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Dinosaur lake site in Morrison Formation explained
From: Ben Creisler
New in May 2011 Palaios:
DAVID L. JEFFERY, JANET L. BERTOG, and JOHN R. BISHOP (2011)
SEQUENCE STRATIGRAPHY OF DINOSAUR LAKE: SMALL SCALE FLUVIO-DELTAIC STRATAL
RELATIONSHIPS OF A DINOSAUR ACCUMULATION AT THE AARON SCOTT QUARRY,
MORRISON FORMATION, SAN RAFAEL SWELL, UTAH.
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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