Limestone is commonly formed from the slow accumulation and lithification of carbonate sediments. In the case of our local rocks, these carbonate sediments accumulated for the most part in and around benthic communities. These communities of organisms developed on the muddy bottom of the warm Ordovician sea.
The shallow ocean environments of the near shore and shore zones, where most of the limestone formed, are often referred to as "carbonate factories." The invertebrate organisms typical of these environments had shells or skeletons made of calcium carbonate, either in the form of the mineral calcite or its polymorph, aragonite.
As the organisms died, some shells and skeletons remained intact, and these remains were added to the coarse and fine particles left by other organisms. The finest-grained portion of these accumulations, generally referred to as carbonate ooze or mud, is thought to have originated from at least three different sources:
- Fragmentation and disintegration of shells and skeletons or organisms.
- Small crystals of carbonate minerals produced by calcareous algae, only a few microns in size.
- Inorganic precipitation directly from seawater.
After the sediment accumulated, and additional layers were deposited on top of it, the sediment was compressed and cemented to form solid rock, a process known as lithification.
Learn about lithification in Cincinnatian Series limestone.