Ordovician Fossils in Ogle County, Illinois

Ordovician Fossils in Ogle County, Illinois
Illinois State Geological Survey bedrock maps help identify fossils and their associated geological units.
Special Thanks to Dr. Dennis Kolata
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Fossilized snails- Maclurites sp.

Middle Ordovician fossilized snails, ~450 MYA. Platteville Group, Hook Larson Prairie at Nachusa Grasslands

Trace fossil invertebrate burrows – Middle Ordovician, Platteville Group ~450 MYA.  Mud Creek, Ogle County, Illinois
Platteville Group / Grand Detour Formation Dolomite, limestone, and shale. Dolomite is orange-brown when weathered, light brown when fresh; limestone is gray when fresh and can have a distinctive dark blue-gray color. The Grand Detour varies vertically from fine- to medium-grained, from thick to thin bedded, and from pure to argillaceous. Packages of thin beds can appear massive. Shale is greenish gray, red-brown, or dark gray; it occurs generally in the upper part as wispy partings, but partings are not strong enough to cause the rock to cleave on those surfaces. The fauna is reported to be abundant and diverse, including the solitary rugose coral Streptelasma, the rhombiferan cystoids Coronocystis durandensis, the lithistid sponges Anthaspidella and Zittelella, and the trace fossils Palaeophycus and Buthotrephis (Chondrites). Hundreds of horn corals can be concentrated on the shaly horizons. 

Solution cavities at the top of the unit were observed in the Steve Benesh and Sons Quarry (NW Sec. 23, T24N, R10E); cavities were filled with greenish gray shale and large, jumbled blocks of dolomite. The shale content makes this a useful horizon for electric log correlation. The basal contact is conformable with the Mifflin and can contain a thin K-bentonite bed [2].

Ordovician Diorama

Ordovician Seafloor Diorama [1]
The bedrock formations that underlie the Oregon (Illinois) Quadrangle are Cambrian and Ordovician magnesian carbonates and siliciclastics. The sediments were all deposited in near-shore or marine environments. At this time, Illinois lay in the tropics south of the equator; a warm, shallow sea alternately flooded and receded for millions of years [2].

Receptaculites / late Ordovician Galena Group, Dunleith formation  / Pine Creek at Mooseheart Camp, Ogle County, Illinois
The dolomites and limestones of the Galena Group produce oil throughout the Illinois Basin. However, oil and gas prospects are unlikely (in the Oregon Quadrangle) because the Galena (equivalent to Trenton or Kimmswick) dolomites outcrop at the surface, and any oil has long since escaped. A lone attempt at drilling for oil near Chana (the Canfield oil well) produced a dry hole to 600 feet when the well casing broke and the prospecting was unceremoniously abandoned [6]. Probable source rocks above the Galena, the Ordovician Maquoketa and Devonian New Albany Shales, also are eroded at the surface [2].

Dunleith Formation Dolomite: Light gray on fresh surface, fine- to coarse-grained, crystalline; dark olive green or pale orange-tan on weathered face. Massive beds comprise thin, wavy, indistinct bedding, and some beds are separated by thin, carbonaceous greenish shale partings. Moldic porosity can occur on fresh faces; a pitted surface, or beehive weathering, commonly occurs on weathered faces. Light gray to white chert occurs in flat slabs or lenses; the chert can be irregularly scattered throughout or form well-defined layers. A few Receptaculites zones occur in this formation but not in the Platteville Group; the occurrence of the calcareous algae

Receptaculites is useful to distinguish Galena dolomites from the Platteville Group. When identifying the formations of the Galena Group in the field, the crystalline texture is characteristic and is the key to telling it apart from the formations of the Platteville Group. Several K-bentonite beds that are one-inch-thick or less have been identified from the Dunleith. Stylolites and hardground omission surfaces occur locally at the basal contact, but the two formations otherwise appear to be conformable [2].

Rugose (solitary) Coral
 Rugose (solitary) coral / Platteville Group / NIU Taft Campus east of Rock River, at Oregon, Illinois.
Rugose corals completely disappeared in the still-mysterious Permian extinction, aka "The Great Dying" ~250 mya [5].
Trace fossil invertebrate burrows
Trace fossil invertebrate burrows / Platteville Group / Stronghold at Mud Creek, Ogle County, Illinois

Ordovician seafloor Brachiopod fragments / Pine Creek at Mooseheart Camp
Brachiopods (brack'-i-oh-pods) are marine animals with two shells, an upper and a lower. The right and left halves of each shell are mirror images, but the two shells are not exactly alike. The shells may be of lime, phosphate, or a horny substance, and the shells range in size from less than a fourth of an inch to several inches.

Most brachiopods live attached to the sea floor by a fleshy stalk that is an extension of the soft body. Some forms lose the stalk when they become adults and either attach themselves directly to the sea floor or lie loose in the mud or sand. Some have spines that serve as anchors.

Brachiopods are not common in most oceans today, but at times in the past they were the most abundant shellfish and sometimes formed large shell banks, much as oysters do today.

The oldest fossil brachiopods are found in Cambrian rocks, which are over 500 million years old. The animals first became abundant in Ordovician time and remained so throughout the Paleozoic Era. In Illinois, the fossils are especially common and well preserved in the limestones and shales of Mississippian age in the Ohio and Mississippi River bluffs, but you can find them easily in almost any part of the state [3].

Further Reading

The Paleontology Portal has detailed bedrock maps
Baltic amber with beetle inclusion
The genus Magnolia is an ancient plant lineage, first appearing in the fossil record about 20 million years ago, while evidence of plants in the family goes back to 90 mya. Having evolved before pollinators in Hymenoptera (bees, wasps & ants) appeared, the progenitors of our modern ornamental magnolias relied on beetles for their sexual gratification. Their large, showy flowers are a direct result of the plant's strengthening its delicate flower parts against the beetle's comparatively "rough handling" while feeding on pollen.

Left: Primitive beetle ancestor inside 50 million year old Baltic amber [4].
My friend Anders in Denmark has a wonderful collection of creatures fossilized in amber, with a nice selection of amazing close-up pictures: www.amber-inclusions.dk


  1. National Museum of Natural History, Washington D.C. "Ordovician Seafloor" Photo by Fritz Geller-Grimm
  2. Special thanks to Mary J. Seid, Illinois State Geological Survey for her help with identification and her
    work on the excellent bedrock maps of the Oregon and Mt. Morris quadrangles in Illinois.
  3. Illinois State Geological Survey, Prairie Research Institute, “Brachiopods
  4. Anders L. Damgaard, File:Baltic amber Coleoptera Scraptiidae.JPG under Creative Commons 3.0 unported
  5. Alanna Mitchel, New York Times, "Life in the Sea finds Fate in Paroxysm of Extinction" April 30, 2012
  6. Ogle County Historical Society, "The Canfield oil well"
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