THE ARGENTA FORMATION: A NEWLY RECOGNIZED CAMBRIAN STRATIGRAPHIC UNIT IN THE ILLINOIS BASIN

A new sedimentary unit has been identified in the Illinois Basin as a result of coring activities during the Illinois Basin–Decatur Project (IBDP), a one million metric tonne carbon capture and storage demonstration project located in Macon County, Illinois. The name Argenta Formation is proposed for this unit, after the nearest town to the type locality that has not been previously used to name a stratigraphic unit. The Argenta Formation is bounded by unconformities—the Lower Mt. Simon Sandstone at its top and the Precambrian basement at its base—and was formerly considered to be part of the Lower Mt. Simon or a granite wash unit, but can now be identified as a distinct formation on the basis of core data from IBDP. This formation is mappable using geophysical logs and is correlated into northeastern Illinois. Other midcontinent pre-Mt. Simon sedimentary units have been described (such as the Middle Run Formation of Ohio and Kentucky) or inferred on the basis of seismic reflection data (Centralia Sequence). However, this is the first pre-Mt. Simon sedimentary unit to be identified from core and named in the Illinois Basin.

The Argenta Formation is interpreted as dominantly shallow-marine shoreface to fan-delta deposits composed of sandstone and conglomerate with rare interbedded mudstone. Conglomerates are dominantly clast supported but may be matrix supported, exhibit inverse and normal grading, and are crudely planar to cross-bedded. Interbedded sandstones are cross-bedded, ripple laminated, and planar laminated. Bioturbation is abundant in some sandstone intervals.

The age of the Argenta Formation is unclear. A Middle Cambrian age for the Upper Mt. Simon is suggested by the presence of Guzhangian index fossils in the overlying Eau Claire/Bonneterre Formation of Iowa and Missouri (the Eau Claire conformably overlies the Upper Mt. Simon at the IBDP site). The age of the Middle and Lower Mt. Simon is ambiguous due to the presence of unconformities. The presence of extensive vertical burrows excludes a Precambrian age for the Argenta Formation, so the formation may be either Lower or Middle Cambrian. Regardless, the marine indicators in the core suggest the Argenta is part of a marine transgression that may have been caused by thermal subsidence during the breakup of the supercontinent Rodinia.