Scaling the Response of Deltas to Relative Sea-level Cycles by Autogenic Space and Time Scales: a Laboratory Study
Relative Sea-Level (RSL) change influences surface processes and stratigraphic architecture of deltaic systems and has been studied extensively for decades. However, we still lack a quantitative framework to define what constitutes a small vs. large or short vs. long RSL cycle. We explore these questions with a suite of physical experiments that shared identical forcing conditions with the exception of sea-level. We utilize two non-dimensional numbers that characterize the magnitude and period of RSL cycles. Magnitude is defined with respect to the maximum autogenic channel depth, while the periodicity is defined with respect to the time required to deposit one channel depth of sediment, on average, everywhere in the basin. The experiments include: 1) a control experiment lacking RSL cycles, used to define autogenic scales, 2) a low magnitude, long period (LMLP) stage, and 3) a high magnitude, short period (HMSP) stage. We observe clear differences in the response of deltas to the forcing in each experiment. The RSL cycles in the HMSP stage induce allogenic surface processes and stratigraphic products with scales that exceed the stochastic variability found in the control stage. These include the generation of rough shorelines and large temporal gaps in the stratigraphy. In contrast, the imprint of LMLP cycles on surface processes and stratigraphy is found in properties that define the mean state of a system. These include the mean shoreline location and extraction of sediment inbound of the mean shoreline. This work demonstrates the effectiveness of defining RSL cycle magnitude and period through autogenic scales and provides insights for generation of forward stratigraphic models influenced by RSL change.