Minor Internal Structures of Some Recent Unconsolidated Sediments

Widespread occurrence of several different minor sedimentary structures in shallow-water Gulf of Mexico sediments has been shown by more than 2,000 cores and bottom samples. Minor internal structures in two areas, one off the east side of the Mississippi River Delta and the other along the central Texas coast, were found to be similar. Deposits in these two areas were subdivided into those containing: (1) regular layers (thin beds or laminations), (2) irregular layers (rough or crude layers and lenses), (3) mottles (discontinuous lumps, tubes, and pockets), and (4) structureless homogeneous sediments, depending on the principal type that occurs. Areal distributions of these different structures in the surficial sediments are mappable. Similar sedimentary structures and sequences of structures occur in the open gulf and in the bays and sounds. In the open gulf the structures occur in wide bands, and the sequences are developed through wide ranges of water depths; in the bays and sounds the bands generally are narrower, and the sequences are compressed into much smaller ranges of water depths. The different types of structures are formed on or near the depositional surface, contemporaneous or nearly contemporaneous with deposition. The differences between depositional environments which produce the various structures are differences in: (1) sediment sources, (2) in physical processes and their intensities, and (3) rates of deposition. Regular layers are characteristic of areas of rapid deposition and/or relatively few bottom-living animals; they are either primary—formed by fluctuations of sediment, or secondary—formed by wave or current winnowing. Irregular layers and mottles are mostly secondary features, formed mainly by bottom-living animals altering existing sediments. Homogeneous deposits are either primary or secondary; they form by extremely rapid deposition, uniform deposition, or by complete secondary reworking. The appearance of many normal marine sediments is due in large measure to burrowing and crawling organisms. Structures similar to those in presently forming sediments of the Gulf of Mexico apparently were developed in rocks of all ages when environmental conditions were proper. Knowledge of how these structures form, why they vary, and the relationships between them furnishes a valuable key for interpreting deposition of ancient rocks.