Abstract

Stratal geometries, cyclical stacking patterns, and taphonomic analysis of the Aaron Scott Quarry in the Jurassic Morrison Formation indicate that sediments accumulated within a prograding fluvio-deltaic lacustrine system. Sediments were deposited during the progressive fill of a large lake. Stratal geometries and stacking patterns display relationships similar to larger-scale deltaic sequences caused by fluctuations in base level. Stratigraphic, paleontologic, and sedimentologic analysis indicates that water-volume-driven, base-level changes were responsible for the accumulation and preservation of the assemblage. The assemblage accumulated during a lowering of base level during the late stages of an overall fall caused by drought and was buried by a rapid rise at the end of the drought. Fluvio-deltaic lake fill consists of numerous off-lapping beds of sparsely bioturbated, coarsening-upward mudstone capped by thin, bioturbated silty mudstones. Erosional surfaces dip to the west and cut older mudstone parasequences. Retrogradation, aggradation, and progradation of strata typify stratigraphic trends above the erosion surface. Coarse braided stream sands cap off successive sequences as accommodation is filled prior to the next lowstand and sequence boundary incisement. The quarry is a large, taxonomically diverse deposit formed by the concentration of animals at the margins of a lake during a drought. Animals congregated near the banks as smaller watering holes dried. Bone locations and orientations indicate reworking by fluvial and wave currents. Vertical preservational trends indicate accumulation over a period of years. Trace fossil, lake invertebrate distributions, and sedimentological evidence indicate that periodic establishment of oxygen stratification within lake waters may have contributed to conditions favorable for bone preservation.

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