“The cenotes of Yucatan in Mexico and the Caribbean are sinkholes containing lakes in a limestone karst landscape that has been dissolved by passing waters. They open onto extensive cave systems and underground rivers, some of which travel many miles underground (some have been explored by divers through 100Km of galleries). This system of underground rivers and aquifers is linked to the sea, and the underground river systems have haloclines, where layers of fresh and sea water meet. We have covered this aspect before at http://tinyurl.com/ohf4cp7.
Their waters are very clear, due to being filtered by the limestone before joining the system They were also one of the main sources of water for the Maya culture. While we have already covered their cultural aspects (see http://tinyurl.com/ppltpcv), some of them have another fascinating connection: A dense ring of them occurs at the edge of the 180 Km wide Chicxulub impact crater, thought to be the mark left behind by the 10 Km astrobleme that killed the dinosaurs and ammonites.
The rock was shattered by the impact, making it more permeable, and the edges were unstable and crumbled in along a set of faults, widening the crater and accounting for the ring of cenotes. Because the impact site was a shallow sea, it is possible that kilometres high waterfalls poured into the still smoking crater. The remnants were buried under several Km of sediment during the next 65 million years, leaving the cenote ring as the only visible surface trace. In fact the crater was found purely incidentally by a geophysical and drilling survey in search of oil, some time after Luis and Walter Alvarez had proposed an impact as the cause of the mass extinction on the basis of an iridium spike in the rocks at the Cretaceous/Tertiary boundary.
The ring is currently under consideration by UNESCO as a world heritage site, since it marks such a significant and unique boundary, as well as having cultural and touristic importance. So, should you visit Mexico and go swimming in one, remember that you are bathing in a remnant of one of the great events of our planet’s recent geological history.”
Loz via [The Earth Story]
Image credit: Paul Nicklen/National Geographic
Original paper, paywall access: http://geology.gsapubs.org/content/23/1/17.abstract