A DINOSAUR-RICH SPLAY FILL, SAN RAFAEL SWELL, UTAH

A significant accumulation of Jurassic vertebrates in the San Rafael Swell of Utah is an overbank splay fill feature within the lower Brushy Basin Member of the Morrison Formation. The vertebrate accumulation near the top of this deposit consists of 2-3m of grey mudstones capped by a highly bioturbated dark grey mudstone with abundant unionid bivalves in life position and gastropods. Preliminary discoveries include: a Diplodocus skull; articulated Diplodocus and Apatosaurus vertebrae as well as numerous bones from several additional individuals; Two Allosaurus individuals, including a very small specimen < 1m tall and numerous teeth including a broken-off tooth penetrating a bone; Othnielia; Camptosaurus; Stegosaurus; crocodile; turtle; a sphenodontid skull and three lower jaws including probable new species; and a large collection of small bones. Bones are in excellent condition, black, and largely disarticulated.

The deposit base lies 20-25m above the Salt Wash Member and ranges to 25m thick. The deposit base is a broadly arcuate scour that cuts into older red and green strata. A discontinuous cross-bedded sandstone at the base up to 8cm thick is overlain by a continuous burrowed limestone 5-10cm thick. Fill sediments consist primarily of dark grey mudstones. The lower half of the deposit consists of banded mudstone beds up to 2m thick, and contains 6 highly bioturbated limestones 2-8cm thick, and two channel-form sand bodies 1m thick and 5m wide. The upper half of the deposit consists of grey to dark grey mudstones with poorly developed banding and very little bioturbation and is overlain by a thick, tabular sand.

The environment is interpreted as an overbank splay scour that intermittently existed as an isolated lake. Limestones indicate periods of alkalinity during isolation, evaporation, and shrinking of the lake environment. The dark color of sediments, scarcity of bioturbation, good condition and black color of the bones indicate dysoxic conditions and lake stratification. Extensive disarticulation of bones indicates a non-catastrophic sediment rate where animals probably sank to the lake floor, decomposed, and bones mixed over a period of time. Lake invertebrates only in the uppermost portions of the deposit may indicate well oxygenated water as the lake shallowed and the waters became well mixed.