Arkell.1956.Jurassic..

SWABIAN AND FRANCONIAN ALB
(figures in Quenstedt, 1886, and Horn, 1908). The Franconian pelecypods
are monographed by Schmidtill (1925).
Zones of Graphoceras concavum and Ludwigia murchisonae. From
Klettgau to Gosheim (Lorcher, 1934, p. 130) and in other places (Frank,
1942, p. 5) the Concavum Zone is united with the Sowerbyi Bed as a
single inseparable stone band (max. 3 m.), just as in the Sherborne district
of Dorset, where the union of these two zones caused Buckman so much
confusion (see p. 33).
In SW. Swabia, Lorcher (1934) found an interesting subzonal succession
with several puzzling anomalies. Instead of Ludwigia murchisonae being
confined to a high horizon with Staufenia staufensis as at Sehnde near
Hannover (see p. 143), he found that it appears much lower, in the
Sehndense Subzone, and occurs in both the next following subzones of
discoideum and staufensis. He attributed this to northward migration of
Ludwigia murchisonae from the Tethys. Whatever the explanation, the
facts are a vindication of Oppel's original concept of a Murchisonae Zone,
embracing the other horizons as subzones. Between Kirchheim and
Metzingen these horizons are all represented by nothing but a pebble bed.
Between Metzingen and Heckingen the gap widens downwards and
embraces everything between the Concavum Zone and the Opalinum
Zone. The subzones where the sequence is fully developed are shown in
table 12 (p. 123).
Zones of Tmetoceras scissum and Leioceras opalinum. Tmetoceras
scissum occurs at Balingen with Costileioceras sinon (Bayle), the index
used by Hoffman in 1913 and identified by Stahlecker (1926, p. 162)
and Lorcher (1934) in the Swabian Alb. Costileioceras tolutarium (Dumortier),
which indicates a subzone above sinon in the Swabian Alb (same
references) was figured by Buckman in 1899 fr° m
the Scissum Zone in
England. The Scissum Zone therefore probably covers both Sinon and
Tolutarium Subzones.
The Wasserfallbank at the base of the Doggersandstein and of Braun
Jura beta contains both Leioceras opalinum and Costileioceras costosum
(Lorcher, 1934, p. 126), and the costosum horizon is therefore a subzone of
the Opalinum Zone, as shown by Lieb (1950, p. 452) in the Swiss Jura.
The rest of the zone, constituting Braun Jura alpha, comprises the
Opalinum clay with nodules, for which a fair average thickness is 25 m.
In Franconia (P. Dorn, 1923, 1936a) there is a gradual passage down into
the Upper Toarcian clays and upwards into the Doggersandstein. Common
fossils of the Opalinum Zone are Pachylytoceras torulosum (Schloth.)
and Trigonia navis Lam., used by Oppel as zonal indices but abandoned
in favour of Leioceras opalinum, introduced as index by Brauns in 1864.
UPPER TOARCIAN (2-6 m.)
Zone of Lytoceras jurense: Lias Zeta. The Jurense marls are sometimes
abruptly followed disconformably by the Opalinum Zone, but
more often they pass up lithologically by gradations of more or less sandy
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