Softgrounds: substrates controlled by sediment gravity flows and the evolution of deep-water trace fossils

Bypassing sediment gravity flows play an important role in turbidite systems because they produce sole structures unconnected with the depositional processes of the casting bed. However, their role in facilitating seafloor colonization is underappreciated. The Aberystwyth Grits–Borth Mudstone turbidite system (Silurian, Welsh Basin) contains a famous ichnoassemblage with common graphoglyptids. This is interpreted to record colonization following erosion of surficial fluidal muds by flows that exposed firmer substrates. The burrows are developed at this level and formed beneath a thin, post-depositional mud blanket. They frequently cross-cut fluted surfaces indicating that basal turbidite bedding surfaces can record at least two, or more, bypassing flow events. Thus, even on a basin floor with thick mudstone deposition, the number of flow events will be under-represented. This Silurian turbidite system also illustrates that substrate conditions, not oxygenation, controlled trace fossil occurrence. Other than the burrows on turbidite soles, the remainder of the succession consists of thinly bedded and laminated strata typical of anoxic deposition but diverse geochemical proxies (e.g. iron speciation, trace metals) indicate full seabed oxygenation. The absence of macrofaunal bioturbation is attributed to a soft fluidal substrate in which only small-scale (meiofaunal) bioturbation is seen.