Abstract

Detailed analyses of the marginal sediments of the Mississippi delta have permitted the development of criteria for recognizing deltaic environments. In order to test the use of these criteria as a means of improving the interpretation of older sediments, 6 borings were jetted into the lower delta to depths up to 45 ft., and the continuous samples along these borings were given intense study. The Foraminifera, the constituents of the coarse fractions, the grain-size parameters, and the sedimentary structures served to diagnose the environments in which most of the sediments were deposited. A boring into the end of Baptiste Collette subdelta showed at least 3 alternations between sound and deltaic deposits. A boring into outer Main Pass, where the water was 26 ft. deep in 1860, showed the advance of the delta over the sound facies. The boring into the end of North Pass is interpreted as showing the muddy fill of an abandoned pass overlying a laminate river-mouth bar, which in turn overlies a probable barrier-island deposit. The other borings into the birdfoot delta pass through deposits characteristic of interdistributary bays and of the platforms bordering the delta margin, but do not reach the clayey foreset beds beyond the delta margin. The borings cast doubt on the concept of bar-finger "sands" forming in front of the advancing deltaic lobes in the birdfoot delta. The sediments encountered were no sandier than those off the present mouths which consist predominantly of silt and clay. The only place where a clean sand was penetrated which could be compared with the thick sand sections shown in lower delta borings by the U. S. Army Engineers yielded sediment differing from that of the present-day river-mouth bars. More borings with detailed studies of the core samples are needed before accepting or rejecting the bar-finger "sand" interpretation.

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