17 research outputs found
Evaluation of bulk carbonate delta C-13 data from Triassic hemipelagites and the initial composition of carbonate mud
No full textBulk carbonate samples of hemipelagic limestone–marl alternations from the
Middle and Upper Triassic of Italy are analysed for their isotopic
compositions. Middle Triassic samples are representative of the Livinallongo
Formation of the Dolomites, while Upper Triassic hemipelagites were sampled
in the Pignola 2 section, within the Calcari con Selce Formation of the
Southern Apennines in Southern Italy. Triassic hemipelagites occur either as
nodular limestones with chert nodules or as plane-bedded limestone–marl
alternations which are locally silicified. In the Middle Triassic Livinallongo
Formation, diagenetic alteration primarily affected the stable isotopic
composition of sediment surrounding carbonate nodules, whereas the latter
show almost pristine compositions. Diagenesis lowered the carbon and oxygen
isotope values of bulk carbonate and introduced a strong correlation between
d13C and d18O values. In the Middle Triassic successions of the Dolomites,
bulk carbonate of nodular limestone facies is most commonly unaltered,
whereas carbonate of the plane-bedded facies is uniformly affected by
diagenetic alteration. In contrast to carbonate nodules, plane-bedded facies
often show compaction features. Although both types of pelagic carbonate
rocks show very similar petrographic characteristics, scanning electron
microscopy studies reveal that nodular limestone consists of micrite (< 5 lm
in diameter), whereas samples of the plane-bedded facies are composed of
calcite crystals ca 10 lm in size showing pitted, polished surfaces. These
observations suggest that nodular and plane-bedded facies underwent
different diagenetic pathways determined by the prevailing mineralogy of
the precursor sediment, i.e. probably high-Mg calcite in the nodular facies and
aragonite in the case of the plane-bedded facies. Similar to Middle Triassic
nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are
also characterized by uncorrelated d13C and d18O values and exhibit small, less
than 5 lm size, crystals. The alternation of calcitic and aragonitic precursors in
the Middle Triassic of the Dolomites is thought to mirror rapid changes in the
type of carbonate production of adjacent platforms. Bioturbation and
dissolution of metastable carbonate grains played a key role during early
lithification of nodular limestone beds, whereby early stabilization recorded
the carbon isotopic composition of sea water. The bulk carbonate d13C values
of Middle and Upper Triassic hemipelagites from Italy agree with those of
Tethyan low-Mg calcite shells of articulate brachiopods, confirming that
Triassic hemipelagites retained the primary carbon isotopic composition of the
bottom sea water. A trend of increasing d13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in
successions from tropical palaeolatitudes elsewhere. The carbon isotopic
composition of Middle and Upper Triassic nodular hemipelagic limestones
can thus be used for chemostratigraphic correlation and palaeoenvironmental
studies
Triassic and Jurassic calcareous nannofossils of the Pizzo Mondello section: a SEM study.
No full textPizzo Mondello is a 500 m thick pelagic-hemipelagic succession cropping out in Sicily, consisting of a nodular cherty limestone facies association of late Carnian to late Norian age. The uppermost portion was attributed to the Rhaetian and is represented by the plane-bedded Portella Gebbia Limestone. The section has been proposed as stratotype for the base of the Norian stage. The calcareous nannofossil content of limestones was studied with Scanning Electron Microscope (SEM) in two portions of the Pizzo Mondello section, one within “La Cava” that encompasses all proposed horizons for the base of the Norian, and one within the Portella Gebbia Limestone in the uppermost part of the section.
Calcareous nannofossil assemblages of the first portion display a low diversity, being constituted exclusively by calcispheres, that may constitute up to 40% of the sediment volume. Species richness increases in the upper portion. Initially, samples are dominated by Prinsiosphaera triassica, a nannolith of unknown taxonomic affinity. Rare calcareous dinocysts (Thoracosphaera cf. geometrica) and coccoliths are present in few samples. Uppermost samples are still dominated by "calcispheres" comparable to Thoracosphaera, but also yield a variety of coccoliths and nannoliths.
