Abstract

The boundary claystone in the western Canada Basin is composed of three intimately associated layers with a regional distribution: a basal brownish-grey hackly claystone, a middle brownish-black satiny claystone, and an upper brown laminated shale. These layers appear to represent a graded succession 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 vegetation. The shocked-quartz-rich satiny claystone is interpreted as being formed 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 miospores. Further, the presence of this flora requires the continuation of light levels required for photosynthesis and temperatures generally above freezing. 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 impact-generated debris, except for postulated elevated CO2 abundances.

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