Paleontology and geology
The widespread limestones of the earliest part of the Carboniferous record the greatest marine flooding of the North American continent. The rocks from the Early Carboniferous (Mississippian) are primarily shales and sandstones formed from sediments eroded from mountains to the west. These rocks are the most fossiliferous in the state and include nearly all groups of marine invertebrates, including foraminiferans, corals, brachiopods, conodonts, bryozoans, snails, clams, and cephalopods. Fish fossils have also been found, although they are rare.
During the Late Carboniferous, a thick sequence of alternating evaporites and shales accumulated in a large basin in southeastern Utah. The evaporite layers were deposited as very shallow marine water evaporated and left behind salts. In the northwestern part of the state, marine sandstones and limestones reach a thickness of over three miles. Fossils are more abundant in the rocks of the northwest basin and consist of fusilinids, brachiopods, corals, bryozoans, and other organisms, as well as trails and burrows made by marine animals.
Although this map only indicates the presence of undifferentiated rocks, there are scattered outcrops of Carboniferous rocks in the western and southwestern parts of the state, as shown on the map provided by the Utah Geological Survey.