Grain Segregation Mechanism in Aeolian Sand Ripples

TL;DR

This paper proposes a physical mechanism explaining layered structures in sedimentary rocks formed by sand ripple migration, focusing on grain segregation processes during aeolian sand transport.

Contribution

It introduces a discrete grain dynamics model that combines size, shape, and hopping length segregation processes to explain layering in sandstone.

Findings

Identifies three key segregation processes influencing layering.

Develops a coupled dynamic model of moving grains and static surface.

Provides a plausible physical explanation for observed sedimentary structures.

Abstract

Many sedimentary rocks are formed by migration of sand ripples. Thin layers of coarse and fine sand are present in these rocks, and understanding how layers in sandstone are created has been a longstanding question. Here, we propose a mechanism for the origin of the most common layered sedimentary structures such as inverse graded climbing ripple lamination and cross-stratification patterns. The mechanism involves a competition between three segregation processes: (i) size-segregation and (ii) shape-segregation during transport and rolling, and (iii) size segregation due to different hopping lengths of the small and large grains. We develop a discrete model of grain dynamics which incorporates the coupling between moving grains and the static sand surface, as well as the different properties of grains, such as size and roughness, in order to test the plausibility of this physical mechanism.