Autostratigraphic modelling of the growth of alluvial-shelf systems during steady base-level cycles: Two-dimensional tank experiments

Autostratigraphy is the stratigraphy generated by large-scale autogenesis, developed based on the full recognition of the non-equilibrium behaviour of depositional systems in response to steady external forcing. The existing autostratigraphic concepts were derived mostly from studies of river deltas growing during a single rise or fall of base level (or relative sea level). The present study challenges to extend the autostratigraphic framework to the alluvial-shelf system growing through steady base-level cycles by two-dimensional tank experiments. During each experimental run, the base level was changed symmetrically, wherein through cycles, the rise and fall had the same constant rate (|R-bl|) and period (T-bl), and thus the same constant amplitude (A(bl)), but with no basin tectonism. In total, nine runs with different combinations of |R-bl| and A(bl) were performed. The experimental results brought the following implications. (i) The shelf-transiting active depositional system takes non-equilibrium responses in earlier base-level cycles, during each of which the system experiences episodes of degradation with base-level fall. (ii) After the system has sufficiently grown through cycles, non-equilibrium responses change into equilibrium responses, whereby the shelf-transiting alluvial system, whether retrogradational or progradational, is free from degradation and continues to aggrade but with a gradually decreasing rate of aggradation. (iii) The alluvial topset river tentatively but autogenically attains a graded state during the falling limb of an intermediate cycle, which separates the earlier degradation-inclusive and later aggradation-sustainable cycles. (iv) The number (or duration) of cycles elapsed prior to this phase-transition is linearly proportional to the amplitude (or the square of the period) of base-level cycles, with a coefficient defined by the rates of base-level change and sediment supply. Such a growth pattern does not necessarily hold when considering long-term tectonic subsidence or uplift. These notions help to understand the stratigraphic architectures of natural alluvial-shelf systems evolved through base level cycles.