Robert R. Curry

OBSERVATION OF ALPINE MUDFLOWS IN THE TENMILE RANGE, CENTRAL COLORADO

On August 18, 1961, at the head of May-flower Gulch in the Tenmile Range of central Colorado, after a 24-hour rainfall of 24.5 cm, a series of alpine mudflows was observed. The flows occurred as a series of pulses which moved at a maximum observed velocity of 980m/min (53 feet/sec) over and through saturated talus on slopes as steep as 41 degrees. Samples of the matrix of the moving debris had a density of 2.53 g/cm 3 , 11 per cent silt and clay, and less than 10 per cent water. Viscosity of this material calculated from flow rates is roughly 3 × 10 4 poises. Four successive ancient mudflow deposits were recognized at the site of the largest mud-flow. Lichenometric analysis indicates that they occurred about once every 150–400 years.

The diamicton at Deadman Pass, central Sierra Nevada, California: A residual lag and colluvial deposit, not a 3 Ma glacial till

A diamicton exposed at Deadman Pass in the central Sierra Nevada has been previously described as glacial till and dated at about 3 Ma. If till, the deposit would document an exceptionally old and previously unrecognized glaciation in the Sierra Nevada. The age and glacial origin of the diamicton at Deadman Pass has been widely cited in the geologic literature. Recent work, however, demonstrates that the diamicton is a residual lag and colluvial deposit formed by weathering of poorly consolidated Pliocene pyroclastic rocks that are unusually rich in coarse lithic basement clasts, including granitic and metamorphic rock types. Evidence that the diamicton at Deadman Pass is not till includes the following: (1) distribution of the diamicton is limited to areas underlain by the distinctive clast-rich lower pyroclastic member of the quartz latite of San Joaquin Ridge, (2) clasts in the diamicton and in the lower pyroclastic member are identical, (3) clast lithologies in the diamicton reflect nearby sources, (4) glacial deposits are absent in well-exposed sections of the lower pyroclastic member, and (5) formation of diamicton from present-day weathering and mass wasting of outcrops of the lower pyroclastic member can be observed locally.