SEQUENCE STRATIGRAPHY OF DINOSAUR LAKE, AARON SCOTT SITE, MORRISON FORMATION, UTAH

Sequence stratigraphic analysis of lake sediments reveals the depositional setting of the Aaron Scott Site, an important Jurassic vertebrate locality in the Morrison Formation of Utah. The lower Brushy Basin Member within the San Rafael Swell consists of cyclic, progradational, fluvio-lacustrine delta sediments. Cyclicity is indicated by the geometry of successive progradational wedges, and the orientation and stratal relationships of delta facies and unconformities.

The 25m thick interval consists of successive westward-stepping sigmoidal wedges separated by erosional surfaces. Each cycle consists of horizontal mudstone strata that contain thin limestones and siltstones. Mudstones coarsen upward and are up to 5m thick. Siltstones and limestones are 1-10cm thick and cap mudstone beds. Erosional surfaces truncate underlying bedding. Strata of the succeeding package onlap the erosional surface.

The deltaic unit is capped by cross-bedded, channelized, discontinuous, coarse, pebbly sandstone. Sandstones appear to correlate across successive wedges, but careful observation reveals that they do not. In addition to the coarse capping sandstones, fine sandstones that can be traced along strike onlap and pinch out against the down-dip portions of the erosional surfaces.

Delta sediments were deposited within accommodation created during high base level and eroded during low base level. In sequence stratigraphic terminology we would call the erosional surface the cycle boundary, the onlapping sandstones on the erosional slope the lowstand systems tract, and the onlapping mudstones the transgressive systems tract. The overlying progradational package is the highstand systems tract. Coarsening upward intervals within the cycles are paracycles caused by higher order fluctuations in lake volumes. The bone beds are situated within several silty sandstones at the top of a paracycle and within onlapping lowstand sands. Mudstones overlying the bone beds contain bivalves in life position. The bone accumulation occurred during low base level indicating a dry period during which animals congregated at the shores of a large lake. Increased water input raised base level and buried the bone beds with fine, impermeable sediments.