Investigating the occurrence of hierarchies of cyclicity in platform carbonates

David Pollitt, Peter Burgess, Paul Wright

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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.
Original languageEnglish
Title of host publicationStrata and Time
Subtitle of host publicationProbing the Gaps in Our Understanding
Place of PublicationBath
PublisherGeological Society of London
Number of pages28
ISBN (Print)978-1-86239-655-5
Publication statusPublished - 2015

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