Petrological and δ13C analyses were undertaken on contiguous specimens of coal and intercalated minor organic-rich clastic sediments collected from coal seams spanning the Cretaceous-Paleogene boundary in the Alberta and Saskatchewan portions of the Western Interior Basin. The generally high smectite content of the coal suggests that the original mires were largely small, disconnected, and rheotrophic, readily receiving abundant waterborne detrital clastic material of largely volcanic origin. Nevertheless, using the distinctive claystone that marks the Cretaceous-Paleogene boundary as a regional datum, it is possible to correlate cycles in the vitrinite and inertinite composition of the coals over >500 km. Estimates of peat accumulation rates suggest that the cycles in vitrinite and inertinite composition represent regional, cyclic fluctuations in wildfire and oxidation of the peatlands and overlying canopy at a frequency of hundreds to thousands of years. The likely causes of these fluctuations were cyclic, regional-scale changes in temperature. The Cretaceous-Paleogene boundary event occurred early during a phase of gradually increasing temperature and/or decreasing rainfall, but peak wildfire and desiccation of peat occurred up to 14,000 yr later than the Cretaceous-Paleogene boundary, and the mires did not experience significant water stress in the immediate aftermath of the extinction event. A persistent, 1.5‰–3.0‰ negative δ13C excursion occurs across the Cretaceous-Paleogene boundary, but it cannot be readily separated from four, further negative excursions later in the earliest Danian. The negative carbon isotope excursion linked to the Cretaceous-Paleogene boundary began a few hundred years before the event itself, and recovery occurred within 21 k.y., and possibly in as little as just a few thousand years, consistent with recently calibrated shallow-marine δ13C records. Hence, the atmospheric and surface ocean carbon pools were coupled at this time. The absence of evidence for catastrophic change in the climatic regime at the time of the Cretaceous-Paleogene extinction in these mires supports the notion that the negative shift in atmospheric δ13C was brought about by changes in the δ13C composition of the surface ocean. This is consistent with the greater magnitude of extinction experienced by marine fauna relative to the terrestrial realm.