by undisturbed Carboniferous Limestone series rocks that overlie it at the eastern end of its

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

. During the Variscan orogeny in the late Carboniferous (about 300 million years ago), the Benton Fault was reactivated, but this time with reverse movement—meaning the southern side was pushed up relative to the northern side. This reactivation was particularly significant in the western portion of the fault. Later in the Variscan deformation, the western portion of the Benton Fault was transected and displaced northward by the Johnston Thrust, a younger fault structure.

References

{{Authority control Geology of Wales

Geological history

See also

References