ILLINOIAN AND PRE-ILLINOIAN PALEOECOLOGY AND PALEOENVIRONMENTS IN CENTRAL AND SOUTHERN ILLINOIS

Several sites with fossiliferous Illinoian or pre-Illinoian lacustrine/loess deposits were studied with an emphasis on sedimentology, gastropods, and plant macrofossils. Illinoian (oxygen isotope stage 6) lacustrine sediments, immediately below glacial till, commonly contain sub-horizontal Picea logs or wood fragments, with occasionally well preserved needles of Picea mariana (black spruce). Illinoian gastropods in southwestern Illinois include species typical of northern forests (e.g., Vertigo eliator), spring-like areas along wooded slopes (Carychium sp.), seasonally wet floodplains or shorelines (Pomatiopsis scalaris), shallow lakes (Fossaria sp.), or perennial lakes (Valvata tricarinata). Species representative of colder (Columella alticola, Vertigo alsperis oughtoni, Valvata sincera) and/or drier (Pupilla muscorum) conditions are more abundant in central Illinois (Leonard et al., 1971), implying a climatic gradient.

Pre-Illinoian fossils, more rarely observed, are found in infilled paleovalleys. Gastropod assemblages, though not unique to the pre-Illinoian, are locally useful for correlation and for interpreting ecological shifts. In east-central Illinois, Picea sp. wood and needles found along with terrestrial gastropods (e.g., Hendersonia occulta, Discus macclintocki, Vertigo hubrichti) in loess imply a cold boreal forest, or moist, north-facing slopes in mixed-boreal regions. Mixed aquatic and terrestrial gastropods, observed in faintly stratified silty deposits, imply changing water levels in a slackwater lake/pond environment and/or periodic washing of terrestrial species and loessal sediment into shallow temporary water bodies.

Preliminary comparison of fossil assemblages suggests a relatively similar cold boreal condition or parkland ecotone in central and southern Illinois during both Illinoian and pre-Illinoian full glacial conditions. However, the records represent only relatively short windows of time near maximum extents of the Laurentide Ice Sheet. The study of stratigraphic and geographic variations in Quaternary fossil assemblages can improve our understanding of local ecological changes, regional paleoclimatic interpretations, depositional environments, and glacial sediment correlations.