Ferruginous oncoids up to 30 cm in diameter are common in the upper Aalenian and lower Bajocian (Middle Jurassic) of southern England (Dorset), in the Inferior Oolite Group. Similarly, ferruginous oncoids are common in the upper Bajocian (Middle Jurassic) in the Oolithe Ferrugineuse de Bayeux Formation in Normandy, France. Unlike similar carbonate structures, the ferruginous oncoids grew by accumulating laminae on one side and then overturning to grow laminae on the opposite side. It has previously been suggested that these iron-oxide laminae formed by microbial action on the undersides of the oncoids, in gloomy, oxic conditions. Numerous sclerobionts encrusted the layers, dominated by serpulid worms, but also including cyclostome bryozoans, foraminiferans, and sponges. These encrusters are common in cryptic environments, supporting the hypothesis of laminae accumulation on the undersides of the snuff-boxes. We present here new observations of similar ferruginous and encrusted laminar accretions inside associated open Thalassinoides burrow systems, also found within the Inferior Oolite of southern England. These laminae are found accumulated near the ceilings of these burrows, hanging down as pendants into what were open, soft-sediment cavities. These laminae were clearly forming in dark cavities. The cryptic growth process of the ferruginous snuff-boxes is again supported by this new evidence. The oncoids were initiated on nuclei of shells (primarily bivalve) and carbonate hardground fragments, namely limestone. Both substrate types show evidence that they were heavily bored by bivalves and polydroid worms, prior to growth of the ferruginous layers, indicating that they had considerable residence time on the seafloor. The cortices of the snuff-boxes often have numerous discontinuous ferruginous laminae, many of which incorporate laminae heavily encrusted by serpulids. These ferruginous oncoids grew in oxic conditions of low sedimentation with periodic overturning by currents or storm activity, which developed their characteristic rounded discoidal shapes. With an influx or precipitation of iron, the ferruginous oncoids, iron material in burrows, and the presence of iron ooids preserved in the system appear to share a similar composition and paleoenvironment. They are a product of the condensed nature of the Bajocian sections in this region.


Wilson, Mark




Geology | Paleontology | Sedimentology


paleontology, Jurassic, England, snuff-box, oncoid, ferruginous oncoid

Publication Date


Degree Granted

Bachelor of Arts

Document Type

Senior Independent Study Thesis



© Copyright 2017 Cassidy D. Jester