11.6: Sedimentary Structures
Sedimentary Structures
Sedimentary rocks often show distinctive patterns, called sedimentary structures Links to an external site., that reflect events or conditions during deposition. These patterns in the rocks can be very informative to geologists attempting to reconstruct the environment in which a sedimentary rock was formed. Many sedimentary structures are useful as “up-indicators”, meaning geologists use them to interpret where the surface of the Earth was in the geologic past. This is particularly useful if the layers have been deformed, bent or broken through tectonics.
Most students are familiar with ripples and dunes (mega-ripples). We would typically see these structures at the beach or in deserts. Ripples and dunes form as a result of a wind or water current. Current or asymmetric ripples Links to an external site. form when a dominant current, which moves in one direction, is present. The current pushes sediment into a pile: on the down-current side the sediment is shadowed and protected from the wind or water current; however, erosion on the up-current side results in a shallow slope and deposition on the steeper, down-current side. If you cut a ripple or dune in half and look at it in profile, you will see inclined layers of sediment building up on the steep down current side of the ripple (Figure 11.11).
Figure 11.11: Modern current ripples forming in Cadiz Dunes Wilderness area in San Bernardino County. Bottom inset image: a cross-section of an asymmetric ripple. In both images the dominant current (wind) moves from right to left. (Public Domain; Bob Wick/BLM Links to an external site.; inset image, CC-BY 4.0, Chloe Branciforte)
We often see multiple layers of beds consisting of these inclined layers, called cross-beds Links to an external site., which represent multiple generations of migrating ripples or dunes. Both ripples and cross-beds indicate the presence and direction of the current in an environment (Figure 11.12). Visit this site Links to an external site. to see cross-beds on a sample from the Precambrian Johnnie Formation in southeastern Death Valley.
Figure 11.12: Formation of cross-beds as a series of ripples or dunes that migrate with the flow. Each ripple advances forward (right to left in this view) as more sediment is deposited on its leading face. (CC-BY 4.0; Steven Earle Links to an external site.)
Wave-formed, symmetric, or oscillation ripples Links to an external site. are commonly found in shallow waters, beaches, or tidal flats Links to an external site. where water oscillates back and forth with wave-action. In cross-section, these ripples are symmetric and have long relatively straight crests (Figure 11.13).
Figure 11.13: Modern oscillation ripples forming near the Ventura Pier in Ventura, California. (CC-BY 4.0, Chloe Branciforte, own work)
Mud cracks Links to an external site., covered and preserved cracks that are the result of the drying of wet mud, and raindrop impressions Links to an external site., covered and preserved impacts of raindrops in soft mud, are both sedimentary structures that inform us about the sediment, water content, and exposure at the surface above water level.
Some sedimentary structures indicate changes in the strength of a current. Imagine a fast-moving current carrying a variety of sediment sizes; if the current slows, it will no longer be able to carry the largest particles and they will be deposited first. Then, as the current continues to slow, progressively smaller particles are deposited on top of the bigger particles, forming a sedimentary deposit called a graded bed Links to an external site.. This graded bed is a sedimentary layer with larger clasts on the bottom and smaller clasts on the top. These types of beds can be the result of floods in a river, storms in the ocean, or turbidite flows Links to an external site..
Related to sedimentary structures are trace fossils Links to an external site., which are patterns in the rocks that are caused by the activity of organisms. Trace fossils indicate different aspects about the environment depending on the trace and the identity of the tracemaker. Traces can be terrestrial (e.g. footprints, burrows or dens, or the traces of roots) and can inform us about the climate, ecosystem, and the development of soils. Traces can also be found in freshwater and marine environments (e.g. burrows, borings, footprints, or feeding traces) and tell us about the sediment, chemistry of the pore water, and the life that lived within it. Visit this site Links to an external site. to see trackways from the Pliocene of Death Valley National Park.