Abstract

Modelling of tidal rhythmites based on modern tidal data, including a spectrum of diurnal to semidiurnal settings, allows a greater understanding of the information that can be encoded in ancient tidal rhythmites and rhythmite-bearing sequences. Based upon such modelling, neap-spring patterns observed within the ancient rhythmites can be directly compared to the complex and hierarchically ordered periods that exist in modern tides. Harmonic analyses of modern data indicate that neap-spring cycles can be related not only to the synodic month (related to lunar phase) but also to the tropical month (related to lunar orbital declination). In addition, close approach of the moon to the earth (lunar perigee during the anomalistic month) results in a slight but significant change in tidal height variability that is superimposed on the aforementioned cycles.

Analysis of modern data indicates that within semidiurnal systems, the principal tide-generating period is related to the synodic month, with smaller effects related to anomalistic and tropical months. In such systems, neap-spring cycles are closely related to the lunar phase. Conversely, in diurnal systems, the principal tide-generating period can be related to the tropical month; such a system would exhibit neap-spring cycles in response to lunar declination rather than lunar phase. Because the tropical month is slightly shorter than the synodic month, a diurnal system can exhibit about 27 neapspring cycles per year rather than the expected 25 cycles produced within a semidiurnal system.

Although such neap-spring cycles are too minor to be recorded in log-based analyses, the larger scale seasonal and yearly cycles can potentially be observed in subsurface logs. A simulation of gamma-log response to these larger scale cycles is discussed, based upon assumptions involving tidal sedimentation.