The diachronism of the Wenlock Limestone

Hurst, John M.

doi:10.1111/j.1502-3931.1975.tb00935.x

Cited by 18 publications

(12 citation statements)

References 25 publications

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“…Based on brachiopod palaeocommunity data, Hurst (1975) concluded that the top of the formation in the West Midlands is the same age as on Wenlock Edge, but that it youngs from north to south. Unambiguous graptolite evidence (Bassett, 1974(Bassett, , 1976(Bassett, , 1989 shows his first conclusion to be incorrect.…”

Section: Diachronism Of the Much Wenlock Limestone Formationmentioning

confidence: 96%

“…Consideration of lithofacies distribution (Sections 2.c.2, 5) indicates that migration was from east to west, supporting the available biostratigraphical data. The basal parts of the overlying Lower Elton Beds in the West Midlands (= Birmingham Siltstone Formation of Hurst, 1975) are the time equivalent of the top of the Much Wenlock Limestone Formation on Wenlock Edge (Fig. 7).…”

Section: Diachronism Of the Much Wenlock Limestone Formationmentioning

confidence: 99%

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Carbonate depositional environments in the late Wenlock of England and Wales

Ratcliffe¹,

Thomas

1999

Geol. Mag.

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Based on outcrop studies and borehole data, six bedded lithofacies and two reef types are recognized within the Much Wenlock Limestone Formation of the English Midlands and Welsh Borderland. The lithofacies are interpreted to represent a series of carbonate shelf environments extending from below storm wave-base to well above fair weather wave-base. In common with many other shallow marine carbonate depositional systems, the principal controls on lithofacies development were hydrodynamic energy, the supply of fine clastic sediment, and patterns of colonization of the sea floor by organisms. Reef distribution was probably controlled by the nature of the substrate, water circulation, and rate of siliciclastic sedimentation. A depositional model is proposed which incorporates biostratigraphical evidence suggesting that the formation youngs to the west on the northern part of the shelf. Deposition of the Much Wenlock Limestone Formation there began in the West Midlands, where 12 m of microbial limestone were lain down in a mid-shelf setting during a local regression. The remainder of the shelf was dominated by low energy siliciclastic deposition at that time. The West Midlands then returned to somewhat deeper water, lower energy deposition, the resulting impure calcareous muds becoming diagenetically changed into the nodular limestone lithofacies. That lithofacies is commonly overlain successively by the interbedded limestone and silty mudstone lithofacies, and then the crinoidal grainstone lithofacies. This vertical lithofacies sequence is uniform over the entire northern part of the shelf, reflecting a gradual decrease in water depth. The crinoidal grainstone lithofacies was deposited as a wave-influenced carbonate sandbody which prograded from east to west. Lithofacies sequences on the southern shelf are laterally impersistent, probably due to greater tectonic instability and topographical variablity. † Author for correspondence: a.t.thomas@bham.ac.uk younging from east to west in the northern part of its outcrop area. We here combine these biostratigraphical data with lithostratigraphy and facies analysis, in order to infer the depositional environments of the Much Wenlock Limestone Formation, and their development through time. ,, siliciclastic mudstones nodular limestones interbedded limestones and silty mudstones crinoidal grainstones thick bedded oncolite-rich lithofacies NBM MWLF Figure 7. Relationship of lithostratigraphical and biostratigraphical units from Wenlock Edge and West Midlands. LQLM = Lower Quarried Limestone Member; NBM = Nodular Beds Member; UQLM = Upper Quarried Limestone Member; LEB = Lower Elton Beds. See text for details.

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Section: Diachronism Of the Much Wenlock Limestone Formationmentioning

confidence: 96%

Section: Diachronism Of the Much Wenlock Limestone Formationmentioning

confidence: 99%

Carbonate depositional environments in the late Wenlock of England and Wales

Ratcliffe¹,

Thomas

1999

Geol. Mag.

