A SEDIMENTOLOGICAL ANALYSIS OF AN UNCONSOLIDATED AIR-FALL ASH BED AT ASHFALL FOSSIL BEDS, NEBRASKA

The Miocene Cap Rock Member of the Ash Hollow Formation exposed at Ashfall Fossil Beds State Historical Park is the site of a spectacular fossil assemblage of hundreds of intact skeletons of horses, camels, birds, turtles, and most notably, a herd of Teleoceras major, a barrel-bodied rhinoceros. The bonebed is encased in approximately 2 meters of unconsolidated volcanic ash that has been linked to activity of the Bruneau-Jarbidge eruptive center 12 million years ago. Despite extensive research on the vertebrate fossils, little attention has been paid to the specific depositional processes responsible for their exquisite preservation. Sedimentological analysis of the ash bed provides new insights into the sequence of events that led to the exceptional preservation of the fossil assemblage.

Though it is composed of nearly 100% pure volcanic glass shards with mean grain size of 64.3 µm, the ash bed contains a large amount of sedimentological detail. The lower 50 cm of the bed are a mixture of ash and muddy sand. Variations in sand and mud content differentiate the intial airfall deposit and a later storm event. No vertebrate fossils are found above this layer of sandy ash. Alternating light and dark gray layers of ash compose the rest of the ash bed. Color variation is a result of grain size difference. Settling tube experiments suggest each light-dark couplet represents a single settling event, caused when dry ash settles into a standing body of water. In addition to sharp horizontal laminations, a number of other sedimentary structures are present, including cross-lamination and flame structures. Additionally, the ash bed is obscured at many intervals by heavy bioturbation. General ichnofabric homogenization ranges throughout the ash bed from 0% - 60%, though no pattern is yet discerned. Petrographic examination reveals 100 micrometer-scale sub-circular void spaces that are defined by tangentially-oriented glass shards. Voids are interpreted to be cross-sections of microinvertebrate burrows. These new observations allow a paleoecological timeline of events to be hypothesized.