Page 88 - Sedimentology and Stratigraphy

Palaeocurrents 75

The direction of dip of the cross-strata in cross-bedding
is measured.
3 Large-scale cross-bedding and cross-stratification
formed by large bars in river channels (9.2.1) and
shallow marine settings (14.3.1), or the progradation
of foresets of Gilbert-type deltas (12.4.2) is an indi-
cator of flow direction. The direction of dip of the
cross-strata is measured. An exception is epsilon
cross-stratification produced by point-bar accumula-
tion, which lies perpendicular to flow direction (9.2.2).
4 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
Fig. 5.5 A field photograph of sedimentary rocks: an irregular upstream when viewed side-on (Fig. 2.9). Note that
lower surface of the thick sandstone unit in the upper part of this is opposite to the measured direction in cross-
the cliff marks the base of a river channel. stratification.
5 Flute casts (4.7) are local scours in the substrata
generated by vortices within a flow. As the turbulent
Further information on the field description of sedi- vortex forms it is carried along by the flow and lifted
mentary rocks is provided in Tucker (2003) and Stow up, away from the basal surface to leave an asym-
(2005). metric 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 mea-
sured.
5.3 PALAEOCURRENTS
Flow axis indicators are structures that provide
information about the axis of the current but do
A palaeocurrent indicator is evidence for the direc- not differentiate between upstream and downstream
tion of flow at the time the sediment was deposited, directions. They are nevertheless useful in combina-
and may also be referred to as the palaeoflow. tion with unidirectional indicators, for example,
Palaeoflow data are used in conjunction with facies grooves and flutes may be associated with turbidites
analysis (5.6.2) and provenance studies (5.4.1)to (4.5.2).
make palaeogeographic reconstructions (5.7). The
1 Primary current lineations (4.3.4) on bedding
data are routinely collected when making a sedimen-
planes are measured by determining the orientation
tary log, but additional palaeocurrent data may also
of the lines of grains.
be collected from localities that have not been logged
2 Groove casts (4.7) are elongate scours caused by
in order to increase the size of the data set.
the indentation of a particle carried within a flow that
give the flow axis.
3 Elongate clast orientation may provide information
5.3.1 Palaeocurrent indicators if needle-like minerals, elongate fossils such as belem-
nites, or pieces of wood show a parallel alignment in
Two groups of palaeocurrent indicators in sedimen- the flow.
tary structures can be distinguished (Miall 1999). 4 Channel and scour margins can be used as indica-
Unidirectional indicators are features that give tors because the cut bank of a channel lies parallel to
the direction of flow. the direction of flow.
1 Cross-lamination (4.3.1) is produced by ripples
migrating in the direction of the flow of the current.
The dip direction of the cross-laminae is measured. 5.3.2 Measuring palaeocurrents
2 Cross-bedding (4.3.2) is formed by the migration of
aeolian and subaqueous dunes and the direction of dip The most commonly used features for determining
of the lee slope is approximately the direction of flow. palaeoflow are cross-stratification, at various scales.