Geology Reference
In-Depth Information
carbonate intervals (Raibl Formation). Oncolite beds
appear in several cycles, but are concentrated at the
base and near the top of the shaly-sandy sequence. These
beds have a thickness of some centimeters to several
meters and alternate with coquina limestones and lime
mudstones. Lower and upper oncolite beds differ in the
size and growth forms of the oncoids, the material used
as nuclei, and the biotic associations (bivalves, fora-
minifera, echinoderms, ammonites). The size of the on-
coids found near the base of the sequence increases
from the base to the top. Most oncoids are cyanoids
and composite oncoids built by
Girvanella
and encrust-
ing foraminifera. The laminae correspond to the lami-
nation Types 1 and 4; calcareous laminae alternate with
clay laminae. The upper horizon is a marly finely bio-
clastic oncoid packstone (Fig. 4.18B) with average
oncoid sizes of rarely > 10 mm. Most oncoids 2-3 mm
thick, between 3 and 15 mm long. The growth forms of
the ellipsoidal porostromate and composite oncoids cor-
respond to Types C, R, and L; the lamination pattern to
Types 1, 2 and 4 (Fig. 4.15). Similar to the lower on-
coid horizon, oncoid sizes increase towards the top.
Oncoid growth was terminated by strong terrigenous
input. Both oncoid horizons can be laterally followed
for more than 200 kilometers.
The depositional environment is interpreted as
deeper subtidal lagoons with reduced sedimentation:
Quiet-water conditions are indicated by the association
with marly sediments and the frequency of large lobate
and asymmetrical oncoids. A deeper setting is indicated
by the common association with crinoids and echinoids
(which also provide most nuclei). In some regions, the
oncoid originated in well-lit areas behind local ooid
shoals. Microfacies and stratigraphic sequence suggest
a general deepening-upward trend for these sections,
whereby the oncoid beds mark regional sequence
boundaries related to global sea-level rises during the
Middle and Late Carnian.
4.2.4.2 Rhodoids and Macroids
Rhodoids are unattached nodules composed predomi-
nantly of free-living encrusting calcareous red algae.
Cenozoic rhodoids are commonly composed of coral-
line algae with High-Mg calcite skeletons (Pl. 54/1),
Fig. 4.19.
Composite red algal-foraminiferal rhodoid
(
A)
. The nucleus is a rhodoid clast exhibiting slightly columnar growth
forms. The outer part of the nodule is characterized by alternating concentric layers of encrusting acervulinid foraminifera
(light laminae;
C, D
) and laminar coralline algae (dark laminae;
E
). The thickness of the foraminiferal laminae varies con-
spicuously. Macroborings (
B
) are restricted to the rhodoid clast, indicating lithification prior to the settlement of encrusting
foraminifera. The borings are infilled with fine-grained and peloidal sediment. The lengths of these commonly ellipsoidal
nodules range from 10 to 70 mm, and are frequently between 30 and 50 mm. The widths range from 5 to 30 mm (frequently
10-20 mm). Modern acervulinid foraminifera and acervulinid macroids preferentially occur in deeper waters of reef slopes
down to about 70 m. Similar water depths are inferred for the rhodoid floatstones represented by the sample. These white- to
cream-colored rhodoid limestones are widely used as decorative stones in the Mediterranean area. Tertiary: Northern Italy.
