The Cretaceous–Paleogene (K–Pg) boundary is the best known and most widely recognized global time horizon in Earth history and coincides with one of the two largest known mass extinctions. We present a series of new high-precision uranium–lead (U–Pb) age determinations by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) method from volcanic ash deposits within a tightly constrained magnetobiostratigraphic framework across the K–Pg boundary in the Denver Basin, Colorado, USA. This new timeline provides a precise interpolated absolute age for the K–Pg boundary of 66.021 ± 0.024 / 0.039 / 0.081 Ma , constrains the ages of magnetic polarity Chrons C28 to C30, and offers a direct and independent test of early Paleogene astronomical and 40Ar/39Ar based timescales. Temporal calibration of paleontological and palynological data from the same deposits shows that the interval between the extinction of the dinosaurs and the appearance of earliest Cenozoic mammals in the Denver Basin lasted ∼185 ky (and no more than 570 ky) and the ‘fern spike’ lasted ∼1 ky (and no more than 71 ky) after the K–Pg boundary layer was deposited, indicating rapid rates of biotic extinction and initial recovery in the Denver Basin during this event.
