Rajchl, Michal¹, David Ulicny² 
(1) Charles University, Prague, Czech Republic 
(2) Czech Academy of Sciences, Prague, Czech Republic

ABSTRACT: Depositional Geometries of an Avulsive Fluvial System Controlled by Peat Compaction (Neogene, Most Basin, Czech Republic)

Large-scale exposures in open-cast mines of the Most Basin (Oligocene-Miocene, Eger Graben, Czech Republic) allow to study the evolution of clastic depositional systems overlying an extensive, up to 45 m thick, lignite seam - especially their syn- and post-depositional interactions with the underlying accumulation of organic material. Deposits of a highly avulsive, mixed-load fluvial system in the Hrabák Mine display a number of characteristics similar to deposits of modern anastomosing systems. Two main processes controlled the evolution of the Hrabák fluvial system: (1) syndepositional compaction of the underlying peat and (2) avulsions of the whole floodplain belt. An idealised autogenic cycle of channel-belt evolution started by strong aggradation of the channel belt folloving avulsion, continued by a gradual increase in lateral migration due to decrease of the compaction potential, and was ended by channel-belt abandonment due to avulsion. Nodal avulsions affecting the whole fluvial system were probably concentrated at the margin of the peat swamp. The study shows that the occurrence of a basinwide peat swamp in an extensional basin has a profound effect on the geomorphic and sedimentological characteristics of the fluvial system that enters the basin. Although the channel migration was not limited by peat growth as is some modern rivers, the extremely low gradient of the floodplain and differential compaction of peat beneath active floodplain belts were the main controls on channel migration and avulsion, at significantly shorter time scales than tectonic deformation.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.