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conditions that represent water depths closer to fair-
weather wave-base rather than storm wave-base.
8. Determine, by examining the nature of the contacts, which
of the conventional four categories the sedimentary
structure falls into: depositional, erosional, biogenic (e.g.
trace fossils) or post-depositional. Biogenic structures of
both plant and animal origin are covered in Chapter 5 and
Appendix A5. If the structure is post-depositional, and you
are interested only in the depositional sedimentary
structures, then you can ignore it. Tables 6.2-6.4
summarize the common depositional, erosional and post-
depositional sedimentary structures and bedforms
respectively.
6
Table 6.2 Some of the common depositional sedimentary structures, bedforms (marked) and their process of
formation. Note: cross-stratifi cation refers to both cross-bedding and cross-lamination with no scale implication.
Figure numbers prefi xed with an A can be found in Appendix A6.
Sedimentary
structure or
bedform
Thickness
or size
Features to observe
Processes
Figure/Table
Lamination
Less than
1 cm
Continuity, variation, colour
Variation in composition
or compaction
2.19b, 12.1,
12.2, 13.2b
Bedding
1 cm to
metres
Continuity, repetitions,
thickness variation
Varying conditions
3.1, 6.2b,
6.15, 6.16
Grading
Variable
Normal or reverse,
gradational or restricted
Waning or waxing
currents
A6.17s,t
Wave-formed
ripples
(bedform)
cm
Three-dimensional. Climbing or
not, associated with HCS or
SCS or other structures
Waves
6.1,
A6.17k,l,m,
Table A6.4
Current-formed
ripples
(bedform)
cm
Three-dimensional. Climbing or
not, associated structures,
palaeocurrent
Unidirectional currents
A6.17i,j,
Table A6.4
Impact ripples
(bedform)
cm
Size, orientation
Aeolian
-
Lenticular
bedding
cm
Wave- or current-formed
ripples? Vertical changes and
whether it is part of a fi ning-
or coarsening-upward trend
Fluctuation between
deposition from traction
currents and settling from
suspension
A6.17
Flaser bedding
cm
Dunes (bedform) 1-10s m
Shape of bedform
Wind and water
-
Current lineation cm
Provides a direct measure of
palaeocurrent. Check
orientation with respect to
other structures
Upper fl ow regime
2.9, A6.17r
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