TOWARD A DETERMINATION OF STEADY STATE VS. EPISODIC PROCESSES IN GEOMORPHOLOGY USING AMS

We are using cosmogenic 10Be and 26Al to study the movement of large rectangular joint-cleaved blocks of a quartz arenite sandstone of early Pennsylvanian age. The blocks have vertical and horizontal faces that are 15 X 30m in size and are located on the northeastern fringe of the Mississippi Embayment, east of the Ozark Uplift, and in the southern half of the Illinois Basin. Lying on the margin of Quaternary glacial advances, this area known locally as Panther's Den, is drained by Devil's Kitchen Lake. The causes of block movement remain uncertain; it may have occurred by creep, slumping, or by catastrophic motion associated with seismic activity. Dating movement of the blocks using cosmogenic 10Be and 26Al may help to distinguish whether block motion was gradual or episodic.

The block we chose to sample has a particularly simple geometry; a fixed vertical planar face that was exposed by movement of an adjacent block parallel to the sampled face. Three samples were taken along a horizontal line 8.3m from the top. These spots were exposed sequentially by movement of the transient block. Quartz from the sandstone was chemically prepared and then 10Be and Al26 were measured by accelerator mass spectrometry at PRIME Lab. Preliminary data supports episodic movement. Samples 2 and 3 are consistent with slow production by muons, yielding an estimated steady-state erosion rate of 6m per million years. Sample 1 shows additional exposure of approximately 50,000 years over 2 and 3, indicating that the blocks were partially separated for a long period of time, and then rapidly separated to their modern configuration. Further studies using more samples are needed to constrain the various episodes of movement.