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Climate Forcing in a Wave-Dominated Delta: The Effects of Drought–Flood Cycles on Delta Progradation

¹ ExxonMobil Upstream Research Company, P.O. Box 2189, CORP-URC-S181, Houston, Texas 77252-2189, U.S.A.; carmen.m.fraticelli{at}exxonmobil.com

The modern Brazos Delta is composed of a series of ridge–trough pairs reflecting episodic delta growth. Previous work has shown that these features represent remnants of channel mouth-bar and back-bar lagoon deposits formed as a consequence of major floods. Subsequent reworking by wave action welds these units to the western flank of the delta plain, forming the next ridge–trough set. Recent work however, has brought to light a primary climate component in the development of these facies, making development of ridge–trough pairs a more complex process than the previous model suggests.

Climate in Texas is characterized by extremes, many of which can be attributed to El Niño and La Niña cycles. El Niño events result in dramatically higher precipitation and flooding, and La Niña events trigger periods of extreme drought. The development of an emergent channel-mouth bar is correlated to high-sediment-yield floods induced by El Niño events. However, only five ridge–trough sets exist on the delta plain and an estimated 20 El Niño events have occurred since 1929, when the delta began to form, indicating that the flood criterion alone is not sufficient to initiate delta progradation. Each of the five episodes of rapid delta growth occurred when a period of extensive drought preceded an El Niño-induced flood. The droughts, associated with La Niña events, were instrumental in removing vegetation and pre-conditioning the drainage basin for erosion. Without a preceding drought, the floods are ineffective in generating sufficient sediment to create a ridge.