Regional sedimentological variations in lower triassic fluvial conglomerates (budleigh salterton pebble beds), southwest england: Some implications for palaeogeography and basin evolution

Abstract

The Budleigh Salterton Pebble Beds (BSPD) are a 20-30 m thick formation of conglomerates and subordinate sandstones which crop out along the western margin of the Wessex Basin. The formation has previously been interpreted as representing a major conduit for southerly-derived (Armorican) detritus and as signalling the start of early Triassic rifting. In this paper the role of the BSPB in the evolution of the Wessex Basin is reassessed.In the south of the outcrop, the lower portions of the BSPB are dominated by extensive (> 50m) sheets or narrow lenses of planar cross-bedded conglomerate (sets 1-3m thick). This reflects deposition from linguoid-shaped bars whose downstream margins were bounded by slipfaces. These accreted at anabranch confluences and as bank-attached bars in relatively confined and channelized gravel-bed streams. Most of the rest of the BSPB consists of couplets of horizontally-bedded conglomerate overlain by large-scale trough cross-bedded sandstone. This style of accretion represents deposition from relatively low-relief gravel bars and large sand dunes in substantial but poorly confined channels. Towards the top of the BSPB in the central parts of the outcrop, there is a change to thinner-bedded units with more lenticular and ribbon-like geometries. This represents deposition in smaller and more flashy streams.The BSPB was deposited either on a braidplain or by an antecedent 'wet' alluvial fan. In both settings, the BSPB streams were exotic, draining basins to the south of the Wessex Basin. This implies that the BSPB did not necessarily develop in response to differential subsidence and rifting within the Wessex Basin.

Rifts, rivers and climate recovery: A new model for the Triassic of England

“…In summary then, it would appear that in the Wessex Basin the coarse-grained deposits of the Budleigh Salterton Pebble Beds and Otter Sandstone Formation formed during episodes of relatively low fault activity, supporting the conclusions of Ruffell and Shelton (1999). It might indeed be expected that major long-range fluvial drainage systems flowing across the structural grain of extensional basins required relative tectonic quiescence to develop (Smith and Edwards, 1991). In the Wessex Basin there is evidence for two particularly marked phases of active extension and subsidence in the Triassic, during the later parts of the Olenekian (Nettlecombe Formation) and in the Carnian (Dorset Halite).…”

“…The magnetostratigraphy and correlation of conglomerate units at the base of the Sherwood Sandstone (Budleigh Salterton Pebble Beds, Kidderminster Formation, Cannock Chase Formation, Chester Pebble Beds) is far from complete (Hounslow et al, 2017) but these units are united by a strong similarity in quartzite-dominated clast composition and sedimentology when traced from the Wessex Basin in the south, through the Worcester Graben and into the Midlands: observations which lead to the concept that they were deposited in a large, connected 'Budleighensis' river system flowing northward from the Armorican Massif (Wills, 1951). In the Wessex Basin and in the Midlands, the planar, trough and horizontally bedded conglomerates are interpreted as the product of gravel bars within substantial but poorly confined braided channels (Smith and Edwards, 1991;Steel and Thompson, 1983). The conglomerates are typically only a few tens of metres thick and are readily identified on geophysical logs by their low gamma-ray values and low sonic travel times suggesting extensive cementation (Figure 8).…”

Evolving stratigraphy of a Middle Triassic fluvial‐dominated sheet sandstone: The Otter Sandstone Formation of the Wessex Basin (UK)

“…The full extent of the aeolian sandstones of the West Down Member has not been mapped, but they appear to be of relatively limited distribution. Evidence from quarries north of the coastal sections (to the east of Exeter) indicates that the aeolian sandstones at the base of the Otter Sandstone Formation are partially or entirely eroded by overlying gravelly fluvial sandstones (Edwards et al, ; Smith & Edwards, ). At the coast, the Monk's Wall Borehole (Figure ) indicates a relatively rapid eastward transition into mudstone.…”

Back‐flow ripples in troughs downstream of unit bars: Formation, preservation and value for interpreting flow conditions

“…Back‐flow ripples have been observed in deposits formed in a range of modern and ancient environments, including fluvial (e.g. Basumallick, ; Boersma, ; Boersma et al ., ; Tillman & Ellis, ; Collinson, ; Gradzinski, ; Karcz, ; Van Beek & Koster, ; Livera & Leeder, ; Kirk, ; Hunter, ; Casshyap & Kumar, ; Smith & Edwards, ; Ashworth et al ., ; Reesink & Bridge, ), fluvioglacial (e.g. Johansson, ; Helm, ; Saunderson & Jopling, ), glaciolacustrine (e.g.…”