Sedimentary Rock Origins

Clues to the Origin of Sedimentary Rocks

• Sedimentary rocks preserve a record of history because of the way they form, comprising bits of pre-existing rocks or minerals and sometimes organic material like plant and animal remains.
• These sediment grains are moved by natural forces (water, ice, and wind) and settle in a new location, forming layers of sediment (strata) that build up over time and get compressed and cemented together to form rock.

Stratification

• Stratification is the layering found in sedimentary rock, with different types of sediment deposited on top of each other.
• Changes in color between layers provide clues about the rock, indicating different minerals and origins from different rocks or locations.

Graded Bedding

• Graded bedding is a sediment deposit characterized by a detectable sorting of grain size from bottom to top.
• Large, coarse sediment grains are deposited at the base of the layer, while small, fine sediments are near the top.
• This arrangement indicates that a current of water loaded with sediment rushed into place, allowing larger and heavier particles to settle out first, followed by lighter particles as the flow lessened.

Cross-Bedding

• Cross-bedding is layering within a rock that does not run parallel to the original horizontal layers.
• This suggests that the sediment was deposited on a slope or an angle, indicating an environment like a beach, river, or sand dunes.
• Forces like wind and water cause the erosion and redepositing of sediment grains in piles, which eventually become unstable and cascade downslope at an angle.

Ripple Marks

• Ripple marks are small wavy ridges of sediment caused by the action of waves or wind.
• These wavy ridges remain after the sediment hardens into rock, preserving the ripples.
• Ripple marks indicate that the rock originated in a stream bed, lake, or on a beach, and can provide information about the direction of water or wind currents.

Mud Cracks

• Mud cracks are multi-sided structures produced in fine-grained, muddy sediments that dry and shrink.
• They form in environments that go through periods of wetting and drying, such as marshes, seasonal rivers, and lake shores.
• Mud cracks fill with other types of sediment, preserving the cracked appearance within the rock and revealing that the sediments were deposited in an environment that was once wet and then dried.