Question

Contrast the patterns of occurrence of sediments and sedimentary structures in the following three kinds of depositional cycles: the kind produced by meandering rivers, the kind produced by deltas, and the kind produced by turbidity currents.

Short answer

In conclusion, the three depositional cycles: meandering rivers, deltas, and turbidity currents exhibit different patterns of occurrence of sediments and sedimentary structures. Meandering rivers show a fining upward sequence with structures like point bars and levees, while deltas display a coarsening upward sequence with foreset, bottomset, and topset beds. Turbidity currents present Bouma sequences with fining upward patterns and structures like graded bedding and flute casts. Recognizing these distinct patterns allows for better interpretation of the sedimentary record and depositional history in different geological settings.

Step-by-step solution

Introduction to Depositional Cycles:

Depositional cycles are the result of changes in sediment supply, rates of subsidence, and sea level fluctuations. Each depositional environment forms its characteristic pattern of sediments and sedimentary structures. In this exercise, we will discuss the characteristics of meandering rivers, deltas, and turbidity currents and compare the patterns of sediments and sedimentary structures in these depositional cycles.

Meandering Rivers:

Meandering rivers are characterized by their sinusoidal paths and consist of alternating pool and riffle sections. Sedimentary structures typically associated with meandering rivers include point bars, levees, oxbow lakes, and floodplain deposits. The pattern of occurrence of sediments in meandering rivers is generally fining upward, with coarser sediments deposited in channels and finer sediments deposited on floodplains.

Deltas:

Deltas form at the mouths of rivers where they meet a standing body of water, such as a lake or an ocean. The sedimentary structures commonly associated with deltas include foreset beds, bottomset beds, and topset beds. The deltaic depositional environment shows a coarsening upward sequence, with finer sediments deposited off the delta front (bottomset beds) and coarser sediments deposited on the delta plain (topset beds).

Turbidity Currents:

Turbidity currents are underwater avalanches of sediments that flow down continental slopes due to the pull of gravity, typically following submarine canyons. Sedimentary structures commonly found in turbidite sequences include graded bedding, flute casts, convolute bedding, and flame structures. The pattern of occurrence of sediments in turbidite systems is characterized by Bouma sequences that show fining upward sequences with a sharp base and gradual top.

: Comparison of Meandering Rivers, Deltas, and Turbidity Currents

: In conclusion, meandering rivers, deltas, and turbidity currents show different patterns of occurrence of sediments and sedimentary structures: 1. Meandering rivers generally show a fining upward sequence, with point bars, levees, oxbow lakes, and floodplain deposits as common sedimentary structures. 2. Deltas exhibit a coarsening upward sequence, with foreset beds, bottomset beds, and topset beds as characteristic sedimentary structures. 3. Turbidity currents display Bouma sequences with fining upward patterns, including graded bedding, flute casts, convolute bedding, and flame structures as typical sedimentary structures. By understanding the patterns of occurrence of sediments and sedimentary structures in these three depositional environments, we can better interpret the sedimentary record and infer the depositional history of a given geological setting.

Key concepts

Depositional Cycles

Depositional cycles refer to the repeated sequences in the layers of sediments laid down in sedimentary environments. These cycles result from varying forces like sediment supply, water depth changes, or subsidence rates. Each environment forms its unique pattern, creating different sedimentary structures. Understanding depositional cycles helps us decipher past environments and the dynamics that shaped them.

• In meandering rivers, deposition occurs as the water slows in the outer bends, leaving heavier sediments.
• Deltas evolve as rivers drop sediment loads into standing bodies of water, like seas or lakes.
• Turbidity currents involve sediment-laden flows down oceanic slopes, driven by gravity.

Observing these patterns allows geologists to interpret the geological history of an area.

Sedimentary Structures

Sedimentary structures are the various forms and arrangements of sediments that indicate the conditions under which they were deposited. These structures can be crucial for interpreting past environments and sedimentary processes.
In meandering rivers, you'll often find features like point bars and levees formed by the gradual deposition of sediments. Meanwhile, deltas display beds of sediments such as foreset, bottomset, and topset beds which reflect different parts of the delta's growth and form. Turbidites, on the other hand, are identified by structures like graded bedding, a sequence from coarse to fine sediments.
Different sedimentary structures give clues about the speed, direction, and medium of sediment movement.

Meandering Rivers

Meandering rivers wind in sinuous paths, creating varied environments along their course. As rivers meander, they create distinctive sedimentary structures.

• Point bars form on the inner bends as the river deposits coarser sediments.
• Oxbow lakes can occur when a river creates a new channel, leaving behind a loop.
• Levees form during floods from overbank deposition of sediments.

Over time, sediments show a fining upward sequence: coarser near the river channel, finer on floodplains. This sequence tells the story of changing water flow and deposition over time.

Deltas

Deltas are formed at the mouth of rivers where they meet a body of water, such as a sea or lake. The interaction between the river and the body of water results in depositional features like:

• Topset beds: Coarse materials deposited near the delta front.
• Foreset beds: Sloping layers that show the main body of deposition.
• Bottomset beds: Fine sediments deposited further from the delta in deep waters.

The sequence of these layers typically shows a coarsening upward pattern. Deltas grow outward and are continuously shaped by the sediment load of the river and interaction with ocean tides and waves.

Turbidity Currents

Turbidity currents are underwater flows of sediment-laden water, moving rapidly down continental slopes due to gravity. Often triggered by seismic activities or rapid sediment loading, these currents leave behind distinct sedimentary structures. Key features include:

• Bouma sequences with a characteristic fining upward pattern.
• Graded bedding marks layers of sediment from coarse at the bottom to fine at the top.
• Flute casts and convolute bedding reflect intense underwater sediment movement.

These structures are clues to diagnosing past turbidity currents and deciphering the history of oceanic or deep-sea environments.