Transcript Impact - Middle Tennessee State University

The Tennessee Impact Craters:
Changing Views on Wells Creek
Jana Ruth Ford
Keith Milam
William Deane
Wayne Orchiston
Centre for Astronomy,
James Cook University,
Townsville, Queensland,
Australia
2. Terrestrial Meteorite Impact Structures:
Astronomical and Geological Evidence
megascopic – radial & concentric fault systems,
Flynn Creek
Wells Creek
morphometry
Lunar Crater
Tycho
macroscopic – impact melt sheets,
shatter cones,
breccias
Wells Creek
microscopic – highly shocked rocks,
high pressure mineral polymorphs
3. Historical Context: Changing
Perspectives in Impact Research
Most astronomers initially assumed that lunar craters were
volcanic due to the fact that most lunar craters are round.
Impact angles greater than 10 °
will result in a circular crater. This
is also true for terrestrial craters.
NASA:
Ranger 8
NASA: Apollo 11
4. Tennessee Meteorite Impact Sites
Geologic Map of Tennessee (1966) by the Tennessee Department of Conservation,
Division of Geology
&Dycus
Mississippi
River
Scale: 160 kilometers
Generalized Physiographic Map of Tennessee showing two proven (Flynn Creek & Wells
Creek) and two suspected (Howell Structure & Dycus Disturbance) impact sites.
4.1 The Wells Creek Structure
Youngest Rock
Oldest Rock
(Map - Tennessee Department of Environment and Conservation, Division of Geology)
The Wells Creek Structure consists of older rock uplifted in the center and
surrounded by younger rock.
Wells Creek Structure Digital Elevation Model
Diameter is ~ 13.7km.
Geology and Structure – complex crater with the characteristic central uplift,
terraced walls, and flat floor surrounded by a circular rim.
Earth Impact Database, 2006. <http://www.unb.ca/passc/ImpactDatabase/>
(Accessed: 7 March 2010)
View is across the central uplift to the distant rim of the Wells Creek Structure.
The Wells Creek area was settled by 1800 and is used as pasture today.
Location outside of Wells Creek Impact Structure
Principle of Original Horizontality – rock materials were originally
deposited in horizontal layers. When rock layers are found in nonhorizontal positions, they must have been tilted to their present
position at some later time.
Principle of Superposition – rock materials are deposited on top of
earlier, older deposits. In any horizontal sequence of rock layers
(strata), the youngest will be at the top and the oldest at the bottom.
Wells Creek overturned rock layers. Note the different angles.
These vertical beds are in the northeast region of inner graben.
An 1855 geological map of Tennessee shows no indication of the Wells Creek Structure.
Around that same time, surveyors and engineers first noticed the deformed rocks and
overturned, vertical beds as they prepared to lay a railroad line through the area.
Wells Creek overturned rock layers.
Some vertical, others at different angles.
Dr. James Stafford, State Geologist of Tennessee, became aware of the structure
sometime between 1855 and 1869 and a detailed map of the Wells Creek Basin was
included in the 1869 Geologic Map of Tennessee.
Same site as two previous photos!
Folded rock layers
Safford and W.T. Lander wrote about the “Circumferential Faulting around Wells Creek
Basin” in a circa 1895 manuscript based on field work done between 1889 and 1893.
The full size of the structure was first recognized during this project.
State of Tennessee Geologist, Marvin Berwind, next to a chevron fold
located on the north side of Wells Creek next to the Cumberland River.
North
Note layers.
Wells Creek
Structure
South
Digital elevation model Generated by Graham Nickerson at Interactive Visualization
Systems, Fredericton, New Brunswick. Earth Impact Database, 2006.
<http://www.unb.ca/passc/ImpactDatabase/> (Accessed: 7 March 2010)
Note layers in photo taken of eastward dipping beds in the Wells Creek Structure.
Wells Creek Structure Fault Lines
From Geologic Map of
Tennessee (1966)
Tennessee Department
of Conservation,
Division of Geology
(Black lines are fault lines.)
The entire area of the
Wells Creek disturbance is
around 13.7 km in diameter
with the central basin being
about 3.2 by 4.8 km.
_______________
8 km
Road cut on the south side of the Wells Creek impact structure showing
fault lines at different angles along with jumbled rocks of different ages.
Keith Milam investigating a small cave that
is forming along a Wells Creek fault line.
Wells Creek Shatter Cones on site
Proof of impact! Shatter cone orientation suggests that the impactor hit the surface at
Wells Creek and then penetrated more than 600 meters before exploding.
