Turbidites and Graded Bedding

April 04, 2016

Turbudites Diagram

Photographer: David K. Lynch
Summary Author: David K. Lynch
A turbidite is a sedimentary formation resulting when sudden underwater landslides send a slurry of gravel, sand and silt down a slope. As the mixture reaches a nearly level seafloor, the flow slows down and rocks begin to settle to the bottom. The first to settle are the largest followed successively by smaller and smaller grains until finally only the smallest, silty particles settle on top. This leads to graded bedding, a formation in which the particles becomes systematically smaller with increasing height.

Graded bedding is commonly seen in sedimentary rocks, but not all of it comes from underwater landslides. Any situation where sediment-laden flows slow down, such as in a flash flood, can produce graded bedding. What makes graded bedding particularly interesting is that it's almost always the result of a sudden event, one with a definite beginning and a more gradual ending. Multiple landslides in the same location can produce successive turbidities layers.

The picture above shows a rock wall in Death Valley National Park near Mesquite Springs. Many turbidites are exposed in the Tertiary sediments and three are highlighted here. They were deposited and lithified millions of years ago when this part of Death Valley was under water. Tectonic activity raised the rocks and a stream cut through them, revealing the turbidites. Photo taken on February 9, 2016.