Paleocurrents
A palaeocurrent indicator is evidence for the direction of flow at the time the sediment was deposited, and may also be referred to as the palaeoflow. Palaeoflow data are used in conjunction with facies analysis and provenance studies to make palaeogeographic reconstructions. The data are routinely collected when making a sedimentary log, but additional palaeocurrent data may also be collected from localities that have not been logged in order to increase the size of the data set.
Palaeocurrent indicators
Two groups of palaeocurrent indicators in sedimentary structures can be distinguished. Unidirectional indicators are features that give the direction of flow.
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| Cross lamination |
- Cross-lamination is produced by ripples migrating in the direction of the flow of the current. The dip direction of the cross-laminae is measured.
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| Cross bedding |
- Cross-bedding is formed by the migration of aeolian and subaqueous dunes and the direction of dip of the lee slope is approximately the direction of flow. The direction of dip of the cross-strata in cross-bedding is measured.
- Large-scale cross-bedding and cross-stratification formed by large bars in river channels and shallow marine settings, or the progradation of foresets of Gilbert-type deltas is an indicator of flow direction. The direction of dip of the cross-strata is measured. An exception is epsilon cross-stratification produced by point-bar accumulation,which lies perpendicular to flow direction.
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| Clast imbrication |
- Clast imbrication is formed when discoid gravel clasts become oriented in strong flows into a stable position with one of the two longer axes dipping upstream when viewed side-on. Note that this is opposite to the measured direction in cross stratification.
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| Flute casts |
- Flute casts are local scours in the substrata generated by vortices within a flow. As the turbulent vortex forms it is carried along by the flow and lifted up, away from the basal surface to leave an asymmetric mark on the floor of the flow, with the steep edge on the upstream side. The direction along the axis of the scour away from the steep edge is measured.
Flow axis indicators are structures that provide information about the axis of the current but do not differentiate between upstream and downstream directions. They are nevertheless useful in combination with unidirectional indicators, for example, grooves and flutes may be associated with turbidites.
- Primary current lineations on bedding planes are measured by determining the orientation of the lines of grains.
- Groove casts are elongate scours caused by the indentation of a particle carried within a flow that give the flow axis.
- Elongate clast orientation may provide information if needle-like minerals, elongate fossils such as belemnites, or pieces of wood show a parallel alignment in the flow.
- Channel and scour margins can be used as indicators because the cut bank of a channel lies parallel to the direction of flow.
Stromatolites
Stromatolites, essentially, can indicate the heading of paleo-streams. They can frame in lengthen shapes that demonstrate the pattern of streams, or they can be slanted and unbalanced and can be utilized to focus the ability to know east from west, fundamentally landward or toward the ocean, and the introduction of the shoreline. The stromatolites in Shark Bay, Australia have been utilized to recreate a paleocurrent framework in proterozoic silt in view of their shape and introductions. It was found that their extend circular, columnar structure was parallel with the bearing of wave scour, and consequently typical to the shoreline. Although most examinations of old and current stromatolites have finished up to shape ordinary to the shoreline, some stromatolites have indicated distinctive introductions. There have been stromatolites depicted from the Proterozoic that are accepted to have framed parallel to the coastline in view of paleocurrent headings that were deciphered from related sedimentary structures. The circular state of the stromatolites are thought to have been the after effect of tidal streams running parallel to the length of a stretched embayment in the Precambrian Sea.
Measuring palaeocurrents
The most commonly used features for determining palaeoflow are cross-stratification, at various scales. The measurement of the direction of dip of an inclined surface is not always straightforward, especially if the surface is curved in three dimensions as is the case with trough cross-stratification. Normally an exposure of cross-bedding that has two vertical faces at right angles is needed, or a horizontal surface cuts through the cross-bedding. In all cases a single vertical cut through the cross-stratification is unsatisfactory because this only gives an apparent dip, which is not necessarily the direction of flow.
Imbrication of discoid pebbles is a useful palaeoflow indicator in conglomerates, and if clasts protrude from the rock face, it is usually possible to directly measure the direction of dip of clasts. It must be remembered that imbricated clasts dip upstream, so the direction of dip of the clasts will be 180 degrees from the direction of palaeoflow. Linear features such as grooves and primary current lineations are the easiest things to measure by recording their direction on the bedding surface, but they do not provide a unidirectional flow indicator. The positions of the edges of scours and channels provide an indication of the orientation of a confined flow: three dimensional exposures are needed to make a satisfactory estimate of a channel orientation, and other features such as cross-bedding will be needed to obtain a flow direction.
The procedure for the collection and interpretation of palaeocurrent data becomes more complex if the strata have been deformed. The direction has to be recorded as a plunge with respect to the orientation of the bedding, and this direction must then be rotated back to the depositional horizontal using stereonet techniques. In answer to the question of how many data points are required to carry out palaeocurrent analysis, it is tempting to say ‘as many as possible’. The statistical validity of the mean will be improved with more data, but if only a general trend of flow is required for the project in hand, then fewer will be required. A detailed palaeoenvironmental analysis is likely to require many tens or hundreds of readings. In general, a mean based on less than 10 readings would be considered to be unreliable, but sometimes only a few data points are available, and any data are better than none. Although every effort should be made to obtain reliable readings, the quality of exposure does not always make this possible, and sometimes the palaeocurrent reading will be known to be rather approximate. Once again, anything may be better than nothing, but the degree of confidence in the data should be noted.
There are several important considerations when collecting palaeocurrent data. Firstly it is absolutely essential to record the nature of the palaeocurrent indicator that has been recorded (trough cross-bedding, flute marks, primary current lineation, and so on). Secondly, the facies of the beds that contain the palaeoflow indicators is also critical: the deposits of a river channel will have current indicators that reflect the river flow, but in overbank deposits the flow may have been perpendicular to the river channel. Lastly, not all palaeoflow indicators have the same ‘rank’: due to the irregularities of flow in a channel, a ripple on a bar may be oriented in almost any direction, but the downstream face of a large sandy or gravelly bar will produce cross-bedding that is close to the direction of flow of the river. It is therefore good practice to separate palaeoflow indicators into their different ranks when carrying out analyses of the data.
Presentation and analysis of directional data
Directional data are commonly collected and used in geology. Palaeocurrents are most frequently encountered in sedimentology, but similar data are collected in structural analyses. Once a set of data has been collected it is useful to be able to determine parameters such as the mean direction and the spread about the mean (or standard deviation). The procedure used for calculating the mean of a set of directional data is described below. Palaeocurrent data are normally plotted on a rose diagram. This is a circular histogram on which directional data are plotted. The calculated mean can also be added. The base used is a circle divided up with radii at 108 or 208 intervals and containing a series of concentric circles. The data are firstly grouped into blocks of 108 or 208 (0018–0208, 021–0408, etc.) and the number that fall within each range is marked by gradations out from the centre of the circular histogram.