Subsurface geological investigation of a Pleistocene braided stream in the northern Coastal Plain, Delaware (U.S.A.)

Primary sedimentary structures and sedimentary textures of the Pleistocene fluvial sediments of the Columbia Formation were investigated both in the outcrops and in the subsurface. Three hundred and thirty-one subsurface samples, taken from forty holes, and fifty-six outcrop samples were analysed in the laboratory and the results were statistically treated. Unknown sedimentary structures were recognized in the subsurface by correlating some textural parameters (mean grain size, skewness, and kurtosis) of the drilling (subsurface) samples with those of the outcrop samples where the sedimentary structures were known. The utilization of both the sedimentary structures and the textures made it possible to recognize major morphologic elements of a Pleistocene stream system (channels, channel bars, floodbasins, and channel-fills) in the subsurface and to reconstruct, in a general way, the events that occurred during the existence of the stream system. It seems that in the initial stages of the Columbia deposition, the streams were confined to the valleys in the underlying surface made up of the Palaeocene-Eocene glauconitic greensands. After the valleys were filled with the Pleistocene sediments, the streams were able to shift their channels freely throughout the study area; at least two new tributary streams were added to the system and a major flood occurred during that time. Although the technique of the recognition of primary sedimentary structures in the subsurface employed in this study has shown useful in the interpretation of the fluvial sedimentary bodies, it needs further testing before it can be accepted as a new geologic tool.