Agglutinated siliceous foraminifera occur in the Middle Ordovician (Blackriveran) Mifflin Formation of the Platteville Group in northern Illinois. The fauna consists of globular saccamminids and a new form Reophax blackriveranus n. sp. which records the oldest validated representative of this genus. This marks the earliest known occurrence of agglutinated foraminifera with multichambered uniserial tests of progressively expanding chambers from the proloculus to the aperture. This early innovation of test morphology was probably developed to control unfavorable changes in the water chemistry of the environment.
Mifflin lithofacies consist of light gray, thin, wavy-bedded, lithographic limestone and fine-grained dolomite with green shale interbeds, thin calcarenite layers with graded bedding, a K-bentonite ash layer and hardground corrosion bedding surfaces. Mifflin biofacies include the foraminiferan fauna, brachiopods and molluscan shelly faunas, bryozoans, trilobites, ostracodes, echinoderms, solitary corals, conodonts, chitinozoans, scolecodonts, sponges and trace fossils particularly Chondrites.
The Mifflin strata were deposited on an exceedingly gentle slope off the Pecatonica carbonate platform which flanked the Wisconsin Arch. Thin Mifflin clinothem limestone beds wedge out into shales in the moderately deep (<200 m) aerobic starved basin in eastern Iowa. This occurred in south tropical seas during a eustatic rise in sealevel and major marine transgression. Reophax is associated with saccamminids in the benthos of the marine upper foreslope. Apparently slope-dwelling Reophax foraminifera of the Ordovician were displaced downslope into the basin by the rapid development and expansion of hyperamminids which occupy the lower and middle foreslope in the Mississippian.