Geology of Alderley Edge - Triassic in Northern England

Triassic in Northern England

The Triassic succession of Great Britain is divided into a number of stratigraphical units that are generally hard to assign to divisions of the Lower, Middle, and Upper Triassic.

The upper boundary of the Triassic is defined by the base of the Jurassic System, which is internationally recognized as being at the base of the planorbis subzone of the planorbis Ammonite Zone of the Hettangian Stage.

The Triassic Period (251Ma – 205Ma) is part of the Mesozoic Era (251Ma – 65Ma) and is so called from its former threefold division in its type locality in Germany. The divisions have been revised in nomenclature so that they do not correspond with most of the older literature. The onshore Triassic in Britain differs from that in the southern North Sea, Germany and other parts of northern Europe, in not being the classic tripartite lithostratigraphic subdivision; The Triassic deposits of Germany form three series. In the Bunter (meaning 'brightly coloured') series, the land was emergent and red sandstone and sandy shales, with some salt and gypsum, were deposited. The Muschelkalk series saw the transgression of the land by the sea and the deposition of marine shale and limestone; the Keuper series saw the land again emergent and shale, sandstone, and gypsum being formed. Instead, the British Triassic is divided into the Sherwood Sandstone Group, the overlying Mercia Mudstone Group (previously described by their quasi Germanic names, the Bunter and the Keuper) and the Penarth Group these together attain a maximum thickness of ca. 3.5 km in the Cheshire Basin.. In Britain the middle division of marine beds is largely absent, giving the original twofold division into Keuper and Bunter, which whilst recognizable as lithological or facies divisions, they are not equivalent to the strata in Germany. The absence of the middle facies, the Muschelkalk, does not mean that Muschelkalk beds of the middle Triassic are absent, but rather that the deposition was continental over this area and not marine as over northern and central Europe. Dating is therefore difficult as there are no fossils and there was no igneous activity, which could have led to the emplacement of isotopes enabling dating. Recent work has shown that the Mercia Mudstones are more complex than originally thought and there are now proposals to give new stratigraphic names to four of the formations.