Questions: Submarine Canyon Sediment Transport and Gravity Flows

Question 1

A sediment core from a deep-sea fan shows alternating beds. One bed has coarse sand at the bottom, grading upward through silt to a clay cap. What does this graded structure indicate?

• A. A slow, continuous rain of particles settling by size from a calm water column
• B. A turbidity current that deposited sediment as it decelerated, with coarser particles settling first
• C. A storm event that transported only fine material into deep water
• D. Bioturbation mixing sediment layers over thousands of years

Answer: (B)

Graded bedding — coarse at the base, fining upward — is the diagnostic signature of a turbidite, deposited by a turbidity current. As the dense sediment-laden flow decelerates, the heaviest particles settle first (sand), followed by silt, then clay. This is not slow pelagic settling (which produces uniform fine sediment throughout) nor bioturbation (which destroys grading rather than creating it).

Question 2

Scientists observe that submarine cables snapped in a specific sequence after an earthquake off the Grand Banks, with cables farther from the canyon head breaking later. What does this sequence demonstrate?

• A. Earthquakes propagate downslope as seismic waves, damaging cables sequentially
• B. Turbidity currents travel as dense, fast-moving flows that can be tracked by their sequential cable breaks
• C. Deep-sea currents travel at uniform speeds in all directions
• D. Canyon-parallel fault lines caused the sequential failures

Answer: (B)

The sequential cable-break timing is direct observational evidence for turbidity currents. By calculating the distance between cables and the time elapsed between breaks, researchers computed flow speeds exceeding 10 m/s — faster than an Olympic sprinter. This is empirical proof that turbidity currents are real, directional, and fast, not just theoretical constructs.

Question 3

A turbidite deposit records one past turbidity current event, and stacked turbidites in a sediment core provide a geological archive of canyon activity.

• T. True
• F. False

Answer: True

Each turbidite bed represents a single flow event: the graded sequence (coarse to fine) is deposited by one turbidity current as it decelerates. Repeated events stack vertically, so a sediment core with multiple turbidite beds is literally a stratigraphic record of past events — earthquakes, storms, floods — over thousands to millions of years. This is one of the primary tools for reconstructing ancient sediment transport history.

Question 4

Submarine canyons maintain a continuous, steady sediment flow from the continental shelf to the deep ocean, similar to how rivers constantly transport sediment.

• T. True
• F. False

Answer: False

Unlike rivers, most submarine canyons are episodic rather than continuous. Turbidity currents are triggered by discrete events — earthquakes, storms, river floods — and then the canyon may be quiet for decades or centuries between events. The common misconception that canyons function like underwater rivers with steady flow overlooks the event-driven, catastrophic nature of gravity-flow processes. Even heavily supplied canyons (e.g., those near river mouths) experience episodic surges rather than constant transport.

Question 5

Why do turbidity currents accelerate and maintain high speeds over long distances once initiated, rather than slowing down immediately?

Model answer: Turbidity currents are self-sustaining because the dense sediment-laden mixture is denser than the surrounding clear seawater, so gravity continuously accelerates the flow downslope. As the flow moves, turbulence keeps sediment suspended; the flow remains denser than ambient water and continues accelerating. The flow only decelerates when the slope decreases (on the abyssal plain) or when energy dissipation finally exceeds gravitational input.

This is the key to understanding why turbidity currents can travel hundreds of kilometers. They are density-driven — the suspended sediment makes the mixture significantly denser than ambient seawater. As long as the slope provides a gravitational component and turbulence maintains the suspension, the flow is self-perpetuating. This differs from a simple landslide that runs out of momentum; here the density contrast with ambient water provides continuous driving force.