Deep Structure and Origin of the Serpent Mound
Disturbance
Doyle
Watts (Department of Earth & Environmental Sciences, Wright State
University, Dayton, Ohio, USA)
Ohio Geological Society meeting
26 February 2001
The
Serpent Mound
monument in southern Ohio has an astronomical alignment (the snake is facing
sunset at solstice, the longest day of the year). It has a ~1060 A.D.
construction date. 1066 A.D. saw a spectacular Halley’s Comet approach,
and it is possible that there might be a connection between the Serpent Mound
monument and that serpent in the sky?
Do
many of the North American deformation structures represent impacts?
An
east-west trending “string of impacts” occurs in south-central Missouri and
eastern Kansas and western Kentucky. But, it turns out that some of them
formed in the Cambrian and some in the Devonian. A volcanic origin is
more consistent with this line of features.
More
of these structures are known south of the Pleistocene glacial maximum line,
and there are bound to be some lurking under glacial sediments. There is
an average of 1 new discovery of this type of structure each year.
The
Serpent
Mound Structure (and
see) is on a belt of gravity and magnetic anomalies that passes through
Ohio; it is also on the Grenville Front (at least where it’s interpreted to
be). The coincidence of these features suggests that an
endogenic/volcanic origin is more appropriate. If it is an impact, it’s
aim was pretty good!
The
Serpent Mound structure shows up in the Ohio Delorme Atlas, vegetation-wise.
The
Serpent Mound (SM) structure has Ordovician, Silurian, Devonian, and
Mississippian rocks in it, including the Berea Sandstone, which shouldn’t be in
this part of Ohio. The SM structure (surface-wise) has a downdropped
outer ring graben (dropped ~1000’, in order to have Mississippian-aged Berea
present), a central uplift, and a transitional zone between these two.
The central uplift rocks have come upward (~1000’ - Ordovician rocks are poking
through the Silurian). Six overturned anticlines are present at the
center, radiating outward from the central uplift. The central uplift,
structure-wise, resembles a napkin pulled through a napkin ring. The SM
structure is in an area with poor exposure, so need to go to stream cuts to see
the rocks. The Berea in the area weathers as a topographic high, which
can be seen on the ground in the field, and is also what shows up as the vegetation
ring in the Ohio map atlas.
The
Kentland Dome structure (Indiana) has analogies with Serpent Mound. At
Kentland Dome (KD), the coesite polymorph has been found in St. Peter Sandstone
quartz. The presence of coesite is indicative of shock
metamorphism. Kentland Dome also has authochthonous breccias - polylithic
dry fluid flow breccias - the whole Indiana Paleozoic section can be found
within these breccias. There has been flow and jumbling of material at
KD. Kentland Dome also has nice shattercones (with a horsetail
appearance), indicative of shock metamorphism. In fact, KD is the type
locality for shattercones. Some here are 6’ in diameter. Robert
Dietz 1st described shattercones from KD. Shattercones are patterns
usually attributed to the presence of a pebble or other inhomogeneity at the
top (peak) of the shattercone Subtly overprinted are concentrically
radiating structures. Shattercones require an abrupt, significant shock
in order to form. Coesite and shattercones are considered diagnostic of
impact structures.
Serpent
Mound evidence - there has been controversy over the claimed presence of
coesite at SM (first identified from X-ray analysis of residue of lots of
rocks; reported in the 1960s). This identification has not been
reproducible yet at SM. But, there is no doubt about the presence of
coesite at KD.
Two
seismic lines are available that go over part of the Serpent Mound
structure. One (Line BV-1-92, a NE-SW trending line done in 1992) goes
almost all the way across, but does cross the outer ring graben, the transition
zone, and the central uplift. The other (Line SM-1, a ~N-S trending line)
skirts the eastern ring graben zone and the outer transitional zone.
Also
available are:
1)
a 2000’ long continuous core (# 3275) in the western transitional zone, done
for mineral prospecting
2)
a 3000’ long core drilled at the center of the SM structure (now at ODNR)
3)
a well log (down to basement) from a well just to the east of the SM structure.
Some
shallow cores are also available from the northern part of the structure.
