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Fraticelli, Carmen M.1, John B. Anderson2
(1) Rice University, Houston, TX (2) Rice University, Houston,

ABSTRACT: The Effects of ENSO Cyclicity on Deltaic Progradation: Examples from the Modern Brazos Delta

The facies architecture of the modern Brazos Delta reflects a multi-process mode of development. A series of ridge/trough pairs representing channel mouth bars and back-bar lagoonal facies make up the delta plain. Current models propose a combination of flood-wave processes to deposit the channel bars and rework the bar shoreward. However, the processes governing the development of these ridge/trough pairs are more complex than this model suggests.
Texas climate is characterized by extremes. There are both frequent droughts and flash-floods. Both extremes can be correlated to extremes in the El Nino-Southern Oscillation. El Nino and La Nina events significantly impact Texas precipitation patterns. El Niño events result in dramatically higher precipitation and flooding, and La Niña events trigger periods of drought. Development of the channel mouth bar facies in the Brazos Delta can be correlated to El Niño induced floods. However, only 5 shoreline ridge events exist in the Brazos Delta, and an estimated 20 El Niño events have occurred since 1929. So, the ridge/trough pair development cannot be tied to flooding alone. A period of extended drought proceeding the development of each ridge/trough pair, in addition to El Niño flooding, appears to be the dominant factor. This association also explains why other strong El Niño floods (1972-73, 1982-83, and 1987-88) did not result in the development of a ridge/trough pair, as these floods occur during a dominantly wet period in Texas. Without the proceeding drought sediment needed to initiate the delta progradation is not available.


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