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- E-ISSN: 1365-2117
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The grade index model as a rationale for autogenic nonequilibrium responses of deltaic clinoform to relative sea‐level rise
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Authors
Junhui Wang1,2,3
, Hajime Naruse4, Tetsuji Muto3
- Source: Basin Research, Volume 32, Issue Clinoforms and Clinothems: Fundamental Elements of Basin Infill, Apr 2020, p. 378 - 387
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- 30 Dec 2018
- 10 Oct 2019
- 02 Nov 2019
Abstract
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, Abstract
Given steadily rising relative sea level, a deltaic clinoform grows in a nonequilibrium pattern. These nonequilibrium behaviours include autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which can be captured by the recently‐proposed grade index model. The regressive‐to‐transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain’s dimensionless basal area (At*, which can be reflected by the dimensionless delta plain radius, x*) encounters a critical value that is expressed in terms of grade index (Gindex): regression and transgression are sustained when At* is below and above the threshold, respectively. During transgression, the transgression mode depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both At* and Gindex decrease as the relative sea‐level rises. Eventually, the depositional system experiences autodrowning when At* = Gindex = 0. If γ < β; on the other hand, both At* and Gindex increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which At* =Gindex = 1.
Grade index (Gindex) is a dimensionless number given as the volume‐in‐unit‐time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the Gindex, using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive‐to‐transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (At*) encounters a critical value that is expressed in terms of Gindex: regression and transgression are sustained when At* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both At* and Gindex decrease as the relative sea‐level rises. Eventually, the depositional system experiences autodrowning when At* = Gindex = 0. If γ < β; on the other hand, both At* and Gindex increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which At* = Gindex = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.
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Basin Research © 2020 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists © 2019 The Authors. Basin Research © 2019 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists