Bedding is the most basic sedimentary structure that can be observed at an outcrop. Bedding surfaces represent a discontinuity between sedimentary units and are indicative of a change of some type in the deposition of the sediment. Often the type of change is uncertain. Most bedding surfaces are diastems, gaps in time that can be as short as a day or as long as thousands of years and represent a period of no sedimentation.
Two basic types, non-directional structures and directional structures characterize other sedimentary structures. When looking at these structures it is very important to note every detail about them as they can give you much information about he depositional environment in which the sandstone was laid. There have been a large number of both types of structures described in many geological journals, books, and papers. I will only cover a couple of the most basic structures that are common to most sandstones.
Non-directional structures often include graded beds and bioturbation. These structures can tell you much about the environment in which the sandstone or conglomerate was deposited, but cannot tell you about the direction of the current or flow. Graded beds are layers of sediment that are marked by a change in grain size from the base of the bed to the top. Usually this grading is coarse at the base and finer at the top, but occasionally you might find an outcrop where the reverse is true. These structures often are interpreted to represent turbidity currents or flows. A turbidity flow is a single mass of debris that contains all sizes of grains that flows rapidly down slope. As the flow reaches the base of the slope, the largest grains fall out, followed by smaller and smaller grains.
Bioturbation is the activity of bottom-dwelling organisms that tend to burrow and crawl through the sediment to feed. Bioturbation destroys many of the bedding structures, but at the same time creates a variety of unique sedimentary structures. Many of these structures are only found in certain zones of the continental slope and are useful as indicators of paleoenvironments. Generally these structures best indicate the depth at which they were formed, shallow water structures tend to have a vertical orientation while deep water structures are characterized by traces along bedding planes.
Directional sedimentary structures are grouped into two categories, bi-directional and unidirectional. From their names you can tell that bi-directional structures offer indications of flow in two directions, unidirectional in only one direction. The two most common bi-directional structures are groove casts and parting lineations. Groove casts are generally long, straight structures on the base of sandstone beds. They are thought to be made by objects being dragged along the surface of the bed. Parting lineations are very faint, linear grooves and ridges with slight relief.
