The Milankovitch Interpretation of the Latemar Platform Cycles (Dolomites, Italy): Implications for Geochronology, Biostratigraphy, and Middle Triassic Carbonate Accumulation
Published:January 01, 2004
Nereo Preto, Linda A. Hinnov, Vittorio De Zanche, Paolo Mietto, Lawrence A. Hardie, 2004. "The Milankovitch Interpretation of the Latemar Platform Cycles (Dolomites, Italy): Implications for Geochronology, Biostratigraphy, and Middle Triassic Carbonate Accumulation", Cyclostratigraphy: Approaches and Case Histories, Bruno D’Argenio, Alfred G. Fischer, Isabella Premoli Silva, Helmut Weissert, Vittoria Ferreri
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A 160-m-long section measured in the lagoonal facies of the Middle Triassic Latemar platform (Dolomites, Italy) reveals a set of frequency components that we interpret as a strong Milankovitch signal. In this interpretation, all principal frequencies associated with the theoretical Middle Triassic precession index, P1 = 1/(21.7 ky), P2 = 1/(17.6 ky), and its modulations, E1 = 1/(400 ky), E2 = 1/(95 ky),and E3 = 1/(125 ky), were detected in a time-frequency evaluation of the cycles. A weak obliquity signal is also present in part ofthe section.Thus, the Latemar cycles appear to have recorded the clearest orbital forcing signal yet found in a carbonate platform. This astronomical calibration indicates that the section was deposited in ca. 3.1 My and therefore that the entire Latemar cyclic succession (~470 m) took at least 9 My to form. However, the calibration also leads to serious conflicts with other interpreted geological data: U/Pb radiometric ages of zircons collected from tuffites within theLatemar lagoon and in coeval basinal sediments point to a timescale that is five times shorter than this astronomically calibrated estimate; similar discrepancies arise when the average duration of Triassic ammonoid biozones or the sedimentation rates of coeval basinal series are considered. Nonetheless,all of the methods that have been used to estimate the time of formation of the Latemar platform continue to have shortcomings, and the contradictions among these different geologicalcalibrations remain unresolved.
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Cyclostratigraphy: Approaches and Case Histories
This volume is derived from an SEPM international workshop entitled Multidisciplinary Approach to Cyclostratigraphy, organized by the editors in May 2001 and held in Sorrento (Naples, Italy). In the Introduction we offer a brief history of how concepts of orbital cyclicity and its effects on the Earth evolved, an appraisal of the present state of research, and an overview of the papers in this volume. The main body of the volume consists of the contributed studies. These include a paper on conceptual and pragmatic approaches to stratification cycles by one of the pioneers of cyclostratigraphy, Walther Schwarzacher, who, in the 1940s, discovered the hierarchical expression of orbital cycles in rocks. The other contributions are specific studies of cyclic sequences, extending from the Quaternary back to the Triassic, covering the range from continental deposits to the deep sea, and employing a wide variety of techniques for extracting and processing the information.