Palaeobotany and petrology of a Middle Jurassic ironstone bed at Wrack Hills, North Yorkshire

SUMMARY The palaeobotany and petrology are outlined of a recently discovered Middle Jurassic (Saltwick Formation) ironstone bed at Wrack Hills, a locality near Runswick Bay, North Yorkshire. The plant fossils are remarkable for their excellent preservation in three dimensions. Similar material was much collected in the last century, including specimens from Runswick Bay, but after 1910 the localities were thought to have been entirely lost. The species diversity at the new or perhaps newly available old locality considerably exceeds that obtained by the old collectors, including about five new species out of the total of 31 collected. Androstrobus cones preserved in three dimensions and with well-preserved pollen in situ are reported; new material of Williamsonia gigas is discussed and a revised reconstruction of its mature ‘flower’ proposed; and three-dimensionally preserved leaves of Solenites are recorded. These prove to be circular in transverse sectional outline rather than elliptical as previously reconstructed. There is an intimate association of clay minerals with the plant remains in such siderite-rich ironstone beds. The clays (kaolinite and dickite) were evidently deposited within voids in the organic material during diagenesis and the temperatures required for their recrystallisation indicate former burial of the sediments to a depth of some 2 km. The three-dimensional preservation of the plant organs despite this burial indicates that the ironstone became lithified early in diagenesis.