THE SEDIMENTOLOGY AND ACCRETIONARY STYLES OF AN ANCIENT GRAVEL-BED STREAM - THE BUDLEIGH SALTERTON PEBBLE BEDS (LOWER TRIASSIC), SOUTHWEST ENGLAND

Three accretionary styles are present in the fluvial conglomerates and pebbly sandstones of the Lower Triassic Budleigh Salterton Pebble Beds (BSPB) of east Devon. The lower portions of this 20-30 m sequence are dominated by extensive (> 50 m) sheets or narrow lenses of planar cross-bedded conglomerate (sets 1-3 m thick). These abruptly pass, both downstream and laterally, into horizontally bedded conglomerates, cross-bedded sandstones and muddy sandstones. This accretionary style reflects deposition from linguoid-shaped macroforms whose distal margins were bounded by slipfaces. They accreted at anabranch confluences and as bank-attached bars in relatively confined and channelised gravel-bed streams. Lateral facies changes represent macroform evolution resulting from changing discharges or abandonment of a channel reach, followed by accretion of a new macroform. The upper portions of the BSPB mainly comprise couplets of horizontally bedded conglomerate overlain by large-scale trough cross-bedded sandstone. The conglomerate shows local channel-macroform relief and lateral and downstream transitions into sets of gently dipping conglomerate. This style of accretion represents deposition in less confined and less channelised fluvial systems by relatively low-relief gravel macroforms and large sand dunes. The third style recognised in the BSPB is much more localised. It consists of wedge-like sets of large-scale trough cross-bedded conglomerate. The sets are formed by erosion and subsequent fill of scours cut into a thick sandy substrate. Techniques of lateral profile analysis and hierarchical ordering of bounding surfaces can usefully be applied to some parts of the BSPB. This is important as the approach was originally developed for the deposits of sand-bed streams. However, this approach is more difficult to apply to fluvial conglomerates because of the cryptic nature of some gravel stratification. The appearance and character of some hierarchies of bounding surfaces may differ from their equivalents in fluvial sandstones. © 1990.