Geology Reference
In-Depth Information
Fig. 7.13.
A dip change across a strati-
graphic discontinuity may be
either a fault or an uncon-
formity.
a
Fault.
b
Unconfor-
mity. (After Hurley 1994)
Fig. 7.14.
The geometry of a low-angle unconformity.
a
Cross section showing a filled paleo-valley.
Vertical exaggeration is about two to three times.
b
Subcrop map of units below the unconformity.
(After Calvert 1974)
An unconformity can be mapped like a stratigraphic horizon, but it is significantly
different because the units both above and below the unconformity can change over
the map area (Fig. 7.14). An unconformity surface should be mapped separately from
the units that it cuts or that terminate against it. The stratigraphic separation across
an unconformity can be mapped and the separation can be expected to die out later-
ally into a continuous stratigraphic sequence (the correlative conformity) or at an-
other unconformity. A subcrop map shows the units present immediately below an
unconformity (Fig. 7.14b). This type of map has several important uses. It provides a
guide to the paleogeology, can indicate the trends of important units, for example
hydrocarbon reservoirs, and can suggest the paleostructure. An analogous map type
is a subcrop map of the units below a thrust fault.
The dip change across an unconformity can be very small, a few degrees or less. A
plot of the cumulative dip versus depth (Hurley 1994) is very sensitive to such small
