Content
Contact
Email
Thilo Bechstädt (Info)
Rainer Zühlke (Info)
Phone
+49 6221 54-8292
+49 6221 54-6055
Fax
+49 6221 54-5503
Thilo Bechstädt (Info)
Rainer Zühlke (Info)
Phone
+49 6221 54-8292
+49 6221 54-6055
Fax
+49 6221 54-5503
Project Login
Cyclostratigraphy and High-Frequency Controls of Carbonate Platform development – Latemar, Triassic, N-ItalyAbstract & Detailed Project Description |
||
 |
||
| Scientists Rainer Zühlke, GeoResources and University of Heidelberg Contributions Thilo Bechstädt, GeoResources and University of Heidelberg Peter Brack, ETH Zürich Roland Mundil, Berkeley Geochronology Center Hans Rieber, ETH Zürich |
||
| Abstract | ||
| An integrated cyclcstratigraphic approach has been 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. Hierarchic cycles include meter-scale shallowing upward microcycles and 2-6 bundled thinning upward macrocycles. Lateral tracing and correlation of micro- and macrocycles provides a high-resolution 2D architectural model of the platform interior. The overwhelming number of micro- and macrocycles is physically persistent over the platform top with only subordinate 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 cycles in the platform interior include little more than a single ammonoid biozone (Secedensis Zone), that the total time interval is shorter than 4.96 My (Ø 2.27 My), and that average microcycle period is shorter than 7.61 ky (Ø 3.41 ky). Microcycles can not be reconciled with precession forcing, but reflect sub-Milankovitch forcing. Spectral analysis is exclusively based on accommodation cycles, which represent the only direct indication for external control on cyclic deposition. Blackman-Tukey spectral, multi-taper spectral and harmonic analyses indicate highly similar and significant frequencies/amplitudes which are largely stationary over all subsets applied to the cyclic series. Ratios and periods indicative of orbital forcing in the Milankovitch band 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 macrocycles short and long precession forcing (18, 21 ky), and higher order cycles bundles short and long obliquity (35, 45 ky) as well as short eccentricity forcing (95-105 ky). Due to significant latitudinal temperature gradients and seasonal climate differences, the Triassic period held a significant potential for sub-Milankovitch fluctuations in coupled ocean-atmospheric 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-dependant 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, the "Latemar Controversy", between biochronostratigraphic and cyclostratigraphical time scales. In contrast the new cyclostratigraphic model of this study is based on a complete 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. |
||
| Detailed Project Description ... |
||
| Virtual Field Trip ... |
||