Keith Milam pointing out a possible ejecta site about 6.4km
from the approximate center of the Wells Creek impact .
The possible ejecta is even more apparent in snow.
4.1.1 Wells Creek Associated
Craters
According to Wilson and Stearns (1968) depressions north of the Wells Creek
Basin were found during drilling and excavation done before or during 1934.
“It seems logical that the four basins, or craters, had a similar origin at the same
time. That origin would have been related to the phenomenon that formed the
Wells Creek Basin structure…(these) craters represent small meteoritic pits, or
craters.” (Wilson 1953)
O’Connell (1965) and Hey (1966) also mention Wells Creek having five craters
that occur in a NNE line. It is interesting to note that the main structure has a
north-northeast axis of bilateral symmetry.
4.1.1
Wells Creek Associated Craters
All five structures lie in a NNE line.
Austin Deposit:
520m north of Indian Mound,
115m in diameter, over 12m deep
Indian Mound:
4.8km north of Cave Spring Hollow
610m in diameter, 80m deep
Cave Spring Hollow:
7.3km NNE of the Wells Creek Basin
1.6km in diameter
Little Elk Creek Deposit:
lies just inside the depressed ring
of the Wells Creek Structure.
Wells Creek
Structure &
Basin
4.2 The Flynn Creek Structure
The structure is 3.6 km in diameter with a 300 to
350m central uplift. Shatter cones were discovered
in the uplift in 1977 confirming an impact origin.
Flynn Creek Topographic Model
The Lunar Crater Pythagoras.
Note structural similarity to Flynn Creek.
Colors represent different rock formations.
The orange in the center is the uplift. This
structure was first noted as a disturbance
by Stafford in 1869.
The rock layer at the very top is relatively horizontal.
Collapsed cave - lower rock layers on the left
and right all dip toward the center of the photo.
At least ten caves are associated with the Flynn
Creek structure, including the only cave known to
occur in the central uplift of an impact structure.
4.2 The Flynn
Creek Structure
Bill Deane examining rock
exposure on Flynn Creek’s
central uplift. Rocks in the
central uplift have been
raised over 300 meters
above their normal position.
4.3 The Dycus Structure
Mitchum (1951) first described Dycus as having a small central
uplift which is the site of the most intense deformation in a very
localized structure roughly circular in plan.
The uplift is not in the center of the structure, though, and the
structure is oval-shaped, not circular. (Deane et al., 2006)
Note in the photo of the Lunar Crater Schiller (right) that the
uplifted area is at one end of the oblique, non-circular crater.
Dycus
Wells
Creek
Howell
Flynn Creek
(NASA/Lunar Orbiter IV)
“Dycus is so close to Flynn Creek … that they may
be the result of a double impact.” (Stratford 2004)
The two structures are 13 km apart.
Dycus is less than 1 km in diameter.
“The most intensely-deformed part of the
structure occurs in the forested area in the
center of this picture.” (Deane et al., 2006)
Bill Deane is seen in the foreground.
4.4 The Howell Structure
Howell was first noted in
1934 by the Tennessee
Division of Geology.
Born and Wilson (1939)
described the structure
as having “highly
disturbed, contorted,
and brecciated strata,”
parts of which were
uplifted about 30m.
The structure is nearly
circular and around 2.5
km in diameter.
Map from Tennessee
Department of
Conservation (1966)
Section of breccia sample
collected in the Howell Structure.
4.4 The Howell Structure
Officer & Carter (1991) regard Howell
as an authentic impact structure.
Breccia from
Howell Structure.
Shatter cone from Howell.
Charles Marsh Woodruff also included photos
of “poorly formed shatter cones” in situ in his
1968 thesis on the Howell Structure.
5. Conclusion
•
Missions to the Moon, Mercury, Mars, and the asteroid belt as well as
satellites of the Jovian planets show that impact cratering is an important
geologic process, in fact the dominant process for many surfaces,
throughout our Solar System.
•
The idea that impacts had also occurred on Earth was considered
preposterous by many during the last two centuries.
•
The origin of the Wells Creek Structure has been interpreted as being either
the result of volcanic blowout or meteorite impact. In 1968, Wilson and
Stearns stated that the impact hypothesis was preferred. It is now
considered a proven impact site.
•
Field work completed by D.J. Roddy in 1977 indicated that Flynn Creek is
also the result of an impact. Milam and Deane are continuing to work on
Flynn Creek as well as the Howell and Dycus Structures. These last two are
suspected, but not proven impact sites.