The stratigraphic sensitivity of rift-basin fills to changing climatic and tectonic conditions during rift evolution

Abstract:

Rift systems have relatively short source-to-sink systems, where sediments eroded from rift shoulders are deposited in adjacent basins. Shorter system lengths minimize sediment-transport buffering, increasing the possibility for recording paleoclimate and tectonic signals in rift-basin fills. This archive provides important clues for understanding feedbacks between lake dynamics, fault movements and sediment-transport processes during rifting under different climate and tectonic conditions. Climate factors (precipitation and temperature) affect runoff, water discharge and sediment discharge in erosional systems, and drive base-level variation in closed lake basins. Tectonic factors (e.g. fault movement and flexure) affect upland erosion rates and accommodation creation in rift basins. The connections between these external forcings and the degree to which they are uniquely expressed in the sedimentary records of rift basins are not fully understood.

Here we evaluate how water and sediment input and basin subsidence interact to control lake level and sedimentary stacking patterns in rift systems under different climatic and tectonic conditions. We built a 2D mass-balance model to generate sediment packages and lakes in a back-tilted basin analogous to rift half-grabens. In order to understand the sensitivity of rift-basin deposits to different types of external forcing, we compared depositional facies, deposit geometry, stratal stacking patterns, and lake-depth generated by models runs with different climatic and tectonic boundary conditions. Our results indicate that sensitive combinations of tectonic and climate conditions determine whether rift basins will have open or closed lake systems. Maintaining closed lake systems generally demands high fault-slip rates, which, in turn, tends to produce long-term retrogradational stacking patterns regardless of climatic conditions. During open-lake conditions, associated with slower fault-slip rates, a host of stacking patterns emerge depending on the sensitive balance of climatic and tectonic factors controlling sediment supply and evapotranspiration. We compare our model results to a range of rift-basin fills and consider which stratigraphic observations are most likely to record high-frequency climate records in these systems.