Beddgelert Fault
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Beddgelert Fault is a geological structure in Snowdonia, North Wales. It is a southwest to northeast running fault formed by volcanic activity and earth movements during the middle Ordovician period (about 460 million years ago), following the general structural direction common to the Caledonian mountain-building event in northern Britain. First documented in 1881, it forms part of a collapsed volcanic basin known as the Snowdon caldera. The fault played a critical role in the development of the volcanic landscape of Snowdonia and provides evidence for mid-Ordovician tectonic activity in Wales.
The Beddgelert Fault separates the Snowdon syncline to the west and northwest from the Gwynant–Glaslyn valley to the east and southeast. These areas represent different stratigraphical successions of Ordovician rocks. The belt of volcanic rocks in the Gwynant-Glaslyn valley is separated from the Snowdon syncline by sediments in an anticlinal core known as the Yr Aran Anticline, with the southeastern limb of this anticline absent due to the mechanical contact along the Beddgelert Fault.[1]
Near the town of Beddgelert, the fault is exposed as a complex zone of lenticles on a cliff known as Craig-yr-Adwy, where the relationships between the different rock successions can be clearly observed. The fault is predominantly a single structure, but in places forms a complex zone.[1]
A key characteristic of the Beddgelert Fault is that it is pre-cleavage, meaning the steeply-inclined cleavage in the rocks is neither crushed nor deflected across it. This indicates that the fault formed before the main Caledonian deformation that produced the cleavage.[1]
The Beddgelert Fault Zone played an important role in the development of mineralisation in the Snowdon caldera. Studies by Lusty and colleagues (2017) indicate that this fault zone was a major structural control for hydrothermal fluid flow during the Caradoc period. The fault zone contains numerous base metal sulfide veins and stockworks, particularly concentrated along an arcuate central corridor within the caldera. These mineralized structures are interpreted to represent volcanogenic pipe-style sulfide mineralization that formed during caldera resurgence, which led to reactivation of existing structures and the opening of normal faults.[2]
Formation and evolution
According to research by Nicholas Rast, the evolution of the Beddgelert Fault during the mid-Caradocian times involved a complex sequence of geological events. The process began with the deposition of shallow-water Glanrafon Grits, followed by a period of uplift and possible erosion. Subsequently, the Pitt's Head Flow, an ignimbrite, was deposited across the area. The next stage involved faulting and uplift of the Snowdon block to the west and northwest of the fault, which resulted in partial erosion of the Pitt's Head Flow, while much of it remained on the downthrow side. This was followed by the deposition of the Flinty Flow on a subaerial surface, although much of this flow was eroded away in the valley that had formed to the southeast of the fault. Later, lahar (volcanic mud-flow) was deposited in this valley, completely filling it. Further faulting then produced a mechanical junction between the Flinty Flow and the Lahar Series. The entire area was subsequently covered by ignimbrites of the Lower Rhyolitic Tuffs. The final stage in this sequence involved additional faulting and the probable penecontemporaneous (nearly contemporary) injection of the Pitt's Head breccias as vertical bosses.[1]