Oncolites in North Horn (Late Cretaceous-Paleocene) and Flagstaff (Paleocene-Eocene) Formations are homologous with recent freshwater and marine oncolites. Geographic, stratigraphic, petrologic, and paleontologic evidence confirms that they are of shallow-water, nearshore lacustrine facies. Locally, autochthonous oncolites are distinguishable from allochthonous forms. Populations with snail-shell cores suggest that algal coatings formed on empty shells, and that a frangible crust developed quickly. Diagenetic sparry calcite filled the snail shells and infused the porous algal laminite.

North Horn and Flagstaff oncolite beds occur preferentially along positive tectonic lineaments and serve to distinguish nearshore facies even in the absence of other indications. The association of nearshore facies, at several stratigraphic levels, with the several positive late-Laramide belts in central Utah indicates the following sequence of events. 1. Elevation of lineaments in Sanpete-Sevier Valleys, in the southern Wasatch Mountains, and along the Canyon Range (Sevier orogenic belt) formed the Pavant-Valley Mountains and the east Gunnison, Thistle, and southern Wasatch Plateau basins of the younger North Horn lakes (early Paleocene ?). 2. Relative stability followed, and upper North Horn fluviatile sediments spread across the east Gunnison and the margin of the southern Wasatch lateau basins (mid-Paleocene). 3. Renewed elevation of the Sanpete-Sevier and Redmond Hills axes formed the lower Flagstaff (Phase 1) lake basins (later mid-Paleocene). 4. Increasing relative elevation of the Sevier orogenic belt provided a new, higher barrier farther west, and formed a larger lake, in the later Paleocene. 5. Infilling and quiescence along tectonic lineaments reduced relief and nearly obliterated the lake in mid-Eocene. 6. Locally, Flagstaff Lake persisted and merged with Green River Lake.