ORIGIN AND PALEOECOLOGY OF FERRUGINOUS ONCOIDS ("SNUFF-BOXES") FROM THE MIDDLE JURASSIC (BAJOCIAN) OF SOUTHERN ENGLAND

Ferruginous oncoids up to 30 cm in diameter are common in the Bajocian (Middle Jurassic) of southern England (Dorset). Unlike similar carbonate structures, they 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. These laminae are found attached to the ceilings of these burrows, hanging down as pendants into what were open 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. Both of these substrate types were heavily bored by bivalves and polydorid 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 are heavily encrusted. These ferruginous oncoids grew in conditions of low sedimentation with periodic overturning by currents or animal activity, which developed their characteristic rounded discoidal shapes. They are a product of the condensed nature of the Bajocian sections in this region.