Now we know that current velocity, or energy, and sediment size are correlated. High velocities only allow large grains to settle out, and only low velocities can allow for very fine grains to settle out and be deposited. So, how do graded beds form? They must represent a transition from a high to low energy current because they possess a transition from large to small grains.

Turbidity currents are water currents that are heavy because they possess a high sediment concentration. They commonly start as slope failures in shallow marine settings like a lake or ocean margin, then travel down the sloped margin, eventually depositing their loads at the bottom where it flattens out. Sediment layers deposited by these currents are called turbidites.

In this activity, choose from statements about turbidity currents and graded-bed formation as exemplified in the Gigapan image. Read the following statements and select which are true. Specifically, focus on the relationships between grain size and velocities in turbidity currents.

Select the two that apply.

Select the two that apply.

a. Sedimentary rocks like these are made from mud only.
b. The fractures in the rock affected the way in which the sedimentary layers were deposited.
c. The sediments exposed in this outcrop were likely deposited in a single, massive turbidity current.
d. Turbidity currents have high concentrations of sediments, making them heavy enough to travel down slopes coherently, even in marine environments.
e. In the Gigapan image, the sedimentary layers can be described as turbidites that were formed by repeated deposition of sediments from turbidity currents in a marine setting.

e. In the Gigapan image, the sedimentary layers can be described as turbidites that were formed by repeated deposition of sediments from turbidity currents in a marine setting.

Explanation:

This outcrop was formed when turbidity currents repeatedly deposited sediments as individual layers in a marine setting. These layers then lithified into sedimentary rock. The rocks were later slightly deformed and metamorphosed and then brought to the surface by erosion for us to investigate.

Investigating the patterns, textures, and sedimentary structures within sedimentary rocks is one of the best ways to reconstruct past environmental conditions from the rock record. Sedimentary rocks that have undergone little to no deformation, like here, provide some of the most critical pieces of information that allow geologists to reconstruct the history and evolution of Earth’s near-surface conditions.