| Pecatonica Formation | |
| Type: | Formation |
| Age: | Ordovician |
| Period: | Ordovician |
| Prilithology: | Limestone |
| Region: | Illinois, Indiana, Iowa and Wisconsin |
| Country: | United States |
| Unitof: | Black River Group |
| Underlies: | Plattin Formation |
| Overlies: | Joachim Dolomite |
The Pecatonica Formation is a geologic formation in Illinois. It preserves fossils dating back to the Ordovician period.