Geology Reference
In-Depth Information
3
Tidal Signatures and Their
Preservation Potential
in Stratigraphic Sequences
Richard A. Davis, Jr.
Abstract
Several indicators of tidal infl uence are preserved in modern and ancient
stratigraphic sequences. These tidal signatures are dominated by cyclic deposi-
tion. The cycles may represent as short as semi-diurnal tides or as long as multiple
years. Most commonly they represent daily or lunar cycles. These cycles are most
commonly represented by some alternation of sand and mud; so-called hetero-
lithic deposits. Some are monolithic. The scale of these rhythmic packages ranges
from a few millimeters to several decimeters.
The depositional environments in which these tidal sequences accumulate
include intertidal and subtidal positions to depths of at least hundreds of meters.
The most common are intertidal fl ats and their contained channels in estuaries and
deltas as well as in coastal bays and open coasts. Preservation potential ranges
from poor to very good. Tidal channels tend to be among the best preserved
whereas the upper intertidal zone is the most poorly preserved.
3.1
Introduction
chapter is the third of four “generic” chapters in this
volume in that they are not tied to a specifi c environ-
ment. The aim here is to acquaint the reader with vari-
ous tidal signatures and the possible environmental
settings in which they occur. Details of their environ-
mental and stratigraphic positions are found in the sub-
sequent chapters where individual tidally-infl uenced
depositional environments are discussed in detail.
The movement of sediments by tidal currents was
recognized in the times of the ancient Greeks and
Romans. Those people noticed that the regularity of
tidal fl uctuations was related to lunar cycles. The
detailed investigation of how tidal currents move sedi-
ment and how this sediment accumulates, however.
has been a subject of sedimentological research for
less than a century. Much of the initial work was on
tidal fl ats on the North Sea coast of Germany and The
Netherlands. Much of the efforts of these investigators
Tides are signifi cant process factors in coastal environ-
ments as discussed in the previous chapter. They pro-
duce currents that move sediment and eventually
deposit it, and in so doing, they create signatures that
may be preserved in the stratigraphic record. These
tidal signatures are important in reconstructing the
ancient environment of deposition in which the strati-
graphic sequences of interest were deposited. This
