Geoscience Reference
In-Depth Information
Table 6.2
Continued
Sedimentary
structure or
bedform
Thickness
or size
Features to observe
Processes
Figure/Table
Planar
lamination
mm to
1 cm
Continuity, nature
(compositional or grain size),
presence of current lineation
Traction currents from
currents or waves (upper
or lower fl ow regime)
A6.17q
6
Cross-lamination
Less than
1 cm
Sedimentary structure resulting
from current- or wave-formed
ripples. Examine continuity
and observe whether ripples
are climbing
Migration of wave- or
current-formed ripples
6.1, A6.17j,l,o
Trough
cross-
stratifi cation
cm to
several
metres
Observe in three dimensions
to confi rm identifi cation;
provides palaeocurrent
direction
Migration of curved
bedforms at particular
speed and depth
6.3a,b,
A6.17c,d
Planar
cross-
stratifi cation
cm to
several
metres
Observe in three dimensions
to confi rm identifi cation;
provides palaeocurrent
direction
Migration of straight
bedforms at particular
speed and depth
6.11b,
A6.17a,b
Hummocky
cross-
stratifi cation
(HCS)
m (but
smaller
than SCS)
Structure should look the same
in all cross-sections; plan view
and cross-section show a 1 : 1
ratio of antiforms and synforms
Storm waves
6.3c,d,
A6.17e,f
Swaley cross-
stratifi cation
(SCS)
m
Structure should look the same
in all cross-sections; plan view
and cross-section show mainly
synforms
Fair-weather waves
A6.17g,h
Tidal bundles
cm
Note the orientation; look for
associated tidal features
(bi-directional cross-
stratifi cation, clay drapes) and
consider possible tidal regime
(diurnal or semi-diurnal)
Cross-stratifi cation usually
picked out by fi ner
grained deposits with the
cross-stratifi cation surfaces
in distinct groups of 7
that change from closely
to widely spaced,
representing neap and
spring tide deposition
respectively. Diagnostic of
tidal currents
A6.17p
Desiccation
cracks
cm
Look for evaporites, palaeosols
and other evidence of
subaerial exposure
Subaerial exposure
A6.17u
Pseudomorphs
cm
Geometrical shapes
Replacement of evaporite
minerals after their
dissolution
A6.17v
