Pennsylvanian1 rocks form the bedrock surfaqe beneath approximately 75 percent of Illinois. The study area (Fig. 1 next-frame) is generally that northwest of the Illinois River, northeast of the LaMoine Riter, but excludes Henryk Bureau, Stark, and LaSalle Counties/ Included, therefore are Rock island, Mercer, Warren, and Knox Counties (the lower units of vhich were studied by Merrill and King, 1971), plus the present coverjage of the .entire stratigraphie column in Knox, Peoria,.Fulton, and Schuyler Gounties. This study area includes the area covered by Wanless definitive works (1957,1958) on thte Beards town, Glas ford, Havana, and. Vermont quadrangles. Also included Is much of the area covered by the Avon-Canton quadrangle, report (Savage, 1922) and the Peoria quadrangle report (Udden, 1912). Knowledge about this area is therefore relatively detailed, The large surface area covered (roughly 2,650 square miles or 7,000 sq km) presents a great many possible sample sites for the several marine units.
The studied marine units that produced conodonts, in descending order (Fig. 2) are the Cramer (Trivoli of literature), Exline, Lonsdale, “Sparland”, Pokeberry, “Sheffield”, Brereton, St. David, Hanover, Oak Grove, Seahorne, “Seville” , and Seville Members. Reasons for the quotations will be explained id the discussions of the units.
Figures & Tables
“Pennsylvanian rocks exposed in Knox, Peoria, Fulton, and Schuyler Counties in northwestern Illinois include 11 major marine units. The oldest marine unit is late Atokan and the youngest is early Missourian; the remainders are Desmoinesian in age. Each unit has produced at least some conodonts, and the more than 200 samples produced an aggregate total in excess of 160,000 conodont specimens. These can be grouped into not fewer than 78 kinds, considered species in disjunct element taxonomy. At least 10 multielement genera and 40 multielement species are represented. Six new species: Diplognathodus illinoisensis, Neognathodus metanodosus, N. polynodosus, N. oligonodsus, N. anodus, and Gondolella pulchra are described. Although all marine beds were formed in shallow water, generally near shore, they represent the products of highly diverse environments. Conodont biofacies are sharply differentiated and mirror this diversity. Ecologic controls that effected conodont distribution are believed to have been salinity, energy, pH, and possibly biologic antagonism. Neognathodus is the most useful conodont genus for biostratigraphy in these rocks. Four zones and subzones are based on species of this genus and it has permitted relatively precise interregional correlations. Secondary zonations can be based on other genera that supplement the Neognathodus zonation and assist in identifying units. In decreasing importance these are Gondolella, the Idiognathodus-Streptognathodus plexus, and Diplognathodus.”