Fossiliferous oolitic ironstone in the Silurian of Ohio, USA. (Orton Geology Museum, Ohio State University, Columbus, Ohio, USA)

This is a scarce sedimentary lithology - fossiliferous oolitic ironstone. It consists of oolites and fossils (including a nice trilobite tail) composed of reddish brown iron oxide.

This unusual rock comes from the Early Silurian-aged Brassfield Formation of Ohio. The Brassfield is a heterolithic unit dominated by limestones, dolostones, and shales. Other lithologies include cherty limestone, fossiliferous chert, and glauconitic chert.

In the subsurface of eastern Ohio, the Brassfield's lateral equivalent is called the Clinton Sandstone, or Clinton Formation. The Clinton is an interbedded succession of quartzose sandstones and shales. The sandstones are frequently charged with economic volumes of petroleum (crude oil & natural gas). The Clinton is one of the most economically significant petroleum systems in all of Ohio (others being the Ordovician-aged Trenton Limestone of northwestern Ohio and the Cambro-Ordovician aged Knox-Rose Run interval).

Oolitic ironstone also occurs in the Lower Silurian of New York State and Alabama.

Stratigraphy: Brassfield Formation, Llandoverian Series, Lower Silurian