THE JURASSIC/CRETACEOUS INTERVAL EAST OF THE SAN RAFAEL SWELL, GRAND COUNTY, UTAH

The basal Yellow Cat Member of the early Cretaceous (Barremian) Cedar Mountain Formation is a complex of fluvio-lacustrine sediments that are characterized by drab green, variegated mudstones. The unconformable contact between the Cedar Mountain Formation and the underlying Jurassic (Tithonian) Morrison Formation has been variably defined.

East of the San Rafael Swell, the contact is often placed at the base of a prominent 0.7-1m thick pedogenic calcrete bed. New field observations in the lower portions of the type section of the Yellow Cat Member, however, highlight the presence of an early Cretaceous interval below this calcrete. This interval is composed of a 3.8–4.5m thick gradational zone of translocated red/green smectite-rich mudstone which preserves vertebrate remains, including abundant iguanodont and ankylosaur teeth and ankylosaur dermal ossicles, indicating an early Cretaceous age. Underlying this mudstone is a thin chert layer accompanied by mounds of agate/chert, possibly of spring-water origin.

Petrographic observations of the calcrete profile show that calcite formed by the pseudomorphic replacement of host silicate minerals after smectite formation and translocation. This has been interpreted to be the result of alternating wet/dry climatic cycles based on geochemical models proposed for the genesis of calcretes. The chert mounds, on the other hand, have been interpreted to be silcretes that precipitated in spring seepages within paleovalleys that developed during earliest Cretaceous time.