Abstract Cambrian and Ordovician strata in Millard and Juab counties, western central Utah, are a thick (17,500 ft [5334 m]) succession that was deposited on a tropical miogeoclinal platform that experienced rapid thermal subsidence after a Neoproterozoic sea-floor spreading ridge formed along the western margin of Laurentia. In this area, which includes the Cricket Mountains, Drum Mountains, Fish Springs Range, House Range, Confusion Range, and Wah Wah Mountains, the Cambrian to Middle Ordovician Sauk megasequence is approximately 15,875 ft (∼4839 m) thick, and the Upper Ordovician part of the Tippecanoe megasequence is approximately 1525 ft thick (∼465m). Basal deposits of the Sauk megasequence are the transgressive Lower Cambrian Prospect Mountain Quartzite, and the top of the Sauk megasequence is the upper Whiterockian Watson Ranch Quartzite. Strata between these sandstones are mostly limestone with several shaly intervals. The Sauk megasequence is divided into four parts, Sauk I to IV, in this area, and these parts have been divided into smaller sequences. The Ordovician part of the Tippecanoe megasequence is mostly dolomite and quartzite. Major influences on the depositional history of these strata include rapid generation of accommodation space caused by thermal subsidence following continental rifting, in-situ generation of tropical carbonates that generally kept pace with accommodation, eustatic fluctuations, influx of siliciclastics during sea level lowstands, and vertical tectonic adjustments of regional tectonic elements inherited from Neoproterozoic rifting: the Wah Wah arch, House Range embayment, Tooele arch, and Ibex Basin. The resulting strata comprise one of the best known Middle Cambrian-Middle Ordovician stratigraphic successions in North America and include the reference sections of the Upper Cambrian Millardan Series and the Cambrian-Ordovician Ibexian Series. Stratigra-phers established a Global boundary Stratotype Section and Point (GSSP) for the base of the Middle Cambrian Drumian Stage in the Drum Mountains and proposed another GSSP for the base of the uppermost Cambrian stage in the Wah Wah Mountains. Middle Cambrian– Middle Ordovician strata are very fossiliferous, and some intervals have incredibly abundant fossils, such as the numerous complete specimens of the Middle Cambrian trilobite Collenia in the central House Range. Trilobites, conodonts, brachiopods, and other fossil groups have been used for biozonation and correlation, and these strata comprise a North American standard for uppermost Cambrian–Middle Ordovician trilobite and conodont zonations. Upper Ordovician dolomites and quartzites are less fossiliferous. These Cambrian and Ordovician strata are the lower half of a Lower Cambrian–Lower Triassic succession that is approximately 34,000 ft (10,300 m) thick and was thrust onto the Jurassic Navajo Sandstone in the southern Wah Wah Mountains during the Sevier orogeny. These strata are exposed in block-faulted mountain ranges resulting from basin and range extension during the late Tertiary.