Paper No. 17
Presentation Time: 1:00 PM


OLSEN, Joshua E.S., Geology and Environmental Geosciences, Northern Illinois University, Davis Hall 312, Normal Rd, DeKalb, IL 60115, CURRY, Brandon, Quaternary Geology Section, Illinois State Geological Survey, 615 E Peabody Dr, Champaign, IL 61820-6964 and MOSHIER, Stephen O., Geology & Environmental Science, Wheaton College, 501 College Ave, Wheaton, IL 60187,

The Perry Mastodon (Mammut americanum) was excavated in Glen Ellyn, Illinois, in October, 1963, immediately after it was discovered during construction of a residential pond. The property is now part of the Churchill Woods Forest Preserve. Little attention was paid to the geological setting during the eight-day excavation by the Wheaton College Geology Department. The skeleton was found in a hydrologically open topographic depression in the valley of the East Branch Du Page River. The site is located within the Valparaiso Morainic System. Our project determined the site’s environment of deposition and chronology through study and interpretation of cored sediments obtained through hand augering and the Illinois State Geological Survey’s direct-push rig. The upward vertical succession is: (a) muddy sand of the Haeger Member of the Lemont Formation, (b) undifferentiated sand and gravel, (c) silt loam diamicton of the Wadsworth Formation, (d) fossiliferous laminated silt of the Equality Formation, (e) fossiliferous marl and peat of the Grayslake Peat and, (f) sterile fluvial sand of the Henry Formation. The surficial sediments are covered by thin deposits of fill.

Correlation to nearby sites indicate the basal fossiliferous silts date from about 16,500 to 14,500 cal yr BP, and the marl, from about 14,500 to 11,000 cal yr BP. The mastodon skeleton was encased in silt loam marl containing abundant cold, freshwater gastropods Helisoma sp. and Physa sp. Bone collagen from the right femur of the Perry Mastodon (extracted with alkali) yielded a radiocarbon age of 11,700 ± 60 yr BP (Beta-270214), which calibrates to 13,640-13,450 cal yr BP (σ1 range, IntCal 09, Calib 6.0). The lake may have formed as a late-stage kettle due to collapse of sand and gravel outwash into a void left by a melting ice block in the valley of the East Branch Du Page River as the Joliet sublobe retreated from the Valparaiso Moranic System. A sand layer in the upper section of the marl and peat unit may have been deposited by a major post-glacial flood event.

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