#turbidites

[happy-geology]

[earthstory]

Submarine canyons

The waters of the Earth’s oceans hide many fascinating features. This image shows the topography offshore of the city of Los Angeles.

Like many areas of the world, the area just offshore is shallowly sloped and part of the continental crust, an area known as the continental shelf. That continent ends at a sharp break known as the continental slope, which slopes more steeply down toward the open ocean. At this boundary off Los Angeles, giant canyons have been cut. These are remnants of the recent glaciations. Sea level was lower, exposing parts of the continental shelf. Rivers including the San Gabriel River and the Santa Ana River migrated along these shelves, eroding into them and fixing their position in the form of these canyons. When the most recent glaciation ended, the oceans returned and submerged the canyons, leaving them stranded and hidden from view.

Despite being beneath the waters, these canyons still are active. Sediment carried by rivers to the ocean accumulates at the canyon heads and on occasion that sediment will tumble down, forming submarine avalanches and deposits of turbidites at the base. Similar canyons are found at the intersection between river valleys and the continental slope around the world, a remnant of the Pleistocene glaciations.

-JBB

Image credit: http://commons.wikimedia.org/wiki/File:Canyons_off_LA.jpg

Read more: http://www.nrdc.org/oceans/canyons/ http://www.britannica.com/EBchecked/topic/570847/submarine-canyon http://sanctuarysimon.org/regional_sections/submarineCanyons/overview.php

[studying-geology-blog]

outcrops of the Smaull Graywacke of the (Neoproterozoic) Colonsay Group. These are turbidites (deposits of submarine “avalanches”) that were later folded and cleaved during the end-Ordovician Caledonian Orogeny (equivalent of east coast North America’s Taconian Orogeny). #geology #geologyrocks #geologylife #sedimentology #turbidites #amazing https://www.instagram.com/p/BsV8glChFSr/?utm_source=ig_tumblr_share&igshid=18h4469y1so3n

[was-a-mistake]

Turbidites were a mistake.

[earthstory]

Turbidites! These rock layers may not look that distinctive, but to geologists these tell a really cool story. These are turbidites, the remnants of debris flows off the shore of an ancient ocean. Turbidites form when sediment piles up just off of a shoreline, often carried to the area by a river. Eventually, even underwater, big enough piles of sediment will collapse and avalanche downslope. Sometimes they do so under their own weight, sometimes an earthquake will set them off. The avalanche of debris produces a recognizable pattern to geologists. The heaviest particles, the biggest grains, settle out at the bottom of the debris flow, and the sequence “fines upward”, meaning the grain sizes get smaller. A typical turbidite will start at the bottom with sandy grains, maybe even larger stuff, and the grain size will decrease going upward as progressively finer grains settle out. Finally, each turbidite is topped by a layer of very fine grained clay particles that can even be a different color from the stuff below it. This sequence even has a name – the “Bouma” sequence. Turbidites show up throughout the geologic record because they’re easily preserved. They form in areas in the ocean that aren’t likely to be eroded and they form in areas with lots of sediment that can bury and protect them afterwards. This sequence photographed here is about 10 separate turbidites; the whole outcrop probably has a lot more. -JBB Image credit: Brian Romans (Creative commons): https://www.flickr.com/photos/bromans/4969233953 Read more: https://courses.washington.edu/sicilia/pdf/JBturbidites_fans.pdf http://trg.leeds.ac.uk/

[functionswithoutfunctions]

Thin bedded turbidites Zumaia, Guipuzcoa

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[guitar-and-pick]

Turbidites - Castle Point, New Zealand

[rockon-ro]

Overturned marine turbidite deposits of the Jurassic Fernie Formation near Banff, Alberta, Canada. The photos show interbedding between the lighter coloured sandy event deposits and the darker shaly deposits. The beds are structurally overturned and younger sediments are to the right.