THE EFFECTS OF ENSO CYCLICITY ON DELTAIC PROGRADATION: EXAMPLES FROM THE MODERN BRAZOS DELTA

The modern Brazos Delta plain is made up of a series of ridge/trough pairs that reflect episodes of climatically-induced delta progradation. The ridge-trough pairs represent the remnants of channel-mouth bar and back-bar lagoonal deposits. Current models invoke a combination of flood and wave processes as the primary drivers in the development of these facies. However, recent work has shown a strong climatic component to the episodes of delta growth and has revealed that the flooding process alone is not a primary driver.

The climate of Texas is characterized by extremes of droughts and floods. These drought-flood cycles can be correlated to shifts between El Niño and La Niña events, which significantly impact precipitation patterns in Texas. El Niño events result in dramatically higher precipitation and, as would be expected, the floods associated with channel-mouth bar development can be correlated to El Niño events. However, only 5 of ridge/trough pairs exist in the Brazos Delta, and an estimated 20 El Niño events have occurred since the delta began forming in 1929, so that the flood criteria alone is not sufficient to explain the development of this unit. Instead, each episode of rapid growth in the Brazos Delta was initiated by a unique combination of an extended period of drought, associated with La Niña, which was subsequently broken by an El Niño induced flood. The preceding droughts are instrumental in removing vegetation and pre-conditioning the drainage basin for erosion, without which delta progradation does not occur.