Non-Isochronous Transgression in Dual-Provenance Sedimentary Basins: Insights From Experimental Studies and Geometric Analysis

Abstract

In conventional sequence stratigraphy, it is commonly believed that transgression occurs simultaneously across the sedimentary basin experiencing relative sea-level (RSL) rise. As a consequence, the maximum flooding surface is widely utilised as a quasi-isochronous chronostratigraphic marker for regional stratigraphic correlation. The concept of shoreline autoretreat demonstrates that the transgression may occur after a precursory regression. How long the precursory regression would last depends on external forcings including the rate of sediment supply (q_s) and rate of RSL rise (R_rsl) and slope features of the basin. This means within a given basin, the onset of transgression varies in locations with different sediment supply rates and/or slope features. This study aims to analyse the influencing factors of the occurrence time of transgression in dual-provenance basins through theoretical modelling and two-dimensional flume experiments. To investigate these factors, two series of tank experiments were designed. The first series is supply-modulated where the two provenances were different in q_s, while the basin slope condition were kept the same. The second series is modulaed where the two provenances varied in hinterland slopes (γ) while q_s were kept the same. Each series included 2 or 3 runs with different multiples of R_rsl or q_s. Furthermore, a reference run was performed where the two provenances were identical in both q_s and γ. The results reveal that: (1) q_s and R_rsl have similar effect on transgression. As q_s increases, the timing of transgression is delayed, while an increase in R_rsl accelerates its occurrence. Both factors affect the timing of transgression by altering the size of the river-delta system. (2) The occurrence of transgression is closely related to the size of the river-delta system. Guided by the autoretreat mechanism, the fluvial-deltaic system maintains its progradational trend during a constant RSL rise, provided it has not yet reached its critical dimensions. A transgressive retreat is fundamentally postponed until the delta evolves to exceed this intrinsic spatial threshold, at which point the required sediment volume for progradation outstrips the supply. Only when the river-delta size exceeds the critical size does transgression take place. The critical size is controlled by a combination of q_s, R_rsl, and basin slope. (3) Hinterland slope (γ) affects subaerial and subaqueous allocations of sediment. Larger γ result in a reduction of subaerial allocations of sediment. As a result, the alluvial realm expands slower but the aggradation rate is higher, by which transgression is delayed. The Canterbury Plain in New Zealand serves as a potential example of asynchronous transgression in a basin with varying provenance. Based on an understanding of autostratigraphic processes, these findings offer a framework for explaining the anisochronicity of transgression in natural basins and provide a novel perspective for reconstructing basin evolution and stratigraphic analysis.