A late pre-Phanerozoic platform, almost continuously blanketed by algal mats, is recorded in the distributional pattern, composition, and textures of the Noonday Dolomite, Death Valley region, California. The platform, apparently L-shaped, was at least 160 km long. It lay west and south of the Nopah upland, established much earlier, and north of a fault-bounded east-trending depression containing units of the older Pahrump Group.

A lower member of the Noonday consists of very finely crystalline, laminated, and relatively pure dolomite. The laminations, although commonly deposited horizontally, also outline mounds having a synoptic relief of a few metres to nearly 200 m. Algal or cryptalgal fabrics are common. Especially large mounds in the Nopah Range lie on the down-dropped south side of a contemporaneous fault, indicating tectonic control of size and location. The laminated mounds are characterized by large-scale vertical stacking, best explained as produced by differential growth of algal mats. The configuration of the laminations indicate that the mats grew to depths of at least 100 m but probably did not extend more than 150 m below the mound crests. The absence in the lower member of features attributable to moving water and the paucity of clearly detrital material indicate quiet water and very low relief in the bordering land.

An upper member consists of (1) discontinuous bodies of thinly and evenly bedded siltstone and silty dolomite that ordinarily occupy the deeper of the intermound lows; (2) a laterally continuous unit of silty dolomite with wavy laminations and small, rippled to subhemispherical, laterally linked stromatolites; and (3) an upper unit of massive to laminated silty dolomite, containing large domal and bosslike stromatolites and associated with bodies of strongly cross-bedded quartz-dolomite sandstone. These features evidence the entry onto the platform of detritus supplied by increasingly stronger uplift of the Nopah Upland and the sweeping of the platform by increasingly stronger currents.

We interpret the carbonate of the lower member and much of the carbonate of the upper member as having been precipitated from ambient waters by removal of CO2 during algal photosynthesis.

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