Day 6: Sediment layers are like pages in a book
August 1, 2013
One of the most exciting discoveries that Charlie Paull’s team made while exploring the large scours at the base of Eel Canyon was a series of underwater cliffs and ledges that expose a beautiful sequence of sedimentary strata. We traversed about 90 vertical meters of alternating beds of sand and mud that at a first glance resemble a barcode pattern. But to a geologist, these layers are like pages in a book that tell the story of how underwater currents transported and deposited sediment through deep submarine canyons and onto the ocean floor.
Dramatic underwater cliffs in the Eel Canyon scour.
Repeated layers of alternating sand and mud reveal that these layers were formed by distinct episodes of intense current activity followed by long periods of quiet.
Underwater avalanches of sand and mud (called turbidity currents) flow down slope, and when the avalanche loses energy, it begins to deposit the sediment it was carrying. Because sand settles faster than mud through water, sand is deposited first, followed by a blanket of mud.
The texture and internal structuring of individual beds form “sedimentary structures” that provide important clues to the process of sedimentation. Many of the sandy layers that we observed today formed as migrating ripples on the ocean floor, just as ripples form in rivers and along beaches by flowing water. In some cases, the ripples actually climb on each other’s backs, forming a distinctive and beautiful sedimentary structure called climbing ripple cross-lamination.
The arrow points to a sandstone bed containing climbing ripples. The lighter colored layers are mud and the darker are sand.
Besides being visually striking, climbing ripples are very useful to geologists because they provide a lot of information about the process of deposition.
Because underwater sediment flows are very difficult to observe directly, the deposits exposed at the base of Eel Canyon may hold some critical clues about the dynamics of sediment delivery from rivers to deep ocean basins.
Today we took six vibracores and 28 push cores. This panorama shows many of the push cores extruded onto trays and laid out in the small lab. Krystle Anderson adds one to the pile!
— Glenn Sharman