FIRST DISCOVERY OF THE CRETACEOUS-TERTIARY (K-T) BOUNDARY CLAY IN UTAH
Until now the K-T boundary in the North Horn Fm (NHF) has been poorly defined and based on biostratigraphy. We report the discovery of lacustrine sediments at North Horn Mountain, Emery County, Utah, that preserve the boundary clay and ejecta debris, including shocked quartz and altered microtektites. Biostratigraphically, the KTB is 4.4 m above the stratigraphically highest dinosaur fossils and 10 m below the lowest occurrence of Paleocene turtles. The KTB lies 208m above the base of the NHF, is characterized by ferruginous stains, a gamma kick, a magnetic susceptibility spike, and is bounded by Maastrichtian and Paleocene palynomorphs.
The following microstratigraphy is present. Unit 1 ( ~20 cm), rooted limestone with bivalves and gastropods. Unit 2 (20 cm), coal. Unit3 (1 cm), pale brown, kaolinite-rich boundary clay. Unit 4 (22 cm), tan-gray mudstone that includes shocked quartz (<350mm), altered microtektites (<0.6 mm), and charcoal (<1cm). Analysis shows microspherules (20-40 mm) range in composition from iron sulfphate to clay. Some microspherules occur in (120 mm) clusters at the bottom of this unit. Higher in the unit is a layer of kaolinitic clay spheres (<0.6 mm in diameter encased in secondary gypsum or calcite shells. Unit 5 (30 cm), caps the ejecta layer and is a tan bioturbated sandstone.
Wet environments represented by gray carbonaceous limestones, mudstones, and thin sandstones prevail up to and include the KTB. Localized mudcracks and gypsum stringers suggest drying and subaerial exposure of the lake margin following KTB deposition. Above the KTB there is a sharp transition to dcm-m scale sandstone beds and variegated mudstones, suggesting environmental change.