Geology Reference
In-Depth Information
Fig. 10.46.
Pelagic limestones with cricoconarid shells.
A
: Styliolinid grainstone. Telescoping in some shells is caused
by currents (arrows). Late Devonian (Frasnian): Eastern Anti-
Atlas, Morocco. After Wendt and Belka (1991).
B
: Tentaculitid wackestone with longitudinal and cross sec-
tions of
Nowakia
characterized by ringed exterior and inte-
rior surfaces. Middle Devonian (Eifelian): Prague Basin,
Czech Republic. Scale is 1 mm.
lites
in possessing a smooth, unchambered, imperfo-
rate shell without concentric thickenings. The shell
usually terminates in a small bulbous extremity. The
microstructure is clear homogeneous calcite. Transverse
sections show an extinction cross under crossed nicols.
The tentaculitids are valuable index fossils used in re-
gional zonations (particularly Devonian).
Ecology and paleoenvironment:
The styliolinids and
dacryoconarids were pelagic (mainly planktonic) in-
habitants of the upper reaches of the oceans contribut-
ing to the formation of pelagic carbonates in basins and
deep ramp settings (Fig. 10.46). The tentaculitids sensu
strictu lived probably as nektobenthos; they are abun-
dant in slightly argillaceous, sometimes nodular pack-
stones and wackestones (e.g. 'Flaserkalk' of the Ger-
man Devonian cephalopod limestone) as well as in
shales formed on deep shelves, submarine highs (swells)
Plate 91 Devonian Tentaculitids and Styliolinids
Small conical calcitic shells including the tentaculitids, styliolinids and other morphologically similar fossils are
common and rock-building constituents of Paleozoic, particularly Silurian and Devonian limestones formed in
open-marine environments. The fossils are interpreted as presumably planktonic organisms allowing regional
and global biozonations e.g. of the upper Late Devonian. Diagnostic criteria are the shell sculpture (-> 2), type
and angle of the apex (-> 3, 5) and shell microstructure (-> 7).
1
Bioclastic wackestone with tentaculitids (T) and trilobites (TR). The sample is from bedded carbonates deposited on a
deep platform. Early Devonian (Emsian): Erfoud, Anti-Atlas, Morocco.
2
Longitudinal section (LS) of
Nowakia
(N) with characteristic ribs on the exterior and interior shell surfaces, and trans-
verse section of
Styliolina
(S). Note stacked cone-in-cone cross section of
Styliolina
(center bottom). The micritic matrix
is rich in fine-grained bioclastic debris (white spots). Middle Devonian (Eifelian): Prague Basin, Czech Republic.
3
Tentaculitid packstone. Variously sized circular and polyhedral cross sections and V-shaped longitudinal sections. Note
'cone-in-cone structure' of some shells (arrows) indicating current activity (Larsson 1979) and the delicate external trans-
versal ornamentation of the shells typical for tentaculitids. SMF 3-T
ENT
. Early Devonian: Frankenwald, Germany.
4
Tentaculites
. Note shells stacked one within the other (arrow) and the current-controlled unidirectional orientation of
some shells. Same locality as -> 3.
5
Styliolina
microcoquina packstone deposited after current transport on a pelagic swell. Note the late diagenetic calcite
cement covering the shells (arrows). Styliolinid coquinas occur in pelagic to hemipelagic settings and are known from the
Lochkovian to the Late Frasnian. Early Devonian (Emsian): Erfoud, Anti-Atlas, southern Morocco.
6
Pelagic styliolinid wackestone.
Styliolinids (arrow) range from the Early Devonian to the Frasnian. The group is charac-
terized by the absence of peripheral ornamentation. Limestones composed entirely of styliolinids are believed to be
ancient analogues of modern pteropod-rich carbonates. E: Echinoderm fragment; O: ostracod. White curved shells are
trilobite fragments. Middle Devonian (Givetian): Menorca Island, Spain.
7
Preservation of styliolinid shells:
The original microstructure of the shell walls is completely replaced by a fibrous calcite
displaying distinct curved twin-lamellae (arrows), probably indicating low-grade metamorphism (Tucker and Kendall
1973). This preservation is common in many styliolinid limestones. The fine-grained matrix is microspar. Middle Devo-
nian pebble within Lower Carboniferous flysch deposits: Betic Cordillera, southern Spain.
