1 Introduction Cyclostratigraphy is emerging as a central focus in

Scuola di Dottorato in Scienze della Terra
Dipartimento di Geoscienze, Università degli Studi di Padova –A.A. 2009-2010
STRATIGRAPHIC ANALYSIS ON MONTE AGNELLO AND LATEMAR PLATFORMS
Ph.D. candidate: ALESSANDRO MARANGON
Tutor: Prof. PAOLO MIETTO
Co-tutor: Prof. NEREO PRETO
Cycle: XXIII
Abstract
Variations of magnetic mineral concentrations in shallow marine environments appear to be in tune with a depth index
derived from facies cyclicity. Two platforms (Latemar and Monte Agnello, Southern Alps, Italy) were chosen to verify this
idea. No magnetic data could be collected from Monte Agnello because of the strong dolomitization of the buildup. In the
Latemar the magnetic signal appears related to aeolian dust, while facies ranking seems a proxy for sea level changes. Facies
ranking and magnetic parameters result uncorrelated even if their spectra are similar. This suggests that two independent
global proxies are beating at the same frequencies, and implicates climate variations as part of the cause of the Latemar
carbonate cyclicity. Lastly, facies analysis revealed the growth history of the so far unknown Agnello platform and a
consistent microbial contribution for the formation of Latemar platform (especially in the slopes).
Introduction
Cyclostratigraphy is emerging as a central focus in stratigraphy, with its impressive record of
global climate changes forced by Earth’s astronomical parameters, and with its capacity to provide
high- resolution information about geologic time. Shallow-marine cyclostratigraphy, principally
from carbonate-rich peritidal facies, could represent a major source of information about
astronomical forcing and global climate change prior to the Jurassic. Mineral magnetic parameters
(MS, ARM, SIRM, S-ratio, ARM/MS and ARM/SIRM ratio) provide new information about
shallow-marine cyclostratigraphy. They reveal a coherent signal indicating magnetic concentration
variations in tune with a depth index derived from facies cyclicity (Mayer & Appel, 1999; Latta et
al, 2006; Kodama et al., 2010).
The Dolomites area of the Southern Alps (Italy) was characterized, at the end of the Anisian
(Middle Triassic), by an episode of exceptionally high subsidence, that caused dramatic aggradation
of isolated carbonate platforms. Some carbonate buildups grew up to 700 m (e.g., Brack et al.,
2007) until subsidence rates dropped and a progradational phase began (Bosellini, 1984). Thanks to
the exceptional preservation and exposure, the sedimentary cyclicity of the Latemar and Monte
Agnello platform interiors, represented by high order peritidal cycles, is evident.
The combined study of facies and magnetic parameters is a powerful tool in investigating
cyclicities and opens new issues about its origin.
Two coeval platforms were choosen to test this new method for cyclostratigraphic purposes:
Monte Agnello and Latemar. Unfortunatly, Monte Agnello resulted strongly affected by
dolomitization and thus results not suitable for magnetic studies. We then focused our attention on
the Latemar platform, which is by far better preserved with respect to the Agnello buildup.
Growth history of the Agnello Platform
Strong dolomitization characterizes the whole platform, making the study of magnetic parameters
impossible. Nonetheless field data allowed to reconstruct the growth history of the buildup, a
carbonate platform never studied before.
A detailed geological mapping of Monte Agnello platform was undertaken and geological data
were draped on a high resolution Digital Terrain Model in order to evaluate the geometrical
parameters of the platform. Stratigraphic sections were logged within the upper slope-marginlagoon progradational system, and the microfacies of the platform interior were compared with
those of the nearby aggradational Latemar platform. A biostratigraphic study of dasycladacean
algae and scattered ammonoids findings was also carried out, but yielded few results. However,
ammonoids of the avisianum and crassus subzones were recovered in the lower-middle part of the
aggradational platform interior. It was possible to reconstruct the growth history of this platform.