The
seismic data has been reprocessed, the geophysical logs have been analyzed and
field studies were done. Also helpful was a velocity log from the Smith
well, about 10 miles away from the SM structure, which helped in analyzing
data.
Reflectors
are bowed downward at the center of the Serpent Mound structure. The
shallower reflectors are more deeply bowed down than the deeper ones.
Is
the downward bowing due to the low velocity of the waves through the central
uplift feature? (a velocity pulldown?) Velocity pulldowns are common
artifacts on seismic lines. Are these artifacts, then?
Looked
at the core drilled at the center of the SM structure. Well log picks for
the top of the Gull River Limestone [Middle Ordovician] were mapped and the
Gull River was identified on logs and on seismic, in order to see if these are
true reflectors, and not a velocity pulldown. The Gull River is 820’
further down than it “should be” in the central uplift area.
The
Gull River Limestone to Conasauga Shale [Middle Cambrian] interval is thinned
below the center of the central uplift.
Experimental
projectile-at-layered-sand studies show the same reflector pattern as seen at
SM (including a decreasing downbowing with increasing depth).
The
other seismic line (along the ring graben) shows a ring anticline in the Gull
River-Knox-Precambrian reflectors.
This
is the paradox of SM - the central uplift (at the surface) has a downdropped
Conasauga in the subsurface. The subsurface expression of SM extends
beyond the limit as shown by surface mapping. Asphalt covers some outcrop
minerals in the SM structure area. Ring anticlines are also present
around the outer graben ring.
The
gravity anomaly at SM was reconfirmed (but it has been long known). A
gravity low coincides with the central uplift. Other impact structures
have similar gravity anomalies. Gravity anomalies are to be expected if
SM is an impact.
The
deep core from the central uplift (3000’ deep) reveals lithologies showing some
tectonic thickening. The Fairview-Kope-Point Pleasant section [Upper
Ordovician] (600’ thick normally) occurs through 2000’ of section (multiple
repetitions). Part of the uplift is due to tectonic thickening.
Autochthonous breccias occur at several levels. There are undoubted
shattercones in the core. Quartz dissolved out of limestone shows PDS
(planar deformation structures). These were sent to Vienna and the planes
of deformation in these crystals were confirmed as being PDS.
So,
the SM structure does fit with an impact origin.
What
is the age of the Serpent Mound Structure?
Mississippian
sandstones here have been deformed and the Illinoisan tills haven’t been
deformed. So, the geology doesn’t help in dating this. There are
some red rocks in the core. Looked at the magnetism in the red rocks
(hematite from the Silurian Brassfield Formation). The core has visible
bedding planes (but unoriented in the core). When SM rocks were
magnetized, they were located on the equator (average 0˚ inclination of
hematite from red rocks in the Brassfield Fm. of the SM core). This is
the result from uncorrected-for-dip measurements. When corrected for dip,
the data scatters. So, it appears that magnetization of these rocks
occurred after the rocks acquired their dip (post-deformation, or
post-impact). The last time SM was on the equator was the Late
Permian. A Late Permian or early Triassic remagnetization event is well
known for this part of the world (affecting central North America). So, SM
was 1st, then the Late Permian remagnetization event. This puts the
timing of the SM impact event at post-Mississippian and pre-Late Permian.
Several
things are consistent with an impact.
But,
what about the magnetic anomaly that passes through the SM structure? Isn’t
this indicative of an endogenic/volcanic origin? A magnetic ridge does
pass through SM. The aim was pretty precise if so!
But,
did the impact create the magnetic anomaly, and was the anomaly really
there before-hand? Whacking rocks does change their magnetic signature.
Conclusion: it is an
impact! Another thing to keep in mind, especially when comparing Serpent
Mound to other impact structures: the SM structure is deeply eroded.
New discoveries: the Muskingum County
anomaly [see
talk] quickly filled in and was buried by a kilometer of sediments.
It has a 2-mile diameter (it’s surprising it has a central peak for this size).
The lower size limit for central peaks being present in impact structures is
about this size. SM is ~10 kilometers across. Chicxulub is ~100
kilometers across.
