The area in western Taiwan and the Taiwan Strait is a foreland basin resulted from the flexure of the Eurasian plate caused by loading of the Taiwan mountain belt. The basin is formed since the overriding of Luzon volcanic arc of the Philippine Sea plate on top of the rifted continental margin of Eurasian plate in late Miocene (˜ 6.5 Ma). In central Taiwan, the foreland sequence is up to 5 km in thickness and exhibits a deepening followed by shallowing upward succession. The basal deepening trend (˜ 6.5-3 Ma) reflects the growing and advancing loads of the proto-Taiwan mountain belt with sediments sourced mostly from continental interior (i.e. China) and paleo-environments evolved from fluvial to outer shelf settings; the upper shallowing and coarsening upward succession (˜3 Ma to present-day) records the rapid growth and denudation of the orogenic belt and filling-up of the foreland basin with paleo-environments evolved from outer shelf to braided river settings. Notably, the presence of the spectacular upper Pleistocene, ˜1 km-thick succession of braided-river conglomerates (Hoyenshan Member of the Toukoshan Formation) at the top of the foreland basin indicates a delicate balance between basin subsidence and sediment supply during the latest stage of foreland basin development.

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 Erosional marks, formed by fluid-mechanical interactions between flows and substrates, corresponding to various ages and environments have been observed on various materials. These marks are similar in terms of their geometry, distributions, and dimensions. Similarities have also been observed in the fluid-mechanical interactions that form erosional marks. Experimental studies on erosional marks have also suggested that development of the marks starts from an area of flat surface (basement) to the surface of close-packed erosional marks through the region including isolated marks. However, in these studies, including numerous studies of flute marks formed by sediment gravity flows, the results regarding the development process of erosional marks have not been applied to interpret natural examples because these studies have mainly focused on the classification or development process of erosional marks.
 In the present study, first, we characterized the development process of erosional marks. Thereafter, we applied our findings to study outcrop examples of flute marks observed in turbidites and interpret the depositional processes of the turbidites as suggested from the statistical characteristics of erosional marks. The parameters we characterized are the distributions of these marks, the area occupied by these marks on the analyzed surfaces, and the dimensions and geometry of these marks. Based on the characterization, erosional marks resulting from long-term erosional action are considered relatively mature, whereas those resulting from short-term erosional action are considered relatively immature. When we applied the characterization to the erosional marks on a turbidite succession in the Nichinan Group, which is interpreted as a levee deposit and a crevasse channel fill, there are differences corresponding to the maturity between analyzed surfaces of the turbidite succession. The differences can be explained by considering the depositional setting of the analyzed turbidites; the turbidite succession of the crevasse splay is considered to have formed by quick deposition in a relatively short duration of flow, whereas the turbidites of the near-crevasse channel show evidence of a relatively longer erosional action resulting from flow convergence. No clear relation between the erosional marks and the sedimentary structures in the turbidites deposited from same sediment gravity flow is observed from the outcrop of the Nichinan Group; however, the characterization of erosional marks has the potential to elucidate the erosional process at the head of a sediment gravity flow and to provide information about their depositional environments.

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