Geology Reference
In-Depth Information
1.3.1
Depositional Contacts
A depositional contact is produced by the accumulation of material adjacent to the
contact (after Bates and Jackson 1987). Sediments, igneous or sedimentary extrusions,
and air-fall igneous rocks have a depositional lower contact which is parallel to the
pre-existing surface. The upper surface of such units is usually, but not always, close to
horizontal. A conformable contact is one in which the strata are in unbroken sequence
and in which the layers are formed one above the other in parallel order, representing
the uninterrupted deposition of the same general type of material, e.g., sedimentary or
volcanic (after Bates and Jackson 1987).
Lithologic boundaries that represent lateral facies transitions (Fig. 1.6a), were prob-
ably not horizontal to begin with. Certain sedimentary deposits drape over pre-exist-
ing topography (Fig. 1.6b) while others are deposited with primary depositional slopes
(Fig. 1.6c). The importance of the lack of original horizontality depends on the scale
of the map relative to the magnitude of the primary dip of the contact. Contacts that
dip only a few degrees might be treated as originally horizontal in the interpretation
of a local map area, but the depositional contact between a reef and the adjacent basin
sediments may be close to vertical (Fig. 1.7), for example, and could not be considered
as originally horizontal at any scale. Depositional contacts that had significant original
topographic relief (Fig. 1.7) should be restored to their original depositional geometry,
not to the horizontal.
Fig. 1.6.
Cross sections showing pri-
mary depositional lithologic
contacts that are not horizon-
tal.
a
Laterally equivalent de-
posits of sandstone and shale.
The depositional surface is a
time line, not the lithologic
boundary.
b
Draped deposition
parallel to a topographic slope.
c
Primary topography associ-
ated with clinoform deposition
Fig. 1.7.
Cross sections showing pri-
mary sedimentary facies re-
lationships and maximum
flooding surface. All time
lines are horizontal in this
example
