Turbidite

Sand and mud slide down steep underwater slopes.

This slide moves like a big wave of mud. It carries much sand into the deep ocean.

As it slows down, the heavy sand lands first. Then, the soft mud lands on top.

This makes many thin layers in the sea. These layers can even hold gold!

Scientists study these layers to learn about Earth.

Imagine a giant slide under the sea.

Sometimes, sand and mud slide down steep slopes. This is called a turbidity current. It is a thick, heavy flow of water and sediment. This flow moves much faster than a normal river. It carries heavy rocks deep into the ocean.

As the flow slows down, it leaves layers behind. The heavy sand lands on the bottom first. Then, the medium sand settles. Finally, the fine mud lands on top. A scientist named Arnold Bouma studied these layers. He found they follow a set pattern. We call this a Bouma sequence.

These layers are very important to us. They can tell us about old earthquakes. They can even hold gold! In Australia, people found much gold in these rocks. Over 2,600 tons of gold came from them. These layers can also hold oil. This helps the petroleum industry find new energy. Scientists use many tools to study these deep sea fans.

A turbidite is a special kind of rock layer. It forms from a turbidity current. This is a heavy flow of water and sediment.

These flows happen in the deep ocean. They can also happen in deep lakes. They often move down steep slopes near the edge of a continental shelf. These underwater avalanches carry huge amounts of material. They move sediment from shallow areas into the deep ocean troughs. This process helps move sand and mud across the seafloor.

How does this flow work? It is different from a normal river. In a river, water pulls rocks along the bottom. This is called tractional flow. A turbidity current uses density-based flow instead. The water becomes a thick slurry of fine particles. This makes the liquid much heavier than plain water. Because the liquid is dense, it can carry large rocks easily.

As the current slows down, the materials settle in layers. The heavy sand and pebbles land first. Then, the medium sand settles. Finally, the very fine mud lands on top.

In 1962, a scientist named Arnold H. Bouma studied these layers. He looked at deep-water sediments in the ocean. He noticed a strange pattern in the rocks. Some layers started with big pebbles and ended with fine mud. This was a surprise to scientists at the time. They thought the deep ocean was too still for heavy rocks.

Bouma described this vertical pattern of changing layers. We now call this a Bouma sequence. It shows how the water flow gets weaker over time.

Scientists have found many different types of these sequences. A complete Bouma sequence includes pebbles, sandstone, and shale.

Some flows are low-density and follow Bouma's pattern. Other flows are high-density. These high-density flows create a different pattern called a Lowe sequence. These massive deposits can build huge shapes called submarine fans. There are about 26 different models used to describe these fan systems. These models help experts understand how sediment moves from a source to its final home.

Turbidites are very useful to people today. They act like a history book for the Earth. They can show us when old earthquakes or big storms happened. They can also be very valuable. In Victoria, Australia, people found gold in these rocks. They extracted more than 2,600 tons of gold from them.

These rock layers can also hold oil. The petroleum industry studies them to find new energy sources. Knowing where these layers are helps them find fuel more efficiently.

A turbidite is a geological deposit created by a turbidity current. These currents are complex mixtures of fluid and sediment gravity flows. They act as a major system for distributing vast amounts of clastic sediment throughout the deep ocean.

This process typically occurs in deep ocean troughs located below the continental shelf. It can also happen in similar structures within deep lakes. These deposits are vital for scientists because they help identify ancient tectonic and depositional settings. They often indicate that rocks formed offshore of a convergent margin.

To understand a turbidite, one must understand how density-based flow differs from tractional flow. In a normal river, water moves particles through frictional drag, known as tractional flow. In that system, water must reach a specific velocity to suspend a particle. The larger or denser the particle, the faster the water must move to carry it. However, turbidity currents rely on density-based flow. This occurs when sediment liquefaction causes a change in the fluid's density. The water becomes a highly turbulent slurry filled with fine particles. This mixture is much denser than plain water, allowing it to transport large rock fragments even at lower velocities.

When a turbidity current reaches a resting point, the materials settle in a specific order. This process creates a vertical succession of layers known as a Bouma sequence. The sequence begins with an erosional contact, often featuring coarse pebble or granule conglomerate. As the flow loses energy, it grades upward through coarse and medium plane-parallel sandstone. This is followed by cross-bedded sandstone and rippled sand or silty sand. Finally, the sequence ends with laminar siltstone and shale.

This pattern represents a flow regime that is transitioning from strong to waning.

Arnold H. Bouma first properly described these sequences in 1962. He studied deep-water sediments and identified anomalous "fining-up intervals." These intervals were strange because they began with coarse pebbles and ended in fine-grained shales. Before his work, scientists assumed the deep ocean lacked the mechanism to carry coarse sediments into abyssal depths.

Bouma's research proved that these gravity-driven flows were responsible for such deposits. His work changed how geologists interpret deep-water facies and sedimentary structures.

Not all turbidites follow the same pattern. The Bouma sequence specifically describes low-density turbidity currents. In these flows, grain-to-grain collisions create dispersive pressures that hinder settling. However, when the sand concentration increases, high-density turbidity currents form. These create a different set of sedimentary structures known as the Lowe sequence.

These high-density flows can lead to the formation of massive submarine fans. Scientists use various models to study these fans, with approximately 26 different models currently recognized. These models help explain how sediment moves from a source to a sink.

Submarine fans are large accumulations of turbidite deposits. These systems are often subdivided into upper, mid, and lower fan sequences. Each section has unique sand-body geometries and lithologic characteristics. The development of these fans is influenced by both allogenic and autogenic factors. Allogenic factors include changes in sea level, tectonic events, and sediment supply rates. Autogenic factors involve the seafloor topography and the steepness of the slope.

By studying these fans, researchers can better understand the architecture of deep-sea environments.

Turbidites are highly significant for both science and industry. They serve as high-resolution records of Earth's history. In lakes and fjords, they can provide evidence of the frequency of landslides or earthquakes. By dating the material above and below a turbidite, scientists can track seismic events. Beyond history, they have immense economic value. In Victoria, Australia, saddle-reef deposits in Cambrian-Ordovician turbidites have yielded more than 2,600 tons of gold. Additionally, lithified turbidites often serve as hydrocarbon reservoirs. The petroleum industry studies these bodies to predict their shape and location for efficient energy extraction.