FROM ROCK TO DUST: DENSITY CHANGES DURING THE TRANSFORMATION OF FRONT RANGE GRANITE TO SAPROLITE AND SOIL

The bulk density of granitic regolith (saprolite, grus and soil) records long-term physical and chemical transformation of Colorado Front Range bedrock and mixing with organic material to form mobile regolith. To help quantify geochemical budgets, we measured the density of granitic bedrock and regolith from soil pits or outcrops at stable sites and on slopes in Colorado and adjacent Wyoming. For granular samples from stable sites we measured density in the field using a 4-cm diameter coring tube or the excavation method; in the laboratory we used paraffin coating and displacement techniques. At >75 slope locations we used coring tubes in soil pits or a bulb planter for the upper 10 to 15 cm of the soil. The density of oxidized, fractured bedrock at stable sites ranged from 2.7 to about 2.2 Tm^-3, saprolite and grus gave densities between ~2.1 and 1.6 Tm^-3 and soil B and BC horizons generally ranged between 1.8 and 1.4 Tm^-3. Soil horizons from slopes averaged ~1.6 Tm^-3 and composite soil O + A horizons gave values of 1.1 ± 0.2 Tm^-3 (n =146). Thin section and geochemical analyses show that chemical weathering rates and clay generation is slow and that strain in Front Range saprolite is driven by formation of fractures and microfractures, enhanced by expansion of weathering biotite. In contrast, saprolites described by M. Pavich from the Appalachian piedmont are rich in neoformed minerals typical of warm, moist climates. On Front Range slopes, where density is less than ~1.7 Tm^-3, bioturbation and incorporation of organic matter produces regolith that creeps downslope and can be eroded by overland flow. Organic-rich horizons on slopes incorporate an average of 36% > 2 mm material, much of it pebble gravel derived from upward-mixed saprolite. Catchments have average ¹⁰Be erosion rates of 2.2 cm ky^-1 and the depth to saprolite on slopes is about 40 cm, implying steady-state residence times of a few tens of thousands of years on most slopes. Areas of thick, low-density saprolite may record a relict landscape that formed in pre-late Pleistocene time.