Sedimentary Structures 2tu44

SEDIMENTARY STRUCTURES

Bedding

Bedding How are they formed? Result of relatively uniform deposition, usually on a flat surface. Bedding planes, Which define the lower and upper boundary of beds, represent breaks in deposition. Thus, a bed may represent a single event of deposition.

Laminations

Laminations How are they formed? Forms in a way similar to beds, but with smaller amounts of sediment resulting to thinner strata. Laminations typically represent short periods or slow rates of deposition.

Graded Bedding - Normal

Graded Bedding - Normal How are they formed? Large amount of sediment with varying grain sizes are deposited to quiet waters. Water allows for coarser sediments to deposit first, which are followed by the finer sediments. This usually happens during underwater landslides, resulting in deposits called turbidites.

Cross Bedding

Cross Bedding How are they formed? Repeated deposition at lee side of dunes results in series of inclined beds. Once buried/overlain by sediments from normal deposition, the observed pattern shows non-parallel planar features.

Mudcracks

Mudcracks How are they formed? Wet muddy sediments experience uneven loss of water as it dries. The upper layer is exposed to the surface which causes it to lose more water and contract. The lower layer remains relatively the same volume which results to uneven tension and produces cracks.

Ripple Marks

Ripple Marks

Ripple Marks How are they formed? Undulations in the unconsolidated sediments are formed as fluid (e.g. water, wind) moves over its surface. Type (asymmetrical vs. symmetrical) depends on the nature of fluid movement

Ripple Marks

Tracks and Trails

Ripple Marks How are they formed? Movement of organisms typically leave impressions on unconsolidated sediments. They are typically preserved on finer sediments.

Bioturbation

Bioturbation

Ripple Marks How are they formed? Organisms dig through the substrate and create borings, which can be filled by latter deposition or by sediments ingested and excreted by the organism.

In Structure 4 1. True or False: Tracks are continuous as opposed to trails. 2. True or False: Laminations always indicate slow sedimentation rates. 3. Deposits formed by underwater landslides are called __________. 4. What type of sedimentary structure is associated with the deposits in #3? 5. What type of ripple marks are found in beach environments?