Sedimentary Rocks in the Field: by Maurice E. Tucker

Author:Maurice E. Tucker
Language: eng
Format: epub
Publisher: A John Wiley & Sons
Published: 2011-06-24T16:00:00+00:00

Figure 5.23 Herring-bone cross-bedding and a reactivation surface in cross-bedding.

5.3.3.8 Tidal cross-bedding

There are several features of cross-bedding that indicate deposition by tidal currents. Herring-bone cross-bedding refers to bipolar crossbedding, where cross-bed dips of adjacent sets are oriented in opposite directions (see Figures 5.23 and 6.6). Herring-bone cross-bedding is produced by reversals of the current, causing dunes and sand-waves to change their direction of migration. It is a characteristic but not ubiquitous feature of tidal sand deposits. Do check that the bipolar, herring-bone appearance is not due to a section through trough crossbedding (Figure 5.17).

In many cases tidal cross-bedding is all unidirectional, since one tidal current is much stronger than the other. However, there may be subtle features to indicate a tidal origin: there may be mud drapes on crossbed surfaces reflecting deposition from slack water during tidal current reversals (Figure 5.24; see also Figure 5.26); there may be thin lenses of ripples and cross-lamination within the cross-beds with a current direction opposite to that of the cross-beds (i.e. up the lee slope of the sandwave/dune), indicating a weak, reverse-flow tidal current (Figure 5.24).

Some larger-scale tidal cross-strata show a regular pattern of varying cross-bed thickness and grain-size along the section, that is, through time (see, e.g., Figures 5.25 and 5.26); these tidal bundles reflect the increasing and decreasing strength of tidal currents through the lunar cycle. You can measure up the thickness of the cross-beds and see how many days there were in the month! Reactivation surfaces (see Section 5.3.3.7 and Figure 5.23) and master bedding planes (see Section 5.3.3.2 and Figure 5.15) are also common in tidal-sand deposits from the effects of storms modifying and eroding the sand-wave/dune.

Figure 5.24 Features of tidal cross-bedding: mud drapes and reverse flow cross-lamination.

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