The origin of depositional cycles in shallow marine carbonates: An approach using coupled computer modelling and time series analysis.

摘要

Although meter-scale shallowing-upwards cycles (SUCs) are a common feature of shallow marine carbonate deposits, their origin remains largely unresolved. The possible mechanisms capable of producing this kind of cyclicity include periodic (orbitally-driven) and random sea level oscillations, Ginsburgian autocyclicity, and random or quasi-periodic tectonic processes. In order to resolve the problem of the SUCs origin an integrated study incorporating field research, advanced time series analysis, and forward modelling of shallow-marine carbonate deposition was undertaken.;Shallow-marine carbonate deposits of the Cambro-Ordovician Aisha-Bibi seamount (Malyi Karatau, Kazakstan) contain tens to hundreds of SUCs composed of (from bottom to top) transgressive lag conglomerates, subtidal cross-stratified grainstones and bioturbated mudstones, intertidal "ribbon rocks", and upper intertidal-supratidal mudcracked cyanobacterial laminites and stromatolites. The time series analysis of the SUC stacking patterns demonstrated the presence of Cambrian precessional periodicities in the spectra, suggesting that depositional cyclicity was driven by Milankovitch-forced eustatic sea level changes. The Aisha-Bibi deposits were compared to coeval sedimentologicaly similar deposits of the Cambrian Conococheague Limestone (Central Appalachians, USA). The time series analysis of the Conococheague cyclic succession revealed no evidence for orbital forcing and, indeed, suggested random character of the depositional cyclicity. A computer modelling study was conducted to resolve this discrepancy.;Two-dimensional forward computer model CYCOPATH 2D was used to simulate shallow-water carbonate deposition. The unique feature of CYCOPATH 2D is that it incorporates lateral transport of sediment from the offshore "carbonate factory" onto the tidal flats and therefore realistically models tidal flat progradation, producing Ginsburgian autocycles. The computer experiments allowed to establish that the width of the carbonate platform is a principal control determining whether the record of sea level changes will be preserved in the shallow-water cyclic deposits. A series of simulations demonstrated that when the platform is relatively narrow (e.g., Aisha-Bibi seamount), high-frequency sea level changes may control depositional cyclicity and can be recovered from the sedimentary record by the time series methods. However, if the width of the platform is large (e.g., the Cambro-Ordovician Great American Bank), autocycles "override" the sea level-driven allocycles. No periodic signature can be recovered from the resulting SUC stacking patterns even when the sea level was perfectly periodic.