Premium
Regional sedimentological variations in lower triassic fluvial conglomerates (budleigh salterton pebble beds), southwest england: Some implications for palaeogeography and basin evolution
Author(s) -
Smith Simon A.,
Edwards Richard A.
Publication year - 1991
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350260105
Subject(s) - conglomerate , geology , outcrop , pebble , fluvial , structural basin , molasse , paleocurrent , paleontology , geomorphology , alluvial fan , clastic rock , overbank , cobble , graben , bedrock , rift , foreland basin , ecology , habitat , biology
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.