TEXTURAL CRITERIA FOR PALEOENVIRONMENTAL INTERPRETATION OF PALUSTRINE FACIES OF THE CRETACEOUS CEDAR MOUNTAIN FORMATION IN DINOSAUR NATIONAL MONUMENT

The Albian (mid- Cretaceous) Cedar Mountain Formation consists primarily of clastic fluvial deposits, but also contains abundant, complexly developed terrestrial carbonate horizons. These horizons contain a wide variety of distinct textures, including:

* bedded micrite and micro-spar, containing occasional gastropod and ostracode fossils * coalesced beds of micritic concretions 10-100 cm diameter (commonly containing calcite-filled septarian fractures) * bladed epitaxial calcite growth on upper surfaces of large (10-100 cm) coalesced micritic concretions * finely laminated micrite typical of algal mat development * abundant burrows and root casts both within and on surfaces of beds * beds of mixed carbonate and very fine-grained (eolian?) sand * brecciated micrite beds, with both crystalline calcite and chalcedony cement * intraclastic fine-pebble to cobble conglomerates, with both crystalline calcite and chalcedony cement * chalcedony as bedding-parallel laminae and lenses 1mm to 20 cm thick * sponge-like calcite crystalline mesh fabrics, possibly developed as a calcite cement in fractured gypsum

From these textures, we interpret the calcareous horizons in the Cedar Mountain Formation to have resulted from multiple processes. Most calcareous horizons display a simple micritic nodular texture developed within fine-grained floodplain deposits, and can be interpreted as primarily pedogenic. Other horizons, particularly the thick calcrete beds that mark the base of the Cedar Mountain Formation, can best be interpreted as resulting from a complex sequence of depositional and early diagenetic processes that includes lacustrine precipitation of calcite and other evaporite phases, followed by pedogenic calcrete and silcrete development, and later phreatic zone recrystallization.

The nature of the depositional and pedogenic features indicates a paleoenvironment for the Cedar Mountain Formation that was largely arid to semi-arid. This interpretation corroborates evidence from fluvial channel characteristics for occasional flashy, extreme precipitation events followed by arid conditions.