Progress in the study of the main characteristics and formation theory of armoured mudballs

Armoured mudballs are clay fragments coated in gravel. They are formed under specific erosion and transport mechanisms and are widely distributed in fluvial, coastal and lacustrine environments. This paper provides a systematic review of the morphological characteristics, formation mechanisms and controlling factors of armoured mudballs in different environments. The main conclusions are as follows: The morphology of mudballs exhibits significant environmental dependence. In fluvial environments, they tend to be larger and more spherical (sphericity >0.95), whereas in shoreline environments, smaller ellipsoids tend to dominate (only 36.7% of mudballs are spherical). Armoured mudballs are formed through processes such as erosion, abrasion and adhesion, and their sphericity is positively correlated with transport distance. The presence of an armoured layer significantly enhances their resistance to disintegration. Key factors in mudball formation are material sources (e.g. collapsed riverbank clay blocks, tidal flat mud layers) and high-energy events (floods, storm surges), while rapid burial is the primary preservation pathway. Mudballs can be used to study flood processes. Future research should integrate experimental simulations and multi-environment comparisons to further elucidate their dynamic mechanisms and significance in paleoenvironmental reconstruction.