The Millstone Grit of Gower

Dix, Emily

doi:10.1017/s0016756800098617

Cited by 7 publications

(1 citation statement)

References 10 publications

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“…The Basal Grits are locally unconformable on underlying strata and may represent a renewed rifting event (George 1970;Powell 1989). In the basin depocentre, the Basal Grits are absent and the lower Millstone Grit comprises a shale-dominated succession conformable with the underlying Dinantian limestones (Dix 1931;George 1970). The overlying Middle Shales (George and Kelling 1982) consist of deep-water, basinal shales, which grade upward into a series of northerly derived, shallow-water deltaic cyclothems.…”

Section: Geological Setting Stratigraphy and Cyclothemsmentioning

confidence: 99%

Evidence for relative sea‐level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Hampson¹

1998

Geol. J.

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Detailed sedimentological facies analysis of the¯uvio-deltaic Millstone Grit succession (Upper Carboniferous) of South Wales reveals that a number of cyclothems bounded by marine¯ooding surfaces (marine bands) in these strata exhibit facies architectures that represent erosion, non-deposition and/or deposition during periods of falling relative sea-level. A major¯uvial complex below the Subcrenatum Marine Band, the Farewell Rock, lies within an incised valley, with a regional unconformity (sequence boundary) at its base. This unconformity is marked by deep erosional relief, an identi®able time gap and an angular discordance in bedding. The Cumbriense Quartzite, a correlative unit containing several mature palaeosols, records a depositional hiatus on a terrace-like inter¯uve that lay beyond the margins of the coeval Farewell Rock valley. Cyclothems in the underlying Middle Shales contain additional surfaces and units of subtler character. Beneath the Cancellatum Marine Band, a thin (15 cm), calcareous siltstone bed (the`Amroth Granule Bed') that directly overlies prodelta shales contains reworked bioclasts, bored phosphorite clasts and quartz granules. Quartz granules in this bed are interpreted to represent relict lowstand,¯uviatile? deposits, which were reworked during later transgression. Three further cyclothems contain sharp-based, storm-reworked mouth bars that record an abrupt lowering of wave base, most probably during periods of falling relative sea-level. One of these cyclothems also contains a distributary channel complex, which records an abrupt in¯ux of coarse-grained sediment of ambiguous origin. The signi®cance of these subtle surfaces and units for intracyclothem stratigraphy has rarely been considered; their prevalence in the Middle Shales provides evidence for numerous, high-frequency relative sea-level falls, which were previously unrecognized. These relative sea-level falls appear to alternate coherently with the widespread sea-level rises recorded by the marine bands, suggesting that glacio-eustasy is their most likely driving mechanism. The notion of glacio-eustatic sea-level falls is supported by the correlation of the basal Farewell Rock sequence boundary with sequence boundaries documented in adjacent basins. The angular unconformity and a change in sediment provenance at the base of the Farewell Rock, however, suggest an additional tectonic control on stratigraphic architecture here, namely a short-lived phase of rifting or inversion prior to widespread¯uvial incision.

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Section: Geological Setting Stratigraphy and Cyclothemsmentioning

confidence: 99%

Evidence for relative sea‐level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Hampson¹

1998

Geol. J.

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show abstract

Evidence for relative sea-level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Hampson

1998

Geol. J.

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Detailed sedimentological facies analysis of the fluvio‐deltaic Millstone Grit succession (Upper Carboniferous) of South Wales reveals that a number of cyclothems bounded by marine flooding surfaces (marine bands) in these strata exhibit facies architectures that represent erosion, non‐deposition and/or deposition during periods of falling relative sea‐level. A major fluvial complex below the Subcrenatum Marine Band, the Farewell Rock, lies within an incised valley, with a regional unconformity (sequence boundary) at its base. This unconformity is marked by deep erosional relief, an identifiable time gap and an angular discordance in bedding. The Cumbriense Quartzite, a correlative unit containing several mature palaeosols, records a depositional hiatus on a terrace‐like interfluve that lay beyond the margins of the coeval Farewell Rock valley. Cyclothems in the underlying Middle Shales contain additional surfaces and units of subtler character. Beneath the Cancellatum Marine Band, a thin (15 cm), calcareous siltstone bed (the ‘Amroth Granule Bed’) that directly overlies prodelta shales contains reworked bioclasts, bored phosphorite clasts and quartz granules. Quartz granules in this bed are interpreted to represent relict lowstand, fluviatile? deposits, which were reworked during later transgression. Three further cyclothems contain sharp‐based, storm‐reworked mouth bars that record an abrupt lowering of wave base, most probably during periods of falling relative sea‐level. One of these cyclothems also contains a distributary channel complex, which records an abrupt influx of coarse‐grained sediment of ambiguous origin. The significance of these subtle surfaces and units for intracyclothem stratigraphy has rarely been considered; their prevalence in the Middle Shales provides evidence for numerous, high‐frequency relative sea‐level falls, which were previously unrecognized. These relative sea‐level falls appear to alternate coherently with the widespread sea‐level rises recorded by the marine bands, suggesting that glacio‐eustasy is their most likely driving mechanism. The notion of glacio‐eustatic sea‐level falls is supported by the correlation of the basal Farewell Rock sequence boundary with sequence boundaries documented in adjacent basins. The angular unconformity and a change in sediment provenance at the base of the Farewell Rock, however, suggest an additional tectonic control on stratigraphic architecture here, namely a short‐lived phase of rifting or inversion prior to widespread fluvial incision. Copyright © 1998 John Wiley & Sons, Ltd.

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The namurian succession between Tenby and Waterwynch, Pembrokeshire

Jones

Eagar²

1969

Geological Journal

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With a report on some bivalve faunas by R. M. c. EAGAR Namurian cliff sections are described in detail. A minimum thickness of about 76 m is ascribed to Namurian A and 198 m and 104 m respectively to Namurian B and C. Faunal marker bands enable the Chokierian. Marsdenian and Yeadonian Stages to be recognised and the detailed structure to be worked out and this confirms the structural interpretation of Dixon.

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The Millstone Grit of Gower

Cited by 7 publications

References 10 publications

Evidence for relative sea‐level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Evidence for relative sea‐level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Evidence for relative sea-level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

The namurian succession between Tenby and Waterwynch, Pembrokeshire

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