Using the Thickness of Pedogenic Carbonate Coatings as a Proxy for Ages of Alluvial Fan Abandonment in the Lost River Range of Eastern Idaho

Determining the age of abandonment of alluvial fan surfaces is crucial in relating fan development to past climate change or using fans as strain markers for neotectonic deformation. Cosmogenic surface exposure dating is not readily applicable to fan surfaces of the Lost River Range where large quartz-rich boulders are scarce, and loess has been deposited and eroded. Prior studies have shown, however, that the thickness of dense, laminated pedogenic carbonate pebble coats is useful for relative age and correlation of alluvial fans and moraines.

Tributaries Northwest of Mackay, Idaho deliver limestone rich sediment from the Lost River Mountains to the Big Lost River before joining the Snake River south of Arco, Idaho. Numerical 230Th/U ages of pedogenic carbonate samples from trenches on abandoned and incised sheetflood dominated alluvial fans will date the time of fan-surface abandonment. Calcium carbonate coat thicknesses are most developed in the upper-most B-horizon but depositional periods mixed with longer periods of temporary abandonment may complicate the depth of maximum accumulation.

Coating thicknesses as a function of depth were measured in a soil profile developed on the Late Pleistocene, Ramshorn fan consisting of coarse sand to cobble size clasts overlain by loess. The CaCO3 coating thicknesses measured from six distinct zones within the subdivided B-horizon can be used to illustrate the zone of maximum accumulation and the downward thinning of coats there after. Depths of 90-100, 100-110,110-125, 125-140, 140-150,and 150-165 cm resulted in coating thicknesses of 1.1±0.7, 1.2±0.4, 0.9±0.2, 0.4±0.2, 0.8±0.3, and 0.5±0.5 mm respectively.

Sites judged most stable will compare numerical ages with pedogenic carbonate coating thicknesses to create a soil chronosequence for the Lost River fans. Calcium carbonate coating thickness will be calibrated using U-series dating of the innermost coatings of milligram-size samples of carbonate analyzed by mass spectrometry.