Article: The Toarcian age of the upper part of the Marlstone Rock Bed of England
The 'Transition Bed' of Oxfordshire, Northamptonshire, and Leicestershire is the weathered or altered top of the Marlstone Rock Bed. In the top 0.05 m-0.3 m of the bed, the green ferrous minerals were oxidized to limonite, partly before deposition of overlying beds, partly recently in some areas. In another type of alteration, best seen at Harston, Leicestershire, much granular iron-pyrites was deposited in a highly irregular zone up to 0.08 m thick at the top of the bed. In these Midland counties the whole of the Tenuicostatum Zone, the basal zone of the Toarcian, is represented in the top 1-3 m of the Marlstone Rock Bed, the lower 3-6 m of which belongs to the Spinatum Zone. Regardless of the depth of weathering or alteration, Tiltoniceras antiquum and Dactylioceras semicelatum of the Semicelatum Subzone occur widely in the top 0.1 m of the bed, D. tenuicostatum occurs more locally at a slightly lower horizon, and lower still one D. crosbeyi is evidence for the Clevelandicum Subzone. Ammonites from the Semicelatum, Tenuicostatum, and Paltum Subzones occur in the Dorset coast Marlstone Rock Bed. North of Lincoln the top of the Bed is at about the top of the Spinatum Zone, while the Tenuicostatum Zone is divided between an overlying hard mudstone and higher grey shales. The change from the underlying ironstone/limestone facies to the overlying clays/shales-with-nodules facies took place at the top of the Spinatum Zone in Yorkshire, but at the top of the Tenuicostatum Zone in the Midland counties.