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Stratigrafia magnetica ad alta risoluzione del limite Eocene-Oligocene nella successione Umbro-Marchigiana

High-resolution magnetostratigraphy of the eocene-oligocene boundary in the umbro-marchean appenine

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Abstract

High-resolution magnetostratigraphy across the Eocene-Oligocene boundary has been employed in a detailed investigation of the nature of low-amplitude, short-wavelength oceanic magnetic anomalies. A core, 39.4mlong and 10 cm in diameter, was drilled through the Eocene-Oligocene boundary near to the Massignano Quarry stratotype section near Ancona, Italy. The stratigraphy of the core, which traverses the Scaglia Variegata and Scaglia Cinerea formations, was correlated precisely to the quarry section by linear regression of the depths of identifiable biotite-rich layers. The good recovery of intact material allowed an average sampling interval of about 12 cm, which is closer than in preceding magnetostratigraphic studies of Umbrian-Marche sequences. The characteristic remanent magnetization was obtained by both progressive alternating field and thermal demagnetizations. The stable component of the natural remanent magnetization could be isolated by thermal demagnetization at temperatures of 300–540°C or by alternating field demagnetization in fields higher than 20 mT. It is probably carried by magnetite in the Scaglia Cinerea marls, while some amount of hematite is present in the underlying Scaglia Variegata. A stratigraphic plot of the ChRM directions shows well-defined magnetozones and the resulting polarity sequence correlates well with polarity chrons C12r to C16n-2. A few single-sample normal magnetozones that do not correspond to the geomagnetic polarity timescale are found within chron 16n.1-r. The magnetozones corresponding to chrons C12r or C13r do not exhibit short subchrons that might account for the low-amplitude and short-wavelength magnetic anomalies reported in this part of the marine magnetic record. In investigation of relative paleointensity fluctuations has been carried out in this part of the core, which embraces the Scaglia Cinerea formation. Anhysteretic remanent magnetization (ARM) has been used to normalize the natural remanent magnetization (NRM), compensating variations in sedimentary input. The ensuing NRM/ARM ratio is taken to be a proxy for relative variation of paleomagnetic field intensity. The paleointensity fluctuates systematically and has minimum values close to the reported positions of low-amplitude, short-wavelength magnetic anomalies in the marine record.

Riassunto

La stratigrafia magnetica ad alta risoluzione del limite Eocene-Oligocene ed il magnetismo delle rocce sono stati studiati in dettaglio per verificare la presenza di eventuali inversioni di polarità brevi (short chrons) e variazioni della intensità del campo geomagnetico che possano essere messe in relazione con le anomalie oceaniche a bassa ampiezza e corto periodo. Una carota della lunghezza di 39,4 m e del diametro di 10 cm è stata perforata attraverso il limite Eocene-Oligocene nei pressi della sezione-tipo di Massignano (Ancona, Italia). La sequenza stratigrafica della carota, costituita dalle formazioni della Scaglie Cinerea e Variegata, è stata correlata con precisione alla sezione-tipo mediante alcuni marker stratigrafici costituiti da straterelli vulcanoclastici. Il buon recupero di materiale intatto della carota ha reso possibile una magnetostratigrafia con un intervallo di campionamento medio di circa 12 cm mai ottenuto nei precedenti lavori sulla successione Umbro-Marchigiana. Le direzioni caratteristiche della magnetizzazione naturale (ChRM) sono state ottenute mediante la demagnetizzazione progressiva sia in campi alternati (AF) che termica. La componente stabile della magnetizzazione rimanente naturale (NRM) è stata isolata per coercività maggiori di 20 mT e temperature comprese fra i 300 °C ed i 540 °C. Il principale minerale magnetico nella formazione della Scaglia Cinerea è risultato essere magnetite mentre nella Scaglia Variegata sono presenti sia magnetite che ematite. La stratigrafia magnetica ottenuta dalle direzioni della ChRM mostra magnetozone ben definite con una sequenza di polarità che si correla bene con ichrons dal C12r al C16n-2 della scala geomagnetica marina ad eccezione di alcune brevi magnetozone normali comprese nelchron C16n.1-r. Le magnetozone corrispondenti aichrons C12r e C13n non mostrano nessuna breve magnetozona che possa essere messa in relazione con le anomalie oceaniche a bassa ampiezza e corto periodo trovate in questa parte del record geomagnetico oceanico. Lo studio delle paleointensità relative nell’intervallo compreso neichrons C12r e C13n mostra invece delle fluttuazioni di intensità del campo geomagnetico con minimi in corrispondenza sia delle inversioni di polarità sia delle anomalie a bassa ampiezza e corto periodo riportate nella scala delle polarità magnetiche [1].

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Authors and Affiliations

  1. Institut für Geophysik, ETH Hönggerberg, 8093, Zurigo, Svizzera

    Luca Lanci & William Lowrie

  2. Istituto di Dinamica Ambientale, Università degli Studi di Urbino, 61029, Urbino

    Luca Lanci

  3. Osservatorio Geologico di Coldigiogo, 62020, Frontale di Apiro MC

    Alessandro Montanari

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  1. Luca Lanci
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  2. William Lowrie
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  3. Alessandro Montanari
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Correspondence to Luca Lanci.

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Nella seduta del 24 aprile 1998.

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Lanci, L., Lowrie, W. & Montanari, A. Stratigrafia magnetica ad alta risoluzione del limite Eocene-Oligocene nella successione Umbro-Marchigiana. Rend. Fis. Acc. Lincei 9, 103–123 (1998). https://doi.org/10.1007/BF02904394

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