IMPLICATION OF FLUVIAL MICROBIOLITES IN THE POISON STRIP AND RUBY RANCH MEMBERS, CEDAR MOUNTAIN FORMATION, EAST-CENTRAL UTAH, USA

Understanding intrinsic controls on microbiolite development in fluvial settings is in its infancy. Two dissimilar microbiolite depositional settings were selected and examined in the Early Cretaceous Cedar Mountain Formation. The two separate settings demonstrate the impact of clastic sediment supply on internal microbiolite morphology development; greater clastic sediment input appears to inhibit the formation of well-developed internal laminations.

A ~30 cm thick and ~150 m wide microbiolite bed is at the top of the Poison Strip Member (PSM) of the Cedar Mountain Formation, a regionally extensive fluvial deposit. The bed is underlain and overlain by meter-thick hyperconcentrated-flow deposits. Microbiolites consist of hemispheres up to 70 cm wide. Internal laminations are not present, but microbial textures, clotted and spherical forms are developed near the upper surface. The unit is crosscut by desiccation features. Nucleation sites are dominated by quartz sands with lesser calcite. A vertical increase in carbonate and reduction in clastics occurs within the hemispheres.

Microbilites are also present near the top of a fining-upward sequence in elevated fluvial channel-fill deposits within the younger Ruby Ranch Member. The microbiolites are ~5-10 cm diameter, laterally connected, microbial hemispheroids. Nucleation site evolution is similar to the PSM microbiolites but well-developed laminations are present at the margins with branching forms. Capping the microbiolites are mudstones that contain palustrine carbonates.

With higher clastic content based the abundance of clastic grains, hemispheric forms are still developed, but without apparent internal laminations or microbial textures. Therefore, the comparison of these two sites suggests that stromatolitic internal lamination development is potentially controlled by clastic input to the growth location; a greater the amount of clastics reduces the probability of stromatolitic lamination development.