GSSP for Tortonian Stage
Definition:The base of the Tortonian Stage is defined as the mid-point of the sapropel layer of basic cycle number 76 in the Monte dei Corvi Beach section, located on the Adriatic coast of central Italy, 5km SE of Ancona. This level has been assigned an astronomical age of 11.625 Ma. It coincides almost exactly with the Last Common Occurrences of the calcareous nannofossil Discoaster kugleri and the planktonic foraminifer Globigerinoides subquadratus, and is associated with the short normal-polarity subchron C5r.2n. This level coincides closely with oxygen isotope event Mi-5 and the associated glacio-eustatic sea-level low-stand of supercycle T3.1 and concurrent deep-sea hiatus NH4.
Sedimentology:The Serravallian and lower Tortonian is exposed along the eastern slopes and in the coastal cliffs of Monte dei Corvi. This section contains the upper part of the marly Schlier Formation and consists of a cyclic alternation of greenish-grey marls, whitish marly limestones and brown colored organic-rich layers (sapropel). In addition two biotite-rich volcanic ash layers, named Respighi and Ancona, are present.
The depositional environment remained (hemi)pelagic throughout the succession. Bottom water conditions changed cyclically between oxic (marls) and anoxic or dysoxic (sapropel layers).
Primary Markers:The Monte dei Corvi Beach section is composed of a cyclic alternation of marls, marly limestones and organic-rich beds (Montanari et al., 1997). The basic small-scale cycle is a couplet (between 0.3 and 1.0 m thick) which consists of an indurated whitish marly limestone and a softer grey to greenish-grey marl. Brownish to blackish colored organic-rich beds termed sapropels are frequently but not always intercalated in the limestones.
Larger-scale cycles can be distinguished in addition and comprise both small-scale and large-scale sapropel clusters. Small-scale clusters contain 2 to 4 sapropels (and 5-6 basic cycles); large-scale clusters contain several small-scale clusters (and up to 20 basic cycles). Finally a cycle of intermediate-scale is recognized by repetitive sapropel patterns in every other basic cycle. Previous studies of marine sections of Late Miocene to Pleistocene age in the Mediterranean (Hilgen, 1991; Hilgen et al., 1995; Lourens et al., 1996) showed that similar sapropel patterns reflect the astronomical cycles of precession (basic cycle), obliquity (intermediate cycle) and eccentricity (larger-scale cycles). Using the same phase relations as established for the younger sapropels, the sedimentary cycles in the Monte dei Corvi Beach section were tuned to the precession and insolation time series of the La93 solution (Laskar, 1990; Laskar et al., 1993). This tuning provides astronomical ages for all the sedimentary cycles, calcareous plankton bioevents, magnetic reversals and ash beds recorded in the section. It shows that the entire beach section ranges from 13.4 to 8.5 Ma and that the Tortonian GSSP has an astronomical age of 11.625 Ma (Hilgen et al., 2003, 2005).
The GSSP is close to the Last Common Occurrence of Globigerinoides subquadratus. The GSSP falls within the Paragloborotalia siakensis Zone of Iaccarino (1985).
The GSSP closely coincides with the Last Common Occurrence of Discoaster kugleri.
The GSSP is closely associated with the base of the short normal subchron C5r.2n.
Correlation Events:Calcareous nannofossil last common occurrence of Discoaster kugleri
Other Locations around the World:An auxiliary boundary stratotype section was designated along the southern slopes of Monte Gibliscemi, Sicily (Italy). This section has been correlated bed by bed to the Monte dei Corvi section. The Last Common Occurrence of Discoaster kugleri and Globigerinoides subquadratus appear to be synchronous between the Mediterranean and the low-latitude open ocean. The GSSP further coincides almost exactly with the Barstovian-Clarendonian Mammal Age boundary in North America.
Notes on Derivation of Age:Astronomical cycles in sediments
Hilgen, F. J., Abdul Aziz, H., Bice, D., Iaccarino, S., Krijgsman, W., Kuiper, K., Montanari, A., Raffi, I., Turco, E., Zachariasse, W. J., 2005. The Global Boundary Stratotype Section and Point (GSSP) of the Tortonian Stage (Upper Miocene) at Monte dei Corvi. Episodes, 28/1, p. 6 - 17.
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