The Latemar platform from the Dolomites of northern Italy is characterized by a superb lagoonal succession where Hinnov and Godhammer (1991) counted ≈600 shallowing-upward cycles in about 470 m of section. These cycles were attributed to a 12 m.y. record of Milankovitch orbital forcing of sea level. However, the Milankovitch origin of the cyclicity was challenged by radiometric age data. Two ash layers located in the Lower Cyclic Facies and Upper Cyclic Facies of the Latemar succession yielded U-Pb ages of 242.8 (±0.4) Ma and 241.5 (±0.4) Ma, respectively, implying a duration of ≈0.5--2 m.y. for the ≈400 cycles embraced by these dates. We contribute to the resolution of the Latemar controversy by presenting magnetostratigraphic data from the lagoonal succession. The Triassic has a long-term reversal frequency of about 2 rev/m.y. Therefore, if the Latemar cyclic succession lasted 12 m.y., we would have to encounter herein as many as 24 polarity reversals, whereas, on the contrary, if it lasted only ≈0.5--2 m.y., we should hardly find any. In fact, one single, clearly normal polarity magnetozone was found, in which one relatively short reversed submagnetozone is embedded towards the top of the section. Talking into account also the available biostratigraphic information, which constrain the Latemar cyclic deposition to levels immediately around the Secedensis Ammonoid Zone, we obtained a satisfactory magnetostratigraphic match with the lowermost portion of the polarity sequence of reversals from the partly coeval basinal Buchenstein Beds from the literature. We infer that Latemar deposition should not have lasted more than 2m.y. Instead, accepting the validity of U-Pb ages, the ≈600 Latemar cycles encompass a total time interval which would associate them with a millennial-scale sub-Milankovitch band.
|Titolo:||Millennial-scale cycles at Latemar|
MUTTONI, GIOVANNI (Primo)
|Data di pubblicazione:||mar-2003|
|Settore Scientifico Disciplinare:||Settore GEO/02 - Geologia Stratigrafica e Sedimentologica|
|Citazione:||Millennial-scale cycles at Latemar / G. Muttoni, D.V. Kent. ((Intervento presentato al convegno Joint Assembly tenutosi a Nice nel 2003.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|