Stratigraphic Signals of Autogenic Processes in a Bimodal Alluvial Fan: Laboratory Experiments

Abstract:

Autogenic processes are intrinsic to the depositional system and occur naturally without changes in external forcing. It has been generally accepted that stratigraphic records could mainly reflect changes in allogenic forcing such as climate, sea level and tectonic movement. However, autogenic processes also record unique stratigraphic signatures. Understanding the relationship between autogenic processes and their associated sedimentary record is critical to correctly interpreting basin-filling history. To interpret lithofacies boundaries clearly, we analyze short-term fluctuations of lithofacies boundaries caused by autogenic processes as a distinct signature. This study uses physical laboratory experiments of an alluvial fan evolution to investigate the short-term migration of a lithofacies boundary as an autogenic signature. The experimental tank used in this study is Delta Basin-1 at the University of Minnesota. A series of eight experiments was conducted to test the effects of sediment discharge (Qs), water discharge (Qw), and the ratio of these discharges (i.e., Qs/Qw) on the autogenic process’ timescale. The sediment mixture was composed of 50 % quartz sand and 50 % crushed walnut sediment in volume. The transitional boundary between the quartz sand and walnut sediment served as the gravel-sand transition proxy in an alluvial fan. After each run, the resulting fan deposits were dried and then sectioned in the radial coordinates. The sliced cross sections were analyzed to capture the migration of the grain size transition. Here, six of the eight experiments show a progradational fan-margin migration during the experiments; the rest show a retreat of the fan. Based on the analyses of fan-margin migration and wet fraction, an increased in sediment supply and water discharge reduced the autogenic timescale as higher sediment supply and water discharge refilled the fluvial sediment buffer faster. The stratigraphic results in the sliced sections also followed the same trend as higher sediment and water discharges developed highly zigzag shazam trajectories of the grain-size transition. The current study will provide a new perspective in interpreting the paleoenvironment using the autogenic signatures recorded in alluvial-fan deposit.