SPRING DEPOSITS WITHIN THE PALUSTRINE CARBONATES OF THE CRETACEOUS CEDAR MOUNTAIN FORMATION, UT

This study reports on calcareous facies within the Lower Cretaceous Cedar Mountain Formation in and around Dinosaur National Monument in northeastern Utah and east central Utah. Common, nodular calcareous beds in the Cedar Mountain Formation, particularly in the Yellow Cat and Ruby Ranch members, have previously been interpreted as calcretes generated by pedogenic processes within fine-grained overbank mudstones and siltstones (e.g. Kirkland et al., 1999). More recently, interpretations based on the complexity of macro-and microscopic textures (e.g., Ludvigson et al., this volume) suggest that many calcrete beds result from pedogenic overprinting of lacustrine deposits. Our observations suggest two additional processes that overprint and modify Cedar Mountain Formation calcrete beds: 1) deposition of calcite and chalcedony by springs, and 2) erosion and dissolution processes due to subaerial exposure of calcareous soil horizons.

We identify four similar and common macro-textures in Cedar Mountain Formation calcretes and in recent travertine deposits of Crystal Geyser, in east central Utah. These macro-textures are: a. radial, fibrous calcite crystal growth surrounding sub-spherical micrite nodules and blanketing the tops of coalesced nodular horizons, b. boxy meshwork textures, c. "bubbly" calcite surfaces, d. botryoidal chalcedony filling voids.

The similarities between these deposits suggests that spring processes were influential in calcrete modification within the Cedar Mountain Formation. Fluid inclusion study and detailed petrographic analyses will further contribute to an understanding of growth of these textures, and constrain the temperature range of spring waters.

Textures resulting from subaerial exposure of calcrete horizons include karstification and silty infilling of voids, brecciation of patchy regions within otherwise homogeneous micritic nodules and coalesced nodule beds, and calcrete-pebble conglomerate lenses scoured into calcrete beds. Each of these textures is expected to result from flow of meteoric water within and over near-surface or exposed calcrete horizons.