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“…Whether this sealevel rise was rapid or more gradual has been the subject of some debate (e.g. Hurst, 1975;Bassett, 1976). The nilssoni-scanicus biozones record the highest late Silurian sea-levels (Johnson, Kaljo & Rong, 1991;Johnson, 1996).…”

Section: N Early Ludlow Sea-level Risementioning

confidence: 98%

Early Silurian sea-level changes

1998

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Of the various mechanisms suggested (Donovan & Jones, 1979), that most likely to be responsible for the eustatic sealevel changes in the early Silurian is variation in the amount of water incorporated in land-based ice-sheets. This conclusion is based first on the apparent rapidity of many of the sea-level changes (see, e.g. Loydell, 1994) and secondly on the direct evidence of glacial deposits present in the Silurian sequences of Brazil (Grahn & Caputo, 1992) and elsewhere in South America (see Hambrey, 1985).Obviously, the third (and possibly fourth) order cycles being discussed here are superimposed upon the longer term second order and first order cycles (Haq, Hardenbol & Vail, 1988) linked to global tectonic processes. It is likely, for example, that the particularly high sea-levels of the Telychian and early Sheinwoodian reflect the coincidence of interglacial 'greenhouse' conditions with an increase in the volume of oceanic ridges. Abstract -Global sea-level fluctuated markedly during the early Silurian, probably as a result of the waxing and waning of ice-sheets in the South American portion of Gondwana. The highest sea-levels of the Silurian are recorded by the Telychian upper crispus-lower griestoniensis and spiralis-lower lapworthi biozones. Other highstands occurred in the early Aeronian, during the convolutus Zone (mid Aeronian), guerichi Zone and late turriculatus Zone (early Telychian), and early Sheinwoodian. Low sea-levels characterized much of the argenteus and sedgwickii zones (Aeronian), the utilis Subzone (late guerichi-early turriculatus zones, early Telychian), the late Telychian (commencing in the mid lapworthi Zone) and, after a period of apparently only small amplitude sea-level fluctuations in the late Sheinwoodian and earliest Homerian, the mid-late Homerian, in particular the early nassa Zone. Facies (and faunal) changes in the Lower Silurian do not support the P and S model of Jeppsson and others, but are consistent with the sea-level changes proposed herein. Mid Telychian marine red beds appear to have been deposited during a minor sea-level fall immediately after a period of very high sea-levels, rather than during a transgressive episode as previously suggested. Comparison of the sea-level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early Silurian deep water (graptolitic) and shallow water (shelly) sequences. Improving the precision of this correlation should be a priority for future research.*

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“…At the outset it can be stated that Hurst's (1975b) postulation of an abrupt deepening, based on community changes close to the Wenlock-Ludlow boundary in the Welsh Borderland, is no more than an unsupported assumption. Nowhere in the data on carbonate community composition (Hurst 1975a) is there evidence of such a sharp change, but, on the contrary, the relevant faunal lists (Hurst 1975a, Tables 1-3) clearly show that the majority of taxa spread across more than one community, indicating complete gradation between them.…”

Section: Community Composition and Depthmentioning

confidence: 99%

“…The recent paper in this journal by Hurst (1975b), on diachronism in the late Wenlock (Silurian) carbonate facies of the Welsh Borderland, mainly uses distributions of ecologically controlled brachiopod communities to draw an unorthodox picture of Wenlock-Ludlow chronostratigraphical correlation throughout the area. In particular, the community relationships suggest to Hurst (1975b: 306) that near the top of the Wenlock 'there was an abrupt deepening of the sea .…”

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confidence: 99%

A critique of diachronism, community distribution and correlation at the Wenlock‐Ludlow boundary

Bassett¹

1976

LET

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This paper reviews conclusions reached by J. M. Hurst in a recent paper in Lethaia regarding abrupt, widespread deepening close to the Wenlock‐Ludlow boundary as a result of possible eustatic or epeirogenically controlled transgression, and use of such an event as a means of stratigraphical correlation. Much of the benthic community evidence on which these conclusions are based is shown to be conflictory and the resulting correlations to be imprecise. Undoubted diachronism of the Much Wenlock Limestone Formation and overlying Lower Elton Beds is evident only between Dudley and Shropshire. Gradual deepening at this level in the Welsh Borderland appears to be a local reflection of tectonic movement. Wider regional evidence, from North America to the Anglo‐Baltic area, rejects the view of a synchronous eustatic or epeirogenic transgression.

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The diachronism of the Wenlock Limestone

Cited by 18 publications

References 25 publications

Carbonate depositional environments in the late Wenlock of England and Wales

Carbonate depositional environments in the late Wenlock of England and Wales

Early Silurian sea-level changes

A critique of diachronism, community distribution and correlation at the Wenlock‐Ludlow boundary

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