CHARACTERIZATION AND INTERPRETATION OF A LACUSTRINE MICROBIALITE UNIT IN THE EARLY CRETACEOUS YELLOW CAT MEMBER, CEDAR MOUNTAIN FORMATION, EAST-CENTRAL UTAH

Within the upper Yellow Cat Member of the Cedar Mountain Formation, 20 km southeast of Green River, Utah, is a 0.5 km wide microbialitic limestone bed. This study of the limestone unit refines the paleoenvironment of the upper Yellow Cat Member, which contains an important dinosaur fauna.

The limestone bed overlies fluvial deposits and consists of a basal micrite, 0-50 cm thick, separated from an upper microbialite unit by up to 20 cm of marl. The microbialite bed is 40-140 cm thick and consists of a massive lower subunit and well-laminated upper subunit. The lower subunit has a patchy appearance with no visible laminae, and in places a 0.5 m diameter domal top. The upper subunit contains continuous mostly subhorizontal laminae, but commonly has 0.5 m diameter domal structures. The transition between subunits marks a change from thrombolites to stromatolites. Thin sections show an increase in the complexity of microstructures from basal silty limestone to the microbialitic subunits. All layers of the limestone bed contain ostracods and charophytes. The micrite contains abundant micritic intraclasts and quartz silt. The lower subunit consists of a cm-scale clotted fabric of fine micrite with peloids, oncoids, and calcite spar filled fenestrae. The upper subunit is characterized by alternating mm-scale intervals of thick and thin micritic laminae with mm- and cm-scale wrinkles. Thinner, darker micritic laminae grade into thicker, coarser grain laminae which have abundant peloids, roll-ups and contorted laminae. The laminae contain mm- to cm-scale fenestrae which are filled with early drusy calcite and dolomite followed by spherulitic chalcedony and late spar calcite.

The vertical transition represents a decrease of siliciclastic input and a change in the lacustrine environment that impacted the development of the microbial community. The increasing presence of stromatolitic structures, micritic and sparry textures, lateral continuity of their calcite structures (from clotting to stromatoid laminae) over time indicates development of a persistent microbial community. The change from thin, finer laminae to coarser laminae result from variations in environmental energy and are likely a response to longer climactic cycles on the order of years. The fenestrae developed from microbial decay and degassing.