Carbon dating only is good for about 50,000 years so that obviously wasn't the case here. Now do you know what type of dating they did use?
Yay! You get a cookie!
No, I'm about to go home and don't have time to look up the article and it's also likely that they did not date it but cited others' research. Another poster replied to you that this strata is quite well known, perhaps you should ask that person for some hints on sources of this information. Or go hit your closest university library. It's quite interesting stuff.
Basically, they measure the proportions of mother/daughter radioisotopes found in those rock layers, including Uranium-238/Lead-206, Uranium-235/Lead-207, Thorium-232/Lead-208, Rubidium-87/Strontium-87, Potassium-40/Argon-40, and Samarium-147/Neodymium-143.