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Sand bars which have features and distribution similar to sand bars on modern coast lines are being recognized as oil reservoirs in many parts of the geologic column, and their occurrence is in many widely spaced areas. A knowledge of modern sand bars, therefore, should aid the discovery and development of oil fields in ancient sand bars.
Systems of sand bars are common features of many coasts of the world. They are particularly well developed on the Atlantic and Gulf coasts of the United States. Except for an attachment, commonly at one end, the bars are separated from the mainland by lagoons or marshes, many of which range from ½ to 5 miles in width. The width of some lagoons, however, is 30 miles or more. The length of many bars on the Atlantic Coast ranges from 4 to 8 miles, others from 15 to 30 miles, and on the Gulf Coast from a few to more than 100 miles. The individual bars are separated by narrow water channels called tidal inlets, which connect the ocean with the lagoons. The inlets are kept open by tidal currents which surge in and out through the inlets twice daily.
Longshore currents, which are produced by waves striking the coast at an angle, operate on the seaward side of the bars. They transport sand along the shore, and lengthen the bars at one end. At the same time the tidal currents at the inlets tend to limit the extension of the bars. The result is that the end of an individual bar is extended along shore beyond the end of the next bar on the coast, but slightly seaward of it; thus producing an en echelon arrangement of the bars. Offset features are particularly well developed on the south shore of Long Island, where a westward longshore current has extended the western end of each bar westward beyond the adjacent bar.
Many bars on the Atlantic Coast are characterized by ridges of sand that trend parallel with the shore; the ridges commonly are capped by dunes and are separated by long marsh strips or "slashes." These are growth ridges and are produced by addition of sand to the seaward side of the bars. Growth
ridges are prominent at Cape Henry, Virginia, Bogue Island, Parramore Island, and elsewhere on the Atlantic Coast and at many places on the Gulf Coast.
Beach sands on modern coasts are well sorted within individual beds or laminae. On the other hand, there is considerable range in grain size from laminae to laminae. The beach sands are composed chiefly of grains of quartz but contain minor amounts of a great variety of minerals.
Beach sands on modern coasts are transitory features that are modified by every storm. It is truly remarkable that many similar sand bodies of the geologic past have been so perfectly preserved in the geologic column. A few examples include the so-called shoestring oil sands in the Cherokee shale of Pennsylvanian age in Kansas and Oklahoma. These sand bodies have many features of the modern coastal sands. The length of one system of these sands is 150 miles. Some of the sand bodies in the Cretaceous system in the Denver basin of Colorado and Nebraska, and in the Powder River basin of Wyoming and the somewhat younger Cretaceous sands in the San Juan basin in New Mexico appear to have had a similar origin.
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