Arkell.1956.Jurassic..

138 WESTERN GERMANY
coralline oolite (Schondorf, 1914a). The formation marks an important
transgression following the Deister phase of movements, which produced
extensive alterations in the geography and the direction of transport of
material (Raecke, 1932, p. 638). In the region of the Dutch frontier the
formation is still more strongly conglomeratic, with conglomerates at top
and bottom and sometimes in the middle, and containing pebbles of
Bunter Sandstone, and borings have proved that it rests disconformably
on Lower Callovian and on various parts of the Lias (Schott, 1951, pp.
218-20).
As the name implies, the Gigas Beds yield various species of the tumid,
involute, giant ammonites of the genus Gravesia. Until the brilliant work
of Salfeld (1914) they were mistaken for Portlandian Titanites. A number
were figured by Struckmann (1887), and two of Struckmann's figures
were misidentified with Valanginian Polyptychites by Pavlow * . Salfeld
(1914, p. 154) recognized two horizons, as follows :—
Above, Zone of Gravesia irius (d'Orb.), with G. gigas (Zieten), which
by priority should be called the zone of G. gigas
Below, Zone of Gravesia gravesiana (d'Orb.)
The genus requires monographic revision, for Salfeld doubted most of
Struckmann's identifications, and G. gigas (Zieten), from South Germany,
has been considered by several authors to be an Aspidoceras of the group of
A. uhlandi (Oppel). The only other ammonite figured from the formation,
'Perisphinctes giganteus Sow.' of Brauns (1874, pi. i, figs. 4-6), cannot be
interpreted from the crude drawings; and Salfeld (1914, p. 157) doubted
its provenance.
LOWER KIMERIDGIAN (usually 30-80 m., locally to 250 m.)
Marls and clays with occasional hard bands, customarily divided into
'Lower, Middle and Upper Kimeridge'. These are highly fossiliferous
and have been subdivided into a number of beds distinguished by species
of gastropods, pelecypods, brachiopods and echinoderms (Credner,
1863, 1864; Struckmann, 1878; Brauns, 1874; Schondorf, 1909; etc.).
Exogyra virgula is common in the upper part but also occurs in the middle
and in the Gigas Beds. Ammonites have been found only in the middle
part; they are Aspidoceras longispinum (Sow.) auct., A. iphicerum (Oppel),
A. caletanum (Oppel), A. acanthicum (Oppel), Aulacostephanusyo (d'Orb.),
Aulacostephanus pseudomutabilis (de Loriol) and A. cf. eudoxus (d'Orb.)
(Lowe, 1913, p. 197). Salfeld (1914, pp. 145-151) proposed for this
faunule a distinct zone of A. yo, but it has not proved to be recognizable
elsewhere and there seems insufficient reason for separating it from the
Pseudomutabilis Zone. For detailed profiles in the Brunswick area see
Schott (1932). A greater variety of Lower Kimeridgian ammonites is
known from Hohnstein near Dresden (Bruder, 1885).
* 1892, Argiles de Speeton, pp. 124, 127 of reprint: lectotype of P. gravesiformis
pi. xiii, figs. 7, 8 ; lectotype of P. lamplughi, pi. xiv, fig. 1, now designated.
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