The Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, St. George, Southwestern Utah

Extensive development in the St. George, Utah area resulted in the temporary exposure of fresh rock at and near the St. George Dinosaur Discovery Site at Johnson Farm (SGDS), facilitating examination and description of the Lower Jurassic stratigraphy of the vicinity in some detail. Since this study, construction has covered much of what we observed, making this the only detailed record of the Moenave Formation in the SGDS area. We measured a total thickness for the Moenave Formation of 73.97 m. The formation is divided into a lower Dinosaur Canyon Member (56.41 m thick) and upper Whitmore Point Member (17.56 m thick). A poorly cemented chert and anhydrite pebble conglomerate approximately 1 m thick overlies an unconformable contact (J-0 unconformity of previous authors) of the Moenave with the older (Late Triassic) Chinle Formation. Overlying the conglomerate is 34.8 m of slope-forming, mudstone-dominated rocks with anhydrite nodules and secondary gypsum veins in the lower portion, and thin, ripple-bedded sandstone layers with mudcracks that become increasingly abundant up-section. Fineto medium-grained sandstone beds, each 10 to 100 cm thick, constitute a ledge-forming sandstone unit 18.9 m thick at the top of the Dinosaur Canyon Member. Medium-scale trough cross-bedding characterizes the thicker beds in the lower half, and ripple-drift cross-bedding that preserves dinosaur tracks dominates the beds near the top. A poorly sorted, yellowish-tan to green sandstone bed 5 m from the top preserves abundant, identifiable plant debris. The Whitmore Point Member has a more varied lithology. The lower, conformable contact is placed at the base of a distinctive, cherty limestone bed above the ledge-forming sandstones at the top of the Dinosaur Canyon Member. The member can be divided into three intervals at the SGDS: (1) a basal, complex interval that includes the main track-bearing sandstone; (2) a middle, sandstone-dominated interval similar to the upper few meters of the Dinosaur Canyon Member; and (3) an upper, thin-bedded interval. The main track layer (MTL) at the base of the main track-bearing sandstone is impressed into a mudstone. Two episodes of dinosaur track emplacement and preservation are recognized on the MTL. To the northwest, the MTL preserves scours (northward-directed currents), tool marks, and abundant dinosaur swim tracks, suggesting deeper water. The top surface of the main track-bearing sandstone is eroded into a series of large (meter-scale) megaripples that are draped by thin, ripple-bedded sandstone beds that preserve additional dinosaur tracks and rare fossil