No abstract

Metre-scale cycles in ancient peritidal carbonate facies have long been thought to represent the product of shallow water carbonate accumulation under orbitally controlled sea-level oscillations. The theory remains somewhat controversial, however, and a contrasting view is that these cycles are the product of intrinsic, and perhaps random, processes. Owing to this debate, it is important to understand the conditions that do, or do not, favour the preservation of orbital forcing, and the precise stratigraphic expression of that forcing. In this work, a one-dimensional forward model of carbonate accumulation is used to test the ability of orbitally paced sea-level changes to reconstruct cyclicities and cycle stacking patterns observed in greenhouse peritidal carbonate successions. Importantly, the modelling specifically tests insolation-based sea-level curves that probably best reflect the pattern and amplitude of sea-level change in the absence of large-scale glacioeustasy. This study found that such sea-level histories can generate precession and eccentricity water depth/facies cycles in models, as well as eccentricity-modulated cycles in precession cycle thicknesses (bundles). Nevertheless, preservation of orbital forcing is highly sensitive to carbonate production rates and amplitudes of sea-level change, and the conditions best suited to preserving orbital cycles in facies/water depth are different to those best suited to preserving eccentricity-scale bundling. In addition, it can be demonstrated that the preservation of orbital forcing is commonly associated with both stratigraphic incompleteness (missing cycles) and complex cycle thickness distributions (for example, exponential), with corresponding implications for the use of peritidal carbonate successions to build accurate astronomical timescales.

The observation that shallow‐marine carbonate strata often have exponential lithofacies thickness distributions is one of the most fundamental results in carbonate stratigraphy in recent years because it is an observation that can be tested for its repeatability in various outcrop and subsurface examples, and also because it raises the question of what sedimentary processes and climatic and oceanographic settings might lead to formation of a particular lithofacies thickness distribution. This observation, in turn, links to the significant issue of how carbonate strata record climatic and oceanographic change through geological time. This study applies a simple one‐dimensional numerical stratigraphic forward model of carbonate platform strata (Dougal) to study how relative sea‐level oscillations could control lithofacies distribution. Dougal records platform‐top carbonate accumulation influenced by water‐depth dependent sediment production in euphotic, oligophotic and aphotic production profiles with a lag‐depth controlling onset of production. Results from five single model runs highlight the issue of non‐stationary behaviour where statistical properties of the strata change with elevation up the section, and show that exponential lithofacies thickness distributions can be generated from an entirely deterministic model. Results of multiple model runs, 27 200 in total, spanning a range of production and accommodation creation rates, demonstrate that the three main controls on carbonate lithofacies distribution in this one‐dimensional model are: (i) complex variations in the rate of creation of accommodation, here due to high‐frequency glacio‐eustatic oscillations; (ii) rate of sediment production, dominated in these model runs by euphotic production rates; and (iii) operation of autocyclic oscillations in deposition driven by a lag‐depth effect and formed during certain high‐frequency rising limbs on the glacio‐eustatic curve. In these multiple model runs, only ca 13% of the total runs created exponential distributions, compared with 28% in the documented outcrop examples, suggesting that other processes not included in this model play an important role. More work is required to determine what these processes might be. Obvious candidates would be the various sedimentary processes that occur in three dimensions on platform tops, such as sediment transport and variations in rate and type of sediment production related to biological variability, and other process that occur in other different depositional settings, for example sediment transport on slopes.

Eocene climate change effect on carbonate platforms • D. A. Pollitt et al.

Hierarchies of cyclicity have been described from a wide variety of carbonate platform strata and are assumed to be a consequence of Milankovitch-forced variations in accommodation, although descriptions of hierarchical strata, including 'cycles' and what they constitute, are typically qualitative, subjective, and in some cases difficult to reproduce. One reason for this is the lack of any detailed definition of what constitutes a hierarchy, as well as the implicit and largely untested nature of the assumptions underpinning most interpretations of hierarchical strata.In this study we aim to investigate the response of depositional systems if they were to behave in the way implied by sequence stratigraphic (hierarchical) models, to clearly state the assumptions of these models, and illustrate the consequences of these assumptions when they are employed in a simple, internally-consistent forward model with plausible parameters.We define hierarchies, in both the time-domain (chronostratigraphic) and thickness-domain (stratigraphic), as two or more high-frequency sequences (HFSs) in which there exists a repeated trend of decreasing high-frequency sequence thickness such that within a single low-frequency sequence (LFS) each high-frequency sequence is thinner than the previous sequence.Based on this definition, results from 110 000 numerical model runs suggest that ordered forcing via cyclical eustatic sea-level oscillations rarely results in an easily identifiable hierarchy of stacked cycles. Hierarchies measured in the chronostratigraphic time-domain occur in only 9% of model run cases, and in 15% of cases when measured in the thickness-domain, suggesting that vertical thickness trends are probably not a useful way to identify products of ordered periodic external forcing. Variability in relative forcing periodicity results in significant variation in both HFS and LFS thickness trends making accurate identification of hierarchy and any forcing controls from thickness data alone very difficult. Comparison between model results and outcrop sections suggests that hierarchies are often assumed to be present despite a lack of adequate supporting evidence and quantitative analysis of these sections suggests that they are not hierarchical in any meaningful sense.