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A new method of recognition and delineation of ancient stream channels is based on the interpretation of subsurface sedimentary structures. A study of the Columbia Formation (Pleistocene) was conducted in a small area in northern Delaware. Forty holes were drilled through the formation and 486 samples were collected. In the absence of definite correlation between sedimentary units in various holes, and in the absence of any dependable horizon markers, the Columbia sediments have been thought of as divided into horizontal layers spaced 5 ft apart, and referred to their height above sea level.
Primary sedimentary structures (crossbedding in sands and gravels, horizontal bedding in coarse sands, and horizontal lamination in clayey silts) determined from the drilling samples were mapped for each layer separately. This mapping made possible the recognition and delineation of Pleistocene stream channels. The behavior of these ancient streams, interpreted from the maps, is suggestive of low, flat topography, easily eroded banks, shallow and wide channels, frequent change in water and sediment discharges, and flooding. All these are the characteristics of a braided stream system. In spite of frequent lateral shifts of stream courses, the channel bodies of the Columbia Formation are vertically continuous.
The method of subsurface investigation of fluvial sedimentary bodies described here could be applied to the exploration for oil and gas in areas where sufficient well control and sampling are available.
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