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Elongate, lenticular sand bodies, termed bar fingers, characterize the Mississippi birdfoot delta sedimentary complex. They underlie the 15-20-mile-long principal distributaries or passes of the river which radiate from Head of Passes, and have formed in response to long continued distributary-mouth-bar deposition. These sand bodies attain a thickness of more than 250 feet and a width of as much as 5 miles. Their thickness results in part from subsidence brought about by compaction of underlying clays, and the width beneath a given pass is comparable to that of the actively forming distributary-mouth bar. The sand bodies comprise beds of well sorted, fine to very fine sand or silt and occasional thin layers of clay and silty clay. Diagnostic features are numerous cross-be ded thin layers in which the principal elements dip gulfward, laminae composed of root and wood fragments, sand-size grains of lignite, and an absence of faunas. The bar-finger sands grade downward into delta-front silty clays which rest upon deeper marine prodelta clay deposits. Laterally they interfinger with extensive thick clay sections which accumulated in delta-front, bay, and marsh environments. The sands are transitional with overlying natural-levee, marsh, and bay deposits. Locally, the bar fingers have been disturbed by the upward movement of clays from underlying deposits to form mud lumps.
Bar fingers are distinguished from other sand bodies in the Recent deposits of the Mississippi deltaic plain by their greater thickness and their pattern of distribution. Their ancient counterparts have been recognized in the Pennsylvanian Booch sands of Oklahoma. Abnormally thick sand masses present locally within the Eocene Wilcox and Sparta formations of Louisiana may also be deltaic bar-finger sands.
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