Arkell.1956.Jurassic..

OXFORDIAN (25-115 m.)
LOWER SAXONY *39
The Upper Oxfordian is developed as Coralline Oolite (20-100 m.)
above, and Heersum Beds (5-15 m.) below. The top part of the Coralline
Oolite is more marly and dolomitic and abounds in ' Terebratula' humeralis,
and should be separated. The Humeralis Beds have yielded Ringsteadiae
(Salfeld, 1914, p. 142), which correlates them with the Pseudocordata
Zone. The upper true Coralline Oolite has not produced
ammonites and may be equivalent to the Sandsfoot Clay. The middle
Coralline Oolite contains Perisphinctes cautisnigrae Arkell (1937, p. 57,
pi. xiii) and therefore correlates with the Dorset Trigonia clavellata Beds.
The lower Coralline Oolite and the Heersum Beds contain the Perisphinctid
fauna of the Plicatilis Zone.
The Heersum Beds bear a strong resemblance, lithologically and
palaeontologically, to the Highworth Limestones of central England.
The ammonites have been revised and figured by Arkell (1935-48, pp.
57-9, 390-3, etc.) and in an excellent memoir by Siegfried (1952). The
Perisphinctids of both Upper and Lower Heersum Beds are typical of the
Plicatilis Zone. They include such common English species as P. cotovui
Sim. and P. maximus (Y. & B.).*
In the Lower Heersum Beds, however, Cardioceratids are much more
numerous than Perisphinctids, and Salfeld consequently made for them
a separate zone of Cardioceras tenuicostatum.
For the most part the Cardioceratids of the Lower Heersum Beds
are similar to those in the Arngrove Stone and Berkshire Oolite Series
about Oxford, namely the Plicatilis Zone, and are therefore early-Upper
Oxfordian. Siegfried believes that C. cordatum (Sow.) also occurs, but
the specimens he figures, with their prominent, tuberculate secondary
ribbing and absence of any tendency to form a large, smooth, bodychamber,
are in my opinion C. (Subvertebriceras) zenaidae and C. (S.)
densiplicatum (Boden?) (as interpreted by Spath and Arkell); and his
C. costellatum is at least in part the later C. costulosum (Buck.).
Nevertheless there seem to be a few earlier elements in this fauna
even in the Hildesheim-Heersum district, showing that the topmost
part of the Cordatum Zone is present in condensed or derived form.
Such elements are rare but seem to be represented by Cardioceras roemeri
Siegfried, which appears to be a Scarburgiceras, and by two specimens of
Goliathiceras more like species of the Cordatum Zone than those of the
Plicatilis Zone.
In the Weser-Wiehengebirge there is a more definite and separate
representation of the Cordatum Zone at the base of the Heersum Beds.
The beds here contain bands of black limestone, from some of which
come perfectly-preserved common English ammonites of the Plicatilis
Zone, such as Perisphinctes pickeringius (Y. & B.). Not far away, however,
* It is inexplicable how Siegfried (p. 302) failed to separate these two quite distinct
species. Moreover, he overlooked P. kranaus and P. apolipon, both figured from the
Heersum Beds (Arkell, 1935-48, pi. xxxviii, fig. 2, pi. xxxv, figs. 4, 5) ; and his P. martelli
is nothing like either Salfeld's or Oppel's (which are two distinct species).
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