Earth, Our Environment - Class Notes
Chapter 4, The Cornerstones of Geology: Rocks - 11
Sedimentary Structures
Earth, Our Environment - Class Notes
In addition to grain size and sorting, sedimentary rocks contain structures that provide clues to their origin. For example:
When sediment is transported by currents of water or air, ripples often result (or dunes on a large scale). Sometimes ripple marks are directly visible on the rock. Other times, you can see the structures of the ripples or dunes in a cross-section. The resulting sedimentary structure is called cross-bedding. By studying sedimentary rocks, we can identify rocks that formed in a current and identify the direction and strength of the paleo-current that produced the rock.
Storm deposits contrast with the steady flow of currents. Storms are episodic events and they result is a sedimentary structure called graded bedding. The process is simple - a storm disturbs the shallow sediment, perhaps causing a submarine flow. The sediment is mixed into the water. The larger grains are heavier and settle out of the water first, then the next largest, ... The result is a sedimentary "bedding" with the largest grains at the bottom and the smallest at the top.
Mud cracks can also be preserved in sedimentary rocks and indicate that the sediment formed in shallow water and an arid climate (that was able to dry out).
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