Megaflora of the Hell Creek and lower Fort Union Formations in the western Dakotas: Vegetational response to climate change, the Cretaceous-Tertiary boundary event, and rapid marine transgression

Abstract

The Hell Creek and Fort Union Formations in southwestern North Dakota and northwestern South Dakota have yielded a diverse megaflora of 380 species from 158 quarry sites. These sites are situated in a stratigraphic framework, delimited by palynology, magnetostratigraphy, and vertebrate paleontology, that contains both the Cretaceous-Tertiary (K-T) boundary event horizon and stratigraphic evidence for rapid base-level increase. The late Maastrichtian Hell Creek flora is dominated by angiosperms; ferns, fern allies, cycads, ginkgo, and conifers represent <10% of total taxa and specimens. Megafloral extinction at the K-T boundary is extensive, effectively eliminating all dominant plant taxa of the upper Hell Creek Formation. K-T survivorship appears to be greatest in plants that occupied Cretaceous mire facies. The Hell Creek megaflora is heterogeneous with respect to stratigraphic position and sedimentary facies, allowing the recognition of three superposed megafloral zones and two facies-controlled megafloras. Hell Creek vegetation represents an angiosperm-dominated woodland composed of small- to medium-sized trees, often with lobed leaves. One exception is a diverse angiosperm-dominated herbaceous vegetation associated with large paleochannels in the middle of the formation. The uppermost Hell Creek megaflora zone (HCIII) first occurs near the base of magnetic polarity subchron C29R. The HCIII megaflora is significantly more diverse than earlier Hell Creek floras, and foliar physiognomy suggests a significant climate warming during the final 300-500 k.y. of the Cretaceous. The Paleocene megaflora is depauperate, less heterogeneous than the Cretaceous megaflora, and dominated, in all facies, by taxa that were most common in Cretaceous mire facies.