Integrated recording of airgun shots and natural seismicity on land array. With increasingly detailed examination and dating of the sedimentary rocks came increasing consensus that a large number of species died about 65 Ma ago when the sediments representing the boundary between the Cretaceous and Tertiary stratigraphic divisions were laid down with an unusually high content of the element Iridium.
Iridium is rare on Earth, but relatively common in meteorites or comets.
Globally, this stratigraphic horizon provided consistent observations, but did not consistently indicate the source of the Iridium, presumed to be an impact crater. Many thought it lay offshore in the deep oceans and therefore probably had not survived crustal recycling during plate subduction. Then in the last few years, Mexican geophysicists assessing petroleum and other natural resource reserves beneath about a kilometre of shallow-water limestones collected higher resolution gravity and magnetic data off the Yucatan peninsula, data which revealed anomalies in the form of concentric circles that straddle the present coastline. Subsequent drilling onshore encountered breccias, tectonites and andesite melts that have confirmed a probable impact structure, today only the exact size and internal geometry of the multi-ringed structure is hotly debated. Neither the full radial extent nor the depth of excavation are well known and the size proves crucial to whether the impact caused mass extinctions.