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.