Finding Ancient Shorelines
For many years the primary function of the Research Committee of the Society of Economic Paleontologists and Minerologists was to organize a symposium concerned with the most pertinent problems in sedimentary geology. This year (1955) the main theme centered on the characteristics of near shore deposition, particularly aimed toward recognition of ancient shorelines. The chairman that year was Dr. Henry W. Menard of the Navy Electronics Laboratory. Committee members selected the topic for the symposium, aided in selection of the speakers, and often participated in the program. The plan was to ask each speaker to present his paper in twenty minutes. Following the formal presentation a discussant, who has had opportunity to examine certain parts of the paper, was asked to comment. This approach led to some lively and focussed comments which benefited the entire presentation and is very interesting to read in that context. Papers included: Continental terrace sediments in the northeastern Gulf of Mexico, Sediments of the eastern Mississippi Delta, Sandless coastal terrain of the Atchafalaya Bay area, Louisiana, Sediment zones bordering the barrier islands of central Texas coast, Dynamic geology of the modern coastal region, northwest Gulf of Mexico, Particle size distribution in nearshore sediments
The Mississippi Delta is building because the rate at which sediments are supplied is faster than the combined effect of the rate at which they are removed by waves and currents and the rate the delta area is subsiding. River load is discharged into a variety of depositional environments in the Gulf of Mexico. These environments are inter-related, gradational, and persist both laterally and through some depth of water. Sedimentary deposits being formed in these environments have different lithologic and petrographic characteristics but also are related and gradational. Differences between environmental factors and processes produce the lithologic and petrographic differences and also produce correlative differences in depositional rates. Continuing delta growth causes seaward migration of these environments and leads to the formation of deposits, or sediment units, of large areal extent which bear some predictable relationships to each other.
Sedimentary units which are almost entirely the products of river-borne material in order of seaward occurrence are: (1) “marsh deposits,” (2) “delta front silts and sands,” (3) “prodelta silty clays,” and (4) “offshore clays.” These units are deposited approximately concentric to distributary mouths and are analogous to the topset, forset, and bottomset beds of classic delta description. Their characteristics depend to a large extent on the distance from source and depth of water. Another unit, “marginal deposits,” generally occurs between the deltaic clays and outlying sands from marine sources. Marginal deposits have characteristics transitional between the clays and the outlying sands.