Applying sediment budget models on rock glaciers in the arid Andes of NW Argentina

MATEO A. MARTINI; JORGE A. STRELIN; RICARDO A. ASTINI

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

Active rock glaciers consist of frozen rock debris bodies, including ice lenses and cores, that extent downslope due to the deformation of the contained ice. Rock glaciers can be considered as closed sediment and ice traps. The sediment budget model developed by Brenning (2005) proposes that the amount of rock material (the sediment/ice rate is considered constant) stored within rock glaciers can be used to estimate their morphodynamics. The equation applied here (pta=sDA) is a mass conservative equation that provides an approximate sediment budget, where ”pta” is the sediment volume stored within a rock glacier (expression to the left in the equation) as the sediment/ice rate content of a rock glacier (p), its thickness (t), and its planimetric area (a); and ”sDA” is the total volume of post-glacial vertical lowering (right-hand side in the equation) as a function of talus shed size (s), talus denudation rate (D) in the sense of a vertical lowering rate, and the rock glacier age (A). Empirical equations are used to estimates p and t. Also an advance rate (C=l/A), is estimated as the ratio of the rock glacier length (l), and the age (A). This model was applied to 25 active rock glaciers in the Palermo and Cachi ranges between 24º 27’ S and 25º 05’ S in NW Argentina where the lower limit of active rock glaciers is about 4500m a.s.l. These rock glaciers are interpreted to be younger than the last glaciations recorded in the area. Zech et al. (2009) studied the Quaternary glaciations in the Sierra de Santa Victoria, further north, and found that the glaciers retracted since ~10Ka. We take this age as the maximum age of the rock glaciers. The model outputs a denudation rate in the source area of 0.6- 0.7mm/yr, a mean age of 6.2Ka (median 3.4Ka), and a mean advance rate of 25 cm/yr (median 17 cm/yr). The denudation rate is similar to the calculated by Azocar and Brenning (2010) for the dry Andes to the side of Chile, while estimated ages are older and the advance rate is much lower than the obtained in this work. To check the model we also applied Lliboutry’s (1965) equation for glacigenic rock glaciers, H = 13.73 m5/y4*(C/sin3a)1/4, where H is the maximum rock glacier thickness and a is the inclination in the ablation zone. The results are not consistent between the two models neither in the ice thickness nor in the advance rate. The discrepancy could be related with a wrong assumption of some of the parameters and/or with the rock glacier type considered in each of the models (periglacial vs ice cored). These results represent a first approach to the study of rock glaciers characteristics in this region, but more specific data are necessary to develop an appropriate model. Future research will provide new results, improving the measurement of morphodynamic and morphometric variables like rock glacier thickness, sediment/ice rate, superficial velocity and surface slope.