The Downtonian and Dittonian Facies of the Welsh Borderland

Allen, J. R. L.; Tarlo, L. B. Halstead

doi:10.1017/s0016756800055345

Cited by 81 publications

(41 citation statements)

References 37 publications

(32 reference statements)

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“…Acanthodians, heterostracans, osteostracans and thelodonts are the most common groups and specific assemblage 'zones' based on a common index species have been recognized (see Bleick and Janvier 1999). The fossils are virtually all transported remains, hence the details of the true natural environment that the fish inhabited (fluvial or marine) has remained problematical (Allen and Tarlo 1963;Allen et al 1968;Halstead 1985). Fossil vertebrates retrieved from Tredomen Quarry by one of us (J.P.B.)…”

Section: Possible Trace-makersmentioning

confidence: 99%

Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales

Morrissey

Braddy

Bennett³

et al. 2004

Geological Journal

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Alluvial deposits of the St Maughans Formation (Lower Old Red Sandstone, Early Devonian) of Tredomen Quarry, near Brecon, southeast Wales, have yielded the oldest known trails of swimming fish as well as body fossils of heterostracans and osteostracans, their inferred producers. Undichna unisulca comprises a single sinusoidal wave (of varying amplitude and wavelength) and is attributed to the caudal lobe or fin of a swimming heterostracan or osteostracan. Variation in the dimensions of U. unisulca trails, together with functional analysis of their inferred producers, suggests different fish sizes and swimming speeds. Undichna cf. simplicitas shows a more complex arrangement of intertwined waves and is interpreted as being produced by a combination of the caudal fin, anal spine and paired pelvic spines of an acanthodian. A new ichnotaxon, Undichna trisulcata isp. nov., consists of three isolated furrows arranged in parallel with associated pectoral fin imprints, and is interpreted as either the trail left by a 'cruising' cephalaspid, intermittently pushing off the substrate with its pectoral fins, or a trail made by a cephalaspid landing on the substrate. The presence of Undichna within these relatively proximal fluvial sediments (displaying no evidence of marine influence) is indicative of an in situ vertebrate freshwater community. Taphonomic constraints on Undichna preservation, in combination with sedimentological analysis, suggest weak bottom currents and/or rapid burial, post-trail formation. Arthropod trackways (Diplichnites gouldi Types A and B, Protichnites isp., and Palmichnium pottsae), a bilobed trail (cf. Cruziana), worm burrows (cf. Cochlichnus) and large meniscate backfilled burrows (Beaconites barretti) contribute invertebrate components to this ichnofauna. The association of Diplichnites trackways on the same surfaces as Undichna supports a semi-aquatic habit of their inferred myriapod producers.

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Section: Possible Trace-makersmentioning

confidence: 99%

Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales

Morrissey

Braddy

Bennett³

et al. 2004

Geological Journal

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“…Their sedimentology is on the whole better known than that of coeval rocks in Pembrokeshire. It was discussed recently by Dineley (1951), Wills (1950), Denison (1956), Allen (1961a, b), Ball and Dineley (1961), and Allen and Tarlo (1963). The succession resembles that of Pembrokeshire and is between 1,000 and 1,500 feet thick.…”

Section: Introductionmentioning

confidence: 93%

“…The unit of this pattern when perfectly developed comprises an intraformational conglomerate followed by sandstone in turn overlain by siltstone with a concretionary bed (Allen and Tarlo, 1963). Thus the cyclothems consist of arenaceous measures below and argillaceous measures above.…”

Section: Sedimentary Organizationmentioning

confidence: 99%

“…The same three penecontemporaneous structures are present in the two areas: sandstone pipes, several forms of slumped bedding, and animal burrows. By comparison with the modern sediments of the Colorado Delta floodplain and Colorado River (Sykes, 1937a, b;McKee, 1938McKee, , 1939, the Dittonian facies of the Welsh Borderland has been interpreted as the deposit of a floodplain complex (Allen and Tarlo, 1963). Some of the details of the comparison are included in Table 1.…”

Section: Structures Penecontemporaneous With Depositionmentioning

confidence: 99%

“…White (1946) recorded Traquairaspis symondsi (Lankester) from Caldy Island in beds mapped at an intermediate to high position in the Lower Marl Group. This form characterizes the lowest zone of the Dittonian as redefined by Allen and Tarlo (1963). Althaspis leachi (White), a late Dittonian zonal species, occurs in Swanlake Bay about the middle of the Upper Marl Group (White, 1938).…”

Section: Introductionmentioning

confidence: 99%

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Depositional Features of Dittonian Rocks: Pembrokeshire compared with the Welsh Borderland

Allen

1963

Geol. Mag.

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The Dittonian Stage in Pembrokeshire comprises the Lower Marl Group (higher beds), the Sandstone-and-Marl Group, and the Upper Marl Group (at least lower half). These formations cannot be distinguished lithologically from Dittonian strata of the Welsh Borderland 100 miles away along the depositional strike.Cyclothems of intraformational conglomerate (scoured surface below) -> sandstone ->• siltstone with concretions recur vertically in each area. The conglomerates in both districts consist of intraformational siltstone and concretionary debris. Flat-bedding, primary current lineation, planar cross-bedding, trough crossbedding, and ripple-drift bedding are common to the sandstones. Suncracked siltstones abound in both Pembrokeshire and the Welsh Borderland. Slumped bedding, sandstone pipes, and animal burrows are the structures penecontemporaneous with deposition in each area.The Dittonian strata of the Welsh Borderland are shown to be probably floodplain deposits, by reason of their close similarity to the modern sediments of the Colorado Delta floodplain and Colorado River. The occurrence of an identical facies in Pembrokeshire suggests the even wider extent of this floodplain in early Lower Old Red Sandstone times, when the sea lay far to the south of both areas.

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Fining‐upwards cycles in alluvial successions

Allen¹

1965

Geological Journal

144

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Fining-upwards cycles on a scale of thickness from a few metres to a few tens of metres, and consisting of a coarse grade member (erosive base) overlain by a fine grade member, are the hall-mark of a distinct non-marine sedimentary facies widely distributed in space and time. Using data on modern alluvial sediments, it is shown that the interpretation of the cycles in terms of an alluvial environment is probably correct. The cyclicity itself cannot yet be accounted for, although three principal factors are likely to have been involved.

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The Downtonian and Dittonian Facies of the Welsh Borderland

Cited by 81 publications

References 37 publications

Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales

Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales

Depositional Features of Dittonian Rocks: Pembrokeshire compared with the Welsh Borderland

Fining‐upwards cycles in alluvial successions

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