Guevara, Edgar¹, Andres Aslan², W. A. White¹, J. A. Raney¹, J. C. Gibeaut¹
(1) University of Texas at Austin, Austin, TX
(2) Mesa State College, Grand Junction, CO

ABSTRACT: The Orinoco Delta, Venezuela: Sedimentation Near an Active Plate Margin

The Orinoco Delta encompasses more than 22,000 km² of nearly pristine coastal and deltaic environments near the South America-Caribbean plate margin. Fluvial and marine interactions and tectonic processes are reflected in the facies character, distribution, and architecture of this poorly known delta system. Two distinct sectors with contrasting geomorphic characteristics, sedimentary processes, and lithofacies are recognized.
The southeastern delta plain is dominated by the straight to anastomosing, main distributary channel (Rio Grande), overbank flooding, and siliciclastic sedimentation. The coastline is dominated by estuaries, interchannel mangrove islands, and tidal sand ridges. An intricate network of distributary channels and tidal creeks is present in the northwest delta plain. Distributary channels produce mud- and peat-encased sand bodies that are up to 1 km wide and up to 15 m thick. Distributaries separate densely vegetated flood basins that contain peat up to 10 m thick and, which cover areas as large as 200 km².
Shoreline progradation in the northwest delta results from progradation of coast-parallel mudcapes that is triggered by distributary avulsion. The mudcapes, comparable to those of the Suriname coast, are present along ~100 km of the coast and are comprised of wave-modified suspended sediment that is largely transported by northwest-directed longshore currents. Mudcapes are typically 5 to 10 km wide and up to 30 km long.
Transpression along the South American-Caribbean plate margin triggers shale diapirism manifested by mud volcanoes along the NW coast of the delta. The shale diapirs locally form hydrocarbon traps.

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