Melanie Lynn McQuinn, Purdue University

Ph.D. Thesis, Purdue University

The development of the late Wisconsinan chronostratigraphy of Indiana has been limited by the lack of detailed time control. The lithostratigraphy of each major region of the state has been adequately described (Wayne, 1963; Gooding, 1963: Johnson & Bleur, 1980), but definitive correlations between regions will not be possible until precise time control is established. Correlation of units between regions assumes synchroneity (Richmond & Fullerton, 1986), and therefore if is not possible to demonstrate that two distinct deposits are synchronous, then correlations are unfounded. One important use of late Wisconsin chronostratigraphies in glaciated regions such as Indiana, is in paleoclimatic reconstructions which facilitate understanding of the dynamics of climate change. Because such reconstructions require knowledge of both the spatial extent and the temporal frequency of glacier ice in any area, the paucity of time control in Indiana, and possibly elsewhere, inhibits truly meaningful reconstructions.

In this study, surface-exposure dating is tested as a possible approach for refining the chronology of Quaternary deposits in Indiana. This new method involves measuring the concentration of Quaternary deposits in Indiana. This new method involves measuring the concentration of nuclides produced by cosmic-rays in in-situ on glacial erratics in Indiana. If the production rate is known, one may find the time since exposure, which is assumed to represent the time since deglaciation, by measuring the concentration of a cosmogenic isotope in a well-exposed rock surface. This method has the advantage that it directly dates till units, in contrast to radiocarbon dating which brackets till units with ages of interill organic deposits. Furthermore, this method makes it possible to date units that are too old to be confined by finite rabiocarbon dates, and areas where bracketing organic matter is absent.

Surface exposure dating contributes ages consistent with independent estimates, though the level of uncertainty will need to be reduced to enable accurate correlation among glacial events. The results demonstrate the importance of etching to remove meteoric 10-Be. Samples from the same erratic yielded similar ages. Wabash terraces samples yielded the youngest exposure ages, whereas a southeastern Indiana sample yielded the oldest age. Ramaining samples are consistent with the existing chronology though a few present evidence of shielding.