The influence of long-wavelength tilting and climatic change on sediment accumulation

Abstract

The elevation of continental interiors over time is demonstrably variable. A major part of change in elevation within the continental interior is likely driven by density changes within the upper mantle and by global mantle convection. For example, upper-mantle flow has been invoked as the cause of Neogene uplift of the interior Rocky Mountains and Colorado Plateau, warping and tilting sediment transport slopes that link to the widespread deposition of gravel units within the Great Plains. These geomorphic and sedimentologic features, however, can also be generated by an increase in runoff, since erosion will promote change in elevation due to isostatic compensation and the loading of the lithosphere by the deposition of sediment. To explore the consequences of change in topography and climate, we use a general length-dependent diffusive sediment transport law to model both erosion and deposition that includes the concentrative effects of river systems. The simplicity of the approach means that we can collapse sediment transport to one dimension and couple erosion and deposition with plate flexure. We find that for a landscape that is gently tilted (slope of order of 10 –3 ), a change in runoff has a minor effect on transport gradient, as sediment transport and associated flexural response maintain topography at a similar elevation. However, there can be a significant change in depositional style when the degree of tilt is altered by, for example, a local change in upper-mantle density. An increase in buoyancy within the upper mantle, which increases slopes, leads to a transient reduction in grain sizes deposited at a fixed location. This behavior is due to a temporary retreat of the zone of erosion into the catchment and a transient increase in accommodation space relative to sediment supply. A reduction in tilt has the opposite effect, the older deposits are eroded, and the erosion-deposition transition rapidly moves down system. There is convincing evidence that the formation of thin and laterally extensive conglomeratic units of the Great Plains was due to a reduced rate of subsidence. Based on the results of our model, we suggest that the deposition of widespread conglomeratic units within continental interiors is generally a consequence of a reduction in slope, as the dynamic support for regions of high topography is reduced.