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].

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 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].

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].

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].

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.

Trace fossil invertebrate burrows / Platteville Group / Stronghold at Mud Creek, Ogle County, Illinois