ARE SILICLASTIC NODULES IN THE UPPER JURASSIC MORRISON FORMATION (EASTERN BIG HORN BASIN, WYOMING) ACCRETIONARY LAPILLI?

Kvale and Vondra (1983) described a lenticular bed of siliceous nodules in the Upper Jurassic Morrison Formation in the Big Horn Basin, Wyoming, and suggested they were accretionary lapilli. This interpretation was based on field observations, and implies close proximity to a volcanic vent. We collected nodules from the upper Morrison Formation near its conformable contact with the Lower Cretaceous Cloverly Formation, and performed a petrographic and scanning electron microscopy study to validate this interpretation. The nodules are 1-2 cm in diameter and are generally found loose on the surface. Two can be accreted together to form a "dumb-bell" shape. Central nuclei are surrounded by concentric layers that can be distinguished by grain-size and color changes. Feldspar grains from 50-400 μm in diameter and occasional 25-100 μm rutile grains are observed in a fine-grained silica-rich matrix. Small veins/fractures are filled with barite, consistent with a volcanic origin for these nodules. Our data support the interpretation that these nodules are accretionary lapilli, suggesting explosive volcanic activity in close proximity to the Big Horn Basin, northern Wyoming, during upper Morrison deposition.