CONGRESSO SGI - ABSTACTS
Key words: Calcareous turbidites, Chattian-Aquitanian,
Northern Apennines
INTRODUCTION
Macigno and Falterona Sandstone are two thick, terrigenous, Oligocene to Lower Miocene turbidite successions of the Tuscan Domain, severely involved in the thrust sheets and in the folded structures of the Northern Apennines. Since they have similar lithologies, weak biostratigraphic signals and are partially coeval, it is problematic to recognize their stratigraphic architecture and correlate the sections on regional scale. Different stratigraphic and tectonic interpretations of these perisutural turbidite units have been proposed, as well as different palinspastic restorations of their sedimentary basins (cfr. ABBATE & BRUNI, 1989). To contribute in solving these problems, here we illustrate and compare unpublished and published biostratigraphic nannoplankton analyses of calcaerous turbidites sampled in two typical and well known sections: M. Rondinaio/M. Gomito section, near Abetone (PT), for the Macigno (belonging to the Tuscan Nappe); Trappola/Croce di Pratomagno, outcropping in the western side of the Pratomagno (FI-AR), for the Falterona Sandstone (belonging to the Falterona Unit).
SAMPLE-COLLECTING AND METHODOLOGY
The very low content of calcareous microfossils (calcareous nannoplankton and forams) in the pelite portion of the siliciclastic beds usually requires a great amount of samples for reliable biostratigraphic analyses. An undoubted difficulties which can be partially overcome for the two turbidite units.
The lithologies of Macigno and of the Monte Falterona
Sandstone are characterized by prevailing sandstones, which alternate with clayey silts, marls and shales. Occasionally there are also graded calcareous turbidites that are decimetric to locally metric in thickness (eg. BRUNI & PANDELI, 1980;
ARUTA et alii, 2007, fig. 5) and lateral traceable at regional
scale. This contribute aims to present the preliminary results of calcareous nannoplankton analysis performed on samples collected in these calcareous turbidite beds (Fig. 1). The samples have been collected in the uppermost part of the nannofossil-rich marly interval. By this way it has been possible to detect a good biostratigraphic signal with low number of samples. According to FORNACIARI & RIO (1996),
two methods of calcareous nannofossil counting were performed: 1) counting index species versus total assemblages (300 specimens) ; 2) abundance patterns of index sphenoliths (100 sphenoliths
RESULTS
The biostratigraphic signal of the calcareous turbidites assignes the lower Macigno (from the base up to 1.900 m) to the Chattian. Upward, for about one thousand meters up to the stratigraphic top of the unit, the biostratigraphic signal is weak and the transition Chattian/Aquitanian is not confirmed.
For the Falterona Sandstone, the analyses indicate the Chattian for the lower half of the formation. Upward, the nannofossils allow to identify the MNN1a-c Zone, i.e. the Oligocene / Miocene boundary (Polvano key bed; see also: PLESI et alii, 2002; BARSELLA et alii, 2009 for the Cortona area).
The results did not improve the uncertainty of biostratigraphic signal of the two sampled formations. However, have shown that the pelitic intervals of the calcareous turbidite layers are usefull to provide an equally-reliable biostratigraphic signal, while requiring a significantly lower number of samples.
The biostratigraphic signal of the calcareous turbidites of Macigno
and Falterona Sandstone (Northern Apennines)
PIERO BRUNI(*), ENRICO PANDELI (*, **) & VIVIANA REALE (*)
1
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(*) Earth Sciences Department, Florence University (**)IGC (Istituto di Geoscienze e Georisorse)-CNR, Florence
CONGRESSO SGI - ABSTRACTS
Fig. 1 – Lithostratigraphy and biostratigraphy of (A) Macigno (Abetone area) and (B) Mt. Falterona Sandstone (Pratomagno, south western side). The two columns synthesize the lithology of the two turbidite formations and indicate the stratigraphic position of the collected samples. The gray strip evidences the Oligocene-Miocene boundary.
REFERENCES
ABBATE E. & BRUNI P. (1989) – Modino-Crvarola o Modino e Cervarola? Torbiditi oligo-mioceniche ed evoluzione del margine Nord-appenninico. Mem. Soc. Geol. It., 39, 19-33.
ARUTA G., BRUNI P., BUCCIANTI A., CECCHI M., CIPRIANI N., MONTI L., NEBBIAI M., PANDELI E., PAPINI M. & REALE V.
(2004) – Integrated stratigraphic and statistical data as a tool
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MENEGHINI F., MOTTI A., CALANDRI S. & PANDOLFI L. (2009) – Oligocene-Miocene foredeep deposits in the Lake
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FORNACIARI E. & RIO D (1996) – Latest Oligocene to Early-Middle Miocene quantitative calcareous nannofossil biostratigraphy in the Mediterranean region. Micropal., 42,
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