Depositional periodicity and the hierarchy of stratigraphic forcing in the Triassic carbonates of the Dolomite Alps, N. Italy
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The Dolomite Alps of northern Italy are a classic field locality in the development carbonate stratigraphic theory. Included in the many discoveries rooted in the geology of the Dolomites is the concept of a hierarchy of stratigraphic forcing in the Alpine Triassic. The hierarchy states that carbonate sedimentation is dominantly a record of eustasy, resulting in organized stacking patterns, and that these stacking patterns reflect the interplay between low frequency (1-10 my) eustatic cycles and their component bundled high-frequency (100 & 20 kyr) eustatic cycles. The overall aim of this study is to further investigate the validity of the hierarchical model after recent dating of Anisian and Ladinian successions called the Milankovitchian periodicity and/or allocyclicity of the cyclic series into question. The study was completed using four sub-studies, 3 based on data collected in the field and a fourth based in cycle theory and computer modeling. First, it can be shown that allocyclic forcing exists in the Anisian/Ladinian platforms of the Dolomites by comparing the stratigraphic sections measured from 2 time-equivalent, independent carbonate platforms, the Latemar and Mendola Pass. Second, computer modeling of Anisian/Ladinian carbonate platform stratigraphy using Milankovitchain solar insolation as a proxy for high-frequency eustasy shows that both pure Milankovitch forcing and mixed Milankovitch/sub-Milankovitch forcing will produce synthetic carbonate platforms with stratigraphic successions comparable to those of the Anisian/Ladinian platforms of the Dolomites. Third, it can be shown that the while the Norian Dolomia Principale (a regional carbonate shelf) was affected by differential subsidence, megacycles systematically increase in their number of component cycles from 2-3:1 in the eastern Dolomites (updip) to 5-6:1 in the western Dolomites (seaward). In conclusion, the concept that carbonate platform stratigraphy is a record of an interplay between eustasy, subsidence, and sedimentation is upheld, while the validity of Milankovitchian forcing acting on all Alpine carbonate cycles is questioned. Instead, cyclic carbonates with sub-Milankovitch periodicities were common in the early and mid-Triassic, while cycles with Milankovitchian periodicities were common in the late Triassic.