Geology Reference
In-Depth Information
diapiric structures. Traps for oil and gas at the boundaries of these
diapiric structures are searched for using geophysical methods and
then targeted by drilling. Salt depositsarealsoconsideredtohave
great potential as nuclear waste repositories because of their low
permeability, although there are also reservations because of poten-
tial dissolution, mobility and in
uence of heat (e.g. Krauskopf,
1988). Salt also provides a cementing medium for sediments in
some environments (sabkha) and can be very important for founding
structures, as in the Gatch underlying Kuwait (Al-Sanad et al.,
1990).
Engineering properties of sediments and sedimentary rocks and their
investigation are addressed in
Chapters 4
and
5.
Metamorphic rocks are rocks that have been changed by heat, pres-
sure or both. By de
nition, they do not include low-temperature and
low-pressure diagenetic processes affecting soils, as discussed in
A general classi
cation of metamorphic rocks is presented in
Table 3.3.
Contact metamorphism occurs at the host boundaries of
igneous bodies, as illustrated in
Figure 3.8.
The metamorphic aureole
surrounding a major granite or gabbro pluton can extend for hundreds
of metres. The greatest effect is on sedimentary rocks; recrystallised,
generally very strong rock found close to large plutonic igneous rock
bodies is called hornfels. At greater distances the effect of the intrusion
is less
often the only indication of change being the growth of new
minerals such as kyanite or cordierite in the otherwise largely unaltered
-
Table 3.3 Simpli
ed classi
cation of metamorphic rocks. Refer to Fry (1984) for more detail.
FOLIATED
NON-FOLIATED
GNEISS
Often widely spaced and irregular foliation
MIGMATITE
Mixed schist and gneiss
SCHIST
Strong foliation
MARBLE
Derived from limestone/dolomite
Coarse to medium
QUARTZITE
Recrystallised sandstone
PHYLLITE
Undulose foliation. Often micaceous, shiny
SLATE
Planar cleavage
MYLONITE
Fault gouge
HORNFELS
Generally recrystallised contact rock
Fine
<0.06mm
SERPENTINITE
Metamorphosed peridotite/norite
