Arkell.1956.Jurassic..

136 WESTERN GERMANY
their limitations by introducing vertical bars where palaeontological
links are still lacking.
It is interesting that the most important of the three phases of Saxonian
folding, that preceding deposition of the Serpulite (pre-Middle Purbeckian),
comes out as contemporaneous with the pre-Upper Tithonian movements
which caused transgressions of the Upper Tithonian in many parts of the
world.
The Purbeckian freshwater limestones of the Hils basin have proved
to contain Middle Purbeckian ostracods and to be one of several local
facies of the Serpulite formation (Schmidt & Wolburg, 1949). Furthermore,
the Miinder Marls, previously correlated with the Lower Purbeckian
only, have yielded in the lower part marine Portlandian ostracods and
accordingly are supposed to bridge the Upper Portlandian and Lower
Purbeckian. The Purbeckian of NW. Germany is therefore as follows :—
Upper Purbeckian . Wealden 2 and 3 of Wolburg
t Wealden 1 of Wolburg
Middle Purbeckian . -j Siisswasserkalke of Hils basin (about 70 m.)
{ Serpulite (in many facies, with conglomerates)
Lower Purbeckian . Miinder Mergel, upper and middle parts (altogether
150 m. at Miinder, and up to 400 m. in other
places, where the marls contain salt)
The most remarkable formation of this series is the Serpulite. It
consists in some places of a mass of Serpula coacervata, in others of a
lumachelle of oysters and other small pelecypods, in others of oolitic
shelly limestone passing into marls or 'plattenkalk' or even sandstone.
In many places it becomes conglomeratic, consisting of Upper Jurassic
limestone pebbles, to which in places are added pebbles of Keuper and
Muschelkalk and even occasionally of dark Palaeozoic rocks (Raecke,
1932, pp. 621-8). The Serpulite is transgressive on to Kimeridgian,
Oxfordian and Triassic rocks. In borings in the German-Dutch frontier
region it transgresses over the 'Cimbrian' land and the Serpula banks
and other facies are aligned with the old coast (Schott, 1951). Figures
and descriptions of the Purbeckian fossils will be found chiefly in Koch
& Dunker (1837), Dunker (1846), Struckmann (1878, 1879, 1892) and
Koert (1898); some of the gastropods have been revised by Arkell (1941)
and the ostracods by Martin (1940).
PORTLANDIAN AND UPPER KIMERIDGIAN?
As stated above, the Miinder Marls are 150 m. thick at Miinder, near
the Deister, but elsewhere expand to some 400 m. Most of this is Lower
Purbeckian, but according to the ostracods the lower part is Portlandian.
Where the thickness is greatest, deposits of salt and gypsum occur
(reminiscent of the Lower Purbeck of Dorset). The lower part is locally
red in colour and passes down imperceptibly into Einbeckhausen Plattenkalk
(maximum about 100 m., in the Hils basin). This formation consists
everywhere of dark grey, compact, ringing flagstones, interbedded with
marls and grey marlstones. There are also occasional beds of oolite,
http://jurassic.ru/