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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 26 (1976), Pages 298-304

Clay Sedimentation in the Pearl River Delta, Louisiana-Mississippi

Jesse O. Snowden, Gary M. Forsthoff (1)

ABSTRACT

The Pearl River, the nearest large river to the modern Mississippi delta, is beginning to deposit fine sediment over the northern margin of the abandoned St. Bernard delta of the Mississippi River. In contrast to the Mississippi, the Pearl River drains only Tertiary and Quaternary sedimentary rocks in Mississippi and Louisiana.

Numerous X-ray diffraction analyses of suspended sediment and river channel cores show that, except in its lower delta, the Pearl River is characterized by a clay mineral suite that has nearly equal amounts of kaolinite and smectite, with lesser amounts of illite, usually less than 10 percent. In the lower delta, as the Pearl River water begins to mix with saline Gulf of Mexico water, there is an increase in the kaolinite content of the channel sediments. However, after this initial increase, the percentage of kaolinite then decreases steadily seaward, until it is only 30 percent of the clay fraction at the mouth of the river. This decrease in kaolinite is accompanied by a proportional increase in smectite. The suspended clay likewise shows a decrease in kaolinite toward the mouth of the river, except during periods of unusually high fresh water runoff. However, the top water suspended sediment begins its decrease much farther downstream, reflecting the salt-wedge effect.

We believe that the changes in clay mineralogy in the Pearl River delta are the result of differential flocculation and settling of kaolinite in the brackish water of the lower delta. The result of this differential settling is smectite contents in the lower deltaic sediments that are very close to those of Mississippi River sediments. However, the low illite content of the Pearl River sediments may still be used to differentiate between the two.


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