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Lower Jurassic

Chronostratigraphy The Lower Jurassic crops out extensively in England, with a significant outlying area in south Wales, Southern, NW and Eastern Scotland. The chronostratigraphically defined Lower Jurassic Series, incorporating the Hettangian to Toarcian stages, corresponds almost exactly to the lithostratigraphically defined Lias Group. The Lias Group, encompasses the entire Lower Jurassic succession together with the uppermost beds of the Triassic System (the ‘Pre-Planorbis Beds’ of earlier accounts) and the lowermost part of the Middle Jurassic Series (in those areas where the upper part of the Bridport Sand Fm is Aalenian in age). To the north and west of the main outcrop much of the former cover of Lower Jurassic sediment has been lost through erosion. The Carlisle and Prees outliers are examples of erosional remnants of outcrops that formerly were much more extensive.

With few exceptions these deposits are fully marine, with mostly highly fossiliferous rocks, which lead to Lower Jurassic strata in southern England being figured in some of the earliest stratigraphical investigations anywhere in the world. The frequent occurrence of ammonites and their wide geographical distribution has led to their use for correlating sequences of standard zones. Other fossil groups, especially microfossils, have been used to construct biozonal schemes (the resolution of these schemes is generally inferior). Smaller, infra-subzonal ammonite divisions, known as ‘horizons’, are also frequently used in Jurassic ammonite stratigraphy to further refine correlations.

The Lias Group was deposited in a series of interconnected sedimentary basins and shelf areas, producing local differences in the sedimentary successions. However, at some stratigraphical levels the same lithostratigraphical formation can be recognized across large areas of Britain (for example the Blue Lias Formation and the Marlstone Rock Formation). During the 19th and 20th centuries many new names were developed for the Lias Group units, with these now being rationalised at a regional level. Although this has unified lithostratigraphical nomenclature across England and Wales, substantial facies differences do exist between some areas, particularly between northern and southern England and between England and NW Scotland (extract from Simms et al. 2004).
Further Information

Simms, M.J., Chidlaw, N., Morton, N. & Page, K.N., (2004), British Lower Jurassic stratigraphy , Geological Conservation Review Series, No. 30, Joint Nature Conservation Committee, Peterborough, 458 pages.

Cox, B.M., Sumbler, M.G. & Ivimey-Cook (1999). A formational framework for the Lower Jurassic of England and Wales (on shore area). BGS research report RR/99/01