STRUCTURAL GEOLOGY

1. UNCONFORMITY

2. Unconformity
• A surface of erosion or non-deposition
separating younger strata from older rocks,
along which there is evidence of subaerial
erosional truncation (and, in some areas,
correlative submarine erosion) or subaerial
exposure, with a significant hiatus indicated.

3. Unconformity
A surface or sequences of strata represent times of
nondeposition and/or erosion separating younger strata
from older rocks, along which there is evidence of
subaerial erosional truncation or subaerial exposure,
that encompass long periods of geologic time, perhaps
millions or tens of millions of years ,with a significant
hiatus indicated .
INTRODUCTION

5. – For 1 million years erosion
occurred and removed 2
MY of rocks
– and giving rise to a 3
million year hiatus
THE ORIGIN OF AN
UNCONFORMITY
• The process of forming an unconformity
– deposition began 12 million years ago (MYA),
– continues until 4 MYA
• The last column
– is the actual stratigraphic
record
– with an unconformity

6. • Three types of surfaces can be
unconformities:
– A disconformity is a surface
separating younger from
older rocks, both of which
are parallel to one another
– A nonconformity is an
erosional surface cut into
metamorphic or intrusive
rocks and covered by
sedimentary rocks
– An angular unconformity
is an erosional surface on
tilted or folded strata over
which younger rocks were
deposited
TYPES OF UNCONFORMITY

7. • A disconformity between sedimentary rocks in California,
with conglomerate deposited upon an erosion surface in
the underlying rocks
A Disconformity

8. An Angular Unconformity
• An angular
unconformity,
Santa Rosa

10. 8_9
(a)
(b)
(c)
Layered
sedimentary
rocks
Nonconformity
Metamorphic
rock
Igneous
intrusive rock
Younger
sedimentary
rocks
Angular
unconformity
Older, folded
sedimentary
rocks
Disconformity
Brachiopod
(290 million years old)
Trilobite
(490 million years old)

11. • A nonconformity in South Dakota separating Precambrian
metamorphic rocks from the overlying Cambrian-aged
Deadwood Formation
A Nonconformity

12. Overlap: younger bed completely covers up and
advances much beyond the limits of the
underlying rocks. Can be seen during
trangression.
Offlap: it is reverse of overlap, lower beds of the
upper series extends further than the younger
ones due to marine regression.

14. • Unconformities of regional extent may change from one
type to another
• They may not represent the same amount of geologic
time everywhere
Types of Unconformities

15. Unconformities
3 types of
unconformities are
seen

16. Map Patterns of Unconformities
Ig/Meta Rx
Sed Rx
Sed Rx
Sed Rx
Sed Rx
Sed Rx

17. CRITERIA FOR RECOGNITION OF
UNCONFORMITIES
Unconformities may be recognized at the surface or on the
geological map by:
1. Discordance of strike
2. Discordance of dip, without change of strike. The
discordance may be by one amount or of direction. It should be
made sure that the structure is not due to folding
3. Transgression of the outcrops . It is a clear test, provided
such structure is not due to faulting.
4. Thinning out of beds (overlap)
5.Absence of beds in one area known to occur in
neighbouring area

18. Recognition:
• Time interval
•Thickness of bed missing from the stratigraphic record
•Structural discordance
•Topographic irregularity
•Thinning out of beds (overlap)
•Presence of basal conglomerate