Sedimentary Features

Features that were part of the sediments when they were deposited are often preserved when the sediments become lithified. These features are very useful in reconstructing how the sediment grains were transported, where they came from, the age relationships of different layers, and what the environment was like when the sediments were deposited.

Bedding. Bedding is often the most obvious feature of a sedimentary rock and consists of lines called bedding planes, which mark the boundaries of different layers of sediment. Most sediments were deposited along a flat surface that was roughly parallel with the depositional surface. An exception is cross‐bedding, where sediments are carried over an edge or slope by a strong surge of water or wind, forming steeper layers. Cross‐bedding tends to occur locally within a larger block of rock, and is overlain and underlain by flat‐lying beds. Herringbone cross‐bedding is a distinctive pattern of alternating cross‐bedding directions that is reflective of a rhythmic, high‐energy environment, such as a tidal zone.

Graded beds are common when a sediment is being deposited by a slow‐moving current. The base of the bed consists of coarser material, which settles to the bottom first. The subsequent beds grade upward through sand and silt, to the finest clay sizes at the top. This pattern is typical in submarine turbidity flows, where sediments are dislodged and tumble down an ocean floor slope.

Fossils. Fossils are the remains of plants or animals buried in sediments that were later lithified into rock. They can be extremely useful in determining the depositional environment and the age of the rock. The most obvious fossils are those parts of an organism that have been preserved by being replaced by calcite or silica during lithification. A fossil can also be a cast that formed when the organic remains dissolved, leaving an opening, or mold, shaped like the organism and later filled with calcite or silica. Other types of fossils include tracks, worm trails, feces, and burrows.

Desiccation cracks and ripple marks. Common structures preserved in sedimentary rocks can be seen forming today along beaches and rivers. Desiccation cracks, or mud cracks, develop when a muddy sediment is exposed to air and begins to dry out, creating a polygonal pattern of cracks. Ripple marks are gentle repeated ridges, usually in sand or silt, that are formed perpendicular to the flow of wind or water.