Geology Reference
In-Depth Information
Fig. 1.10.
Cross sections of intrusions.
Intrusive material is
patterned
and
lines
represent layering in
the country rock.
a
Concor-
dant.
b
Discordant
A vein is a relatively thin, normally tabular, rock mass of distinctive lithologic char-
acter, usually crosscutting the structure of the host rock. Many veins are depositional
and represent the filling of a fracture, whereas others are the result of replacement of
the country rock. Veins are mentioned here with intrusions because the contact rela-
tionships and unit geometries may be similar to those of some intrusions.
1.3.6
Other Boundaries
Many other attributes of the rock units and their contained fluids can be mapped, for
example, the porosity, the oil-water contact, or the grades of mineral deposits. Most of
the mapping techniques to be discussed will apply to any type of unit or contact. Some
interpretation techniques, particularly those for fold interpretation, depend on the
contacts being originally planar boundaries and so those methods may not apply to
nonstratigraphic boundaries.
1.4
Thickness
The thickness of a unit is the perpendicular distance between its bounding surfaces
(Fig. 1.11a). The true thickness does not depend on the orientation of the bounding
surfaces. If a unit has variable thickness, various alternative measurements might be
used, such as the shortest distance between upper and lower surfaces or the distance
measured perpendicular to either the upper or lower surface. The definition used here
is based on the premise that if the unit was deposited with a horizontal surface but a
variable thickness, then the logical measurement direction would be the thickness
measured perpendicular to the upper surface, regardless of the structural dip of the
surface (Fig. 1.11b).
Thickness variations can be due to a variety of stratigraphic and structural causes.
Growth of a structure during the deposition of sediment typically results in thinner
stratigraphy on the structural highs and thicker stratigraphy in the lows. Both growth
folds and growth faults occur. A sedimentary package with its thickness influenced by
an active structure is known as a growth unit or growth sequence. The high part of a
growth structure may be erosional at the same time that the lower parts are deposi-
tional. Thickness variations may be the result of differential compaction during and
after deposition. If the composition of a unit undergoes a facies change from relatively
uncompactable (i.e., sand) to relatively compactable (i.e., shale) then after burial and