The Agnello massif preserves a portion of a carbonate platform that was prograding towards North,
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Scuola di Dottorato in Scienze della Terra
Dipartimento di Geoscienze, Università degli Studi di Padova –A.A. 2009-2010
although it is impossible to determine whether the platform was isolated or attached to a putative
southern structural high. It grew nearly 600 m until subsidence rates suddenly dropped, and then
prograded at least 3.5 km; the buildup reached a total thickness of about 700 m. Clinoforms are
steep, 30° on average. The platform sediments are sealed by a subaerial pyroclastic succession that
lies on a slightly karstified surface. Extended microbial crusts (including common Tubiphytes),
and corals characterized margin and upper slope
during the progradational phase. The inner platform is constituted by submetric peritidal
sedimentary cycles with prevailing subtidal facies. Microfacies are more micritic, and grains more
deeply micritized than those of the aggrading Latemar platform, reflecting longer residence time of
lagoonal sediments before burial. Well developed tepee belts as those of the Latemar platform are
absent. Thin sections analysis reveals that sedimentary environments changed significantly in the
lagoon at the switch from aggradation to progradation. The thickness of the platform is comparable
or higher than that of other coeval platforms in the Southern Alps, including those that underwent
drowning in the Late Anisian. This suggests that strong subsidence was not the primary cause of
drowning, although it may have enhanced the effects of paleoceanographic or climatic factors as
suggested by Preto et al. (2005) and Brack et al., (2007).
The petrological and magnetic mineral composition of the Latemar cycles
Several authors studied the Latemar platform: its exceptional exposure allows a detailed
investigation above all for cyclostratigraphic purposes. Biostratigraphic and radiometric data appear
in strong contrast with sequence stratigraphy. A 5:1 bundling is well evident in the Latemar cycles,
which was always interpreted as the response of short eccentricity and long precession, occurring in
the Middle Triassic every 100 and 22 Kyrs. This implies a 10 Myrs time span for the whole
Latemar platform. Ammonoids findings and radiometric data instead give less than 1 Myr time span
for the buildup development. In this case, the 5:1 bundling should refer to sub-Milankovitch cycles
of unknown origin. This is the so called “Latemar Paradox”. Using the magnetic properties of the
Latemar limestones, we tried to find a solution to this controversy.
The Latemar massif appears to be more suitable than Monte Agnello for magnetic analysis
because several portions of the platform are not affected by dolomitization. One hundred and two m
of inner platform series were sampled at Cimon del Latemar in order to investigate MS, ARM,
SIRM, S-ratio, ARM/MS and ARM/SIRM ratio. About 500 samples were collected. The chosen
sampling rate was 20 cm on average, we tried to sample at least 4 samples per cycle. SEM
observation and Lowrie Test were carried out on a subset of samples to determine the mineralogy of
the magnetic grains. All the measurements were made in collaboration with prof. Ken Kodama
(Lehigh University, Bethlehem, USA) while the spectral analysis was performed with the
collaboration of prof. Linda Hinnov (Johns Hopkins University, Baltimore, USA).
The results obtained are here summarized:
 Facies measurements exhibit a cyclic pattern. The spectra obtained from the facies rank are
anyway quite noisy: that happens because facies ranking implies interpretation, and because facies
ranks are discretized to only 4 possible values. Problems are still present in the definition of a
sedimentary cycle and in its recognition in the field.
 A clear cyclic signal emerges from the spectra related to the magnetic parameters. Some of the
parameters chosen are more suitable than others for a cyclostratigraphic purpose, depending on
what each parameter is measuring. For example, MS, measuring all the magnetic components of the
rock (diamagnetic, paramagnetic and ferromagnetic), is not a good tool in the case of carbonate
rocks. The diamagnetic component of carbonate is prevailing on the ferromagnetic minerals,
masking in this way their contribution. ARM and SIRM instead measure only the ferromagnetic
component (all the minerals in the matter of SIRM, low coercivity components in the matter of
ARM).
 A comparison between the spectra from rank series and magnetic parameters reveals similarities.
Two meaningful peaks describing a 5:1 ratio emerge in both cases and they are in the same
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Scuola di Dottorato in Scienze della Terra
Dipartimento di Geoscienze, Università degli Studi di Padova –A.A. 2009-2010
frequency range.
 Facies ranking and magnetic parameters appear uncorrelated, with a correlation coefficient of
nearly zero. This suggests they may be related to different causes, or a same process triggered both
ciclicities producing cycles that are out of phase. Facies ranking was so far interpreted as a proxy
for sea level changes (exhibiting an alternation of subtidal and supratidal facies), while magnetic
parameters reveal values falling in the range of aeolian dust (Oldfield et al. 1985, Hounslow and
Maher, 1999).
 We can observe in the field two kind of cycles: a first one on the order of 1 m and a second
one on the order of 5 m. They are the physical expression of the 5:1 ratio clearly visible also in the
spectra. Magnetic data do not reveal if the cyclic pattern refers to a Milankovitch or a subMilankovitch signal, even if calculations made on the basis of the periodograms suggest a subMilankovitch forcing for the 1 m cycle, giving thus reason to radiometric data.
