Oxford Clay

Oxford Clay
Stratigraphic range: Middle to Late Jurassic (Callovian-Oxfordian), 166–160 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Coastal exposure of the Oxford Clay Formation near Weymouth
Type	Geological formation
Unit of	Ancholme Group
Sub-units	Peterborough Member, Stewartby Member, Weymouth Member
Underlies	West Walton Formation, Corallian Group
Overlies	Kellaways Formation, Osgodby Formation
Thickness	Up to 185 metres, typically 50 to 70 m on East Midlands Shelf
Lithology
Primary	Claystone
Other	Mudstone
Location
Region	Oxford, Peterborough, Dorset, Yorkshire
Country	England
Type section
Named for	Oxford

The Oxford Clay (or Oxford Clay Formation) is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay Formation dates to the Jurassic, specifically, the Callovian and Oxfordian ages,^[1] and comprises two main facies. The lower facies comprises the Peterborough Member, a fossiliferous organic-rich mudstone. This facies and its rocks are commonly known as lower Oxford Clay. The upper facies comprises the middle Oxford Clay, the Stewartby Member, and the upper Oxford Clay, the Weymouth Member. The upper facies is a fossil poor assemblage of calcareous mudstones.

Oxford Clay appears at the surface around Oxford, Peterborough and Weymouth and is exposed in many quarries around these areas. The top of the Lower Oxford Clay shows a lithological change, where fissile shale changes to grey mudstone. The Middle and Upper Oxford Clays differ slightly, as they are separated by an argillaceous limestone in the South Midlands.