SEDIMENTOLOGY AND TAPHONOMY OF RED CANYON RANCH DINOSAUR QUARRY JURASSIC MORRISON FORMATION, BIG HORN BASIN, WYOMING

The upper quarry at Red Canyon Ranch, located within the Late Jurassic (Kimmerdigian-Tithonian) Morrison Formation in the Big Horn Basin, Wyoming, has produced over two hundred sauropod bone fragments. An investigation of the depositional environments based on taphonomic and sedimentologic data was conducted for the upper quarry. Data, including sedimentary facies, vertebrate fossils, quarry assemblage, and bone modification, were used in the paleodepositional interpretation. During excavation GPS position of bone fragments were taken to create a quarry map.

The excavation exposed an area of 42 square meters. The major vertebrate bone layer was found at the interface between a siltstone facies and two sandstone facies. The siltstone facies is structureless becoming siltier up section. The two sandstone facies consist of a lower cross-bedded and upper ripple cross-laminated facies. The lower cross-bedded sandstone facies contains the bone layer and consists of medium scale trough crossbeds with mud rip-up clasts and iron concretions. The upper ripple cross-laminated sandstone facies contained ripple cross laminations, and small layers (<10 cm) of mudstone rip-up clasts and shows an overall fining upward sequence. The cross-bedded sandstone facies is a higher energy deposit while the ripple cross-laminated sandstone facies is a lower energy deposit. The vertical facies stacking is best interpreted as the migration of a point-bar in the meandering fluvial setting.

Paleocurrents taken from sedimentary structures in the upper quarry and surrounding strata show an orientation averaging 158 degrees. Bone orientation data taken from the long axis of bones show a bimodal distribution to the south and southeast which parallels the paleocurrent data. Bone modification evidence in excavated bones includes significant abrasion, scratch marks, and rounding at edges. Bone fragmentation was so extensive that very few identifiable bones were found. Sedimentologic and taphonomic data are consistent with a channel-lag taphonomic mode.