Integrated Cyclostratigraphy of a Model Mesozoic Carbonate Platform—The Latemar (Middle Triassic, Italy)
Published:January 01, 2004
Rainer Zühlke, 2004. "Integrated Cyclostratigraphy of a Model Mesozoic Carbonate Platform—The Latemar (Middle Triassic, Italy)", Cyclostratigraphy: Approaches and Case Histories, Bruno D’Argenio, Alfred G. Fischer, Isabella Premoli Silva, Helmut Weissert, Vittoria Ferreri
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An integrated cyclostratigraphic approach was applied to the 460-m-thick succession in the Latemar platform interior. The approach uses new high-resolution cyclostratigraphic data from vertical sections, lateral tracing of physical surfaces over the platform top, new and existing biostratigraphic data, existing isotopic ages from volcanic ash layers, and new spectral analyses in order to develop a genetic cyclostratigraphic model. Hierarchical cycles include meter-scale shallowing-upward microcycles and 2–6 bundled thinning-upward macrocycles. Lateral tracing and correlation of microcycles and macrocycles provides a high-resolution 2D architectural model of the platform interior. The large majority of microcycles and macrocycles is physically persistent over the platform top with only moderate changes in thickness and internal facies. The platform top showed simultaneous vertical aggradation controlled by low-amplitude, high-frequency sea-level changes. Tied-in cyclostratigraphic and biochronostratigraphic data indicate that the 619–701 microcycles in the platform interior include little more than a single ammonoid biozone (Secedensis Zone), that the total time interval is shorter than 4.10 Ma(average total time interval =1.88 My), and that the interpolated microcycle period is shorter than 5.85 ky (average interpolated microcycle period = 2.68 ky). Microcycles cannot be reconciled with precession forcing but reflect sub-Milankovitch forcing. Spectral analysis is based exclusively on accommodation cycles, which represent the only direct indication of external control on cyclic deposition. Blackman-Tukey spectral, multi-taper spectral, and harmonic analyses indicate highly similar and significant frequencies and amplitudes which are largely stationary over all subsets applied to the cyclic series. Ratios and periods indicative of orbital forcing in the Milankovitch band potentially exist at (very) high significance points with Δt = 4.2 ky. In the Latemar cyclic succession, basic microcycles represent sub-Milankovitch forcing (4.2 ky), thinning-upward, 2–6 bundled macrocycles short- and long-precession forcing (18, 21 ky), and higher-order cycle bundles short and long obliquity (35, 45 ky) as well as short-eccentricity forcing (95–105 ky). Because of significant latitudinal temperature gradient and seasonal climate differences, the Triassic period held a significant potential for sub-Milankovitch fluctuations in coupled ocean-atmosphere circulation. They probably triggered low-amplitude, high-frequency changes in sea level and controlled the deposition of sub-Milankovitch microcycles. Previous studies of the Latemar carbonate platform favored a model-dependent approach based on smaller cyclostratigraphic datasets from single sections and spectral analyses. The resulting orbital-forcing models could not be reconciled with the existing biochronostratigraphic framework for the Triassic and the Anisian to Ladinian stages. They left a widely noted deep disagreement between biochronostratigraphic and cyclostratigraphic time scales. In contrast, the new forcing model of this study is based on a complete 2D cyclostratigraphic dataset, considers all biochronostratigraphic constraints, and includes time-calibrated spectral analyses. The model reconciles biochronostratigraphic and cyclostratigraphic time scales. The Latemar cyclic series includes the oldest explicit sub-Milankovitch signal and the oldest set of both sub-Milankovitch and Milankovitch signals yet observed in the geologic record.
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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.