Geology Reference
In-Depth Information
Limestones with abundant bivalves representing al-
lochthonous shell concentrations or autochthonous oc-
currences of bivalves (Pl. 87/4, Pl. 119/1) are key ele-
ments of characteristic facies types which sometimes
can be found in worldwide distribution as demonstrated
by the following examples:
position within a fine-grained sediment corresponding
to wackestone and packstone with bioclastic debris and
in places with some corals and dasyclad green algae.
The
Megalodon
limestones represent the subtidal part
of the Lofer cycles (see Pl. 140/4) . The depositional
setting of Late Triassic megalodontid limestones is con-
fined to the standard facies zone 8 (see Sect. 14.3).
Triassic
Megalodon limestones:
The Late Triassic
Dachstein limestone of the Northern Calcareous Alps
as well as time-equivalent platform carbonates all over
the Tethys (Fig. 10.40) are characterized by cyclic plat-
form carbonates containing thick-bedded limestones
with abundant shells of pachydont megalodontids.
These equal-sized bivalves may become several tens
of centimeters in size. They occur commonly in life
Liassic Lithiotis
limestones:
A specific limestone
type characterized by the mass occurrence of
Lithiotis
,
a thick-shelled heterodont bivalve several centimeters
in size, and related genera, developed during the Early
Liassic (Sinemurian, abundant in Pliensbachian and
Toarcian) in carbonate platforms of Tethys and Pan-
thalassa forming a low-latitude belt (Nauss and Smith
Plate 87 Bivalves
This plate displays thin sections of Mesozoic and Cenozoic bivalve shells. Shell shape (-> 1) and microstruc-
tures (-> 5-7, 9) are used to differentiate bivalve shells from other mollusk shells and brachiopods.
1
Complete bivalve.
Radial section through joined calcitic valves of an equivalved megalodontid bivalve. D: Dorsal, V: Ventral.
The interior of the shell is filled partly with sediment, partly with recrystallized calcite cement. Late Triassic: Northern
Alps, Austria.
2
Growth lines.
Radial section through a recent aragonitic
Donax
shell. Note the widely and densely spaced growth lines
(GL) and the crystals of the prismatic microstructure (white arrow). R: Rib area. Recent: North Sea.
3
'Filaments'.
The fossils are larval or juvenile shells of pelagic bivalves. Filaments are common constituents of Mesozoic
deep-marine basinal limestones, e.g. in the Ammonitico Rosso facies of the Mediterranean region. Note dense packing,
bioclast-support and common bioclast/bioclast contacts. More than 70% of bioclasts lie within two adjacent size classes.
High degree of close packing reflects episodes of gregarious deposition of pelagic shells on the sea bottom within a very
short time interval. Early Jurassic: Korfu Island, Greece.
4
Bivalve reef.
Densely spaced calcitic valves of fossil 'oysters' (
Placunopsis
ostracina
) forming reef-like structures by
attachment of the right valve to hard substrate. Example of a 'primary biogenic shell concentration' (Fürsich and Oschmann
1993) resulting from a gregarious habit and characterized by epifaunal bivalves growing one shell above the other. Note
growth lamellae (GL) characterizing the 'foliated microstructure' of the shells, and sediment-filled borings (B). The high
frequency of life-positioned oysters in the densely packed well sorted fabric together with the lenticular shape of the
deposit indicates an accretionary buildup. Compare this fabric with the example shown in -> 3 and note the almost
complete absence of carbonate mud due to failure in sediment supply. Middle Triassic (Muschelkalk): Berlichingen,
southwestern Germany.
5
Microstructure
. Bivalve shell fragment showing a finely prismatic calcite microstructure. Small circular black structures
are microborings. Late Jurassic: Untersberg, Salzburg, Austria.
6
Microstructure
. Fossil 'oysters'. Note the microstructural differentiation of the calcitic valve into a prismatic outer layer
and a foliated layer.
Late Tertiary (Miocene): Ekisce, southern Turkey.
7
Microstructure
. The
Ostrea
shell exhibits alternating foliated (F) and vesicular (V) layers characteristic of pycnodont
bivalves. Late Tertiary (Miocene): Mesarevlev, southern Turkey.
8
Inoceramid bivalves:
Longitudinal and cross sections through the outer layers (ostracum) of calcitic
Inoceramus
shells
exhibiting the characteristic prismatic microstructure. Note the honeycomb pattern in plan view (center top) and the
difference in prism size in the outer and inner shell layer. Individual prisms act as single crystals displaying unit extinction
between crossed nicols. Inoceramid bivalves are known from the Jurassic to the Late Cretaceous. They were common
during the Late Cretaceous and became extinct in the Maastrichtian well before the Cretaceous-Tertiary boundary. The
fossils are important Cretaceous index fossils (McLeod and Ward 1990), provide interesting paleoclimatic proxies (Dhondt
and Dieni 1992) and are used as diagenetic markers (Elorza and Garcia-Garmilla 1998). Disarticulated calcitic prisms of
the ostracum are important constituents of Cretaceous open-marine limestones. Note growth lines (top left). Small spar-
filled structures in the micritic matrix are calcispheres (see Pl. 66/7). Late Cretaceous (Cenomanian): Teutoburger Wald,
Germany.
9
Microstructure.
Bivalve wackestone with angular fragments of pycnodont shells showing vesicular and foliated micro-
structures. Middle Cretaceous (Aptian): Subsurface, Ras al Khaimah, United Arabian Emirates.
