Archive for October 30th, 2015

Wooster’s Pseudofossils of the Week: Cone-in-cone structures from southern Ohio

October 30th, 2015

1 ShatterCones 585

Author’s note: James Chesire convinced me through the comments and later correspondence that what we actually have here are cone-in-cone structures, not shatter cones. I’ve thus changed the title but have left the post below in its original form. They are still pseudofossils. I’ll link here later for a full update. Thanks, James!

This complex rock was collected decades ago in Adams County, Ohio, by the late Professor Frank L. Koucky of The College of Wooster. He was at the time studying a strange geological feature in that part of the state known then as the Serpent Mound Cryptoexplosion Structure. He thought that the ring-like disturbance in the bedrock nearly 10 km wide was a place where “mantle gases” explosively erupted from below. The rock shown was going to be a key to deciphering the energy of these cataclysmic events. It is a set of shatter cones formed when enormous, high velocity pressures were applied to a micritic (fine-grained) limestone. Professor Koucky knew what these features represented, but they are still collected in that region and elsewhere as “fossils” by some because of their resemblance to corals. They are thus fine examples of pseudofossils, or inorganic features resembling fossils.

These shatter cones ended up showing conclusively that the event that caused the “bedrock disturbance” in southern Ohio was actually an ancient meteor impact, and the site is now known as the Serpent Mound Crater. This ancient crater (it may be as much as 320 million years old) has a central uplift surrounded by a ring graben (circular down-dropped rocks). It took a lot of clever geology to sort this out because known of it is now visible on the surface.
2 Shatter cones closerThe Serpent Mound shatter cones have a multiple long fractures running parallel to the cones, resembling hair or “horsetails”. The cones have horizontal step-like fractures on their broken surfaces. You can simulate this kind of structure by firing a BB or small rock at thick glass, which produces a conical fracture and perpendicular steps. To do this in a limestone requires between 20 and 200 kbar of pressure, which can only be achieved by a large meteorite impact or a nuclear explosion underground. More likely it was the former!
3 Shatter cones plan viewHere is what these shatter cones look like in plan view. The hole in the upper left is the tip of a cone that is not preserved.

So, shatter cones, despite their fine and repeatable details, are inorganic and not fossils of any kind. They represent enormous shock waves that left their marks as they passed through this limestone many millions of years ago.

References:

Carlton, R.W., Koeberl, C., Baranoski, M.T. and Schumacher, G.A. 1998. Discovery of microscopic evidence for shock metamorphism at the Serpent Mound structure, south-central Ohio: confirmation of an origin by impact. Earth and Planetary Science Letters 162: 177-185.

Kenkmann, T., Poelchau, M.H., Trullenque, G., Hoerth, T., Schäfer, F., Thoma, K. and Deutsch, A. 2012. Shatter cones formed in a MEMIN impact cratering experiment. Meteoritics and Planetary Science Supplement 75: 5092.

Milton, D.J. 1977. Shatter cones – an outstanding problem in shock mechanics. In: Impact and Explosion Cratering: Planetary and Terrestrial Implications 1: 703-714.

Sagy, A., Fineberg, J. and Reches, Z. 2004. Shatter cones: Branched, rapid fractures formed by shock impact. Journal of Geophysical Research 109: B10209.