Depositional cycles, composite sea-level changes, cycle stacking patterns, and the hierarchy of stratigraphic forcing: Examples from Alpine Triassic platform carbonates

Abstract

Carbonate platform deposits record a complex interplay of numerous geodynamic variables, of which eustasy, subsidence, and sediment accumulation are prime factors in determining both the kilometer-scale (depositional sequence scale) and meter-scale (depositional cycle scale) stratigraphic packaging. In this study, we looked particularly at composite eustasy, that is, superimposed sea-level fluctuations with different frequencies (defined as orders) and different amplitudes, and the role it plays in the linkage between meter-scale cyclic stratigraphy and kilometer-scale sequence stratigraphy. Specifically, we have investigated the relationship between low-frequency, third-order (1-10 m.y. period) depositional cycles and their component high-frequency, fourth-(0.1-1 m.y.) and fifth-(0.01-0.1 m.y.) order cycles through detailed stratigraphic analyses of Alpine Triassic platforms, complemented by computer modeling. On the basis of our results, we suggest that in general, there exists a hierarchy of stratigraphic forcing driven by composite eustasy that results in organized stacking patterns (thickness, subfacies character, early diagenetic attributes) of high-frequency, typically fourth- and fifth-order, shallow-water carbonate cycles dictated by low-frequency, third-order relative sea-level effects. We suggest that systematic vertical changes in stacking patterns of high-frequency cycles across a larger depositional sequence are due to systematic and predictable differences in depositional space available during the rising and falling stages of a relative third-order sea-level change. We also suggest that these systematic variations in cycle stacking patterns will exist regardless of the mechanism responsible for generating the high-frequency cycles, be it an autocyclic or allocyclic mechanism. This approach has major implications for the use of high-frequency, fourth- and fifth-order cycle characteristics to identify third-order cycles in outcrops and cores of shallow-water carbonates, where stratigraphic control may be less than desirable. This would constitute a valuable bridge between cyclic stratigraphy at the meter scale and sequence stratigraphy at the seismic scale. We present two examples from Triassic buildups of the Alps (the Ladinian Latemar buildup and the Norian Dolomia Principale) where a systematic succession of high-frequency cycle stacking patterns and early diagenetic features exists within an overriding third-order cycle (sequence) reflecting the interplay of short-term, high-frequency (fourth, fifth order) eustasy and long-term, low-frequency (third order) eustasy in accordance with the hierarchy of stratigraphic forcing. Central to the interpretation of these examples is the demonstration that true eustatic rhythms are recorded in the high-frequency cyclicity, as verified by time-series analyses and the use of Fischer plots. These examples can be modeled by computer under conditions of lag-depth-constrained sedimentation, uniform subsidence, and composite eustasy. We also present two examples, one from the Alpine Triassic (the Norian Lofer cyclothems) and one from the Pleistocene of south Florida, that lack both a systematic succession of high-frequency cycle stacking patterns and identifiable composite rhythms in the stratigraphic record, despite the existence of composite high-frequency eustasy. In these examples, we call upon (1) local tectonic forcing in the form of short-term deviations in subsidence via faulting (Lofer example) and (2) large differences in the relative amplitudes of different orders of high-frequency sea-level oscillations (Pleistocene example) to explain the lack of composite rhythms, and we present computer simulations to illustrate the concepts. An understanding of composite relative sea-level changes and the potential for a hierarchy of stratigraphic forcing provides the link between cyclostratigraphy and sequence stratigraphy and also has important implications regarding the stratigraphy of early diagenesis.