Goethite ooids: growth mechanism and sandwave transport in the Lower Greensand (early Cretaceous, southern England)

Abstract

AbstractThe Woburn Sands (Lower Greensand Group, Aptian–Albian age), in Billington Crossing quarry, Leighton Buzzard, Bedfordshire, England, show, along bedding planes of tabular cross-stratification, foreset alternations of quartz grains and goethitic ooids. Field and petrographic evidence suggest that: (a) the goethite in the layers and cores is primary; (b) the ooids formed by mechanical accretion of original Fe-hydroxide gels around nuclei on the sea-floor in shallower nearshore adjacent environments; (c) goethite cores of some ooids represent an earlier stage of iron deposition within the sediment where silica adsorbed by the iron-gels segregated in inclusions; (d) short-term variations in current strength and iron supply are responsible for the different arrangements of goethite platelets (tangential to random to radial) in the ferruginous layers in connection with the degree of crystallization of the colloidal iron during the accretion stage; (e) the ooids were then swept away in deeper waters by tidal action where their segregation in the lower parts of each foreset lamina is believed to have been hydraulically controlled.