Constrasting Styles of Holocene Avulsion, Texas Gulf Coastal Plain, USA

Abstract

Examination of outcrops, satellite imagery and shallow (< 25 m long) floodplain cores shows that rivers of the Texas Gulf Coastal Plain undergo two distinct avulsion styles: (i) avulsion by channel reoccupation and (ii) avulsion by diversion into flood basins. Holocene avulsion histories of rivers with large sediment supplies, such as the Colorado, and rivers with small sediment supplies, such as the Trinity and Nueces rivers, further suggest that different styles of avulsion occur during different stages of incised-valley filling. The Nueces and Trinity river valleys represent early stages of filling, in response to the Holocene transgression, and these rivers avulse by reoccupying segments of Late Pleistocene falling-stage and lowstand channel courses that are buried by thin veneers of Holocene sediment. Because these rivers have small sediment supplies, floodplain aggradation is slow and this factor, in addition to the abundance and large size of Late Pleistocene palaeochannels, favours channel reoccupation. This style of avulsion is accomplished primarily by erosion and reworking of channel sediments and avulsion deposits are rare, especially in the vicinity of the avulsion node. In contrast to the Trinity and Nueces rivers, the Colorado River has almost completely filled the accommodation space produced during the Holocene transgression and avulsion deposits comprise a significant portion (c. 50%) of the fill. Repeated episodes of channel diversion into flood-basin depressions accompanied rapid floodplain aggradation and valley filling and produced metres-thick successions of massive or laminated flood-basin muds that encase crevasse-splay sands. Deposits of individual avulsions are separated by slickensided muds or buried A horizons of soils, which represent periods of floodplain stability between episodes of avulsive deposition. The modern Colorado River represents a late stage of valley filling and avulses by channel reoccupation. During the most recent event, the Colorado River abandoned its alluvial valley and reoccupied a Pleistocene channel belt of the previous interglacial highstand. Similarly, during the preceding avulsion, the Colorado River incised and reoccupied a buried Pleistocene palaeochannel, that was active during the falling stage of the last glacial cycle. In a river with a large sediment supply, such as the Colorado, late stages of valley filling coincide with sea-level highstand, which limits accommodation space and slow floodplain aggradation leading to avulsions by channel reoccupation. The significance of avulsion during valley filling by these coastal-plain rivers is threefold. First, avulsion deposits represent a large portion of the valley fills, especially of rivers with large sediment supplies such as the Colorado. Second, avulsion by channel reoccupation during early and late stages of valley filling produces multilateral and multistorey sheet sands. Third, the most recent avulsion of the Colorado River demonstrates that avulsion during late stages of valley filling can determine the location of future incised alluvial valleys and controls the preservation of older valley fills.