Non-marine oolitic ironstones in the Lower Cretaceous Wealden sediments of southeast England

Abstract

AbstractOolitic ironstones are described from the early Cretaceous non-marine Wealden sediments of southeast England. The recognition of non-marine oolitic ironstones in the geological record is rare and, therefore, warrants further study. The oolitic ironstones described take two forms, named here type-I and type-2 ironstones.Type-1 ironstones contain pisoids and ooids of berthierine together with sandstone fragments and detrital quartz grains. The pisoids (up to 0.5 cm in size) vary from subspherical to highly irregular. The smaller ooids (up to 1 mm in size) are generally ellipsoidal but strongly asymmetrical forms are also present. The form of these pisoids and ooids suggest that mechanical accretion was not the dominant mechanism controlling their formation. It is proposed that this ironstone type formed from the reworking and redeposition of local soil material.Type-2 ironstones, of which only one unequivocal example has been studied, is composed of iron oxide ooids set in a detrital matrix. The ooids are most commonly regularly ellipsoidal and exhibit a decrease in iron at their centres. It is proposed here that the ooids suffered post-depositional iron depletion at their centres, in a similar fashion to that proposed for the recent Lake Chad oolites. There is no unequivocal evidence as to the origin of the ooids.This study is important in that it shows that different ironstones can be formed by different processes essentially within the same environment. Comparison of non-marine oolitic ironstones with the better-developed marine examples should prove a valuable exercise.