Page 89 - Sedimentology and Stratigraphy

76 Field Sedimentology, Facies and Environments

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Fig. 5.6 The true direction of dip of planes (e.g. planar
cross-beds) cannot be determined from a single vertical face Fig. 5.7 Trough cross-bedding seen in plan view: flow is
(faces A or B): a true dip can be calculated from two different interpreted as being away from the camera.
apparent dip measurements or measured directly from the
horizontal surface (T).
recorded as a plunge with respect to the orientation of
the bedding, and this direction must then be rotated
The measurement of the direction of dip of an inclined back to the depositional horizontal using stereonet
surface is not always straightforward, especially if the techniques (Collinson et al. 2006).
surface is curved in three dimensions as is the case In answer to the question of how many data points
with trough cross-stratification. Normally an expo- are required to carry out palaeocurrent analysis, it is
sure of cross-bedding that has two vertical faces at tempting to say ‘as many as possible’. The statistical
right angles is needed (Fig. 5.6), or a horizontal sur- validity of the mean will be improved with more data,
face cuts through the cross-bedding (Fig. 5.7). In all but if only a general trend of flow is required for the
cases a single vertical cut through the cross-stratifica- project in hand, then fewer will be required. A detailed
tion is unsatisfactory because this only gives an palaeoenvironmental analysis (5.7) is likely to require
apparent dip, which is not necessarily the direction many tens or hundreds of readings. In general, a
of flow. mean based on less than 10 readings would be con-
Imbrication of discoid pebbles is a useful palaeoflow sidered to be unreliable, but sometimes only a few
indicator in conglomerates, and if clasts protrude data points are available, and any data are better
from the rock face, it is usually possible to directly than none. Although every effort should be made to
measure the direction of dip of clasts. It must be obtain reliable readings, the quality of exposure does
remembered that imbricated clasts dip upstream, so not always make this possible, and sometimes the
the direction of dip of the clasts will be 180 degrees palaeocurrent reading will be known to be rather
from the direction of palaeoflow (Figs 2.9 & 2.10). approximate. Once again, anything may be better
Linear features such as grooves and primary current than nothing, but the degree of confidence in the
lineations are the easiest things to measure by record- data should be noted. (One technique is to use num-
ing their direction on the bedding surface, but they do bers for good quality flow indicators, e.g. 2758, 2908,
not provide a unidirectional flow indicator. The posi- etc., but use points of the compass for the less reliable
tions of the edges of scours and channels provide an readings, e.g. WNW.)
indication of the orientation of a confined flow: three- There are several important considerations when
dimensional exposures are needed to make a satisfac- collecting palaeocurrent data. Firstly it is absolutely
tory estimate of a channel orientation, and other essential to record the nature of the palaeocurrent
features such as cross-bedding will be needed to indicator that has been recorded (trough cross-bed-
obtain a flow direction. ding, flute marks, primary current lineation, and
The procedure for the collection and interpretation so on). Secondly, the facies (5.6.1) of the beds that
of palaeocurrent data becomes more complex if the contain the palaeoflow indicators is also critical:
strata have been deformed. The direction has to be the deposits of a river channel will have current