GRAINSIZE DEPENDENT TRANSPORT AND DEPOSITION IN A LANDSCAPE EVOLUTION MODEL

The sedimentary deposits within continental basins frequently exhibit spatially-variable sorting, with coarser clastics (gravel and sand) proximal to river outlets and fine-grained clastics located at greater distances into lakes and seas. This dependency on grain-size produces patterns that can have a significant effect on patterns of subsurface fluid transport, mineralization, and spring effluence. Here, we incorporate grain-size dependent transport and deposition into TISC (Tectonics, Isostasy, Surface processes, and Climate), a large-scale finite-difference landscape evolution model, in order to reproduce sediment sorting along streams and within lake-filled basins. For each fluvial transport model in TISC, clastic sediments of two, user-defined, grain sizes are transported and deposited according to one or two criteria, the first representing differing abilities to be entrained by streams and the second that represents differing grain settling distances. As each incremental coarse and fine-grained sediment layer is deposited, we calculate basic petrophysical properties from user-defined grain size and porosity. From this, we calculate intrinsic permeability, using the Kozeny-Carman equation assuming spherical grains. As sedimentary units are defined by age horizon within TISC, we calculate effective (upscaled) horizontal and vertical permeability for each incremental coarse and/or fine grain depositional event, maintaining our ability to detect and record thin aquitard layers which can affect hydrologic anisotropy. We find that with the additional capability to separate two clastic grain sizes, we can reproduce more realistic sedimentation patterns, where coarse grains settle near river outflows and fine grains settle at greater distances within a basin. We also find that in the regions where significant coarse and fine grain sediment are both deposited, predicted horizontal-to-vertical permeability ratios increase by 1-2 orders of magnitude. We show results from a range of tests for simplified continental basin structures, varying erosional, transport, and depositional parameters, surface runoff, and topographic relief.