Plio-Quaternary sediment budget between thrust belt erosion and foreland deposition in the central Andes, southern Bolivia

摘要

Estimates of the physical boundary conditions on sediment source and sink regions and the flux between them provide insights into the evolution of topography and associated sedimentary basins. We present a regional-scale, Plio-Quaternary to recent sediment budget analysis of the Grande, Parapeti and Pilcomayo drainages of the central Andean fold-thrust belt and related deposits in the Chaco foreland of southern Bolivia (18-23 degrees S). We constrain source-sink dimensions, fluxes and their errors with topographic maps, satellite imagery, a hydrologically conditioned digital elevation model, reconstructions of the San Juan del Oro (SJDO) erosion surface, foreland sediment isopachs and estimated denudation rates. Modern drainages range from 7453 to 86798 km(2) for a total source area of 153 632 km(2). Palaeo-drainage areas range from 9336 to 52 620 km(2) and total 100 706 km(2), suggesting basin source area growth of similar to 50% since similar to 10 Ma. About 2.4-3.1 x 10(4) km(3) were excavated from below the SJDO surface since similar to 3 Ma. The modern foredeep is 132 080 km(2) with fluvial megafan areas and volumes ranging from 6142 to 22511 km(2) and from 1511 to 3332 km(3), respectively. Since Emborozu Formation deposition beginning 2.1 +/- 0.2 Ma, the foreland has a fill of similar to 6.4 x 10(4) km(3). The volume and rate of deposition require that at least similar to 40-60% of additional sediment be supplied beyond that incised from below the SJDO. The data also place a lower limit of >= 0.2 mm year(-1) (perhaps >= 0.4 mm year(-1)) on the time- and space-averaged source area denudation rate since similar to 2-3 Ma. These rates are within the median range measured for the Neogene, but are up to 2 orders of magnitude higher than some observations, as well as analytic solutions for basin topography and stratigraphy using a two-dimensional mathematical model of foreland basin evolution. Source-to-sink sediment budget analyses and associated interpretations must explicitly and quantitatively reconcile all available area, volume and rate observations because of their inherent imprecision and the potential for magnification when they are convolved.