A high-resolution megafloral biostratigraphy spanning the Cretaceous-Tertiary boundary in the northern Great Plains. (Volumes I and II).

摘要

This is a two part study designed to document floral change at the Cretaceous-Tertiary (K-T) boundary in the Western Interior of North America. The first part is an attempt to quantify leaf litter composition in different facies of a modern floodplain forest of known composition. This is used in the second part to analyze collections of fossil leaves from above and below the K-T boundary in the northern Great Plains.; A riparian forest on Quinnipiac River floodplain in North Haven, Connecticut was studied to assess the reliability of using leaf-litter counts to predict actual forest composition. Results indicate that species composition of leaf litter is directly related to forest composition but is influenced by sedimentary facies. This provides a method for interpreting ancient floral communities from fossil leaf census data.; The K-T boundary in the northern Rocky Mountains and Great Plains is preserved near the contact of the terrestrial Hell Creek-Lance (mid-late Maestrichtian) and the Fort Union (Paleocene) Formations. Nearly 14,000 fossil leaves were collected from over 100 quarries in six different study areas in Montana, Wyoming, and the Dakotas. The primary area, near Marmarth, southwestern North Dakota, produced 11,503 specimens from 87 localities in a 200-meter-thick lithostratigraphic framework. This framework is based on detailed measured sections and sedimentary facies analyses and incorporates pollen, vertebrate, and ammonite biostratigraphy, magnetostratigraphy, and the presence of the pollen/iridium/shocked mineral K-T boundary datum. A morphotype system based on leaf architectural analysis was used to recognize 247 megafloral taxa that were then systematically described. When quantified by census and described by sedimentary facies, the fossil localities reflect ancient floral communities. Placed in the stratigraphic framework, they produce a floral zonation (four zones and two subzones) that is used to assess floral change over time. Some of these floral zones are of regional extent and are consistent with chronologies based on the K-T datum and ammonite zonations.; Results indicate that there were major floral changes before and at the K-T boundary. Changes before the boundary were apparently caused by plant migration due warming climate but plant extinctions at the boundary were too large and widespread to have been caused by climate or facies change alone. These data combined with the iridium anomaly, shocked quartz, and the abruptness of pollen extinctions implicate a bolide impact as a causative mechanism for floral devastation at the K-T boundary.