Carbonate depositional environments in the late Wenlock of England and Wales

Abstract

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.

Process regime change on a Silurian siliciclastic shelf: controlling influences on deposition of the Gray Sandstone Formation, Pembrokeshire, UK

“…It is probable that this regime resulted from its open-coast location, proximal to deep water, promoting wave/storm dominance (Porębski and Steel 2006). Ratcliffe and Thomas (1999) similarly interpreted Homerian age tectonism across the southern shelf in their study of carbonate depositional environments across the Midland Platform (Figure 3a). The early Ludlow rise in eustatic sea-level (the Ludlow 'deepening') is not observed in south Pembrokeshire, the area being swamped with detritus comprising the continental deposits of the embryonic Old Red Sandstone Red Cliff Formation.…”

“…Brachiopod faunas from high in the Coralliferous Formation indicate a lower Sheinwoodian riccartonensis Biozone age (Siveter 2000). To the east, carbonate-rich mudrocks and bioherms accumulated across the Midland Platform (Bassett et al 1992;Ratcliffe and Thomas 1999;Siveter 2000;Cherns et al 2006).…”

Harnessing stratigraphic bias at the section scale: conodont diversity in the Homerian (Silurian) of the Midland Platform, England

“…Lithofacies in the Wenlock of the study areas have been previously characterized by Butler (), Phipps & Reeve (), Shergold & Bassett (), Oliver (), Ratcliffe & Thomas (), Ray & Thomas (), and Ray et al . (, ).…”

Aphanitic buildup from the onset of the Mulde Event (Homerian, middle Silurian) at Whitman’s Hill, Herefordshire, UK: ultrastructural insights into proposed microbial fabrics; pp. 287–292

“…This is expressed as decreased species richness and colony sizes of skeletal reef-building organisms (Klaamann & Einasto 1977;Kaljo et al 1995;Calner et al 2000;Kershaw & Da Silva 2013) and an aphanitic structure dominated by micrite, e.g. in the Muksha thrombolitic buildups in Podolia (Jarochowska et al 2014a) and the LQLM reefs (Penn 1971;Ratcliffe & Thomas 1999). The combination of an aphanitic, mottled, matrix-supported structure with scattered peloidal fabrics and with the overall increase in the proportion of microbial carbonates in the form of oncolites and floating porostromate problematica in the carbonate buildup at Whitman's Hill may suggest that the bulk of the buildup framework was leiolitic (Schmid 1996).…”

“…It is associated with the first peak of a double-peaked positive stable carbon isotope excursion (CIE) recognized in lowto mid-latitudes across different palaeocontinents (summarized in Munnecke et al 2010). The event is associated with faunal turnover, the proliferation of microbial carbonates (e.g., Kõrts 1991;Calner 2005;Jarochowska et al 2014a) and the occurrence of lowdiversity, aphanitic reefs (e.g., Kaljo et al 1995;Ratcliffe & Thomas 1999;Calner et al 2000;Kershaw & Da Silva 2013). Among the explanations for the observed changes in reef composition are reduced grazing, increased nutrient input or elevated seawater alkalinity, each of which is interpreted to favour microbiallymediated carbonate precipitation (e.g., Munnecke et al 2010;Jarochowska et al 2014a).…”