Comment by apw

The dead forests you mentioned may have became massive coal beds via the "charcoal route"; i.e. a global firestorm may have incinerated essentially the entire biosphere.

    The global debris layer created by the end-Cretaceous 
    impact at Chicxulub contained enough soot to indicate
    that the entire terrestrial biosphere had burned.

Source:

K-Pg extinction: Reevaluation of the heat-fire hypothesis; Robertson, Lewis, Sheehan & Toon, 2013; Journal of Geophysical Research: Biogeosciences

http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20018/abstra...