Sediment Formation in the Late Ordovician

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 3 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.

Cincinnatian Series Limestone

Cincinnatian Series limestone has very distinct characteristics. The most abundant limestone in the Cincinnatian Series is a coarse grained rock with poorly sorted grains consisting of 30% to 40% fossil allochems surrounded by 60% to 70% matrix developed from what was originally carbonate ooze.

The average Cincinnatian limestone is a borderline rock between a partially recrystallized skeletal wackestone (WACK-y stone) and a packstone, but more commonly a wackestone. Wackestone is limestone that consists mostly of hardened ooze with grains supported throughout the stone.

In constrast, packstone has more grains that actually touch each other, forming a self-supporting framework containing hardened ooze only in the pores. The diagrams below show the difference between wackestone and packstone.

diagrammatic sketches showing wackestone (matrix supported) and packstone (grain supported)