A fault-controlled depositional model for the Aberystwyth Grits turbidite system

Abstract

The Aberystwyth Grits Group was the first in a series of southerly-supplied sandstone-rich turbidite systems that invaded the Welsh Basin during the Telychian Stage (late Llandovery; Silurian). Structural and stratigraphical evidence indicates that deposition of the sand-dominated parts of the system was largely restricted to the east by a zone of syndepositional faulting, now marked by a major vergence divide. Downfaulting and reverse drag along the fault hangingwall created a bathymetric low, which focused deposition of high-density, sand-carrying turbidite flows and flow components, allowing substantial thicknesses of sediment to accumulate. In contrast, low-density, mud-rich turbidites were deposited over a wider area, enhancing the topographic relief generated by uplift above the fault footwall, to form a feature which restricted the spread of the coeval easterly-supplied turbidites of the Devil's Bridge Formation.Peer reviewe

Palaeotopography of the northern portion of the Telychian (Silurian) turbidite basin in central Wales

“…The evident diversion of flow to the south along a line coincident with the footwall crest of the northerly extension of the Bronnant Fault towards the Machynlleth area is consistent with and extends the BGS conclusions to the south (see Wilson et al 1992). It seems likely that synsedimentary movement on this fault southwest of Corris is responsible for the localized west-facing slumping in the Devil's Bridge Formation seen in Cwm Cadian (Figure 1; see also Pratt et al 1995, p. 65).…”

“…Both facies are progradationally transported from the southwest along the basin axis and are relatively 'distal' in the Aberystwyth area (Talling et al 2004). A detailed depositional model for the Aberystwyth Grits, invoking their eastward confinement by an active Bronnant Fault, was erected by Wilson et al (1992) and is accepted herein (see Figure 3) with two provisos. First, the interpretation of the Borth Mudstones along the line of the fault as levee deposits equivalent to the Grits is as yet unsupported by palaeocurrent evidence: moreover these mudstones are similar to the 'background' facies association far away from the fault and have yet to be shown to contain features typical of levee facies (e.g.…”

“…The framework of the southern fill and eastern margin of the turbidite troughs in the Silurian is firmly established, particularly during deposition of the Llandovery Series (Wood and Smith 1959;Kelling and Woollands 1969;Wilson et al 1992). Current understanding is summarized in detail for the important mid-Wales transect (1:50 000 sheets 178 and 179) between Aberystwyth and Rhayader ( Figure 1) by Davies et al (1997) who give full supporting references and facies descriptions.…”

Process regime change on a Silurian siliciclastic shelf: controlling influences on deposition of the Gray Sandstone Formation, Pembrokeshire, UK

“…This transgression was punctuated by an episode of local tectonic uplift during latest Aeronian through to earliest Telychian times (Walmsley and Bassett 1976;Clayton 1994). This event correlates with a switch from lateral to axial sediment influx into the basin (Wilson et al 1992;Woodcock et al 1996;Davies et al 1997), being possibly related to the initial docking of Eastern Avalonia with Laurentia (Soper and Woodcock 1990). Shelf exposure occurred in southwest Pembrokeshire at this time (Hillier 2002), with a lowsinuosity fluvial network feeding lowstand submarine fans in the northerly Welsh Basin.…”

“…It is likely that growth within the Skomer Sub-basin was synchronous during latest Llandovery times, extending well into the Wenlock. Tectonic uplift and exposure of the southern limb of the Marloes-Lindsway Anticline occurred during latest Aeronian through to earliest Telychian times (Hillier 2002), and correlates with a switch from lateral to axial sediment influx into the basin (Wilson et al 1992;Woodcock et al 1996;Smith 2004). Such tectonism was probably related to the initial docking of Eastern Avalonia with Laurentia (Soper and Woodcock 1990).…”