--> ABSTRACT: Mississippi Delta-Lobe Switching During Holocene Eustatic Fluctuations, by Rhodes W. Fairbridge and Allen Lowrie; #91030 (2010)

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Mississippi Delta-Lobe Switching During Holocene Eustatic Fluctuations

Rhodes W. Fairbridge, Allen Lowrie

Delta formation plays an integral role in basin development at a passive continental margin, with depocenters under eustatic control shifting alternately landward and seaward during time frames ranging from decades to 108 years. The classic Gilbert delta model was hydraulic and climatogenetic, based on his Lake Bonneville experience. We challenge the Gilbert model applied to the Mississippi delta, in that the model requires synchronous climatic fluctuations over a 3,327,000 km2 drainage basin.

From the Mississippi delta apex to the Gulf of Mexico, the dynamic gradient is 1:55,000, although the direct slope, over ^approx 110 km, is 1:20,000. Holocene sea levels fluctuated 1-2 m. During cool periods, as in the Little Ice Age (Maximum AD 1650-1750), sea level dropped 0.5-1 m, changing the dynamic and direct gradients to 1:50,000 and 1:18,000, respectively. During warm periods, as in the Viking times 1,000 years ago, sea level rose 0.5-0.6 m, changing the gradients to 1:78,000 and 1:28,000, respectively. Such large gradient changes increase the opportunities for river entrenchment and stream stability during cool periods and delta-lobe switching during warm periods. Available radiometric dates for the 16 individual delta lobes developed since 6,000 Ma concentrated in six delta omplexes and revealed that the 1,000-1,500 yr delta-switching cyclicity roughly coincides with concurrent glacial advances and retreats. C14 dates of peats indicate regressions, comparable to those of the North Sea. Gaps in peat-derived dates indicate transgressions.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.