THE AFTERMATH OF THE K/T EXTINCTION: EASTERN MT AND WESTERN ND

Study of the K/T extinction at the top of the dinosaur-rich Hell Creek Formation in the upper Great Plains has focused on changes in plants, tetrapods, insects, and to a lesser degree aquatic animals. Changing patterns of sedimentation and changes in the overall ecosystem have been less studied. We suggest that facies changes across the K/T boundary, including an increase in deposition due to ponding, incision of river systems, changes in sediment composition and size, change from gray to yellow hues of the formations, and the abrupt appearance of coal beds, are related directly to the K/T event.

Impact scenarios suggest that most vegetation in North America was destroyed by an initial blast of superheated debris from the crater that was followed by months of darkness. A variety of studies have documented a loss of virtually all herbivorous tetrapods, a major extinction of insects and a marked decline in insect folivory.

We propose that for several years after the impact, the loss of vegetation must have resulted in an enormous increase in erosion, especially in upland areas to the west. This erosion clogged rivers and flooded landscapes after the impact. An increase in pond related facies and changes in sediment size, color, composition and sedimentary structures resulted.

Coal formation was common in the late Cretaceous in some regions adjacent to the study area, so the appearance of coal in the study region is not remarkable. However, the fact that coal deposition began nearly simultaneously with the impact needs explanation. Post-extinction ecology was characterized by several millions of years when there were no herbivorous tetrapods, and insect folivory was depressed substantially. Low levels of herbivory likely increased the amount of plant matter reaching the ground. We suggest that the increased plant matter, together with the documented preferential survival of mire plants and an abundance of water from widespread ponding combined to promote coal deposition.