Article 67

Knowledge Of The Margins Of The Tethys Ocean – The Jurassic Period.

Peter Toghill’s Chapter 10 notes that the Jurassic Period, was named after the Jura Mountains on the border of France and Switzerland; and that it lasted around 60 million years from 208 Ma to 146 Ma. It saw the onset of tectonic activity in the North Sea with the establishment of a rift valley, a triple junction and associated volcanism. The Tethys Ocean spread a shallow epicontinental sea over most of Britain which laid down clays and thin limestones, the Lias, at the beginning of the period. Important chamosite and siderite ironstones formed in some areas. the early Jurassic seas were full of ammonites and marine reptiles. A mid-Jurassic uplift produced a positive area in the middle of the North Sea which fed deltaic and fluviatile sediments, the Estuarine Series, into the north of England, Scotland, and the north-east midlands during the middle of the period. Meanwhile, the south of England was covered by shallow water lagoons starved of land-derived sediment and extensive carbonates developed in the form of oolites, forming the Inferior and Great Oolites. Open marine areas only existed at this time in south-west England. Ammonites were abundant in the Lias, but were not so common in the oolites, and were absent in the deltaic sediments of the north. Mid-Jurassic earth movements led to an unconformity within the Inferior Oolite and local areas (axes) of up-lift affected Oolite sequences and thicknesses during the Lower and Middle Jurassic. A Marine transgression produced a widespread limestone, the Cornbrash, at the end of the Middle Jurassic. The Upper Jurassic saw the establishment of open marine conditions over all of Britain and thick organic-rich clays, the Oxford and Kimmeridge Clays were laid Down, separated by Corallian limestones, the Kimmeridge Clay, being an important source-rock for North Sea oil. The Upper Jurassic seas were full of ammonites, fish, ichthyosaurs and plesiosaurs. The collapse of the North Sea uplifted area and the associated graben, led to great thicknesses of the Upper Jurassic coarse sediments in the faulted basins of the North Sea. At the end of the Jurassic, limestones, sometimes evaporitic, were restricted to Southern England due to the uplift of the northern areas and a fall in sea level, but marine conditions continued into the Cretaceous in the Yorkshire Basin.