I'll take a shot at this. My guess from one photo is that this a ball and pillow structure. These are caused by slumping or landslides particularly in submarine environments where denser material slides down slope on saturated less dense material. As seen in the photo, such structures are rounded and wrapped in softer beds. The structure in the photo is further deformed by a thrust fault and a beautiful drag fold where the bed is turned over on itself. Pillow structures and localized thrust faulting are not uncommon. Neat site.
thanks, Josh On Sun, Jan 24, 2010 at 11:33 AM, Mixon Bill <bmixon...@austin.rr.com>wrote: > Here's what Jim Conrad posted in his Naturalist Newsletter about the > roadcut breccia that Bruce Morgan was asking about. It includes a link to a > photograph. -- Mixon > > MYSTERIOUS ROADCUT > Each week when I hike to Pisté to buy fruit, on Hwy > 180 between Mérida and Cancún and about half a mile > from the center of the ruins of Chichén Itzá, I pass > within ten feet of the roadcut through limestone seen > at http://www.backyardnature.net/n/10/100124rx.jpg > > That's a vertical section of the roadcut about six > feet high. Note the weeds at the lower right for > scale. > > About 65 millions years ago the entire Yucatan > Peninsula was covered by sea, as was much of the US > Southeastern Coastal Plain. At that time, at the end > of the Cretaceous Period, an object from space at > least six-miles wide (10 km) crashed into the sea at a > spot now located -- after the Yucatan Peninsula has > risen above sea level -- a few miles off the Yucatan's > northwestern coast. The crater caused by that impact, > today known as the Chicxulub Crater, was about 112 > miles in diameter (180 kms). The Wikipedia page > describing the Chicxulub Crater is at > http://en.wikipedia.org/wiki/Chicxulub_crater > > Long after the crater was formed, during the Oligocene > about 25 million years ago, this part of the Yucatan > Peninsula began rising, pushed upwards by forces from > within the Earth. What earlier had been the carbonate- > rich mud of the ocean floor, and limestone rock below > it lithified from that mud, gradually rose and kept > rising until today it stands above sea level, but not > by much. During those millions of years ocean currents > gradually buried the Chicxulub Crater beneath mud that > eventually hardened into limestone rock. Today if you > stand where earlier the crater was formed, you'll see > no signs of a crater at all. It's all buried beneath > limestone deposited since the impact 65 million years > ago. At Chichén Itzá we're well outside the crater's > former location, but close enough for the ocean floor > here to have been very disrupted. > > The mysterious thing about the roadcut is that what > you see there suggests a great deal of turmoil. > Sediment deposited in calm, seabed conditions is > finely grained and the layering is even. The picture > shows very uneven layering, some layers tilted and > others not, and fragments of fractured rock appear to > be embedded in what once was flowing mud. Maybe > there's even a near-vertical fault cutting across the > layers at the picture's right. I've seen layering like > this in ancient mudflows beside volcanoes, but never > in limestone areas that have been as geologically > quiet as this one -- quiet since the Chicxulub Crater > was formed. In fact, I can't think of anything in the > Yucatan Peninsula's geological history that could > have created such a story of geological turmoil > as this picture suggests, except the Chicxulub Impact. > > It seems that if such a wonderful exhibition of the > effects of the Chicxulub Impact were known, it'd > appear at websites dealing with the event -- would > even be an important tourist attraction. The Chicxulub > Impact, after all, is often regarded as having killed > off the dinosaurs worldwide, thus enabling mammals to > begin their evolutionary ascendancy, eventually making > possible humanity. > > Is there anyone out there who can confirm that what's > in the picture is or is not evidence of the Chicxulub > Impact? > ---------------------------------------- > A bore is a person who talks when you wish him to listen. > ---------------------------------------- > You may "reply" to the address this message > came from, but for long-term use, save: > Personal: bmi...@alumni.uchicago.edu > AMCS: edi...@amcs-pubs.org or sa...@amcs-pubs.org > > > --------------------------------------------------------------------- > Visit our website: http://texascavers.com > To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com > For additional commands, e-mail: texascavers-h...@texascavers.com > >
