The depositional environment of some Middle Jurassic Ironshot Limestones of southern England.

Condensed beds in the Jurassic of northen Europe are usually characterized by limonite eoliths in a matrix of limestone, marl, or clay. Several of these beds have been studied in this country, particularly those in S.W, England; namely the Inferior Oolite and the Twinhoe Ironshot, (Upper Bathonian). The aim of this investigation was to determine the depositional environment of this facies. It is concluded that these ironshot limestones were deposited in very shallow water conditions, in areas of reduced sedimentation and reworking. In the case of the Inferior Oolite, areas of extreme condensation are separated by areas of less condensed deposits, suggesting the existence of several topographic highs on the sea floor on which the thinnest beds were laid down. Prom borehole data the Inferior Oolite of Dorset and Somerset has been found to lie on a wide shelf, on the western side of a large basin covering southern England. The Twinhoe Beds are considered to have been deposited in a transitional area, between the Fullers Earth Clay facies and the Great Oolite Limestone shelf facies; the Twinhoe Ironshot being formed during a period of reduced sedimentation. Both the Inferior Oolite and the Twinhoe Ironshot contain limonite ooliths, and these have been studied in order to decide whether the- ooliths were once chamosite. This was done means of x-ray diffraction and by electron microprobe analysis of individual ooliths and concretions. The results suggest that the limonite is a primary precipitate, which in the Middle Inferior Oolite has been found to pass eastwards into chamosite oolite, and finally into glauconitic limestones. The origin of the large limonitic conretions frequently found in condensed beds was also studied. It is considered that these are large oncolites on which limonite was precipitated, possibly due to selective adsorption by the algae themselves.