FORCED REGRESSION AND LACUSTRINE TO FLUVIAL TRANSITION OF A JURASSIC LAKE, AARON SCOTT DINOSAUR QUARRY, SAN RAFAEL SWELL, UTAH

The Aaron Scott Quarry lies within gray mudstones of the Lower Brushy Basin Member of the Jurassic Morrison Formation that are interpreted to be prograding fluvio-deltaic lacustrine facies. The Upper Brusy Basin Member strata are red and gray banded mudstones with discontinuous cross-bedded sand bodies. The Lower Brushy Basin gray, lacustrine strata contain distinct sediment trends and stratal geometries that display sequence stratigraphic relationships. Lake sediments consist primarily of sparsely bioturbated coarsening upward mudstone parasequences capped by a thin, calcite-cemented, bioturbated silty mudstone. These silty mudstones indicate brief periods of shallowing and increased alkalinity of the lake prior to increased water volume and formation of the next parasequence. The rich accumulation of dinosaur bones of the Aaron Scott Quarry lies within sands lapping onto stratal surfaces formed during a forced regression, most likely caused by drought.

After the drought and reestablishment of lake levels, accommodation at the site filled and facies transitioned into the red and gray banded Upper Brushy Basin composed of river and floodplain facies with numerous paleosols. This transition appears at a consistent stratigraphic level across several offlapping sequences and may indicate the maximum fill or spill level of the lake basin. The transition between the two facies contains numerous well cemented, very fine sands and organic rich mudstones that contain finely comminuted coaly material up to several mm in diameter. Abundant silicified wood and one conifer cone is also associated with this transition.

The extensive three dimensional exposure provides an opportunity for further delineation of sequence stratigraphic relationships within the lake deposits and the marshy transition into floodplain facies. Within the Aaron Scott Quarry and associated sediments, it is clear that climatic fluctuations drove lake levels and dinosaur traffic, and thus controlled the lake fill progression and the distribution of dinosaur-rich accumulations. From these interpretations we predict that the search for additional rich dinosaur accumulations would be well focused in lowstand and drought-forced regressive facies.