Previously unreported silicified trilobite faunas occur in a narrow stratigraphic interval of the Upper Cambrian (Sunwaptan) St. Charles Formation in the Bear River Range of southeastern Idaho. The faunas occur in four closely spaced rudstones and trilobite packstones indicating deposition in a shallow subtidal setting above storm wave base. At least 23 species are represented, included two undescribed genera and several undescribed species. The faunas are notable for their high trilobite abundance and pervasive silicification. Most coeval faunas have been described on the basis of small numbers of "crack-out" specimens, and the new material reveals many details of anatomy, including knowledge of most exoskeletal sclerites. The four trilobite-yielding beds contain markedly different taxon-abundance profiles, yet most species are shared between them. This suggests multiple, taphonomically-controlled samples of a similar underlying distribution, though true ecological variation cannot be discounted.

This report presents the results of a stratigraphic study of the Worm Creek quartzite, the basal member of the St. Charles formation of Upper Cambrian age. The member is present in the Bear River Range, the Malad Range, and the Promontory Range of Northern Utah and southeastern Idaho. The Worm Creek quartzite was differential as the basal unit of the St. Charles formation by Richardson (1913) and named for its occurrence in Worm Creek Canyon in the Bear River Range, Bear Lake County, Idaho.

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.