Volcanic bedrock morphology as a control on fluvial pattern in a semi-arid setting.

ALLARD, JOSÉ OSCAR; MONTES, ALEJANDRO; PAREDES, JOSÉ MATILDO; BUJALESKY, GUSTAVO GABRIEL

San Miguel de Tucumán

Congreso; 9th International Conference on Fluvial Sedimentology; 2009

IESGLO, Facultad de Ciencias Naturales e IML ? Universidad Nacional de Tucumán

Downstream changes in the fluvial pattern can be imposed by lateral changes in the nature of the substrate, or bedrock proximity. This study analyzes an ephemeral, braided channel that cross an uplifted block of Jurassic basement at bahía Bustamante, in the northern margin of the Golfo San Jorge basin. Ground Penetrating Radar and vertical trenches were carried out in the valley to characterize the subsurface stratigraphy. GPR transects allowed us to identify six radar facies: 1) subhorizontal, 2) low angle, 3) high angle, 4) concave, 5) convex, 6) irregular and discontinuos. Three radar facies associations were defined: 1) channel, 2) channel fill deposits and 3) bar deposits. The three-dimensional organization of radar facies and superficial features (e.g. braid bars) are consistent with a braided pattern. The width of the river varies to 40 m to 260 m braid bars ranges from 50 m to 79 m in width and are 79.266 long. Specific stream power index (SSP) is defined as the Stream Power/Width of the channel belt. SSP data were obtained in five positions along the course of the river. Although the river behaves as a braided river along their entire course, it shows systematical change in the braiding index and SSP, defining aggradational and degradational zones along the course. Degradational zones are characterized by narrow valley widths, they have values of SSP of 0.0156 m2/s and 0.00547 m2/s. The largest SSP value is obtained where the river crosses the volcanic basement, which is also the place where the valley river is narrowest. Aggradational zones show wider valleys, and have SSP values of 0.00049 m2/s, 0.00315 m2/s and 0.000317 m2/s. The spatial distribution of aggradational and degradational zones along the course of the braided river is interpreted as a response of the river to the location of the volcanic basement along the course. Both the braided pattern and the SSP values evidence the developement of degradational zones where the resisting powerstream power.