Modelling of cyclic tidal rhythmites (Carboniferous of Indiana and Kansas, Precambrian of Utah, USA) as a basis for reconstruction of intertidal positioning and palaeotidal regimes



Analyses are presented for a variety of ancient cyclic tidal rhythmites, which exhibit well developed neap-spring tidal periods. Many such rhythmites were formed within the upper intertidal zone and exhibit truncated cycles that contain relatively few discrete lamina per neap-spring cycle. In such cases it can be difficult to determine if the originated palaeotidal system was diurnal or semidiurnal.

Based on sedimentological controls observed in modern analogues, the development of cyclic tidal rhythmites can be modelled by use of predicted tidal-height information. The modelling used a 19-year-long series of tidal heights, which were subsequently used to approximate tidal velocities and rhythmite sedimentation. Modelling was based on a range of diurnal to semidiurnal regimes and comparisons were performed at a number of levels within the intertidal zone. This modelling produced a long series of simulated laminae-thickness series which could be cross-correlated with ancient laminae-thickness series measured directly from ancient rhythmites.

This approach involved 1.6 × 106 comparisons of each ancient rhythmite series to the series simulated from the predictive tidal-height information. The high correlations derived for such comparisons indicate that reasonable approximations to the palaeotidal systems can be made. In some cases, it is possible to determine the diurnal or semidiurnal nature of the originating palaeotidal system.