Calcareous nannofossil biohorizons was calibrated with conodont and radiolarian biostratigraphy. On this basis of this integrated work, specimens attribuited to cf. Thoracosphaera, observed in the lower portion of the section, are late Carnian to early Norian, while samples dominated by Prinsiosphaera, with rare Thoracosphaera and coccoliths are Rhaetian. The calcareous nannofossil assemblage of the uppermost samples, along with radiolarians and the absence of conodonts, point to a Jurassic age (Pliensbachian) for the uppermost Portella Gebbia Limestone at Pizzo Mondello. In conclusion, the age of the uppermost part of the Pizzo Mondello section is Jurassic, i.e., younger than previously thought. Calcareous nannofossils are present from the base of section and exhibit a significant taxonomic diversity, thus providing an auxiliary biostratigraphic frame for the Rhaetian - Jurassic interval in this area
Bio-chronostratigraphic calibration of the Upper Carnian-Lower Norian magnetostratigraphic scale at Pizzo Mondello (Sicani Mountains, Sicily)
No full textPizzo Mondello section is known since 15 years because
of the continuous Late Triassic pelagic record of great
significance for the establishment of an integrated chronostratigraphy
of the Late Triassic (Gullo et al. 1996;
Muttoni et al. 2001, 2004).
During the last 4 years, Pizzo Mondello section has been
studied in detail to provide a new and high resolution integrated
bio-chronostratigraphy for the calibration of the
magnetostratigraphy and chemostratigraphy proposed by
Muttoni et al. (2001, 2004), and now it is one of the GSSP
candidates for the definition of the base of the Norian.
The lowest 143 m of the Cherty Limestone, straddling the
C/N boundary have been studied in detail. The preliminary
data of the ongoing research have been presented in all the
meetings of the STS from Albuquerque 2007 and here we
summarize the final results.
The key correlation to the standard marine Triassic Scale
is provided by the ammonoids. They are relatively rare,
however the available collections document the Upper
Carnian Discotropites plinii and Gonionotites italicus
Subzones, from meter 15 to meter 80 from the base of the
section. The following 15 meters are poor in ammonoids,
while higher up the lower part of the Lower Norian Guembelites
jandianus Zone is documented by Dimorphites cf.
n. sp.1 of Krystyn, 1980.
Conodonts are very abundant and have a great potential
as practical tool for global correlations. The abundance
of specimens at Pizzo Mondello gave the opportunity to
point out clear relationships among the five most widespread
Upper Camian/Lower Norian conodont genera
(Paragondolella, Carnepigondolella, Metapolygnathus,
Epigondolella and Norigondolella) and to identify trends
of the generic turnovers (Mazza et al. 2010). The two biomarkers
so far proposed as possible marker events for the
GSSP were the FAD of E. quadrata (sample FNP88A) and
the FAD of M. communisti (sample NA35). However, the
FAD of E. quadrata occurs within the Gonionotites italicus
Subzone, while the FAD of M. communisti is on its top.
Halobiids are extremely common in the Cherty Limestone
and they have also a great potential for large scale correlations.
Nine species of Halobia have been recognized:
Halobia carnica, H. lenticularis, H. simplex, H. superba,
H. cf. rugosa, H. radiata, H. austriaca, H. styriaca and H.
mediterranea. The best possible marker for the base of the
Norian is the first occurrence of Halobia austriaca, that is
recorded in the middle of the interval between the record
of the Gonionotites italicus Subzone and the Guembelites
jandianus Zone.
Radiolarians were found in few samples but with high
diversity assemblages. In the upper Gonionotites italicus
Subzone to the Guembelites jandianus Zone there is an
overlap of species previously considered Late Carnian
with species usually regarded as Early Norian. About 4 m
above the FAD of E. quadrata, in the Gonionotites italicus
Subzone, the first assemblage with Capnuchosphaera
deweveri Kozur & Mostler, Capnuchosphaera tricornis
De Wever, Kahlerosphaera norica Kozur & Mock and
Xiphothecaella longa Kozur & Mock, usually referred to
Early Norian, occurs.
These integrated bio-chronostratigraphic studies lead to
identify some possible GSSP marker events especially on
conodonts and halobiids, which occur in the upper part of
magnetozone PM 4n, within PM 4r and in the lower part
of PM 5n.
Possibly the most suitable magnetostratigraphic event to
recognize the basal Norian is the base of magnetozone
PM 5n, as already suggested by Krystyn et al. 2002 and
Muttoni et al. 2004
Bio-chronostratigraphic calibration of the Upper Carnian-Lower Norian magnetostratigraphic scale at Pizzo Mondello (Sicani Mountains, Sicily).
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Integrated stratigraphy of the Norian GSSP Candidate Pizzo Mondello Section (Sicani Mountains, Sicily)
No full textNew biostratigraphic data around the Carnian/Norian boundary from the Pizzo Mondelllo Section, Sicani Mountains, Sicily.
No full textNew Biostratigraphic data around the Carnian/Norian boundary from the Pizzo Mondello sction, Sicani mountains, Sicily.
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