Muddy subaqueous clinoforms are a common feature of many modern deltas, particularly those developed in basins with strong waves, tides, or oceanographic currents. Ancient examples have only rarely been reported, implying that they are under-recognized. Herein, the sedimentological characteristics of muddy subaqueous-deltaic clinoforms from the Lower Jurassic Down Cliff Clay Member of the Bridport Sand Formation, Wessex Basin, UK, are described and interpreted, as a reference to re-evaluate other ancient shallow-marine mudstone successions.
Deposits below the Down Cliff Clay clinoforms consist of erosionally based, bioclastic sandy limestone beds intercalated with upward-thickening fossiliferous claystones and siltstones (facies association A), which record upward-increasing water depth under conditions of minor input of clastic sediment and reworking by storm waves. These deposits are downlapped by foreset-to-toeset deposits of the subaqueous-deltaic clinoforms, which comprise claystones and siltstones that contain calcareous nodules and are variably bioturbated by a low-diversity trace-fossil assemblages dominated by Chondrites (facies association B). These foreset-to-toeset deposits record slow deposition (undecompacted sediment accumulation rate of c. 4.4 m/Myr) from suspension fall-out and distal sediment gravity flows under conditions of intermittent and/or poor oxygenation of bottom waters. Clinoform foreset deposits dip paleoseaward at 2°, and consist of siltstones and sandy siltstones that are moderately to completely bioturbated by a high-diversity trace-fossil assemblage (facies association C). Thin (< 1 cm), erosionally based, parallel-laminated and current-ripple cross-laminated siltstone and very fine-grained sandstone beds in the foreset deposits record episodic sediment gravity flows and tractional currents. Foreset deposits record relatively rapid deposition (undecompacted sediment accumulation rate of c. 194 m/Myr) above effective storm wave base in well-oxygenated, fully marine bottom waters. Clinoform topset deposits comprise iron-stained and chloritic siltstones that contain iron-stained and phosphatic ooids, phosphatic pebbles, and fragmented and abraded body fossils (facies association D); these deposits record prolonged physical reworking, winnowing, and sediment bypass. The long-term progradation rate of the subaqueous-deltaic clinoforms was c. 5.6 km/Myr (c. 6.7 km/Myr accounting for decompaction). The bed-scale sedimentologic characteristics of clinoform topsets, foresets, and bottomsets are consistent with variations in sedimentation rate resolved in high-resolution biostratigraphic data and with seismic-geomorphic relationships, and provide criteria to aid identification of other subaqueous deltaic clinoforms in the stratigraphic record.