From spiculites to glass ramps – palaeoenvironmental reassessment of Upper Jurassic shallow‐marine sediments dominated by siliceous sponges (Fulmar Formation, UK Central North Sea)

This study adds a new variant to the small, but steadily growing portfolio of work on ‘glass ramps’: coastal/shallow‐marine seaward‐dipping strata, comprising sediments rich in spiculite derived via disaggregation of siliceous sponge skeletons. It may be the first fully documented example of a glass ramp where the spiculite component was sourced from a donor community of (now extinct) rhaxellid sponges. Since glass ramps are based on non‐actualistic facies models, their recognition is both challenging and necessary for future attempts at their classification. Historically, the stratigraphic unit chosen for this study (Upper Jurassic Fulmar Formation, UK Central Graben) has been modelled as a terrigenous sandstone accumulation, despite acceptance that thick spiculite deposits occur locally. Key cored sections through these spiculites were examined using combined sedimentological, palaeontological and petrographical techniques. Analysis indicates that periodically ‘glass ramp’ conditions evolved when the proximal parts of the ramps became extensively colonised by rhaxellid sponge meadows (with associated faunas). These meadows anchored in sand‐sized sediment; initially this was wholly terrigenous in nature, before becoming enriched in sponge spicules via post‐mortem processes. Initiation of glass ramp conditions, with attendant spiculite production, was principally triggered by reduced rates of terrigenous sediment influx, itself a function of interacting higher‐order tectonic and eustatic controls. In this scenario, glass ramps appear to have transitioned into terrigenous ramps (and vice versa), both spatially and temporally. In the Fulmar Formation, there is no evidence to suggest that post‐mortem silica liberation (via early diagenetic processes) back into the water column was a prerequisite to new generations of sponges becoming established. This is in line with much higher postulated dissolved silica concentrations (DSi) in Late Jurassic seawater compared to today. The outcome of this work is a new facies model for spiculitic Fulmar Formation deposits which may have applications elsewhere in the geological record.