Firmgrounds – key surfaces in the recognition of parasequences in the Aptian Lower Greensand Group, Isle of Wight (southern England)

Abstract

Aptian Lower Greensand Group exposures in the cliffs of the Isle of Wight (southern England) display a consistent coarsening‐up cyclicity on the scale of centimetres to tens of metres that reflects the bed, bed‐set, parasequence, parasequence set and sequence hierarchy. These coarsening‐up cycles are most commonly recognized at the scale of parasequences (20 cm to 10 m thick), genetically related groups of which form parasequence sets. Both parasequences and parasequence sets contain the succession of biofacies that culminate in firmground development. Numerous episodes of erosion, deposition and colonization are recorded, reflecting multiple erosion/bypass events. The increase in mean grain‐size through each cycle is reflected by changes in physical sedimentary structures; ichnofauna or bioturbational fabric; fossil fauna and diagenesis. Interbedded mudstones, siltstones and sandstones in the lower beds of each cycle display a variety of structures ranging from low‐angle, hummocky, or tabular cross‐strata, sandstone‐filled erosional gutters and planar lamination. The cleaner sandstones found in the upper parts to each cycle are often completely bioturbated with only rare stratification and pebble/plant debris‐filled scours preserved. Bioturbational fabrics change upward through each cycle from small, subhorizontal, mud‐ or sandstone‐filled burrows to large, branching, clay‐filled or cemented burrow systems. The top surface of each cycle is marked by a fossil epifauna indicative of firm to hard substrate conditions. Concentrations of bivalves, brachiopods, bryozoa, crinoids and corals are preferentially cemented by iron oxide, carbonate or phosphate. Such cements were early and thus utilized by firm or hard substrate dwellers. This fossiliferous, cemented sandstone is overlain by a flooding surface marked by the mudstone and silt‐rich sandstones at the base of the next cycle. Together, the fauna and ichnofauna in each cycle represent the gradual development of firm substrate conditions, culminating in the diverse firmground fauna preserved at the top of each cycle. The fauna and changing substrate conditions reflect the hiatuses developed during successive episodes of marine flooding. High species diversity is matched by complex patterns of taphonomic feedback in the mature firmground faunas that mark major flooding surfaces. Increasing faunal maturity allows recognition of a hierarchy of hiatuses. This hierarchy is analogous to the parasequence–parasequence set division. The stratigraphic condensation of firmgrounds can be used to empirically define the condensed section, the thickness of sediment between firmgrounds being a function of sediment supply and water depth (accommodation space).