Wessex Formation

Position of the Wessex Formation within the Wessex Basin succession

The Wessex Formation forms part of the Wealden Group within the Wessex Basin, an area of subsidence since Permo-Triassic times. The basin is located along southern half of the Isle of Wight and Purbeck, extending offshore into the English Channel. The Wealden Group is also exposed significantly in the Weald Basin, which has a separate stratigraphic succession. The Wealden Group is not widely present elsewhere in Britain, as these areas were tectonic highs where no sediment deposition was taking place. The formation has limited exposure as it has been deeply buried beneath the subsequent Lower Greensand. Selbourne and Chalk Groups, as well as being very vulnerable to erosion. It has been exposed at the surface due to the creation of anticlinal structures as a distant effect of the formation of the Pyrenees as part of the Alpine Orogeny during the Paleogene.[5] The major source rocks for the sediments were from the Cornubian Massif to the west, an upland region roughly equivalent to the extent of Cornwall and Devon, with occasional large dropstones transported in tree roots being found in Wealden sediments over 100 kilometres from where they originated.[6]

The exposure in of the Wessex Formation in the Isle of Purbeck is largely confined to a thin belt on the south side of the Purbeck Ridge and is best exposed at Swanage,[7] Lulworth Cove[8] and Worbarrow Bay.[9] One notable persistent horizon within the Purbeck sections of the formation is the "Coarse Quartz Grit", an up to 6 metre thick sequence of conglomeratic ironstone, with many beds including numerous centimetre sized subangular to rounded pebbles predominantly of vein derived quartz, hence the name.[9] This horizon is present throughout the Purbeck outcrops of the Wessex Formation.

The Isle of Wight succession has two primary exposures, The major one being the several kilometre long section along the South West coastline around Brighstone Bay, and another smaller exposure on the South East coast near Yaverland. While the formation taken as a whole dates from the Berriasian to the Barremian, only the uppermost part of the formation is exposed on the Isle of Wight. With less than 200 metres of exposed composite stratigraphic thickness, and which dates from latest Hauterivian to Barremian. The boundary between the two ages lies near the Pine Raft horizon.[10] This makes the formation coeval with upper portion of the Weald Clay in the Weald Basin. The primary lithology of the exposed portion of the formation on the Isle of Wight consists of featureless purple-red overbank mudstone, interbedded with sandstones. The environment of deposition was a floodplain within a narrow. east-west oriented valley.[11] The climate at the time of deposition is considered to be semi-arid, based on the presence of pedogenic calcrete nodules within the mudstones.[12] The aforementioned "Pine Raft" horizon found near the base of the exposed portion of the formation includes calcitized conifer trunks up to metre in diameter and 2–3 metres long.[13]

Close up of L6 plant debris bed, showing charcoalised plant matter suspended in grey mudstone

A notable feature of the formation are the so-called "Plant debris beds". These consist of a basal matrix supported conglomerate, grading upwards into grey mudstone with lignitic plant debris, including large trunk fragments of the extinct conifer Pseudofrenelopsis present in the upper portion. These were formed by sheet flood deposits induced by storms that filled pre-existing topographic lows like oxbow lakes and abandoned channels in the floodplain depositional environment.[14] The debris beds do not form a continuous horizon throughout the formation, but are laterally extensive over tens of metres. Many of the wood fragments in the debris beds are cemented together with large nodules of pyrite, suggesting depositional conditions were anoxic.[15] Most fossils within the formation are associated with the debris beds, but are mostly disarticulated individual bones, suggesting a long subaerial exposure prior to burial. Many of the larger bones are encased in hard nodules of sideritized mudstone. Plant debris beds also exist within the Swanage section, but are noticeably sparse in fossils.

While most fossils are associated with the plant debris beds, a notable exception is the "Hypsilophodon bed" present near the top of the formation, an up to 1 metre thick bed of silty red-green mudstone, with two separate horzions that have produced almost exclusively over a hundred complete and articulated skeletons of Hypsilophodon, sometimes even with preserved tail tendons. The bed is laterally extensive, being persistent for over a kilometre. It has been recently suggested that the accumulation of skeletons were a mass mortality event caused by a crevasse splay.[16] Just above the "Hypsilophodon bed" the red mudstones of the Wessex Formation change to the transitional light coloured sandstone "White rock" and overlying laminatinated grey mudstones of the Vectis Formation, caused by the changing of environmental conditions from that of a floodplain to coastal lagoon conditions, initiated by the early Aptian marine transgression. The transition to the Vectis Formation is also present in Swanage but thins out and disappears westward, having an erosive unconformity with the Lower Greensand Group or the Gault in these areas, often with a basal pebble bed.

Invertebrates are commonly preserved in the Wessex Formation. Freshwater bivalves can be found including unionids such as Margaritifera, Nippononaia, and Unio. These bivalves are helpful in reconstructing what the freshwater paleoenvironment may have been like during the formation's deposition. Specimens of Viviparus, a genus of freshwater snail, have also been found. While compression fossils of insects are found in the overlying Vectis Formation, all insect fossils in the Wessex formation are found as inclusions in amber. Amber can be found present as a rare component in other plant debris beds in the Wessex formation both on the Isle of Wight and the Isle of Purbeck, however the only significant concentration and where all of the inclusions have been found is a lag channel in the L6 plant debris horizon just south-east of Chilton Chine. Only four species from the amber have been formally described, Cretamygale chasei a mygalomorph spider, and Dungeyella gavini[19] and Libanodiamesa simpsoni, both chironomid midges, as well as Embolemopsis maryannae, a embolemid parasitic wasp. Most of the other taxa in the table come from mentions in the paper describing the latter. However several images of some of the undescribed taxa have been released from various sources, including multiple chironomids, and therevid dipteran.[20]