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Wessex Formation
Stratigraphic range: Late Berriasian–Barremian,
Exposure of the Wessex Formation west of Chilton Chine
Type	Geological formation
Unit of	Wealden Group
Sub-units	Coarse Quartz Grit (In Dorset)
Underlies	Vectis Formation
Overlies	Durlston Formation
Thickness	up to 1000 m near Swanage
Lithology
Primary	mudstone
Other	sandstone, ironstone & conglomerate
Location
Region	Southern England
Country	UK
Extent	Dorset, Isle of Wight, offshore Wessex Basin
Type section
Named for	Wessex
Location	Bacon Hole, Mupe Bay
Exposure of the Wessex and Vectis Formations on the South Coast of the Isle of Wight, shown in turquoise, exposures in Dorset not shown.

The Wessex Formation is a fossil-rich English geological formation that dates from the Berriasian to Barremian stages (about 145–125 million years ago) of the Early Cretaceous. It forms part of the Wealden Group and underlies the younger Vectis Formation and overlies the Durlston Formation. The dominant lithology of this unit is mudstone with some interbedded sandstones. It is exposed in both the Isle of Purbeck and the Isle of Wight. While the Purbeck sections are largely barren of vertebrate remains, the Isle of Wight sections are well known for producing the richest and most diverse fauna in Early Cretaceous Europe.

Stratigraphy and Lithology

Stratigraphy and Structural Geology

The Wessex Formation forms part of the Wealden Group within the Wessex Basin. The Wealden Group is also exposed significantly in the Weald Basin, which has a separate stratigraphic succession. 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. Its exposure 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, Lulworth Cove and Worbarrow Bay. 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. 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. This makes the formation coeval with upper portion of the Weald Clay in the Weald Basin.

Lithology and depositional environment

The primary lithology of the exposed portion of the formation on the Isle of Wight consists of featureless purple-red mudstone, interbedded with sandstones. The environment of deposition was a semi-arid floodplain within a narrow. east-west oriented valley. 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. 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. 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. 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. 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. 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, caused by the early Aptian marine transgression.