Sub‐Milankovitch and Milankovitch forcing on a model Mesozoic carbonate platform – the Latemar (Middle Triassic, Italy)

Abstract

Basic shallowing‐upward cycles (shu‐cycles) and five‐bundled megacycles in the Latemar platform have been widely regarded as a model example for precessional and eccentricity forcing in the Mesozoic. Based on bio‐/chronostratigraphic data, recent studies questioned this particular type of Milankovitch forcing on the Latemar cyclic series. We present an integrated model that incorporates (i) new cyclostratigraphic data, (ii) new and existing bio‐/chronostratigraphic data and (iii) new spectral analyses. The basic shu‐cycles in the Latemar reflect sub‐Milankovitch control. Cycle bundles of 1 : 4–5 (megacycles : shu‐cycles) indicate precessional forcing. They do not reflect eccentricity superimposed on precessional forcing. Spectral analyses reveal highly significant ratios in the large‐scale cycle bundlings. Stacking patterns of 1 : 9.9 and 1 : 24.0 shu‐cycles represent obliquity and short eccentricity forcing. Both sub‐Milankovitch and Milankovitch forcing potentially controlled shallow subtidal carbonate deposition in Mesozoic greenhouse times. Cyclostratigraphic models require an integrated approach including bio‐ and chronostratigraphic data.

Are beds in shelf carbonates millennial-scale cycles? An example from the mid-Carboniferous of northern England…

“…These classic metre-scale peritidal cycles have been shown to be sub-orbital in origin (Zühlke et al, 2003;Mundil et al, 2003), with a duration of ~4000 years each, in contrast to earlier interpretations of a precession origin. In the context of this paper then, at least some high-frequency peritidal cycles are themselves on a millennial scale and their constituent beds will be millennial scale too, or centennial.…”

High-frequency sea-level changes recorded in deep-water carbonates of the Upper Cretaceous Dol Formation (island of Brač, Croatia)

“…Orbitally controlled waxing and waning of the ice caps but also thermal expansion and retraction of the ocean surface waters, translate into high-frequency sea-level changes that, on the platform, may lead to meter-scale shallowing-upward cycles (e.g. Strasser 1991;Goldhammer et al 1993;Strasser 1994;D'Argenio et al 1997;Buonocunto et al 1999;Strasser et al 1999;Yang & Lehrmann 2003;Zühlke et al 2003;Strasser et al 2006). Miller et al (2003) have shown that the Late Cretaceous ice sheets had large and rapid influence on sea-level changes.…”

Sea level changes versus hydrothermal diagenesis: Origin of Triassic carbonate platform cycles in the Dolomites, Italy: Discussion

“…These fluids are said to produce stratiform diagenetic features including tepees, and they favoured early dolomitization and the growth of cyanobacterial mats. This interpretation deviates strongly from the conventional models where these cycles are considered to be the result of relative sea-level fluctuations in a shallow-marine to subaerially exposed environment (Hardie et al, 1986;Goldhammer et al, 1987Goldhammer et al, , 1990Goldhammer et al, , 1993Brack et al, 1996;Mundil et al, 1996;Egenhoff et al, 1999;Preto et al, 2001;Zühlke et al, 2003).We welcome this entirely new approach, as it further stimulates a discussion on the validity of criteria used to detect subaerial exposure from carbonate facies. Furthermore, it highlights the importance of microbial growth and cementation processes for the development and the geometrical maintenance of post-extinction carbonate platforms such as the Latemàr.…”