Another problem emerged during sampling the Latemar platform: field observations suggested a
much higher microbial component compared to literature data. This was evident especially in the
fore-reef/upper slope zone, were microbials clearly extended for thousands of m up to 350/400 m
deep. Point counting analysis in thin sections of reef, slope and inner platform confirmed field
evidence. All components were grouped in 5 categories: skeletal grains, allomicrite, microbialite,
cements and voids. Quantitative analysis was carried out for each portion of the platform: inner
platform, reef and slope. Cements and microbialite are the most represented categories. The
percentages of all components were recalculated omitting cements (i.e., the abiotic component), the
results are shown in the table below (Tab.1). Our results compare well to those reported for the
Sella platform by Keim and Schlager (2001). We thus suggest that the Latemar buildup developed
following the M-Factory model (Schlager, 2005). Microbial contribution is higher on the slope,
although the whole platform is dominated by microbial carbonates.
PERCENTAGE
COMPONENTS
Skeletal grains
Allomicrite
Microbialite
Voids
Cements
Inner
7.3
20
30.7
0.2
39.6
With cements
Reef
Slope
8.4
7.9
6.4
16.7
33.4
41.6
2
0.1
49.6
33.1
Inner
6.07
37.53
56.25
0.14
-
Without cements
Reef
Slope
16.71
11.95
12.81
25.20
66.2
62.7
3.86
0.15
-
References
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carbonate buildups of the Dolomites, Northern Italy. In Reefs in Time and space, Soc. Econ.
Paleont. Min., Spec. Publ., 18, 209-233, Tulsa.
BRACK P., RIEBER H., MUNDIL R., BLENDINGER W. & MAURER F. 2007 Geometry and
chronology of growth and drowning of Middle Triassic carbonate platforms (Cernera and
Bivera/Clapsavon) in the Southern Alps (northern Italy). Swiss Journal of Geosciences 100 (3),
327-347.
HOUNSLOW M.W. AND MAHER B.A. 1999, Source of the cli- mate signal recorded by magnetic
susceptibility variations in Indian Ocean sediments, J. Geophys. Res., 104, 5047–5061.
KODAMA K.P., ANASTASIO D.J., NEWTON M.L., PARES J.M., AND HINNOV L.A. 2010,
High resolution rock magnetic cyclostratigraphy in an Eocene flysch, Spanish Pyrenees, Geochem.
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Scuola di Dottorato in Scienze della Terra
Dipartimento di Geoscienze, Università degli Studi di Padova –A.A. 2009-2010
Geophys. Geosyst., 11.
LATTA, D.K., ANASTASIO D.J., HINNOV L.A., ELRICK M. AND KODAMA K.P. 2006,
Magnetic record of Milankovitch rhythms in lithologically non cyclic marine carbonates, Geology,
34(1), 29–32.
MAYER, H. AND APPEL, E. 1999, Milankovitch cyclicity and rock-magnetic signatures of
palaeoclimatic change in the Early Cretaceous Biacone Formation of the Southern Alps, Italy.
Cretaceous Research, 20, 189-214.
OLDFIELD F., HUNT A., JONES M. D. H., CHESTER R., DEARGIN J. A., OLSSON L. AND
PROSPERO J. M. 1985, Magnetic differen- tiation of atmospheric dusts, Nature, 317, 516–518.
PRETO N., SPÖTL C., MIETTO P., GIANOLLA P., RIVA A., MANFRIN S., 2005. Aragonite
dissolution, sedimentation rates and carbon isotopes in deep-water hemipelagites (Livinallongo
Formation, Middle Triassic, northern Italy). Sediment. Geol. 181, 173–194.
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Scuola di Dottorato in Scienze della Terra
Dipartimento di Geoscienze, Università degli Studi di Padova –A.A. 2009-2010
SUMMARY Ph.D ACTIVITY
Courses
2007-2008
ARTIOLI G., DI TORO G., FIORETTI A.: “Corso di comunicazione scientifica” Università degli
studi di Padova
FLORIS M.: “Introduction to GIS Tecniques”, Università degli Studi di Padova
JENKYNS H.: Short course “Chemostratigraphy: applications, limitations and implications for
global environmental change” , DISGAM, Univesità di Trieste
PESARIN F., SALMASO L.: “Introduction to statistical methods”, Università degli studi di Padova
VOLPI B., GERBASIO D.: “Geologia dei giacimenti petroliferi”, DISGAM, Università di Trieste.
