Influence of downstream changes in hydrodynamics on floodplain sedimentation and resultant channel avulsion in stream-dominated alluvial fans: An example from the Cretaceous Duwon Formation in the southern Korea

Abstract

To understand the influence of downstream changes in hydrodynamics on floodplain sedimentation and associated channel avulsion in stream-dominated alluvial fans, we studied the Cretaceous Duwon Formation in the southern Korea (Goheung area). The Duwon Formation unconformably overlies the Paleoproterozoic basements and mainly consists of braided stream and calcrete-bearing floodplain deposits. Close to the basements, channel deposits show radial paleoflow patterns, and the sediment size and ratio of channel to floodplain deposits decrease downstream over a relatively short distance (< 7 km). These lines of evidence indicate that the Duwon Formation was deposited in the stream-dominated alluvial fans under arid to semi-arid climatic conditions, which can be classified into the proximal, medial, and distal zones. In the medial zone, floodplain deposits are mainly composed of purple sandstones and compensational stacked crevasse channel and splay deposits with unsystematic paleoflow directions. Whereas in the distal zone, floodplain deposits are mainly composed of purple mudstones with calcretes and vegetation traces. They are interbedded with and overlain by progradational stacked (coarsening- and thickening-upward trends) crevasse channel and splay deposits with constant paleoflow directions. These floodplain deposits are finally overlain by braided stream deposits as a result of channel avulsion. In the medial zone, as the relatively high stream flood discharge caused frequent overbank flooding and formed multiple breaching points in channel levees, crevasse channel and splay deposits were compensational accumulated via these breaching points. On the other hand, in the distal zone, the relatively low stream flood discharge commonly supplied fine-grained sediments into floodplains, resulting in the deposition of purple mudstones. Once channel levees collapsed, vegetation stabilized the breaching point of channel levees, progressively incised by the multiple flooding events. Overbank floodwaters through the relatively stabilized-breaching point led to a continuous supply of coarse-grained sediments, resulting in progradational stacked crevasse channel and splay deposits. Thus, floodplain sedimentation, accumulation of crevasse channel and splay deposits, causes rapid filling of accommodation space in the proximal floodplains. With in-channel bed aggradation, their accumulation resulted in the growth of alluvial ridges and a gradual increase in the cross-floodplain topographic gradient. When the channel bed is sufficiently perched above the distal floodplains by alluvial ridge growth, the gradient advantages caused channel pathway to shift on floodplains with the formation of a new channel (channel avulsion). This study suggests that downstream changes in hydrodynamics and associated floodplain sedimentation are important for channel avulsion and thus, ultimately influence sediment dispersal patterns on the stream-dominated alluvial fans (fan-building processes).