Arkell.1956.Jurassic..

LOWER SAXONY 133
which was probably an island, and certain Upper Jurassic formations
overlap on to Trias (Bentz, 1926; Schott, 1951; Wolburg, 1954). Intra-
Jurassic movements that preceded these overlaps also caused uplift and
shallowing between the Weser and Elbe, where the Lias is thick, for parts
of the Upper Jurassic there rest disconformably on eroded or incomplete
Middle Jurassic (as under Liineburg Heath). East of the lower Elbe was
another non-subsiding area against which the Jurassic formations wedge
out and are overlapped by Cretaceous: this is known as Pompeckj's Swell;
it stands on the site of an inferred island of Cambrian times to which
Pompeckj first drew attention (Seitz, 1949).
The southern boundary of the basin of Lower Saxony is formed by the
ancient masses of the Rhenish slate mountains, which are an extension
of the Ardennes ridge, and the Harz, an extension of the Bohemian
massif. Along the northern margin of these masses and at the entrance to
the intervening 'Hessian strait' the Jurassic system crops out in an interrupted
band over 150 miles long, between the River Ems and the River
Aller, around Osnabriick, Hanover, and Brunswick (fig. 16). In these
outcrops the rocks vary greatly in thickness and facies and are nearly all
in shallow-water marginal development, contrasting with the open
basin facies to the north.
This is the classic area of the Saxonian folding (Stille, 1910, 1913,
1922, 1925, 1932, 1949, 1953, etc.). The Mesozoic rocks occupy troughs
of subsidence between resistant swells and massifs and have been compressed
between them as within a frame, whence Stille's concept of
'frame folding'. Tectonic complication increases near the junction
between the basins and their frames. In general there has been overriding
movement towards the south or south-west, often with overturning and
sometimes, as in the Osning range, low-angle overthrusting magnified in its
effects by decollement (Stille, 1953). The tectonics are locally complicated
by, or even largely caused by, salt domes. Maximum compression
in the Tertiary was preceded and often conditioned by folding and faulting
at several periods in the Jurassic and Cretaceous. The principal phases
were (Dahlgriin, 1923):—
pre-Upper Valanginian (Hils phase)
pre-Middle Purbeckian (Osterwald phase) (main phase)
pre-Middle Kimeridgian (Deister phase)
Each is marked by unconformities and conglomerates. In addition to
folding there was faulting during the Mesozoic. The Rhenish massif
extended in Jurassic times over what is now the Munster basin, and its
northern margin seems to have broken down progressively during Upper
Jurassic and Lower Cretaceous times in a series of step faults, each step
descending farther into the trough to the north (Stille, 1936; Wolburg,
1952). This recalls the behaviour of the southern margin of the Vindelician
ridge bordering the Alpine geosyncline (see p. 155).
The anticlines and periclines in which the Jurassic rocks crop out
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