The upper member of the Nounan Formation and the Worm Creek Member of the St. Charles Formation, both late Cambrian in age, were studied in the Bear River Range and the Fish Creek Range in southeast Idaho. Lithology and sedimentary structures of these units were compared with characteristics of similar modern sediments and ancient rocks, to determine the environments of deposition and effects of diagenesis for the interval studied. On the basis of widely traced marker horizons, the two-member interval is divided into three parts, with parts 1 and 2 comprising the upper member of the Nounan Formation, and part 3 equal to the Worm Creek Member. A marker of mixed-fossil lime packstone at the base of part 1 is overlain by mixed-fossil and lithiclast lime grainstones and cryptalgal boundstones. Sedimentary structures within these units suggest that part 1 was deposited in shallow subtidal and intertidal environments. An oolitic lime and/or dolomite grainstone marks the base of part 2, and suggests shallow subtidal conditions. Part 2 is comprised of interbedded limestones and dolostones, with dolostone becoming predominant up section. The mixed-fossil, oolitic, and lithiclast packstones and grainstones, and cryptalgal boundstones of this part include sedimentary structures which indicate shallow, subtidal accumulation. The percentage of non-carbonate sand increases near the top of part 2, and sedimentary features suggest that water depth decreased slightly as terrigenous influx increased. The base of part 3 (Worm Creek Member) is marked by sandstones and quartzites, cemented with carbonate minerals and/or quartz over-growths. Carbonate deposition resumed above these terrigenous units in the southern and central parts of the study area, while terrigenous sediments continued to accumulate in the north and northwest. This suggests that the source of terrigenous sand was north or northwest of the study area. Radial-fibrous cement rims on carbonate grains indicate early subsea cementation in limestones. Dolomite in "birdseye" structures and in reworked lithiclasts, both in limestones, suggests that a minor amount of syngenetic dolomite formed, although there are no beds of primary dolomite. Dolostone units do not have the sedimentary structures typical of supratidal environments of syngenetic dolomitization, and have the coarsely crystalline texture and other characteristics of secondary dolomite. Dolomitization in a zone of mixing of fresh water and sea water is a probable explanation of all dolostones in the sequence . Dolomite-embayed quartz and feldspar grains and overgrowths in some quartzites of part 3 suggest that dolomitization continued after lithification of some rock units.

The Nounan Formation in north-central Utah thickens northward from 696 feet near Causey Dam to 1147 feet at High Creek in the Bear River Range, and northwestward to 1149 feet at Dry Canyon in Wellsville Mountain. The basal contact of the Nounan Formation is sharp, but dolomite extends irregularly downward into limestones of the Bloomington Formation as much as 6 feet. The Nounan Formation is divided into three members based on lithologic characters: (1) a lower member composed of dark, medium-crystalline dolomite; (2) a middle member composed of white, coarse-crystalline dolomite with tongues of dark dolomite; and (3) an upper member of interbedded light and dark dolomites and limestones with local arenites and sandy carbonates. The lower member was deposited in a high-energy, shallow-marine subtidal to intertidal environment. Evidence includes sets of low-angle cross stratification (dunes), oncolites, oolites, and rip-up clasts. The middle member forms distinctive ledges and cliffs. The presence of thinly laminated algal stromatolites and relict structures seen also in the lower member indicate a subtidal to intertidal environment similar to that inferred for the lower member. The white color and coarse crystallinity may have resulted from recrystallization of the dark, finer grained dolomite that comprises the lower member. The upper member is characterized by lithologic variability. Thicknesses of limestone are greatest in the north, and decrease to only a few feet in the south. Quartz and other terrigenous minerals are scattered at intervals throughout the upper member, with a marker of sandy (arenaceous) dolomites at the base and near the middle and an increase of sand near the top also. The upper contact, with quartz-rich arenites (subarkosic quartzites) of the Worm Creek Member of tho St. Charles Formation, is gradational overall, but is sharp and planar in each section and readily located. In the upper member, algal mats trapped a varying but overall increasing influx of quartz and feldspar, probably in shallow subtidal environments, and vertically stacked hemispheroids suggest that depositional conditions may have included intertidal. Virtually all of the dolomite in the Nounan Formation must have formed by replacement of lime sediments by downward-moving high-magnesium brines. It is that these brines originated in restricted, shallow, subtidal evaporating basins, such as the Great Bahama Banks today, and associated supratidal flats. Lateral changes from limestone to dolomite overall and also in individual beds of the upper member indicate that the brines travelled laterally as well as vertically, and dolomitization may have been limited as much by prior diagenetic alteration and cementation as by the volume, concentration, and proximity of the brine itself.