WALTON G.: “Corso di inglese scientifico”, Università degli studi di Padova
2008-2009
PAGANI M., Global climate dynamics and evolution, Dipartimento di Geoscienze, Università degli
Studi di Padova
BOESSO S., Corso di introduzione alla Biblioteca, Dipartimento di Geoscienze, Università degli
Studi di Padova
REMONDINO F., (ETH Zeurich)., Photogrammetry, Dipartimento di Geoscienze, Università degli
Studi di Padova
TAVIANI M., MORSILLI M., BOSELLINI F:, SILVESTRI G., Coral Reef Biogelogy, Marsa
Alam, Mar Rosso, Egitto
2009-2010
HINNOV L.A., Climates of the past, Johns Hopkins University, Baltimore (MD), U.S.A.
Course of seminars for Ph.D. school, Johns Hopkins University, Baltimore (MD), U.S.A.
Course of seminars for Ph.D. school, Lehigh University, Bethlehem (PA), U.S.A.
Ph.D work group on Tectonic and Stratigraphy, Lehigh University, Bethlehem (PA), U.S.A.
Lecture on “Paleomagnetism Laboratory: metholdologies and utilization”, Ken Kodama, Lehigh
University, Bethelehem (PA), U.S.A.
Communications:
MARANGON A., Latemar microfacies: microbial contribution to carbonate production, San
Donato Milanese, 10 luglio 2009
MARANGON A., MIETTO P., PRETO N., Stratigraphic analysis on Monte Agnello and Latemar
platforms, Stratigraphy group, Johns Hopkins University, Baltimore, (MD), U.S.A., 12 febbraio
2010
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Scuola di Dottorato in Scienze della Terra
Dipartimento di Geoscienze, Università degli Studi di Padova –A.A. 2009-2010
MARANGON A., MIETTO P., PRETO N., Stratigraphic analysis on Monte Agnello and Latemar
platforms, Magnetostratigraphy group, Lehigh University, Bethlehem (PA), U.S.A., 5 marzo 2010
PRETO N., MARANGON A., FRANCESCHI M., The Latemar platform: a sedimentological
overview and 3D facies distribution, Sand Donato Milanese, 2 novembre 2010
Posters:
MARANGON A., MANFRIN S., MIETTO P. & PRETO N. - Anisian paleogeography of Adige
Valley from facies analysis of Carbonate Platforms, Sassari, settembre 2008
MARANGON A., RIVA A., PRETO N., MIETTO P. - The growth history of the middle triassic
Agnello Platofrm 27th IAS meeting of sedimentologists, Algehro, 20-23 settembre 2009
Publications:
MARANGON A., MANFRIN S., MIETTO P. & PRETO N. – Anisian paleogeography of Adige
Valley from facies analysis of Carbonate Platforms – Rend. Online Soc. Geol. It., 3, 519-520.
Teaching activities:
Teaching assistant: 25 hours, “Laboratorio di Cartografia geologica”, Laurea di primo livello in
Scienze geologiche (2008/2009).
Teaching assistant: 25 hours, “Rilevamento Geologico”, Laurea magistrale in Scienze della natura
(2008/2009).
Teaching assistant: 25 hours,
(2009/2010).
“Paleontologia”, Laurea di primo livello in Scienze geologiche
Foreign activities:
Febbraio-Giugno 2010 (5 mesi) Research collaborator at the Johns Hopkins University (Baltimore,
MD, U.S.A.) with Prof. Linda Hinnov and at the Lehigh University (Bethlehem, PA, U.S.A.) with
Prof Ken Kodama.
Others
Meeting annuale 2008 del Gruppo Italiano Carbonati, San Donato Milanese, 28-30 aprile 2008
Workshop on Triassic plaeoclimatology “The Triassic climate”, Bolzano, 3-7 giugno 2008
7th International Triassic Field Workshop “Dolomites – Field trip to the World Heritage Site of the
Tethyan Triassic”, Dolomiti, 5-10 Settembre 2010
Agosto 2010: Field work on Rio Sacuz section in collaboration with Prof. Ken Kodama (Lehigh
University, Bethlehem), Zachary Spahn (graduate student, Lehigh University, Bethlehem) and Dr.
Alessio Ponza (Università di Bologna)
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