LEIOLITES FROM THE EARLY CRETACEOUS POISON STRIP MEMBER OF THE CEDAR MOUNTAIN FORMATION: FROM MACROFORMS TO MICROBIOLOGY

Leiolites, a rare type of microbial carbonate, are under recognized in both marine and continental sedimentary successions. Leiolites form in environments with steady clastic sediment input and the presence of a robust microbial community- resulting in a structureless, dome-shaped micrite that differs from both stromatolites and thrombolites. The Early Cretaceous Cedar Mountain Formation is comprised of fluvial and lacustrine deposits, with the Poison Strip Member specifically recording medium to low sinuosity bedload dominated fluvial systems. At the Yellow Cat Road locality a ~1 m thick leiolite bed is present in the Poison Strip Member at the contact with the overlying Ruby Ranch Member. Documentation of the macroscopic and microscopic features, micropaleontology, and diagenesis of the leiolites provides improved understanding of processes during development, adds to the catalogue of leiolite settings, and aids in their recognition. The Poison Strip Member leiolites consist of up to 0.5 m diameter domes with superimposed cm scale irregular bumps. The leiolite bed thickness varies from 25 to 50 cm. Apparent internal structures such as clots or laminations are absent. A high percentage of floating quartz grains encased in micrite was confirmed in thin sections and slabs by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses. Cross-cutting and parallel to the mound surfaces are fractures up to 3 mm wide that contain minor intraclasts and fine quartz fill. Cracks are subsequently filled with sparry cement or chert. Bioturbation cross cuts the upper surface. SEM analysis of leiolite surfaces identified twisted and simple fibers (probable cyanobacteria), single and clusters of ~5 um rough surfaced spheres (fungal spores), micron-scale capsules as individual and strands (bacillus-type bacteria), irregular circular-shape masses (spherical bacterial), massive sheet-like features with irregular surfaces (plant cuticles), and masses with smooth surfaces (EPS; extra-polymeric substance). Poison Strip leiolites developed in a setting with high siliciclastic input and a diverse biological community that was subaerially exposed during its development.