CLAY MINERALOGY AND PETROLOGY ASSOCIATED WITH A DINOSAUR LAKE (AARON SCOTT QUARRY, MORRISON FORMATION, JURASSIC), UTAH

The Aaron Scott Quarry in the Morrison Formation of south-central Utah is an accumulation of vertebrate fossils representing the shoreline of a large lake during a moderate drought. Vertebrate fossils in the assemblage include largely fragmented dinosaur bones, isolated teeth and podial elements of Allosaurus, a diplodocid sauropod, a small ornithopod, and a diverse microfauna. Sequence stratigraphic analysis has shown that the fossil-bearing layer is within a forced-regression of a sequence. Within the quarry, the sediment is composed of a greenish-gray cemented siltstone with clay rip-up clasts near the base. The presence of rip-up clasts in the sediment is consistent with a drought condition, where the shoreline clays dried up, desiccated and cracked. Later, reworking of this desiccated clay produced rip-up clasts. Clay mineralogy of samples taken through the stratigraphic section of the sequence supports changing environmental conditions, with a dry period during the deposition of the fossil-bearing layer.