Imposter versus other types of unconformities. An unconformity

Imposter versus other types of unconformities.
An unconformity surface is a contact between rocks of
significantly different ages. An unconformity develops
from either non-deposition or erosion of older rocks before
the deposition of younger ones. An unconformity takes a
long period of time for its development; but that time is
commonly shorter than the time gap estimated from the
ages of rocks bounding the unconformity surface. Yet, in
the case of an unconformity which is developed by nondeposition the disconformity may have the same time for
its development as the gap in time estimated from the rocks
bounding the unconformity surface. One that takes short
period of time for its development and which is limited to a
sedimentary sequence is called a diastem or non-sequence.
An imposter unconformity is not a diastem, for it requires
uplift erosion and denudational faulting, but takes a shorter
duration for its development than the conventional
unconformity (e.g., angular unconformity or
nonconformity) does.
(continued in the next page)
Imposter versus other types of unconformities
(continued)
1). Suppose the ages of rocks A and B bounding an
unconformity surface are 800 (A) and 200 (B) million years
old. The gap in time between A and B represents 600 (800200) million years of missing record. However, the
development of an unconformity (involving uplift, erosion
and weathering followed by subsidence, or nondeposition)
might have taken a 100 million years, which is much less
than the 600 million years time gap suggested by the ages
of rocks bounding the unconformity surface.
2) Suppose there is a fault contact between rock A and rock
B, and the (denudational) faulting occurred 200 million
years ago. Though there is a gap in age of 600 million years
between the two rocks, the contact between A and B did
not take 600 million years to develop. May be it lasted less
than 10 million years. Such is the nature of faults that
juxtapose rocks of different ages comparatively rapidly.
(continued on the next page)
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Imposter versus other types of unconformities
(continued)
3) Suppose an unconformity surface is traceable to a fault,
such as the surface of a rollover anticline on a listric
(denudational) fault, the combined (fault plus surface on
rollover anticline) surface is called an imposter
unconformity. Sediments would lap on the fault surface
(buttress unconformity). Sedimentary layer may lie nearly
parallel to the rollover anticline surface (nonconformity or
disconformity). Similar to an angular unconformity, the
development of an imposter unconformity involves uplift,
erosion and subsidence (by denudational faulting). Since
the subsidence is by faulting, the development of an
imposter unconformity takes much shorter time than and
the geometry of rocks about the imposter unconformity is
different from that of an angular unconformity.
The significance of identifying an imposter unconformity is
the recognition of the much shorter duration of time taken
for the juxtaposing of rocks of different ages (e.g., rocks A
and B). It follows that the geologic history of a region must
be examined in detail before determining if
nonconformities and disconformities might not be part of
an imposter unconformity.
Imposter versus other types of
unconformities (continued)
The subsequent pages develop the concept
of missing record and unconformities by
using yearbooks as examples
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Menz High Yearbook: Missing & Incomplete Record
Yearbook
1973 Menz High
1972 Menz High
1971 Menz High
1960 Menz High
unconformity
1959 Menz High
1958 Menz High
1957 Menz High
ß Each yearbook is an incomplete record, for it does not contain
all school activities of a year.
ß Yearbooks 1961 through 1970 are missing = Unconformity.
Is the record for1961-1970 missing because yearbooks were
not prepared for these years, or because they were destroyed?
ß Is there necessary and sufficient data to make valid inferences.
For example, can we determine the ratio of black/white soccer
players, if that were what a study involved.
1990 McBride High at Gatwick
1989 McBride High at Gatwick
1988 McBride High at Gatwick
Imposter
Unconformity
1987 McBride High at Gatwick
1986 McBride High at Gatwick
1960 McBride High upon Perth
1959 McBride High upon Perth
At Gatwick
In 1985, pursuant to a decision to relocate McBride High,
the mayor of McBride City lifted old yearbooks, threw
away younger ones (1961 through1984) from McBride
High upon Perth, and placed only the 1959 & 1960
yearbooks at Gatwick. In subsequent years, yearbooks
from McBride at Gatwick were placed on top of those as
they were published.
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Summary
1) The process represented by an unconformity might be much
shorter in duration than implied by the ages of rocks on
either side of an unconformity as in the analogous case of
McBride High. Such an unconformity is called an Imposter
Unconformity
2) Discussion on imposter unconformity alerts us to pay
attention to geologic processes at a locality, rather than to
emphasize on the ages of rocks on either side of the
unconformity.
3) Good examples, such as using yearbooks, enable the student
to grasp the difficult idea of unconformity.
4) Incomplete or missing record when viewed globally may
contain necessary and sufficient data for a particular study.
Foreword to subsequent pages
Photographs of the angular unconformity at Siccar Point,
all kinds of unconformities at the Grand Canyon, and the
nature of an imposter unconformity is provided with the
aid of photos and sketches of Death Valley.
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Angular Unconformity: Siccar Point, Scotland
Devonian
Old Red SS
Silurian
Turbidites
Hutton’s impact: Shattered a belief that Earth was created 4004 BC.
Unleashed human thought to ponder vast time with its immense
possibilities.
Grand Canyon. View from Tonto Plateau,Arizona across
the Colorado to Utah.
Mississippian
Dis.
Cambrian
Non.
Angular
Grand canyon
Supergroup.
715-1,250 MA
Imposter
Vishnu and
Zoroaster.
1,400 to 2,000 MA
Unconformities : Angular, Disconformity (Dis.), Nonconformity (Non.),
and Imposter Unconformity
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Death Valley, CA. View of Black Mountains from Panamint Range.
Imposter Unconformity surface between Precambrian wall rock
and Cenozoic sediment fill. Death Valley is a Pull-apart basin
formed subsequent to uplift, erosion, and denudational faulting
that propagated westward from Spring Mountains..
USGS Photo. Bahada on Panamint Range, western boundary of Death
Valley. Contact between Precambrian and overlying young sediments is
an imposter unconformity. Ocoee likely is submarine equivalent.
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R
S
Q
P
R
H
G
F
E
D
C
D
B
Q
C
B
A
a. Angular unconformity
P
A
b. Disconformity
T
T
S
R
S
Q
c. Nonconformity
R
Q
P
d. Imposter unconformity
Conclusion
1) An imposter unconformity represents a missing record,
which is much shorter in duration than the gap in age
estimated from rock ages bounding the unconformity
surface.
2) When a surface separates rock layers in obvious
discordance (fault-like) on one side of an inferred
depositional basin, and its continuation appears to be a
nonconformity, that surface is an imposter unconformity.
For further discussion go to the following URL
http://www.utc.edu/Faculty/Habte-Churnet/ImposterU-Short.pdf
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