ROCK MAGNETIC PROPERTIES OF AN ALLUVIAL SOIL AT BUTTERMILK CREEK, TEXAS, USA AND THEIR BEARING ON THE PROPOSED 12.9 KA COMETARY IMPACT

The evolution of magnetization within a floodplain soil begins with initial deposition of magnetic particles during sedimentation and continues via subsequent alteration and growth of iron-bearing compounds by pedogenic and biologic processes. Measurements of soil magnetic properties capture information about the developmental history of the soil, and are a convenient method by which to investigate sedimentation rate, environmental change, and pedogenesis. A comprehensive scenario for soil development was constructed for floodplain sediments at the Friedkin archeological site near Buttermilk Creek, Texas using a comprehensive suite of rock magnetic measurements. Floodplain deposits have traditionally been avoided for soil magnetism studies because it is thought that the episodic input of sediment would form soils characterized by discrete sedimentary units rather than a continuous record of pedogenesis. We demonstrate that alluvial deposits can sometimes carry a straightforwardly interpretable magnetic signal similar to those typically seen in loess deposits. Smooth variation of rock magnetic parameters as a function of depth also leads us to conclude that the soil at this site is largely undisturbed, and that the age of lithic artifacts found within the soil may be interpreted within stratigraphic context. Age constraints from optically stimulated luminescence provide an absolute temporal framework for the artifacts, sediment deposition and pedogenesis. This chronological framework also allows us to evaluate the site for cultural, geologic, and geophysical evidence of the hypothesized 12.9 ka impact layer across North America.