Geology Reference
In-Depth Information
mentation rates and periods of non-deposition. Silurian
cephalopod limestones ('
Orthoceras
limestones') ac-
cumulated on paleo-highs below wave base at depths
of several tens to several hundreds of meters (Ferretti
and Kriz 1996).
Paleozoic cephalopod limestones are bioclastic
wackestones characterized by the predominance of pe-
lagic organisms including ammonoids and nautiloids,
dacryoconarids, and thin-shelled ostracods. Mesozoic
cephalopod wackestones exhibit ammonoids, thin-
shelled pelagic bivalves and ostracods as well as radi-
olarians.
Aptychus limestones:
Basinal sequences consisting
of fine-grained pelagic limestones with aptychi are com-
mon in Late Jurassic and Early Cretaceous sediments
of the Alpine-Mediterranean region. Aptychi are present
throughout the sequence although not present in all
beds. The fossils are concentrated at the base of some
beds in striking shell pavements (Zapfe 1963), indicat-
ing the existence of bottom currents, or they occur ir-
regularly distributed as debris (Fig. 10.44). The car-
bonates originated from the deposition of nannoplank-
ton at depths of several hundreds of meters and more
(see Fig. 4.4). The depositional setting of the
Aptychus
Plate 90 Cephalopods
The plate demonstrates the appearance and common preservation of ammonoid shells (-> 1-3, 5), aptychi (-> 4,
6), nautiloid shells (-> 8) and belemnites (-> 7) as seen in thin sections of Paleozoic and Mesozoic limestones.
1
Mesozoic cephalopod limestone.
The sample shows various sections through internal molds of ammonoid shells embed-
ded within micrite (black): (1) Median section through the early whorls, (2) transverse section to the plane of the planispirally
coiled shell, (3) oblique section, (4) relicts of the shell. Earlier chambers of the shell are filled with calcite cement, later
chambers with micritic sediment. Note that the sedimentary infilling of the chambers with pelagic micrite is the same as
the enclosing matrix. This indicates infilling of the shells on the substrate surface before burial (Ferdinandez-Lopez and
Melendez 1994). The diagnostically important lines of junction of septa and shell walls (suture, S) are still preserved. The
originally aragonitic walls and septa have recrystallized to a calcite mosaic. Basinal facies. Late Triassic (Norian, Hall-
statt limestone): Northern Alps, Austria.
2
Paleozoic cephalopod limestone.
Bedding-parallel surface of a cephalopod coquina consisting of ammonoids (goniatites:
Tornoceras
, T) and orthocone nautiloids (ON). Note the preservation of the inner prismatic microstructure of the shell
(center). The sample characterizes a 'condensed shell concentration' defined by high-diversity skeletal elements carrying
different taphonomic signatures (pristine shells as well as abraded shells, various stages of shell solution). Open-marine
pelagic platform. Late Devonian (Early Famennian): Tafilalt, eastern Anti-Atlas, Morocco.
3
Ammonite preservation.
Hardground with molds of ammonoid shells. The sections are transverse to the plane of coiling.
Early Jurassic: Murcia region, southern Spain.
4
Aptychi.
These Low-Mg
calcitic plates are found associated with ammonites and interpreted as parts of the jaws or an
opercular structure. Normally the plates occur as pairs and have an ornamented outer surface. Note the cellular-tubular
microstructure (white arrow) and the ornamentation of the shell surface (black arrow). Classification of aptychi is based
on the kind of ornamentation formed by fine lines, ribs (this sample), tubercles or spines. The matrix of the radiolarian
mudstone and the aptychi are selectively silicified (white). Late Jurassic: Monte Generoso, Ticino, Switzerland.
5
Ammonite preservation
. The shell has been infilled with dark pelagic micrite (PC) and after breakage and dissolution of
the shell, was later embedded in forereef carbonates (FC) with fine-grained bioclastic sediment. The septa were replaced
by coarse granular calcite cements followed by the burial filling of the chambers with blocky calcite. O: Ophiurid ele-
ment. The sample comes from a locality showing the interfingering between basinal sediments and forereef facies. Late
Triassic (Norian): Hoher Göll, Northern Alps, Bavaria, Germany.
6
Aptychus.
The thin section shows the
characteristic ornamentation. White spheres are radiolarians. This microfacies is
common in limestones deposited during pre- or post-radiolarite phases of deep-sea sedimentation. Same locality as -> 4.
7
Belemnite guards.
Transverse section (TS) and longitudinal section (LS) of the Low-Mg calcite guards (rostrum). In
transverse section the rostrum shows a fibrous structure with tiny calcite prisms radiating from an eccentric point. Rings
concentric about this point indicate growth stages. Under crossed nicols an extinction cross can be seen resulting from the
radial orientation of the individual prisms making up the rostrum. The microstructure of belemnite shells seen in SEM can
be used to distinguish belemnite fragments from other mollusk remains (Dauphin 1986). Late Jurassic (Late Oxfordian):
High Atlas Mountains, Morocco.
8
Nautiloid shell.
Longitudinal section of a straight orthocone shell embedded within a bioclastic wackestone matrix. Note
preservation similar to the ammonite shell shown in -> 5. Open-marine deep-water ramp. Early Devonian (Emsian):
Erfoud, Morocco.
-> 2: Wendt and Belka 1991; 6: Kiessling 1996
