Geoscience Reference
In-Depth Information
Table 6.4
Continued
Structure
Size
Features to observe
Processes
Figure
-
Sandstone sills,
dykes and mud
volcanoes
m-km
Direction of extension,
associated features, whether
the infi ll sedimentary
deposits are part of the
'normal' succession
Synsedimentary
movement. Often found
close to faults
6
Dish and pillar
structures
cm
Small concave-up shapes.
Look for other associated
dewatering features
Water escape structures
form where sedimentation
rate is high and the
underlying sediments
have not had chance to
compact before more
sediment is laid down on
top.
A6.17dd,ff
Load casts
cm
Look for other associated
dewatering features
A6.17cc
Ball and pillow
cm
Other evidence of high
sedimentation rate
A6.17dd,ff
Late post-depositional structures
Nodules (late
diagenetic)
cm-m
Size, composition. Late
diagenetic nodules have a
high sphericity. They often
preserve other sedimentary
features
Change in pore-water
chemistry 10s to 100s m
below the sediment-water
interface
A6.17bb
Pressure solution
structures (e.g.
stylolites, cone-in-
cone calcite)
cm-m
Extent laterally and vertically
Sediment compaction and
movement of fl uids.
Presence depends on the
chemistry of the
sedimentary deposits and
pore waters as well as
the amount of pressure
8.5
Leisegang rings
m
Other evidence for diagensis
Late stage pore-fl uid
movement
A6.17hh
Dendrites
cm
A6.17hh
6.3 Graphic logs
The preceding section should have provided you with the
tools to start recording the features of individual units that
make up sedimentary rock successions in the form of written
notes and also sketches of both large- and small-scale
features. However, the standard way to record and summarize
data on sedimentary rock successions is by using a graphic
log (e.g. Figure 6.4, p. 118). This is an idealized and pictorial
summary of each of the sedimentary rock units as they were
laid down stratigraphically on top of each other and preserved.
