Global Influence of Lowland Tectonic Depressions on Fluvial Morphology and Sediment Storage
Based on a global survey of 29 floodplains, the surface morphology of Holocene floodplains is examined, wherein areas with a propensity for river flooding and sediment trapping were identified. Fifty-five percent of these floodplains contain lowland depressions. Such depressions have many origins: foreland basins (peripheral, retroarc) including foredeep and backbulge; graben, half-graben and other fault complexes; mantle-induced warping; other isostatic and flexural depressions; and those related to salt or mud tectonics. Lowland depressions are characterized as including some of the following identifiers: statistically flatter down-valley slopes; greatly expanded widths compared to the container valley widths; multiple secondary or overflow river channels similar in nature to deltaic distributary channels; regional zones of swamps and lakes connected to the main river channel. None of these features alone are diagnostic. Almost all of the lowland depressions flood annually. Satellite surveys show that many of these lakes are tied to the main river through tie-channels, allowing the seasonal flood wave to push sediment laden water into the lakes. Because these river-connected lakes exist over millennia this suggests that the rate of subsidence (thermal or tectonic) is greater than the rate of aggradation. Some examples include: 1) The Mompox depression located upstream of the Magdalena Delta an area that has experienced an area-averaged aggradation rate of 3-4 mm/y; 2) The Poyang inland delta feeding Lake Poyang where sediment from the Gan and Xiu Rivers is filtered before discharging into the main stem Yangtze River; and 3) the Dong Ting depression located at the intersection of the Yangtze and Xiangjiang Rivers, that during the 20th century has sequestered 128 Mt/y, largely through overflow channels. These tectonically controlled lakes and swamps greatly affect the flux of sediment within the source to sink continuum. This continental sink of sediment is not generally recognized as a petroleum target.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California