ASLAN, ANDRES¹ and MICHAEL D. BLUM^2
1Bureau of Economic Geology, The University of Texas at Austin, Austin, TX
²Department of Geosciences, University of Nebraska, Lincoln, NE

Abstract: The Significance of Convulsion during Valley Filling (Transgressive and Highstand Systems Tracts), Louisiana and Texas Coast

Studies of late Quaternary Mississippi and Colorado River deposits in Louisiana and Texas, respectively, illustrate the significance of avulsion during rising sea level and valley filling. Avulsion not only influences the types and distribution of alluvial lithofacies in valley fills and the locus of deltaic sedimentation but also controls the regional distribution and preservation of valley fills.

In rivers with large sediment supplies such as the Mississippi and Colorado, the transgressive systems tract (TST) is dominated by avulsive deposits that consist of muds and isolated sand bodies of varying size and shape. The deposits represent sediments that rapidly filled flood basins and lakes during avulsions. Frequent avulsion by diversion into floodplain depressions accompanies rapid relative sea-level rise and floodplain aggradation. The highstand systems tract (HST) is characterized by large interconnected sand bodies and muds with common soil features. These deposits represent channel belts and overbank sediments and reflect slow rates of floodplain aggradation and decelerating sea-level rise. During highstand conditions, avulsion occurs primarily by channel-belt reoccupation, especially in areas where TST muds are thick. This avulsion style leads to the development of multilateral, multistory sand bodies.

Once a valley fills completely in response to sea-level highstand conditions, avulsion can lead to valley abandonment. If this happens, TST and HST deposits of the recent valley fill have high preservation potentials during subsequent falling stage and lowstand conditions. Thus avulsion can lead to the development of coastal plains characterized by offset and overlapping valley fills.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas