Lithostratigraphy: Top of Upper Bringewood Formation and ‘Aymestry Limestone’ facies. The section is the stratotype for Lower Leintwardine Formation; Lower and Upper Leintwardine formations and Lower Whitcliffe Formation. Divisions across the boundary interval A to H3 are from Holland et al. (1963).
Locality: Disused quarries north of Sunnydingle Cottage, Mortimer Forest and exposures on the N side of the forestry track running SE (grid refs: SO 4950 7255 – 4974 7244). Also known as section C of White and Lawson (1978) and Sunnyhill section (E Ludlow) of Cherns (1988, faunal log figure 5) and Siveter (2000).
Lithology and fossil constituents
The section exposes 40-50 m of succession in fairly continuous outcrops. The GSSP level is within the top of the ‘Aymestry Limestone’ nodular limestone facies, at the ‘C’ -band which is the lower of two very thin shale bands (Holland et al., 1963; Cherns, 1988). The upper shale band, and higher, thicker clay are bentonitic (beds SHQ3 and SHQ4) (Holland et al., 1963, ‘E’ and ‘G’). The GSSP level for the base of the Ludfordian Stage is at the base of the Lower Leintwardine Formation.
The Upper Bringewood limestones (~4.7 m thick in the section) comprise nodular, silty wackestones and mudstones. Fossils are fairly uncommon but typically comprise large brachiopods (e.g. Kirkidium knightii, Strophonella euglypha, Amphistrophia funiculata, Gypidula lata, Atrypa reticularis), tabulate and rugose corals, and large gastropods (Poleumita globosa).
Above the GSSP level, limestone facies continue for ~2.5 m before a shift into more thinly bedded calcareous siltstones. The Lower Leintwardine Fm succession is ~28 m thick with most of the common fossils being long-ranging continuing through from the Upper Bringewood Fm. In the limestone facies of the Lower Leintwardine Formation, fossils are fairly sparse but include A. reticularis and Isorthis orbicularis. Diversity and abundance of macrofossils increase upwards into the siltstones, where the brachiopods I. orbicularis, Dayia navicula, Sphaerirhynchia wilsoni, Microsphaeridiorhynchus nucula and Shagamella ludloviensis are all common (Cherns 1988). The thinly bedded calcareous siltstone facies includes fairly numerous, thin laminated siltstone sheets, shelly limestone beds and lenses. In many beds, a mottled texture indicates intensive bioturbation. Faunas are diverse on bedding surfaces and in shell lenses. D. navicula and I. orbicularis are the dominant species in many of the typically brachiopod-dominated assemblages. Finely laminated siltstone beds and lenses become more common and thicker in the middle to upper parts of the Lower Leintwardine Fm. Interpreted as distal storm sheets, the beds fine up from a coarser, commonly shelly basal layer, and a few narrow burrows often disturb the mm-scale bedding fabric. At Sunnyhill the brachiopod Shaleria ornatella (which in the shelf inliers has its acme of abundance in the Upper Leintwardine Formation) becomes common through the upper part (~8 m) of the Lower Leintwardine Formation (Cherns, 1988).
The Upper Leintwardine Formation at its stratotype at Whitcliffe (grid ref. SO 5071 7428), is a calcareous siltstone facies similar to the Lower Leintwardine Formation, but is defined faunally by the appearance of the brachiopod Aegiria grayi, often associated with trilobites Calymene neointermedia and Encrinurus stubblefieldi (Holland et al., 1963). These trilobites, and the ostracode Neobeyrichia lauensis, are common only in the Upper Leintwardine Formation, but this ostracod is not recorded from the Whitcliffe stratotype. At Sunnyhill Quarry, a N. lauensis–Neobeyrichia scissa ostracod assemblage is recorded at the level where the trilobites also appear, and A. grayi is common in the Upper Leintwardine Fm.
Biostratigraphy
The graptolite Saetograptus leintwardinensis was recorded from bed D within the basal limestones of the Lower Leintwardine Fm (Cherns, 1988), becoming common higher in the formation. Below the GSSP level graptolites are absent, but in the Lower Bringewood Fm at other sections graptolites indicative of the P. tumescens/S. incipiens Zone occur (Melchin et al., 2012). Therefore the GSSP level approximates the base of the S. leintwardinensis Zone, although the GSSP level may be within the lower part of this zone.
The Upper Bringewood Fm at SunnyHill contains the thelodont Paralogogania kaarmisensis along with the conodont Kockelella variabilis and rarer Coryssognathus dubius. At 0.85- 1m above the GSSP level Phlebolepis elegans occurs with Thelodus sp. (Marss & Miller, 2004). K. variabilis and P. elegans together allow correlation to the P. elegans Biozone of the Baltic in the mid parts of the Paadla regional stage.
The base of the Leoniella vilis acritarch Biozone, in the Ludlow area occurs 3.0- 4.5 m above the base of the Upper Bringewood Formation and ranges across the GSSP level. The stratigraphically younger Triangulina sanpetrensis acritarch Biozone begins 15.5 m above the base of the Lower Leintwardine Fm and continues through the Upper Leintwardine (Mullins, 2004; Richards & Mullins 2003). There are also other changes in palynology across the formational contact (Lawson & White, 1989).
Sequence stratigraphy
At Ludlow the transition into the Lower Leintwardine Formation is conformable, followed by a facies shift from carbonates to calcareous siliciclastics. However, limestone conglomerates indicate a widespread, short lived episode of non-deposition across inshore shelf areas at the base of the Lower Leintwardine Formation, while at the shelf margins local, eroded hardground horizon marks the sharp, disconformable boundary between the ‘Aymestry Limestone’ and overlying thin shaly siltstones of the Lower Leintwardine Formation (Cherns 1988). Limited transgression occurs across the shelf at the base of the Lower Leintwardine Formation, which represents a transgressive surface/sequence boundary. Shelf margin instability led to westward slumping and valley incision later in the S. leintwardinensis Zone (Whitaker, 1994).
Isotope stratigraphy
δ13Ccarb values from the Upper Bringewood Fm range from -1.07‰ to 0.37‰ and are generally lower than those of the Lower Leintwardine Formation (-0.50‰ to 0.76‰; Cherns, 2011). This minor positive shift in isotope values begins some 0.5 m below the top of the Upper Bringewood Formation. Longer duration δ13Ccarb datasets need to be collected from the type Ludlow area to make comparisons to the better studied Baltic Basin.
Other stratigraphy
A preliminary magnetostratigraphy has been obtained by Hounslow and others from the Sunny Hill section the results of which will be presented later. At Sunnyhill Quarry four bentonites occur (SHQ1-4) above and below the GSSP level. Bentonites SHQ2 is the thickest (20 cm) and occurs 3.5 m below the top of the Upper Bringewood Formation and consists of four distinctive rusty-orange bands. Zircons extracted from SHQ2 have been fission-track dated at 407 ± 14 Ma (Bassett, 1984), but need re-evaluating using modern techniques. Sufficient zircons in SHQ1, SHQ 2 and SHQ4 indicates that all three bentonites may be suitable for dating (Cherns, 2011).
Source: Mostly derived directly from Cherns (2011) with additional diagrams from Melchin et al. (2012)
(MWH)
Bibliography
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