Overbank sedimentation and its influence on channel avulsion: examples of the Cretaceous Tando and Namyang basins in the mid-western part of the Korean Peninsula

The channel arrangements on alluvial plains control the sediment dispersal patterns and stratigraphic architectures, and understanding of alluvial ridge growth and resultant channel avulsion is therefore important to reconstruct the stratigraphic evolution of alluvial sedimentation as well as to assess the quality of non-marine reservoirs. We analyzed the alluvial successions in the Cretaceous Tando and Namyang basins to understand the genetic relationships among overbank sedimentation, alluvial ridge growth, and the resultant channel avulsion. These successions are classified into four facies associations, which are interpreted to have been deposited in gravel-bed streams (FA-1), floodplain fines (FA-2), crevasse channels (FA-3), and crevasse splays (FA-4). Stratigraphically below the gravel-bed stream deposits (FA-1), the following systematic changes in depositional features are observed in the stacked beds of overbank sediments (FA-2, FA-3, and FA-4): (1) a vertical facies variation from laterally continuous floodplain fines to compensational (lower) and progradational (upper) stacked beds of crevasse channel and splay deposits and (2) the coarsening-upward trends in successive crevasse channel deposits with invariable paleoflow patterns which matched those of the overlying gravel-bed stream deposits. The compensational stacking pattern indicates rapid filling of the accommodation space provided close to trunk channels by continuous accumulation of crevasse channel and splay deposits and, with in-channel bed aggradation, their accumulation resulted in the growth of alluvial ridges and a gradual increase in the cross-floodplain topographic gradient. The increased gradient in turn caused crevasse channels to advance toward the distal floodplains, resulting in the development of coarsening-upward trends with the progradational stacking pattern. Additionally, the growing alluvial ridges allowed the channel bed to be sufficiently perched above the distal floodplains, and the concomitant gradient advantages caused the channel pathway to shift with the formation of a new channel (channel avulsion) on the sand-prone, stacked overbank sediments, as indicated by the overlying FA-1. This study reveals that overbank sedimentation plays a critical role in the growth of alluvial ridges and that the concomitant development of cross-floodplain topographic gradients changes the morphodynamics of nearby floodplains, thereby changing the stacking patterns of the overbank sediments and causing channel avulsions.