Historical Geology/Tidal rhythmites and dating

In this article I shall explain what tidal rhythmites are, how they are formed, and their implications for dating.

As this article re-uses ideas introduced in the previous article on sclerochronology, the reader will need to have read that article first.

Rhythms and rhythmites

Rhythmites are sedimentary rocks which display a repetitive vertical succession of types of sediment. We have already discussed varves, which are a kind of rhythmite. In this article we shall be interested in rhythmites produced by the action of the tide.

Tidal cycles include:

• A semidiurnal cycle. In most locations the moon produces two high tides and two low tides a day.

• A diurnal cycle. Some locations, such as the Gulf of Mexico, get only one high tide and one low tide per day.

• A mixed cycle. This can be seen in many locations on the west coast of America. In a mixed cycle, there are two high tides and two low tides per day, but one high tide is higher than the other, and one low tide is lower than the other.

• A fortnightly cycle. The highest high tides and lowest low tides (spring tides) occur at new moons and full moons, when the moon is either in line with or opposite the sun.

• Tidal cycles such as the apsidal cycle (presently lasting 8.85 years) and the nodal cycle (18.6 years) can also be distinguished; for the purposes of this article we may overlook them.

In additional to these tidal cycles, the rhythm of the seasons can also have their effect on sedimentary deposition. Varves are a special case of this, though typically varves are so thin that cycles of shorter duration are not discernible. Some sediments, however, will display a full range of cycles from semidiurnal to annual.

Rhythmites and dating

Any or all of the cycles mentioned above can be recorded in nearshore sediments. So it is possible to look at nearshore sedimentary rocks and, depending on which rhythms are recorded in the rock, to find out how many days there were in a month, or days in a year, or months in a year, or all of these facts, at the time when the rhythmite was deposited.

This allows us to subject these rhythmites to the same analysis as is used in sclerochronology. The fact that there is close agreement between the number of days in a year as calculated on the basis of rhythmites and by the use of sclerochronology is a reason to have confidence in both methods, since it is hard to see how both could be wrong and yet coincidentally in agreement.

The same caveats apply to the use of this type of rhythmite for dating as apply to sclerochronology, and for just the same reasons. Also as with sclerochronology, the agreement of data from rhythmites with dates produced by radiometric dating is a reason to have confidence in radiometric methods.