Article: Observations on the palaeoecology and ammonite sequence of the Frodingham Ironstone (Lower Jurassic)

The Frodingham Ironstone is shown from the ammonite evidence to range from the top of the semicostatum Zone to the top of the obtusum Zone of the Lower Lias. The rich invertebrate fauna, which is considered from a palaeoecological viewpoint, includes traces of sediment-burrowing and shell-boring organisms which are described for the first time. Certain lamellibranchs in a pyrite-bearing bed are shown to be probably dwarfed. Data for such features as shell size, orientation, disarticulation, and wear are given and relationships bstween the shells and their sedimentary matrix described. The faunal characteristics of the different types of ironstone are discussed and an attempt is made, using both the petrological and palaeontological evidence, to outline the salient features of the environments of deposition. It is concluded that the Frodingham Ironstone was deposited on a marine shoal area isolated from the land and that it records a series of alternating episodes during which the degree of water agitation and the Eh varied considerably.