Glacial Sediment Dating

Period: January 30, 2005 – April 15, 2010
Contact: Charles Rovey, Michael Chalfant, Fred Young
Organization: Missouri State University
Funding Source: Prairie Fork Trust

Objectives: The primary objective of this research is to measure the age of glacial sediments at Prairie Fork Conservation Area (PFCA) using the cosmogenic-isotope technique. These ages or dates, combined with similar results from nearby areas, will provide a complete chronology of Missouri’s ice ages, which included multiple ice or glacial advances to the same general southern terminus approximately 15 km south of the PFCA. In addition, opal phytolith analyses from upper horizons of each glacial material may reveal the general vegetation during the times between the glacial advances. These results will then be incorporated into the Deep Core display and poster, currently under development.

In the recent geologic past northen Missouri was glaciated five times (Figure 1). Each ice advance was stopped by the topographic reversal (uphill grade) along the northern margin of the Ozark Dome, which is just a few kilometers south of PFCA. Each ice advance deposited a distinct type of glacial sediment called (glacial) Atill.@ In the (interglacial) time between ice advances, soils formed atop the most recently deposited till as the climate warmed and vegetation colonized the landscape. These soils were then buried by younger tills during the next glaciation. Thus, the glacial sediments at PFCA consist of a series of tills capped by buried soils or paleosols. These paleosols afford a means of dating the overlying tills.

The glaciations that reached northern Missouri are grouped within a highly generalized time interval known informally as the Apre-Illinoian.@ The five tills present in northern Missouri record the southernmost ice advance during the pre-Illinoian, and thus represent major glacial and climatic events. Unfortunately, we do not know exactly when most of these events occurred.

Based on a regional analysis of such deposits, the pre-Illinoian is older than ~3 x 105 years, and may extend back in time as far as ~2.6 x 106 years. The precise ages of most pre-Illinoian glaciations, however, remain unknown, because these deposits are too old for radiocarbon or luminescence dating techniques. Until recently the only constraint on the age of the tills in northern Missouri was their magnetic polarity. The youngest three tills (within the McCredie Formation, Figure 1) have a normal remanent magnitization. Thus, they are younger than the Brunhes/Matuyama magnetic reversal at 0.78 Ma. The older two tills (Moberly and Atlanta Formations) have a reversed remanent magnetization and are thus older than 0.78 Ma.

When cosmic radiation strikes grains of the mineral quartz at or near the ground surface it produces 26Al and 10Be at a fixed ratio. If the soil material is exposed long enough to develop a mature profile (at least 104 years, but ideally closer to 105 years), these isotopes reach a constant ratio within the quartz grains, which reflects a balance between production, loss by decay, and loss by erosion. If the soil is then buried to a sufficient depth (at least 5m) and shielded from cosmic radiation, the two nuclides remain trapped within the quartz mineral structure, but decay at different rates. Therefore, the ratio of these two nuclides within quartz in a paleosol gives the length of time since burial, which is also the age of the overlying material, in this case the age of the overlying or younger till.

The cosmogenic-isotope method was originally developed for dating sediment buried in caves, but has recently been extended for dating buried soils between 2 x 105 and 3 x 106 years in age. Thus, the technique is ideal for dating pre-Illinoian sediments, and is in fact, the only method presently capable of doing so. We propose to apply this method to paleosols developed in tills at PFCA. This site is critical. As discussed above, the technique must be applied to maturely developed paleosols which meet specific criteria. Unfortunately, there are no current surface exposures where all three tills of the McCredie Formation are preserved with intact soil profiles between the tills. However, in 2003 the Missouri Department of Natural Resources (MDNR), in cooperation with the USDA Natural Resources Conservation Service (NRCS) and Missouri Depatment of Transportation (MoDOT) completed a deep borehole on the northern part of PFCA. Fortuitously, the glacial tills in the subsurface at PFCA have exactly the right characteristics for applying the cosmogenic-isotope method. Below ground surface, all three tills of the McCredie Formation are preserved in direct superposition, the upper two tills are thicker than 5 meters, and each preserves a thick, well developed soil profile with an argillic horizon > 1m in thickness. An argillic horizon of this thickness indicates a long duration of landscape stability and soil formation, generally approaching or exceeding a minimum of 104 years. As discussed above, a duration of this magnitude is ideal for applying the cosmogenic-isotope method. Thus, PFCA is the only known location with the necessary conditions for successfully dating tills of the McCredie Formation.

The dynamic climate changes during the Quaternary (Ice Age) Period in northern Missouri were undoubtedly accompanied by a succession of vegetative associations. The Apaleo-vegetation@ can often be inferred by examining the opal phytoliths left in paleosols from the decayed vegetation that was contemporaneous with the paleosurface. Biogenic opal closely conforms to the biological cell or structure in which it originated. For example, opal of forest origin often consists of cellular incrustations with numerous thin sheet structures, while opal of grass origin consists mostly of solid polyhedral structures. Opal phytoliths persist in the silt fraction of soils. Although most prevalent in the AA@ horizons (topsoil), they also occur in upper subsoil layers, which are present in the four buried paleosurfaces in the PFCA deep core.