File talk:Smith fossils2.jpg

On the position of the “Clay over the Upper Oolite” according to modern stratigraphic concepts

This clay, though considered a major stratigraphic unit by Smith, is hard to find in modern stratigraphic tables of England. Smith placed it on top of his „Upper Oolite“ and below his „Forest Marble“ (see Strata identified by organized fossils pp. 27–30, [1]). The modern Forest Marble Formation is assigned to the Great Oolite Group (GOG) and is dated to the Bathonian, i.e. upper Middle Jurassic (see BGS Lexicon of Named Rock Units [2]). The lithologic description of the Forest Marble given by Smith fairly fits the description of the BGS (mainly shelly clayey, silty, sandy and marly lithologies, and some lignite), and the position of the outcrop of the GOG in modern geologic maps (e.g. [3]) fits the position of the outcrop of the Forest Marble and “Great Oolyte” (apparently an alternative term for the Upper Oolite) in Smith’s map quite well. Furthermore, French specimens of the clam referred to as Avicula costata overleaf are present in the collections of the MNHN (in fact assigned to two different taxa, A. costata Sowerby and A. costata Smith!), where their age ist given as “Bathonien” [4]. Hence, there is sufficient evidence for the “Clay over the Upper Oolite” being a stratum (or several strata) within the Bathonian part of the Great Oolite Group of modern nomenclature. --Gretarsson (トーク) 09:15, 22 February 2019 (UTC)

Update: Based on the “Pear Encrinus” in Figs. 1–3 it can be concluded, that the “Clay over the Upper Oolite” corresponds to the stratigraphic level that is more commonly known as the Bradford clay and today is considered to be the basal subunit of the Forest Marble (Formation), so the Bathonian age, as suggested above, is correct. Hence I have modified the file description accordingly, including a reference that proves the identity of the “Clay over the Upper Oolite” and the “Bradford clay”. --Gretarsson (トーク) 08:04, 19 June 2020 (UTC)