Cyclic Tidal Rhythmites: Environmental, Astronomical, and Sequence-Stratigraphic Significance

Cyclic tidal rhythmites exhibit systematic variations in lamina thickness. These variations can be directly equated to short-term tidal cycles. Such cycle-bearing rhythmites are common in modern hypertidal (tidal range greater than 8 m) environments, such as the Bay of Fundy in Canada, Bay of Mont Saint Michel in France, Cook Inlet in Alaska, and the Atlantic coast of Patagonia. Such modern analogs provide a useful framework to help understand ancient counterparts of fine-scale tidal deposition.

Within the spectrum of astronomically forced cycles, short-term periods exhibited by tidal rhythmites include semi-daily, daily, half lunar month (neap-spring), and several lunar monthly cycles. All of these periodicities have been reported from modern and ancient of tidal rhythmites. Over geologic time, however, tidal periods have changed. Examples of Proterozoic rhythmites indicate that the paleotidal day was significantly shorter.

Carboniferous examples of cyclic rhythmites are notably widespread and probably relate to a unique series of sequence-stratigraphic controls. The aggregation of the continents into Pangea would have resulted in the development of very large-scale fluvial drainage systems. Concurrent Gondwanaland glaciations resulted in significant and periodic changes in sealevel. During sealevel low-stands, large incised valleys were excavated by the extensive fluvial systems. These fluvially excavated valleys were subsequently converted to estuaries during subsequent rise in sealevel. Within such estuarine settings tidal rhythmites are commonly associated with coal-bearing sequences. In addition during the late Paleozoic, a very large ocean covered most of the Earth. This huge ocean could have resonated strongly with the astronomical tide-producing forces. Ancient seaways, which have been termed epicontinental, epeiric, and inland seas, were once thought to be virtually tideless. However, the growing recognition of tidal facies and rhythmites within modern analogs and ancient counterparts suggest that such facies might be more common than conventionally recognized.