COSMOGENIC ANALYSIS OF THE ROCKY FLATS ALLUVIUM NEAR BOULDER, COLORADO

Analysis of a 4 m deep soil profile from Rocky Flats Alluvium near Boulder, Colorado shows that nuclide activity decreases with depth, consistent with a variety of interpretive models.

The Rocky Flats is the most extensively exposed older piedmont alluvium in the vicinity of Denver. Correlative alluvial deposits and remnant erosional surfaces extend along the Front Range and tens of km east into the High Plains ca.100 m above modern channels. Stratigraphic and geomorphic relations demonstrate that Rocky Flats Alluvium is younger than middle Pliocene and older than ca. 0.6 Ma. Regional paleontologic evidence and ages inferred from soil carbonate accumulation imply that deposits are early Pleistocene. Most workers suggest that the Rocky Flats and younger alluvial sequences correlate with glaciation in the Front Range, but alluvium in the type area was derived from an unglaciated 48 km² catchment.

We collected a 9-sample sequence from the surface to —4.2 m in oxidized, clast to matrix-supported gravel exposed temporarily during realignment of an irrigation canal. ¹⁰Be activity in quartz grains from the sand fraction ranges from 3.12 *10⁶ atoms/gram in a strongly developed Bt horizon buried 0.25 m below the surface to 0.77 *10⁶ at -3.7 m. The A-horizon sample has lower than expected nuclide activity, consistent with mobile, recently emplaced surface sediment.

Several scenarios constrained by muon production <2% and SL, >60^{o 10}Be production of 5.17 atoms g^-1 y^-1 fit the nuclide data equally well. A simple, steady erosion model (3.7 m My^-1) and time since deposition of 2 My require high inheritance (2.1 *10⁶ atoms g^-1), equivalent to source basin erosion of 9.4 m My^-1. A no-erosion, exposure model implies deposition of alluvium at 0.2 Ma with inheritance (9*10⁵ atoms g^{-1 10}Be), equivalent to source basin erosion at 22 m My^-1. Multi-stage models (stable fan surface, episodic erosion and deposition, then stable again) also fit the data well and are supported by geologic and pedologic evidence. The fit of such models is optimized by original deposition at ca. 1.5 Ma, stripping between 0.1 and 0.2 Ma, and inheritance of ca.10⁶ atoms g^{-1 10}Be. In all models, high density (2.4 g cm³) and significant inheritance (>10⁶ atoms g^-1) are key to good fits.