Coupled channel–floodplain dynamics and resulting stratigraphic architecture viewed through a mass-balance lens

Abstract

ABSTRACT Basin-wide accommodation production and associated sediment mass deposition exert fundamental controls on stratigraphic architecture, but the details of this relationship are not fully understood. This is because it is unknown how accommodation production directly influences morphodynamics both in terms of channel process (i.e., channel migration, channel avulsion) and floodplain process, both of which are themselves coupled dynamically and are critical to the nature of stratigraphic architecture. To address this, we expand on existing theory that links sediment mass balance and resultant stratigraphic architecture. We use two fan-delta experiments that each experience different rates of accommodation production to measure key surface morphometrics and subsurface sedimentary characteristics. Importantly, sediment was transported in bedload and suspension in these experiments, resulting in construction of strata characterized by channel bodies surrounded by overbank strata deposited from suspension fallout. From these data we use three key timescales to capture the overall behavior of the system when placed into mass-balance space: avulsion setup timescales (TA) and channel mobility timescales (TV) that define short-term surface autogenics, and an accretion timescale (TC) that incorporates longer-term deposition. We find that the ratio of both TC/TA and TC/TV are independent of accommodation production rate in mass-balance space, which supports a self-organized response of channel dynamics to environmental boundary conditions. The fraction of strata generated from key depositional environments largely supports this behavior, particularly for channel sand bodies that resulted in deposition from bedload transport. As such, our results suggest that channel-body density is independent of accommodation production rate in a mass-balance space. We found that, although contributing to a significant fraction of the basin strata, far-field overbank deposition rates are insensitive to accommodation production and that differences in autogenic timescales between experiments largely resulted from differences in channel deposition rates, highlighting the close coupling between channel dynamics and accommodation generation. More generally the observed self-organized response of surface morphodynamics to accommodation production in mass-balance space provides a process-based framework to explain the utility of balancing mass for the prediction of down-system sediment size fractionation and sedimentary architecture.