Question

What sediments compose a deep-sea fan, and what sediments compose an alluvial fan? If you found a strata of one of these, how could you distinguish between them? What is their main physiographic distinction?

Short answer

Deep-sea fans contain fine-grained sediments from turbidity currents, while alluvial fans have coarser sediments from rivers. They differ in location: deep-sea fans are underwater, alluvial fans are on land.

Step-by-step solution

Understand Sediments in a Deep-Sea Fan

A deep-sea fan, which extends outward from the continental slope into the ocean basin, is primarily composed of fine-grained sediments such as fine sands, silts, and clays. The origin of these sediments is often from turbidity currents, which carry material from the continental shelf.

Understand Sediments in an Alluvial Fan

An alluvial fan is a cone-shaped deposit found at the base of mountain ranges where a river deposits sediments as it flows out onto a flatter valley. The sediments in an alluvial fan are typically coarser, including gravel, sand, and larger pebbles, due to the high energy associated with their deposition.

Identify Strata Characteristics

To distinguish between strata from a deep-sea fan and an alluvial fan, look at the grain size and sorting of the sediments. Deep-sea fan sediments will be finer-grained and better sorted than those in alluvial fans, which have coarser, less sorted sediments. Additionally, sedimentary structures in alluvial fans may include cross-bedding and imbricated clasts.

Physiographic Distinction

The main physiographic distinction between a deep-sea fan and an alluvial fan is their location. Deep-sea fans are located under the ocean, extending from the slope into the ocean basin, while alluvial fans are found on land at the base of mountains where a river exits the mountainous area.

Key concepts

Deep-sea fan

Deep-sea fans are fascinating geological features found in marine environments. They resemble underwater deltas and are located at the edges of continental slopes that plunge into the deep ocean basin. These fans are primarily built from fine-grained sediments like fine sands, silts, and clays.

These sediments are transported by powerful underwater flows known as turbidity currents. These currents originate on the continental shelf and cascade down the slope into the abyssal plains, depositing materials as they go. As a result, deep-sea fans are characterized by their fine sediment composition and distinctive fan-like shape, spreading out across the ocean floor.

Alluvial fan

Alluvial fans are found at the transition zone where river streams leave mountainous regions and spread out into flatter valleys. These geologic formations are cone-shaped, often visibly protruding from the base of mountain ranges. An alluvial fan is built from coarser sediments compared to its deep-sea counterpart.

The high-energy environments from which alluvial fans are derived mean that they typically contain a mix of gravels, sands, and larger pebbles. These materials are deposited by swift river flows that slow down when they reach the open valley floor, dropping their sediment load in a fan-shaped deposit. This coarseness and the presence of cross-bedding and imbricated clasts make alluvial fans distinct in their sediment characteristics.

Physiographic distinction

The primary physiographic difference between deep-sea and alluvial fans lies in their location. Deep-sea fans are part of the underwater landscape, forming at the junction of continental slopes and ocean basins. These structures extend from the continents into the much deeper parts of the sea, often covering vast areas of the ocean floor.

Conversely, alluvial fans are terrestrial features situated at the base of mountain ranges. Here, rivers transition from narrow mountain channels to broader valleys, spreading sediments in a fan-like pattern. Recognizing these geographical and environmental settings is essential when distinguishing between these two types of fans.

Turbidity currents

Turbidity currents are the primary mechanism for sediment transport in deep-sea fans. These are fast-moving, density-driven flows that carry suspended sediments down the continental slope. As they rush into the deeper ocean, they can transport vast amounts of material, contributing significantly to the formation and expansion of deep-sea fans.

Turbidity currents are initiated by disturbances like earthquakes or underwater landslides, which dislodge sediments on continental shelves. As these sediments are deposited on the ocean floor, they create layers that are finer and more sorted compared to those in terrestrial alluvial fans. Understanding these currents helps explain why deep-sea fan sediments differ so much from the coarser materials found in alluvial fans.