Cambrian Period

The Cambrian Period began about 542 million years ago and ended about 488 million years ago. Cambrian rocks underlie the state, but like Precambrian rocks, they are nowhere exposed at the surface and have been characterized by deep drilling for hydrocarbons and by geophysical methods.

At the beginning of the Cambrian, the core of the North American continent, including Ohio, was emergent and consisted of a gently rolling surface covered by highly weathered granite and other Precambrian crystalline rocks. The Iapetus ocean had opened to the east and a series of down-dropped basins formed along the continental margin. One of these, the Rome Trough, formed on the southern edge of Ohio, stretching through Kentucky, West Virginia, and Pennsylvania. This basin accumulated a thick sequence of Cambrian rocks.

As sea level began to rise during the middle Cambrian, the transgressing sea reworked the weathered sediments forming a high -- purity quartzose sandstone known as the Mt. Simon. This is the basal Cambrian unit in Ohio and ranges up to 400 feet in thickness. The precise age of the Mt. Simon is uncertain but it is probably Lower or Middle Cambrian.

As sea level continued to rise, carbonate sediments began to accumulate in eastern Ohio along with silty and sandy sediments. By the end of Middle Cambrian time, a wedge of sandy deltaic sediments began to spread from north to south across the central portion of the state. By Late Cambrian time and extensive and thick carbonate (Knox Dolomite) accumulated across the state. This unit continued to accumulate into Early Ordovician time.

Cambrian rocks have been important economically because of the abundant oil and gas deposits in some units. In particular, the Knox Dolomite has been productive. The Morrow County field, discovered in 1959, has produced approximately 38 million barrels of oil and 35 billion cubic feet of natural gas.