Sequential palynological changes across the composite Cretaceous-Tertiary (K-T) boundary claystone and contiguous strata, western Canada and Montana,USA

The boundary claystone in the western Canada Basin is composed of three int imately associated layers with a regional distribution: a basal brownish-gr ey hackly claystone, a middle brownish-black satiny claystone, and an upper brown laminated shale. These layers appear to represent a graded successio n in terms of grain size and depositional rate. An abrupt reduction in the relative abundance of gymnospermous pollen, representing canopy vegetation, immediately precedes the hackly layer. The hackly claystone is accepted as being originally formed of microtektites from a ballistically transported ejecta blanket deposited within minutes or hours of the bolide's impact and associated with a heat pulse. The variable miospore content of the hackly layer is considered to be from local, reproductively active understorey veg etation. The shocked-quartz-rich satiny claystone is interpreted as being f ormed by fine debris deposited over days, weeks, or months. Its homogeneous texture suggests a short depositional event, but must have involved enough time for a residual Cyathidites-Ulmoideipites survival flora to release mi ospores. Further, the presence of this flora requires the continuation of l ight levels required for photosynthesis and temperatures generally above fr eezing. A settling time of several years for very fine debris and aerosols would fit with the apparent seasonal interlayering of the organic tissue in the laminated shale. The abruptly overlying Laevigatosporites-Kurtzipites recovery flora initiates the succession leading to the return of a canopied swamp, and its presence may signal the clearing of the atmosphere of all i mpact-generated debris, except for postulated elevated CO2 abundances.