Where sedimentology meets structural geology

April 21st, 2014

A seismite? 042114MITZPE RAMON, ISRAEL–”Like a hot dog in a bun.” Late this afternoon, while exploring the Eocene (Lutetian) Horsha Formation near the Nabatean/Roman/Byzantine city of Avdat, Yoav Avni and I ran across these odd features in a limestone layer within the chalks (near N 30.79119°, E 34.75494°). They consist of an elongate core of coarse, bioclastic sediment (the hot dog) in chalky sediments folded around them (the bun). They are all oriented in the same direction.

Another seismite type 042114Some are as big as canoes; others like gravy boats. We suspect that these are seismites — sedimentary sturctures formed by seismic shaking. The chalky, water-saturated sediment would have responded plastically as the slightly denser bioclastic sediments above collected in troughs and then began to descend down into the chalk. This is just an idea. If someone else has seen structures like these, please let us know!

Byzantine Cistern 042114Just below these funny structures is this nice Byzantine cistern filled with water. It is on the edge of a wadi, with about a half-meter step above the wadi base. It has this narrow doorway that leads into an interior cavern, all hand-carved. During a flood, the water reaches a level in the wadi where it begins to decant into the cistern, reducing the amount of sediment that would otherwise fill the cavity quickly. A couple of years ago Yoav organized a team to excavate centuries of silt from this cistern. Now it is full from the winter rains, providing a water hole for the local Bedouin children. It also shows that the Byzantine water storage and conservation techniques of 1500 years ago still work fine today.

5 Responses to “Where sedimentology meets structural geology”

  1. Ellenon 22 Apr 2014 at 4:12 pm

    I am not a geologist, just someone who reads all the blogs on all-geo religiously because I find geology fascinating. But your photo reminds me strongly of a formation I fancifully nicknamed “the bones of the sea” on a small southern California beach called Little Corona Del Mar (here’s the only photo that I’ve uploaded):

    http://i1.squidoocdn.com/resize_square/squidoo_images/320/draft_lens8013441module67991131photo_3_1257927502corona5.jpg

    The whole beach and the cliffs behind it are made up of upraised sedimentary layers, tilted up on their sides or twisted and folded. Most appear to be limestone or sandstone — I am not a geologist but I think I’m safe in identifying those! — but the “bones” section down in the shallows puzzles me; the layers seem to be a denser material as if they’ve been more compressed, or as if they were originally more of a mudstone.

    Corona del Mar has been shallow sea bottom off and on, and since it’s the coast of California, there is a LOT of faulting and frequent earthquake shaking.

    Sorry for the longwinded comment, but I’ve wondered for almost 20 years what could’ve caused this weird formation at my local beach, and “seismites” sound to me like that might explain it!

  2. Bill Reinthalon 23 Apr 2014 at 12:37 pm

    Although it’s not possible to verify from the photos, there was an article in Scientific American (I believe) many years ago, which popularized the idea of the “Brazil Nut Effect,” best exemplified by the fact that, in a carton of mixed nuts, after shipping, the Brazil Nuts always end up on top. So, to migrate this analogy to coarse bioclastic sediments and a fine clay-sized fraction, perhaps it’s not so hard to imagine that the two size clusters could be segregated by the winnowing effect of repeated seismic events. Here’s a video from YouTube showing the physics in action: http://youtu.be/wbfV16Ykl4o. Here’s a reference to the effect in a comments section of Scientific American: http://www.scientificamerican.com/article/why-does-shaking-a-can-of/ and another description from the University of Chicago: http://jfi.uchicago.edu/~jaeger/granular2/convection.html. These seem like plausible explanations for segregating particles the way shown in the photos. Add a little de-watering of the finer sediments and you might actually be able to sculpt that hot dog. Those interested should search for “Granular Convection,” or simply, “The Brazil Nut Effect.”

  3. Mark Wilsonon 24 Apr 2014 at 9:08 am

    Very good, Bill. This will be in the nucleus of a new IS proposal for work on these seismites next year. Thank you for the links. I have reading to do!

  4. Mark Wilsonon 24 Apr 2014 at 9:10 am

    Hello Ellen: Those Little Corona Del Mar structures certainly do look similar! Thank you for sharing. As this set of outcrops develops into a project, I’ll do research on seismites along the California coast. Thanks for the tip!

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