Numerical modelling of drainage basin evolution and three-dimensional alluvial fan stratigraphy

Abstract

A forward numerical model is presented to study the effects of bedrock erodability, thrust displacement rate, pulsating tectonic activity and sea-level fluctuations on drainage basin morphology and stratigraphy of large alluvial fan systems. A low value of the bedrock erodability coefficient Kb (0.5×10−4 year−2/3) vs. thrust rate (2.0 m/ky) is associated with a time lag between cessation of tectonic activity and maximum sedimentation rates observed on the alluvial fans. Applying higher Kb values, ranging between 1.0 and 8.0×10−4 year−2/3, results in higher sediment yields due to more rapid headward catchment erosion and elaborate sideways branching of drainage networks. Pulsating tectonic activity is reflected in a stratigraphic alternation of prograding and retrograding alluvial fan gravels. During tectonic activity, fan gravel fronts and coastlines retreat because catchment yields are insufficient to fill the accommodation space created by the flexural response due to thrust loading. Phases of tectonic quiescence and cessation of flexural subsidence are indicated by progradation of the gravel front. Depending on the position in the basin, the lag time for arrival of the gravel associated with tectonic cessation is several tens to hundred thousands of years. A combination of pulsed tectonic activity with sinuous sea-level fluctuation leads to a more complex stratigraphic pattern. In that case, stratigraphic response to tectonic pulses is masked by a similar but higher-order frequency-stacking pattern, especially in the more distal parts.