STRATIGRAPHY, SEDIMENTOLOGY, AND DEPOSITIONAL FACIES OF THE MORRISON FORMATION 5ES QUARRY OF CENTRAL MONTANA

Some of the northernmost dinosaur quarries of the Upper Jurassic Morrison Formation are found in strata exposed along the flanks of Spindletop Dome in central Montana. Several dinosaur quarries discovered there have been excavated over the past 19 years. 5ES Quarry contained fossil remains of 6 stegosaurs of likely different sexes and ages (Saitta, 2015) and a single partially articulated unidentified Macronarian sauropod with shed Allosaurus teeth. No microvertebrate or invertebrate fauna were found. Only a few charophyte gyrogonites (Aclistochara), freshwater diatoms, and algae (unidentified calcispheres) were recovered.

The bonebed is located at the base of a 70-cm thick gray mudstone bed that is stratigraphically 47 m above the Swift Formation. The mudstone lithofacies transitions from a basal indurated yellow-brown mudstone bed to a 15-cm thick reddish mudstone bed, to the 70-cm thick gray mudstone bed, and finally a 140-cm thick greenish mudstone bed. Overlying the greenish mudstone bed is a 20-cm thick sandstone bed. The sandstone is a subangular, well sorted (0.35), fine-grained (2.83 Φ), quartz arenite. The sandstone bed is overlain by a 20-cm thick mudstone bed followed by 70 cm of numerous stacked, thin bedded (< 5 cm) sandstone beds. The sandstones are subangular, moderately sorted (0.50), fine-grained (2.98 Φ) quartz arenites.

XRD analyses of the mudstone beds show they are composed of illite (54%), kaolinite (35%) and smectite (11%). XRF analysis of the lowest mudstone bed recorded a high molybdenum (Mo) concentration (144 ppm) suggesting anoxic conditions. Background Mo concentrations are 8 ppm. Mo is found in humic or dystrophic lake or pond environments filled with decaying organic material. Clumped oxygen isotope analysis of the quarry bed yielded a paleotemperature of 44^o C (111^o F).

The sedimentary sequence records the transition from a humic pond overlain by an oxidized mudstone to an organic-rich pond followed by floodplain mudstone deposition, all capped by repeated distal splay sandstone beds. Very thin laminae (< 1 cm) of a microbial carbonate film often developed on the upper surface of the splay beds indicating a prolonged high groundwater table after deposition. The sedimentary succession indicates a paleoclimate with a high precipitation:evaporation ratio and elevated temperatures.