The estimation of palaeohydrodynamic processes from subtidal deposits using time series analysis methods

Tidal bundle sequences are the characteristic large-scale cross-bedded sets with mud drapes deposited by strongly asymmetrical bidirectional tidal currents. By means of time series analysis of the bundle thickness of such sequences, the palaeohydrodynamic processes can be reconstructed. This technique involves: (1) Fourier analysis to test the periodicity of the bundle-thickness sequence and to estimate the periods and the phases of the most important periodic components; (2) filtering analysis to resolve the bundle-thickness sequence into different components (i.e. diurnal components, random variations, neap/spring components and longer period variations) and also to estimate the relative importance (amplitude) of each component. From these analyses, useful information can be derived as to the palaeotidal regime (whether semidiurnal or mixed) and also about the possible influence of non-tidal processes, such as storms (their strength, duration, frequency and direction). Results from a subrecent and an ancient example show that tidal currents and storm-induced currents are the most important hydrodynamic processes in the transport and deposition of sediment in these shallow marine environments.