SLOW WEATHERING OF GRANITIC ROCKS AND THE LOW-RELIEF LANDSCAPE OF THE BOULDER CREEK CATCHMENT, COLORADO

A low-relief landscape, incised by steep tributaries to the S. Platte River, persists locally between glaciated headwaters and the steep eastern margin of the Front Range, Colorado. Measurement of slow weathering processes and erosion rates suggest that the landscape is unstable in the late Cenozoic climatic and tectonic regime. Field studies and compilation of data from 1850 drilled wells in a 350 km2 part of the low relief area show that in many places it is underlain by a weathered zone 4 to 8 m thick, consisting of oxidized bedrock, saprolite, and grus, capped by a thin soil. We use the geochemistry and mineralogy of weathered materials, modern solute chemistry, and erosion rates calculated from cosmogenic radionuclide (CRN) measurements to evaluate stability of the regolith. Plagioclase and biotite are the major altered minerals in weathering profiles, suggesting the weathering sequence plagioclase>biotite>microcline>quartz. Clay mineralogy indicates that small amounts of illite and chlorite were inherited from Precambrian bedrock and that smectite, illite-smectite intergrades and minor amounts of kaolinite have formed by weathering. Locally abundant kaolinite and illite probably record hydrothermal alteration during early Tertiary time. In the present climate, chemical alteration of the low-relief, regolith-mantled landscape occurs slowly and erosion rates are typical of post-orogenic landscapes. Mass balance analyses demonstrate that elemental loss increases up through the weathering profile as Ca>Mg>>Na>Si>K. Chemical weathering releases dissolved cations + silica to surface water at rates of 2 to 5 T m-2 kyr-1 where runoff varies from 10 to nearly 160 cm; average CRN erosion rates from small catchments are ~2.2 cm kyr-1. If alteration is mainly by volume expansion, erosion may balance the conversion of saprolite to grus + soil. Transformation of oxidized bedrock to saprolite probably is too slow to be in a steady-state with erosion at late Quaternary rates. Topographic evolution of the low-relief landscape, however, is buffered by slow fluvial incision into the resistant bedrock of the Boulder Creek catchment. Better modeling of episodic and long-term landscape change will require a clearer understanding of mineral transformation and solute origin in Front Range catchments.