Reconstruction of bedform dynamics controlled by supercritical flow in the channel–lobe transition zone of a deep-water delta (Sant Llorenç del Munt, north-east Spain, Eocene)

Abstract

Stable supercritical-flow bedform phases under two-dimensional steady flow are geometrically simple and include long-wavelength cyclic steps at high Froude numbers and antidunes characterized by in-phase flow that is near critical. Less well understood are the transitional bedform phases at the boundaries of the stable bedform fields and bedforms developing in complex flow geometries like the channel–lobe transition zone. This complexity is exacerbated by the fact that natural flows are never steady. Stable antidune bedforms may be reworked by temporally increasing discharge into chute and pool, and cyclic step and chute and pool fields will be reworked into antidunes if discharge is sufficiently decreasing. In addition, the channel–lobe transition zone is continuously evolving in space and time due to the influence of solitary hydraulic jumps at the channel mouth on channel extension and back stepping. This detailed outcrop study of a deep-water delta slope belonging to the Eocene Sant Llorenç del Munt clastic wedge exposed near El Pont de Vilomara (north-east Spain), tackles the complex bedform architecture problems by applying a method previously developed for fluvial deposits. Analysis of surfaces traced on high-definition, drone-derived in-strike images combines architectural studies with facies analysis. Set boundaries of the bedforms were thus established, revealing the upslope migration of hydraulic jump zones and the intricate stacking of antidunes and solitary, mouthbar related chute and pool like structures. Further analysis of the stacking of bedforms and bounding surfaces provide evidence that deposition occurred in a relatively short (few hundreds of metres) channel–lobe transition zone at the base of the delta slope. The usefulness of the bounding surface hierarchy approach for turbidite deposits lies in the careful evaluation of the spatial extent of bounding surfaces, which are easily overlooked in complex architectures such as those created in the channel–lobe transition zone.